Data Sheet - Analog Devices
LT3973/LT3973-3.3/LT3973-5 42V, 750mA Step-Down Regulator with 2.5µA Quiescent Current and Integrated Diodes FEATURES
DESCRIPTION
Ultralow Quiescent Current nn 2.5µA I at 12V to 3.3V Q IN OUT nn Low Ripple Burst Mode® Operation nn Output Ripple < 10mV P-P nn Wide Input Voltage Range: 4.2V to 42V Operating nn Adjustable Switching Frequency: 200kHz to 2.2MHz nn Integrated Boost and Catch Diodes nn 750mA Output Current nn Excellent Start-Up and Dropout Performance nn Fixed Output Voltages: 3.3V, 5V nn 1.9µA I at 12V Q IN nn Accurate Programmable Undervoltage Lockout nn Low Shutdown Current: I = 0.75µA Q nn Internal Catch Diode Current Limit nn Power Good Flag nn Thermal Shutdown nn Small, Thermally Enhanced 10-Lead MSOP and (3mm × 3mm) DFN Packages
The LT®3973 is an adjustable frequency monolithic buck switching regulator that accepts a wide input voltage range up to 42V, and consumes only 2.5µA of quiescent current. A high efficiency switch is included on the die along with the catch diode, boost diode, and the necessary oscillator, control and logic circuitry. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping the output ripple below 10mV in a typical application. A minimum dropout voltage of 530mV is maintained when the input voltage drops below the programmed output voltage, such as during automotive cold crank. Current mode topology is used for fast transient response and good loop stability. A catch diode current limit provides protection against shorted outputs and overvoltage conditions, with thermal shutdown providing additional fault protection. An accurate programmable undervoltage lockout feature is available, producing a low shutdown current of 0.75µA. A power good flag signals when VOUT reaches 90% of the programmed output voltage. The LT3973 is available in small, thermally enhanced 10-lead MSOP and 3mm × 3mm DFN packages.
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APPLICATIONS Automotive Battery Regulation Power for Portable Products nn Industrial Supplies nn Gate Drive Bias nn
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
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TYPICAL APPLICATION
Efficiency 90
C3 0.47µF VIN
BOOST LT3973
OFF ON
C1 4.7µF
EN/UVLO PG RT
215k f = 600kHz
GND
L1 15µH
VOUT 5V 750mA
SW BD OUT
15pF
1M
FB
C2 22µF
1000
VIN = 12V
80
100
70
10
60
1
50
0.1
POWER LOSS (mW)
VIN 5.6V TO 42V
EFFICIENCY (%)
5V Step-Down Converter
316k 3973 TA01a
40 0.01
0.1
1 100 10 LOAD CURRENT (mA) 3973 TA01b
For more information www.linear.com/LT3973
0.01
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LT3973/LT3973-3.3/LT3973-5 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, EN/UVLO Voltage................................................42V BOOST Pin Voltage....................................................55V BOOST Pin Above SW Pin..........................................25V FB/VOUT, RT, PG Voltage..............................................6V BD Voltage.................................................................25V OUT Voltage...............................................................14V
Operating Junction Temperature Range (Note 2) LT3973E/LT3973E-X........................... –40°C to 125°C LT3973I/LT3973I-X............................. –40°C to 125°C LT3973H/LT3973H-X.......................... –40°C to 150°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) MSE Only........................................................... 300°C
PIN CONFIGURATION TOP VIEW *FB/VOUT
1
OUT
2
EN/UVLO
3
VIN
4
GND
5
TOP VIEW
10 RT 11 GND
1 2 3 4 5
*FB/VOUT OUT EN/UVLO VIN GND
9 PG 8 BD 7 BOOST 6 SW
11 GND
10 9 8 7 6
RT PG BD BOOST SW
MSE PACKAGE 10-LEAD PLASTIC MSOP
DD PACKAGE 10-LEAD (3mm × 3mm) PLASTIC DFN
θJA = 40°C/W, θJC = 5°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
θJA = 45°C/W, θJC = 10°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
* FB for LT3973, VOUT for LT3973-3.3, LT3973-5.
ORDER INFORMATION LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3973EDD#PBF
LT3973EDD#TRPBF
LGCH
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973IDD#PBF
LT3973IDD#TRPBF
LGCH
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973HDD#PBF
LT3973HDD#TRPBF
LGCH
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 150°C
LT3973EMSE#PBF
LT3973EMSE#TRPBF
LTFYS
10-Lead Plastic MSOP
–40°C to 125°C
LT3973IMSE#PBF
LT3973IMSE#TRPBF
LTFYS
10-Lead Plastic MSOP
–40°C to 125°C
LT3973HMSE#PBF
LT3973HMSE#TRPBF
LTFYS
10-Lead Plastic MSOP
–40°C to 150°C
LT3973EMSE-3.3#PBF
LT3973EMSE-3.3#TRPBF LTGGB
10-Lead Plastic MSOP
–40°C to 125°C
LT3973IMSE-3.3#PBF
LT3973IMSE-3.3#TRPBF
10-Lead Plastic MSOP
–40°C to 125°C
LT3973HMSE-3.3#PBF
LT3973HMSE-3.3#TRPBF LTGGB
10-Lead Plastic MSOP
–40°C to 150°C
LT3973EDD-3.3#PBF
LT3973EDD-3.3#TRPBF
LGGC
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973IDD-3.3#PBF
LT3973IDD-3.3#TRPBF
LGGC
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973HDD-3.3#PBF
LT3973HDD-3.3#TRPBF
LGGC
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 150°C
LT3973EMSE-5#PBF
LT3973EMSE-5#TRPBF
LTGGD
10-Lead Plastic MSOP
–40°C to 125°C
LT3973IMSE-5#PBF
LT3973IMSE-5#TRPBF
LTGGD
10-Lead Plastic MSOP
–40°C to 125°C
LT3973HMSE-5#PBF
LT3973HMSE-5#TRPBF
LTGGD
10-Lead Plastic MSOP
–40°C to 150°C
2
LTGGB
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LT3973/LT3973-3.3/LT3973-5 ORDER INFORMATION LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3973EDD-5#PBF
LT3973EDD-5#TRPBF
LGGF
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973IDD-5#PBF
LT3973IDD-5#TRPBF
LGGF
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973HDD-5#PBF
LT3973HDD-5#TRPBF
LGGF
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBD = 3.3V unless otherwise noted. (Note 2) PARAMETER
CONDITIONS
Minimum Input Voltage (Note 3) Quiescent Current from VIN
MIN l
VEN/UVLO Low VEN/UVLO High VEN/UVLO High, –40°C to 125°C VEN/UVLO High, –40°C to 150°C
4.2V < VIN < 40V
Switching Frequency
RT = 41.2k, VIN = 6V RT = 158k, VIN = 6V RT = 768k, VIN = 6V
Switch Current Limit
VIN = 5V, VFB = 0V
Catch Schottky Current Limit
VIN = 5V
Switch VCESAT
ISW = 500mA
V
0.75 1.8
1.3 2.8 6 12
µA µA µA µA
1.21 1.21
1.225 1.235
V V
l
3.26 3.234
3.3 3.3
3.34 3.366
V V
l
4.94 4.9
5 5
5.06 5.1
V V
0.1
20
nA
0.0002
0.01
%/V
1.72 632 156
2.15 790 195
2.58 948 234
MHz kHz kHz
l
1.237
1.65
1.98
A
l
0.92
1.15
1.44
A
l
250
Switch Leakage Current
0.05 ISCH = 200mA, VIN = VBD = NC
550
Catch Schottky Reverse Leakage
VSW = 12V
0.05
Boost Schottky Forward Voltage
ISCH = 50mA, VIN = NC, VBOOST = 0V
820
Catch Schottky Forward Voltage
Boost Schottky Reverse Leakage
VREVERSE = 12V
Minimum Boost Voltage (Note 4)
VIN = 5V
BOOST Pin Current
ISW = 500mA, VBOOST = 15V
Dropout Comparator Threshold
(VIN - OUT) Falling, VIN = 5V
Dropout Comparator Hysteresis
UNITS
1.195 1.185
LT3973-5 Output Voltage
FB/Output Voltage Line Regulation
4.2
l
LT3973-3.3 Output Voltage
VFB = 1.21V
MAX
3.8
l l
LT3973 Feedback Voltage
LT3973 FB Pin Bias Current
TYP
l
l
400
mV 2
µA mV
2
µA mV
0.02
2
µA
1.4
1.8
V
10
13
mA
490
580
mV
40
mV
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LT3973/LT3973-3.3/LT3973-5 ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBD = 3.3V unless otherwise noted. (Note 2) EN/UVLO Pin Current
VEN/UVLO = 12V
EN/UVLO Voltage Threshold
EN/UVLO Falling, VIN ≥ 4.2V
l
EN/UVLO Voltage Threshold
EN/UVLO Rising, VIN ≥ 4.2V
l
1
30
nA
1.09
1.16
1.23
V
1.12
1.19
1.28
30
45
mV
10
13.5
%
1
µA
EN/UVLO Voltage Hysteresis PG Threshold Offset from Feedback Voltage
VFB Rising
6.5
PG Hysteresis as % of Output Voltage
0.8
PG Leakage
VPG = 3V
PG Sink Current
VPG = 0.4V
0.01 l
Minimum Switch On-Time Minimum Switch Off-Time (Note 5)
VIN = 10V
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3973E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization, and correlation with statistical process controls. The LT3973I is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT3973H is guaranteed over the full –40°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125°C. The junction temperature (TJ, in °C) is calculated from the ambient temperature (TA in °C) and power dissipation (PD, in Watts) according to the formula: TJ = TA + (PD • θJA) where θJA (in °C/W) is the package thermal impedance.
4
220
V
%
350
µA
70
ns
130
180
ns
Note 3: This is the minimum input voltage for operation with accurate FB reference voltage. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. Note 5: The LT3973 contains circuitry that extends the maximum duty cycle if there is sufficient voltage across the boost capacitor. See the Application Information section for more details. Note 6: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability or permanently damage the device.
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LT3973/LT3973-3.3/LT3973-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Efficiency, VOUT = 3.3V
90
VIN = 12V
80
VIN = 12V
80
60 50
FRONT PAGE APPLICATION VOUT = 3.3V R1 = 1M R2 = 576k
30
0
0.1
0.2 0.3 0.4 0.5 LOAD CURRENT (A)
0.6
VIN = 24V
70
VIN = 36V
60 50
0.7
VIN = 24V
FRONT PAGE APPLICATION 0
0.1
0.2 0.3 0.4 0.5 LOAD CURRENT (A)
0.6
20 0.01
0.7
VIN = 36V
50
FRONT PAGE APPLICATION VOUT = 3.3V R1 = 1M R2 = 576k
30
3973 G01
0.1
1 10 100 LOAD CURRENT (mA)
3973 G02
Efficiency, VOUT = 5V
3973 G03
LT3973 Feedback Voltage
90
LT3973-3.3 Output Voltage
1.220
3.32
1.215
3.31
VIN = 24V
60
VIN = 36V
50
OUTPUT VOLTAGE (V)
70
FEEDBACK VOLTAGE (V)
VIN = 12V
80
1.210
1.205
1.200
40 30 0.01
FRONT PAGE APPLICATION 0.1
1 10 100 LOAD CURRENT (mA)
1.195 –50 –25
LT3973-5 Output Voltage
0
3.27 –50 –25
25 50 75 100 125 150 TEMPERATURE (°C)
5.02 SUPPLY CURRENT (µA)
4.98
4.96
3.0 2.5 2.0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G07
25 50 75 100 125 150 TEMPERATURE (°C)
No-Load Supply Current
FRONT PAGE APPLICATION VOUT = 3.3V R1 = 1M R2 = 576k LT3973-3.3
3.5
5.00
0
3973 G06
No-Load Supply Current 4.0
0
3.29
3973 G05
5.04
4.94 –50 –25
3.30
3.28
3973 G04
OUTPUT VOLTAGE (V)
60
40
40 30
VIN = 12V
70 EFFICIENCY (%)
VIN = 36V
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 24V
40
EFFICIENCY (%)
90
80
70
20
Efficiency, VOUT = 3.3V
Efficiency, VOUT = 5V
1.5
35 30 SUPPLY CURRENT (µA)
90
25
FRONT PAGE APPLICATION VIN = 12V VOUT = 3.3V R1 = 1M R2 = 576k
20 15 10 5
5
10
30 15 25 20 INPUT VOLTAGE (V)
35
40
3973 G08
0 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G09
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LT3973/LT3973-3.3/LT3973-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Maximum Load Current
Maximum Load Current TYPICAL
1.2
LOAD CURRENT (A)
MINIMUM
1.0 0.8 0.6 0.4
LOAD CURRENT (A)
1.2
1.2
MINIMUM
0.9
0.6
0.3 FRONT PAGE APPLICATION VOUT = 3.3V
0.2 5
10
15
20 25 30 35 INPUT VOLTAGE (V)
40
0
45
5
10
15
20 25 30 35 INPUT VOLTAGE (V)
40
3973 G10
Load Regulation
0.4
Switch Current Limit
SWITCH CURRENT LIMIT (A)
LOAD REGULATION (%)
0.6
0.10 0.05 0 –0.05 –0.10
LIMITED BY MAXIMUM JUNCTION TEMPERATURE; θJA = 45°C/W
FRONT PAGE APPLICATION VIN = 12V VOUT = 5V 0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G12
Switch Current Limit
2.0
0.20
–0.15 FRONT PAGE APPLICATION REFERENCED FROM VOUT AT 100mA LOAD –0.20 100 200 300 400 500 600 700 0
2.0
1.8 SWITCH PEAK CURRENT LIMIT
1.6 1.4 1.2
CATCH DIODE VALLEY CURRENT LIMIT 1.0 0.8
0
20
40 60 DUTY CYCLE (%)
LOAD CURRENT (mA) 3973 G13
SWITCH PEAK CURRENT LIMIT
1.6 1.4 1.2
CATCH DIODE VALLEY CURRENT LIMIT
1.0 0.8 –50 –25
100
80
1.8
0
25 50 75 100 125 150 TEMPERATURE (°C)
3973 G14
Switching Frequency
3973 G15
Switch VCESAT (ISW = 500mA) vs Temperature
Minimum Switch On-Time 150
2.4 2.2
350
LOAD CURRENT = 375mA
SWITCH ON-TIME (ns)
1.8 1.6 1.4 1.2 1.0 0.8 0.6
SWITCH VCESAT (mV)
125
2.0 FREQUENCY (MHz)
0.8
0 –50 –25
45
H-GRADE
3973 G11
0.25
0.15
LIMITED BY CURRENT LIMIT
1.0
0.2
FRONT PAGE APPLICATION VOUT = 5V
SWITCH CURRENT LIMIT (A)
LOAD CURRENT (A)
1.4
TYPICAL
1.4
0
Maximum Load Current
1.5
1.6
100 75 50
300
250
25
0.4 0.2 0 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G16
6
0 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G17
200 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G18
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LT3973/LT3973-3.3/LT3973-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
BOOST PIN CURRENT (mA)
SWITCH VCESAT (mV)
500 400 300 200
200
400 600 800 1000 SWITCH CURRENT (mA)
5.0
20
4.5
15 10 5
100
0
25
INPUT VOLTAGE (V)
600
0
Minimum Input Voltage, VOUT = 3.3V
BOOST Pin Current
Switch VCESAT
0
1200
Minimum Input Voltage, VOUT = 5V
200
0
400
600
800 1000 SWITCH CURRENT (mA)
2.5
1200
0
100 200 300 400 500 600 700 LOAD CURRENT (mA) 3973 G21
Start-Up and Dropout Performance 9
FRONT PAGE APPLICATION VOUT = 5V
Minimum Input Voltage to Switch 4.0
FRONT PAGE APPLICATION
8 7
5.0
5
INPUT VOLTAGE (V)
TO START/TO RUN
3.5
VIN
6
VOLTAGE (V)
INPUT VOLTAGE (V)
TO START/TO RUN 3.5
3973 G20
6.0
5.5
4.0
3.0
3973 G19
6.5
FRONT PAGE APPLICATION VOUT = 3.3V
VOUT
4 3
4.5
2
4.0
0
3.0
2.5
1 0
100 200 300 400 500 600 700 LOAD CURRENT (mA)
2.0 –50 –25 TIME
0
25 50 75 100 125 150 TEMPERATURE (°C)
3973 G23
3973 G24
3973 G22
1.4
VFB Regulation Voltage
Boost Diode Forward Voltage
Catch Diode Forward Voltage
1.2
1.0
1.0
0.8
1.0
0.8
CATCH DIODE, VF (V)
BOOST DIODE VF (V)
VFB (V)
1.2 0.8 0.6 0.4 150°C 125°C 25°C –50°C
0.2 0.6 2.0
2.5
3.0 3.5 4.0 INPUT VOLTAGE (V)
4.5
5.0 3973 G25
0
0
50 100 150 BOOST DIODE CURRENT (mA)
200 3973 G26
0.6 0.4 150°C 125°C 25°C –50°C
0.2
0
0
600 200 400 800 1000 CATCH DIODE CURRENT (mA)
1200
3973 G27
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LT3973/LT3973-3.3/LT3973-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Catch Diode Leakage
Power Good Threshold
200 150
100
1.245
91
1.220
90
89
50 0 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C)
88 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C)
3973 G28
1.195
1.170
1.145 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G30
Transient Load Response; Load Current is Stepped from 250mA to 500mA
VOUT 100mV/DIV
VOUT 100mV/DIV
IL 200mA/DIV
IL 200mA/DIV
50µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V
EN/UVLO RISING
3973 G29
Transient Load Response; Load Current is Stepped from 50mA (Burst Mode Operation) to 300mA
50µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V
3973 G31
Switching Waveforms, Burst Mode Operation
3973 G32
Switching Waveforms, Full Frequency Continuous Operation VSW 5V/DIV
VSW 5V/DIV IL 200mA/DIV
IL 200mA/DIV
VOUT 10mV/DIV
VOUT 5mV/DIV 5µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V ILOAD = 15mA f = 600kHz
8
EN/UVLO Threshold
92
THRESHOLD VOLTAGE (V)
VR = 12V
THRESHOLD (%)
CATCH DIODE LEAKAGE (µA)
250
3973 G33
1µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V ILOAD = 750mA f = 600kHz
3973 G34
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LT3973/LT3973-3.3/LT3973-5 PIN FUNCTIONS FB (Pin 1, LT3973 Only): The LT3973 regulates the FB pin to 1.21V. Connect the feedback resistor divider tap to this pin. VOUT (Pin 1, LT3973-3.3 and LT3973-5 Only): The LT3973-3.3 and LT3973-5 regulate the VOUT pin to 3.3V and 5V, respectively. This pin connects to the internal feedback divider that programs the fixed output voltage. OUT (Pin 2): The LT3973 regulates the VIN to VOUT voltage for dropout conditions. It will also pull current from this pin to charge the boost capacitor when needed. Connect this pin to the output. If programmed output is greater than 14V, tie this pin to GND. EN/UVLO (Pin 3): The part is in shutdown when this pin is low and active when this pin is high. The threshold voltage is 1.19V going up with 30mV of hysteresis. Tie to VIN if shutdown feature is not used. The EN/UVLO threshold is accurate only when VIN is above 4.2V. If VIN is lower than 4.2V, ground EN/UVLO to place the part in shutdown.
GND (Pin 5, Exposed Pad Pin 11): Ground. The exposed pad must be soldered to the PCB. SW (Pin 6): The SW pin is the output of an internal power switch. Connect this pin to the inductor. BOOST (Pin 7): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. BD (Pin 8): This pin connects to the anode of the boost diode. This pin also supplies current to the LT3973’s internal regulator when BD is above 3.2V. PG (Pin 9): The PG pin is the open-drain output of an internal comparator. PG remains low until the FB pin is within 10% of the final regulation voltage. PG is valid when VIN is above 4.2V and EN/UVLO is high. RT (Pin 10): A resistor is tied between RT and ground to set the switching frequency.
VIN (Pin 4): The VIN pin supplies current to the LT3973’s internal circuitry and to the internal power switch. This pin must be locally bypassed.
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9
10
C1
VIN
RT
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–
+
GND
–
+
LT3973 ONLY
R2
1.09V
SHDN
FB R1
–
ERROR AMP
+
INTERNAL 1.21V REF
* LT3973-3.3: R1 = 12.72M, R2 = 7.39M LT3973-5: R1 = 15.23M, R2 = 4.88M
PG
RT
EN/UVLO
1.21V
VIN
Burst Mode DETECT
R1
LT3973-3.3 AND LT3973-5 ONLY*
R2
VC
VOUT
OSCILLATOR 200kHz TO 2.2MHz
SLOPE COMP
+
–
S
R Q
SWITCH LATCH
+
– + –
DCATCH
DBOOST
SW
BOOST
BD
OUT
C2
L1
C3
3973 BD
VOUT
LT3973/LT3973-3.3/LT3973-5
BLOCK DIAGRAM
3973fb
LT3973/LT3973-3.3/LT3973-5 OPERATION The LT3973 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, sets an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch-off when this current reaches a level determined by the voltage at VC (see Block Diagram). An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC node. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. Another comparator monitors the current flowing through the catch diode and reduces the operating frequency when the current exceeds the 1.15A bottom current limit. This foldback in frequency helps to control the output current in fault conditions such as a shorted output with high input voltage. Maximum deliverable current to the output is therefore limited by both switch current limit and catch diode current limit. An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the BD pin is connected to an external voltage higher than 3.2V, bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. If the EN/UVLO pin is low, the LT3973 is shut down and draws 0.75µA from the input. When the EN/UVLO pin exceeds 1.19V, the switching regulator will become active. Undervoltage lockout is programmable via this pin.
The switch driver operates from either VIN or from the BOOST pin. An external capacitor is used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. To further optimize efficiency, the LT3973 automatically switches to Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down reducing the input supply current to 1.8µA. If the input voltage decreases towards the programmed output voltage, the LT3973 will start to skip switch-off times and decrease the switching frequency to maintain output regulation up to a maximum duty cycle of approximately 97.5%. When the OUT pin is tied to VOUT, the LT3973 regulates the output such that it stays more than 530mV below VIN; this sets a minimum dropout voltage. This enforced minimum dropout voltage limits the duty cycle and keeps the boost capacitor charged during dropout conditions. Since sufficient boost voltage is maintained, the internal switch can fully saturate yielding good dropout performance. The LT3973 contains a power good comparator which trips when the FB pin is at 90% of its regulated value. The PG output is an open-drain transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT3973 is enabled and VIN is above 4.2V.
3973fb
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11
LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to:
V R1= R2 OUT – 1 1.21
Reference designators refer to the Block Diagram. Note that choosing larger resistors will decrease the quiescent current of the application circuit. Setting the Switching Frequency The LT3973 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.2MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Table 1. Table 1. Switching Frequency vs RT Value SWITCHING FREQUENCY (MHz)
RT VALUE (kΩ)
0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
732 475 340 267 215 150 115 90.9 73.2 61.9 51.1 43.2 36.5
Operating Frequency Trade-Offs Selection of the operating frequency is a trade-off between efficiency, component size, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency, and narrower input voltage range at constant-frequency. The highest acceptable switching frequency (fSW(MAX)) for a given application can be calculated as follows:
fSW(MAX) =
12
VOUT + VD tON(MIN) ( VIN – VSW + VD )
where VIN is the typical input voltage, VOUT is the output voltage, VD is the integrated catch diode drop (~0.7V), and VSW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to accommodate high VIN/VOUT ratio. This is due to the limitation on the LT3973’s minimum on-time. The minimum on-time is a strong function of temperature. Use the minimum switch on-time curve (see Typical Performance Characteristics) to design for an application’s maximum temperature, while adding about 30% for part-to-part variation. The minimum duty cycle that can be achieved taking this on-time into account is: DCMIN = tON(MIN) • fSW where fSW is the switching frequency, and the tON(MIN) is the minimum switch on-time. A good choice of switching frequency should allow adequate input voltage range (see next two sections) and keep the inductor and capacitor values small. Minimum Input Voltage Range The minimum input voltage for regulation is determined by either the LT3973’s minimum operating voltage of 4.2V, its maximum duty cycle, or the enforced minimum dropout voltage. See the typical performance characteristics section for the minimum input voltage across load for outputs of 3.3V and 5V. The duty cycle is the fraction of time that the internal switch is on during a clock cycle. Unlike many fixed frequency regulators, the LT3973 can extend its duty cycle by remaining on for multiple clock cycles. The LT3973 will not switch off at the end of each clock cycle if there is sufficient voltage across the boost capacitor (C3 in the Block Diagram). Eventually, the voltage on the boost capacitor falls and requires refreshing. When this occurs, the switch will turn off, allowing the inductor current to recharge the boost capacitor. This places a limitation on the maximum duty cycle as follows: DCMAX = 1/(1+1/ βSW) where βSW is equal to the SW pin current divided by the BOOST pin current (see the Typical Performance Characteristics section), generally leading to a DCMAX of 3973fb
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LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION about 97.5%. This leads to a minimum input voltage of approximately: VIN(MIN1) =
VOUT + VD – VD + VSW DCMAX
Inductor Selection
where VOUT is the output voltage, VD is the catch diode drop (~0.7V), VSW is the internal switch drop (~0.5V at max load), and DCMAX is the maximum duty cycle. The final factor affecting the minimum input voltage is the minimum dropout voltage. When the OUT pin is tied to VOUT, the LT3973 regulates the output such that it stays more than 530mV below VIN. This enforced minimum dropout voltage is due to reasons that are covered in a later section. This places a limitation on the minimum input voltage as follows: VIN(MIN2) = VOUT + VDROPOUT(MIN) where VOUT is the output voltage and VDROPOUT(MIN) is the minimum dropout voltage (530mV). Combining these factors leads to the overall minimum input voltage: VIN(MIN) = max(VIN(MIN1), VIN(MIN2), 4.2V) Note that the LT3973 will begin switching at a lower input voltage (typically 3V) but will regulate to a lower FB voltage in this region of operation (see the Typical Performance Characteristics section). Maximum Input Voltage Range The highest allowed VIN during normal operation (VIN(OPMAX)) is limited by minimum duty cycle and can be calculated by the following equation: VIN(OP-MAX) =
be reduced below the programmed frequency to prevent damage to the part. The output voltage ripple and inductor current ripple may also be higher than in typical operation, however the output will still be in regulation.
VOUT + VD – VD + VSW fSW • tON(MIN)
where tON(MIN) is the minimum switch on time. However, the circuit will tolerate inputs up to the absolute maximum ratings of the VIN and BOOST pins, regardless of chosen switching frequency. During such transients where VIN is higher than VIN(OP-MAX), the switching frequency will
For a given input and output voltage, the inductor value and switching frequency will determine the ripple current. The ripple current increases with higher VIN or VOUT and decreases with higher inductance and faster switching frequency. A good starting point for selecting the inductor value is:
L = 1.5
VOUT + VD fSW
where VD is the voltage drop of the catch diode (~0.7V), L is in µH and fSW is in MHz. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions (start-up or short circuit) and high input voltage (>30V), the saturation current should be above 1.5A. To keep the efficiency high, the series resistance (DCR) should be less than 0.1Ω, and the core material should be intended for high frequency applications. Table 2 lists several inductor vendors. Table 2. Inductor Vendors VENDOR
URL
Coilcraft
www.coilcraft.com
Sumida
www.sumida.com
Toko
www.tokoam.com
Würth Elektronik
www.we-online.com
Coiltronics
www.cooperet.com
Murata
www.murata.com
This simple design guide will not always result in the optimum inductor selection for a given application. As a general rule, lower output voltages and higher switching frequency will require smaller inductor values. If the application requires less than 750mA load current, then a lesser inductor value may be acceptable. This allows use of a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. There are several graphs in the Typical Performance Characteristics section of this data 3973fb
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13
LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION
Input Capacitor Bypass the input of the LT3973 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 4.7µF ceramic capacitor is adequate to bypass the LT3973 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used (due to longer on-times). If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3973 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 4.7µF capacitor is capable of this task, but only if it is placed close to the LT3973 (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3973. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3973 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3973’s voltage rating. This situation is easily avoided (see the Hot Plugging Safely section). Output Capacitor and Output Ripple The output capacitor has two essential functions. It stores energy in order to satisfy transient loads and stabilize the LT3973’s control loop. Ceramic capacitors have very low
14
equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is:
COUT =
50 VOUT • fSW
where fSW is in MHz and COUT is the recommended output capacitance in μF. Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value capacitor if combined with a phase lead capacitor (typically 15pF) between the output and the feedback pin. A lower value of output capacitor can be used to save space and cost but transient performance will suffer. The second function is that the output capacitor, along with the inductor, filters the square wave generated by the LT3973 to produce the DC output. In this role it determines the output ripple, so low impedance (at the switching frequency) is important. The output ripple decreases with increasing output capacitance, down to approximately 1mV. See Figure 1. Note that a larger phase lead capacitor should be used with a large output capacitor. 16 WORST-CASE OUTPUT RIPPLE (mV)
sheet that show the maximum load current as a function of input voltage for several popular output voltages. Low inductance may result in discontinuous mode operation, which is acceptable but reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See Application Note 19.
FRONT PAGE APPLICATION
14 12 10 8 6
VIN = 24V VIN = 12V
4 2 0
0
20
60 40 COUT (µF)
80
100 3973 F01
Figure 1. Worst-Case Output Ripple Across Full Load Range
When choosing a capacitor, look carefully through the data sheet to find out what the actual capacitance is under operating conditions (applied voltage and temperature). A physically larger capacitor or one with a higher voltage rating may be required. Table 3 lists several capacitor vendors. 3973fb
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LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION MANUFACTURER
WEBSITE
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3973 due to their piezoelectric nature. When in Burst Mode operation, the LT3973’s switching frequency depends on the load current, and at very light loads the LT3973 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3973 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3973. As previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3973 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3973’s rating. This situation is easily avoided (see the Hot Plugging Safely section). Low Ripple Burst Mode Operation To enhance efficiency at light loads, the LT3973 operates in low ripple Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode operation, the LT3973 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor. Because the LT3973 delivers power to the output with single, low current pulses, the output ripple is kept below 10mV for a typical application. See Figure 2. As the load current decreases towards a no load condition, the percentage of time that the LT3973 operates in sleep mode increases and the average input current is
greatly reduced resulting in high efficiency even at very low loads. Note that during Burst Mode operation, the switching frequency will be lower than the programmed switching frequency. See Figure 3. At higher output loads (above 90mA for the front page application) the LT3973 will be running at the frequency programmed by the RT resistor, and will be operating in standard PWM mode. The transition between PWM and low ripple Burst Mode is seamless, and will not disturb the output voltage. VSW 5V/DIV IL 200mA/DIV VOUT 10mV/DIV 3973 F02
5µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V ILOAD = 15mA f = 600kHz
Figure 2. Burst Mode Operation 700
SWITCHING FREQUENCY (kHz)
Table 3. Recommended Ceramic Capacitor Vendors
FRONT PAGE APPLICATION
600 500 400 300 200 100 0
0
100
200 300 400 500 600 LOAD CURRENT (mA)
700 3973 F03
Figure 3. Switching Frequency in Burst Mode Operation
BOOST and BD Pin Considerations Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.47µF capacitor will work well. Figure 4 shows two ways to arrange the boost circuit. The BOOST pin must be more 3973fb
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15
LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION 5.0
FRONT PAGE APPLICATION VOUT = 3.3V
4.5 INPUT VOLTAGE (V)
than 1.9V above the SW pin for best efficiency. For outputs of 2.2V and above, the standard circuit (Figure 4a) is best. For outputs between 2.2V and 2.5V, use a 1µF boost capacitor. For output voltages below 2.2V, the boost diode can be tied to the input (Figure 4b), or to another external supply greater than 2.2V. However, the circuit in Figure 4a is more efficient because the BOOST pin current and BD pin quiescent current come from a lower voltage source. You must also be sure that the maximum voltage ratings of the BOOST and BD pins are not exceeded.
4.0 TO START/TO RUN 3.5
3.0
2.5
VOUT
0
100 200 300 400 500 600 700 LOAD CURRENT (mA)
BD VIN
VIN
BOOST 6.5
C3
LT3973
FRONT PAGE APPLICATION VOUT = 5V
SW GND INPUT VOLTAGE (V)
6.0
(4a) For VOUT ≥ 2.2V
BD VIN
VIN
5.5
TO START/TO RUN
5.0
4.5
BOOST C3
LT3973 SW
VOUT
GND
4.0
0
100 200 300 400 500 600 700 LOAD CURRENT (mA) 3973 F05
3973 F04
(4b) For VOUT < 2.2V; VIN < 25V
Figure 4. Two Circuits for Generating the Boost Voltage
Figure 5. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
Minimum Dropout Voltage The LT3973 monitors the boost capacitor for sufficient voltage such that the switch is allowed to fully saturate. During start-up conditions when the boost capacitor may not be fully charged, the switch will operate with about 1V of drop, and an internal current source will begin to pull 70mA (typical) from the OUT pin which is typically connected to VOUT. This current forces the LT3973 to switch more often and with more inductor current, which recharges the boost capacitor. When the boost capacitor is sufficiently charged, the current source turns off, and the part may enter Burst Mode. See Figure 5 for minimum input voltage for outputs of 3.3V and 5V.
16
When the OUT pin is tied to VOUT, the LT3973 regulates the output such that: VIN – VOUT > VDROPOUT(MIN) where VDROPOUT(MIN) is 530mV. This enforced minimum dropout voltage keeps the boost capacitor charged regardless of load during dropout conditions. The LT3973 achieves this by limiting the duty cycle and forcing the switch to turn off regularly to charge the boost capacitor. Since sufficient voltage across the boost capacitor is maintained, the switch is allowed to fully saturate and the internal switch drop stays low for good dropout performance. Figure 6 shows the overall VIN to VOUT performance during start-up and dropout conditions. 3973fb
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LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION During dropout conditions when the output is below regulation, the output ripple may increase. At very high loads, this ripple can increase to approximately 200mV for the front page application. If lower output ripple is desired during such conditions, a larger output capacitor can be used. In order to not exceed the maximum voltage rating, tie the OUT pin to GND for programmed outputs greater than 14V. Note that this will result in degraded start-up and dropout performance. 9
VUVLO =
R3 + R4 • 1.19V R4
where switching should not start until VIN is above VUVLO. Note that due to the comparator’s hysteresis, switching will not stop until the input falls slightly below VUVLO. Undervoltage lockout is functional only when VUVLO is greater than 5.5V.
VIN
FRONT PAGE APPLICATION
8
VOLTAGE (V)
7 6 5 4
LT3973
VIN R3
1.19V EN/UVLO
VIN
+ –
SHDN
R4 VOUT
3973 F07
Figure 7. Undervoltage Lockout
3 2 1 0
D4 TIME
3973 F06
VIN
Enable and Undervoltage Lockout
Figure 7 shows how to add undervoltage lockout (UVLO) to the LT3973. Typically, UVLO is used in situations where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. UVLO prevents the regulator from operating at source voltages where the problems might occur. The UVLO threshold can be adjusted by setting the values R3 and R4 such that they satisfy the following equation:
BOOST LT3973
Figure 6. VIN to VOUT Performance
The LT3973 is in shutdown when the EN/UVLO pin is low and active when the pin is high. The rising threshold of the EN/UVLO comparator is 1.19V, with a 30mV hysteresis. This threshold is accurate when VIN is above 4.2V. If VIN is lower than 4.2V, tie EN/UVLO pin to GND to place the part in shutdown.
BD VIN
EN/UVLO
SW
GND
FB
VOUT
+
BACKUP
3973 F08
Figure 8. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects the Circuit from a Reversed Input. The LT3973 Runs Only When the Input Is Present
Shorted and Reversed Input Protection If the inductor is chosen so that it won’t saturate excessively, a LT3973 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT3973 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode ORed with the LT3973’s output. If the VIN pin is allowed to float and the EN/UVLO pin is held high (either by a logic signal or because it is tied to VIN), then the LT3973’s internal circuitry will pull its quiescent current through its SW pin. This is fine if the system can tolerate 3973fb
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17
LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION a few µA in this state. If the EN/UVLO pin is grounded, the SW pin current will drop to 0.75µA. However, if the VIN pin is grounded while the output is held high, regardless of EN/ UVLO, parasitic diodes inside the LT3973 can pull current from the output through the SW pin and the VIN pin. Figure 8 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input.
GND GND 1
10
2
9
EN/UVLO
3
8
VIN
4
7
5
6
PG
PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 9 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3973’s VIN and SW pins, the internal catch diode and the input capacitor. The loop formed by these components should be as small as possible. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components. The SW and BOOST nodes should be as small as possible. Finally, keep the FB nodes small so that the ground traces will shield them from the SW and BOOST nodes. The exposed pad on the bottom must be soldered to ground so that the pad acts as a heat sink. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT3973 to additional ground planes within the circuit board and on the bottom side. Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3973 circuits. However, these capacitors can cause problems if the LT3973 is plugged into a live supply. The low loss ceramic capacitor, combined with stray inductance in series with the power source, forms an under damped tank circuit, and the voltage at the VIN pin of the LT3973 can ring to twice the nominal input voltage, possibly exceeding the LT3973’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3973 into an energized supply, the input network should be designed to prevent this overshoot. See Application Note 88 for a complete discussion.
18
VOUT
GND VIAS TO LOCAL GROUND PLANE VIAS TO VOUT
3973 F09
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
High Temperature Considerations For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT3973. The exposed pad on the bottom must be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the LT3973. Placing additional vias can reduce thermal resistance further. The maximum load current should be derated as the ambient temperature approaches the maximum junction rating. Power dissipation within the LT3973 can be estimated by calculating the total power loss from an efficiency measurement and subtracting inductor loss. The die temperature is calculated by multiplying the LT3973 power dissipation by the thermal resistance from junction to ambient. Finally, be aware that at high ambient temperatures the internal Schottky diode will have significant leakage current (see the Typical Performance Characteristics section) increasing the quiescent current of the LT3973 converter. Other Linear Technology Publications Application Notes 19, 35 and 44 contain more detailed descriptions and design information for buck regulators and other switching regulators. The LT1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note 100 shows how to generate a bipolar output supply using a buck regulator. 3973fb
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LT3973/LT3973-3.3/LT3973-5 TYPICAL APPLICATIONS 3.3V Step-Down Converter VIN 4.2V TO 42V
C3 0.47µF VIN
BOOST LT3973
OFF ON
C1 4.7µF
5V Step-Down Converter VIN 5.6V TO 42V
EN/UVLO PG RT
BD OUT
15pF
1M
C2 22µF
FB
OFF ON
C1 4.7µF
RT
BOOST
LT3973-3.3 OFF ON
C1 4.7µF
EN/UVLO PG
GND
215k
VOUT 3.3V 750mA
BD
OFF ON
BOOST
OFF ON
C1 4.7µF
EN/UVLO PG RT
215k
215k
R1 1M
22pF
R2 931k
C2 47µF
OFF ON
C1 4.7µF
215k
C1 4.7µF
EN/ULVO PG RT
215k f = 600kHz
GND
FB
BOOST
GND
C3 0.47µF L1 10µH
22pF
FB
VOUT 1.8V 750mA R1 487k R2 1M
C2 47µF
3973 TA07
5V, 2MHz Step-Down Converter VIN 5.6V TO 28V TRANSIENTS TO 42V
C3 0.47µF L1 22µH
15pF
R1 1M R2 113k
C2 22µF
C3 0.47µF VIN
BOOST LT3973
VOUT 12V 750mA
SW BD OUT
3973 TA05
f = 600kHz
12V Step-Down Converter
OFF ON
C2 22µF
VOUT
EN/UVLO SW BD OUT PG RT
3973 TA06
BOOST
BD
LT3973
VOUT 2.5V 750mA
FB
LT3973
GND
VIN
L1 10µH
f = 600kHz
VIN
VOUT 5V 750mA
SW
VIN 4.2V TO 25V
BD
VIN 12.6V TO 42V
L1 15µH
1.8V Step-Down Converter
SW
GND
C3 0.47µF
f = 600kHz
C3 1µF
OUT
EN/UVLO PG RT
2.5V Step-Down Converter
LT3973
C2 22µF
R2 316k
OUT
C1 4.7µF
C2 22µF
VOUT
BOOST
LT3973-5
3973 TA04
VIN
FB
R1 1M
3973 TA03
VIN
L1 15µH
SW
VIN 4.2V TO 42V
15pF
5V Step-Down Converter
C3 0.47µF
f = 600kHz
BD
VIN 5.6V TO 42V
OUT RT
VOUT 5V 750mA
f = 600kHz
3.3V Step-Down Converter
VIN
L1 15µH
SW
GND
215k
3973 TA02
VIN 4.2V TO 42V
EN/UVLO PG
OUT
576k
f = 600kHz
BOOST LT3973
VOUT 3.3V 750mA
SW
GND
215k
VIN
L1 15µH
C3 0.47µF
OFF ON
C1 2.2µF
3973 TA08
EN/UVLO PG RT
43.2k f = 2MHz
GND
L1 10µH
VOUT 5V 750mA
SW BD OUT FB
10pF
R1 1M R2 316k
C2 10µF
3973 TA09
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19
LT3973/LT3973-3.3/LT3973-5 TYPICAL APPLICATIONS 5V Step-Down Converter with Undervoltage Lockout VIN 6V TO 42V
kΩ
+
0.47µF
–
VIN
3.9M
BOOST LT3973
976k
EN/UVLO PG
4.7µF
RT
15µH
VOUT 5V 750mA
SW BD 15pF
215k
FB GND OUT
22µF 316k 3973 TA10a
f = 600kHz
Input Current During Start-Up
Start-Up from High Impedance Input Source
60
UVLO PROGRAMMED TO 6V
INPUT CURRENT (mA)
50 40
INPUT CURRENT DROPOUT CONDITIONS
30
FRONT PAGE APPLICATION
10 0
20
VIN 5V/DIV VOUT 2V/DIV
FRONT PAGE APPLICATION WITH UVLO PROGRAMMED TO 6V
20
–10
1M
0
2
6 8 4 INPUT VOLTAGE (V)
10
12
5ms/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V 1k INPUT SOURCE RESISTANCE 2.5mA LOAD
3973 TA10c
3973 TA10b
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LT3973/LT3973-3.3/LT3973-5 PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DD Package DD Package Plastic DFN (3mm × 3mm) 10-Lead10-Lead Plastic DFN (3mm × 3mm)
(Reference DWG # 05-08-1699 (Reference LTC DWGLTC # 05-08-1699 Rev C) Rev C)
0.70 ±0.05
3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ±0.05
0.50 BSC 2.38 ±0.05 (2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 ±0.10 (4 SIDES)
R = 0.125 TYP 6
0.40 ±0.10 10
1.65 ±0.10 (2 SIDES)
PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER
PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF
5
0.75 ±0.05
0.00 – 0.05
1
(DD) DFN REV C 0310
0.25 ±0.05 0.50 BSC
2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD
NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
3973fb
For more information www.linear.com/LT3973
21
LT3973/LT3973-3.3/LT3973-5 PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev I)
BOTTOM VIEW OF EXPOSED PAD OPTION
1.88 ±0.102 (.074 ±.004)
5.10 (.201) MIN
1
0.889 ±0.127 (.035 ±.005)
1.68 ±0.102 (.066 ±.004)
0.05 REF
10
0.305 ± 0.038 (.0120 ±.0015) TYP RECOMMENDED SOLDER PAD LAYOUT
3.00 ±0.102 (.118 ±.004) (NOTE 3)
DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY NO MEASUREMENT PURPOSE
10 9 8 7 6
DETAIL “A” 0° – 6° TYP
1 2 3 4 5
GAUGE PLANE
0.53 ±0.152 (.021 ±.006) DETAIL “A”
0.18 (.007)
SEATING PLANE
1.10 (.043) MAX
0.17 – 0.27 (.007 – .011) TYP
0.50 (.0197) NOTE: BSC 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL NOT EXCEED 0.254mm (.010") PER SIDE.
22
0.497 ±0.076 (.0196 ±.003) REF
3.00 ±0.102 (.118 ±.004) (NOTE 4)
4.90 ±0.152 (.193 ±.006) 0.254 (.010)
0.29 REF
1.68 (.066)
3.20 – 3.45 (.126 – .136)
0.50 (.0197) BSC
1.88 (.074)
0.86 (.034) REF
0.1016 ±0.0508 (.004 ±.002) MSOP (MSE) 0213 REV I
3973fb
For more information www.linear.com/LT3973
LT3973/LT3973-3.3/LT3973-5 REVISION HISTORY REV
DATE
DESCRIPTION
A
4/12
Title and Features modified to include fixed output versions.
PAGE NUMBER 1
Absolute Maximum Ratings, Pin Configuration, and Order Information sections modified to include fixed output versions.
2
Electrical Characteristics table modified to include fixed output versions.
3
Graphs modified to include fixed output versions. Pin Functions and Block Diagram modified to include fixed output versions. B
3/15
5 9, 10
Applications for fixed output versions added.
19
Clarified package designator from MS to MSE – All
2
Clarified Electrical Characteristics
3
3973fb
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. For more information www.linear.com/LT3973
23
LT3973/LT3973-3.3/LT3973-5 TYPICAL APPLICATION 1.21V Step-Down Converter VIN 4.2V TO 25V VIN
BOOST LT3973
OFF ON C1 4.7µF
340k
C3 0.47µF L1 10µH
EN/UVLO SW BD FB PG OUT RT GND
f = 400kHz
VOUT 1.21V 750mA C2 47µF
3973 TA10
RELATED PARTS PART NUMBER
DESCRIPTION
COMMENTS
LT3970/LT39703.3/LT3970-5
40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5µA
VIN = 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD < 1µA, 3mm × 2mm DFN-10, MSOP-10
LT3990
62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5µA
VIN = 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD < 1µA, 3mm × 3mm DFN-16, MSOP-16E
LT3971
38V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.8µA
VIN = 4.3V to 38V, VOUT(MIN) = 1.2V, IQ = 2.8µA, ISD < 1µA, 3mm × 3mm DFN-10, MSOPE-10
LT3991
55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.8µA
VIN = 4.3V to 55V, VOUT(MIN) = 1.2V, IQ = 2.8µA, ISD < 1µA, 3mm × 3mm DFN-10, MSOPE-10
LT3682
36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter
VIN = 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD < 1µA, 3mm × 3mm DFN-12
24 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LT3973 (408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT3973
3973fb LT 0315 REV B • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2011
DESCRIPTION
Ultralow Quiescent Current nn 2.5µA I at 12V to 3.3V Q IN OUT nn Low Ripple Burst Mode® Operation nn Output Ripple < 10mV P-P nn Wide Input Voltage Range: 4.2V to 42V Operating nn Adjustable Switching Frequency: 200kHz to 2.2MHz nn Integrated Boost and Catch Diodes nn 750mA Output Current nn Excellent Start-Up and Dropout Performance nn Fixed Output Voltages: 3.3V, 5V nn 1.9µA I at 12V Q IN nn Accurate Programmable Undervoltage Lockout nn Low Shutdown Current: I = 0.75µA Q nn Internal Catch Diode Current Limit nn Power Good Flag nn Thermal Shutdown nn Small, Thermally Enhanced 10-Lead MSOP and (3mm × 3mm) DFN Packages
The LT®3973 is an adjustable frequency monolithic buck switching regulator that accepts a wide input voltage range up to 42V, and consumes only 2.5µA of quiescent current. A high efficiency switch is included on the die along with the catch diode, boost diode, and the necessary oscillator, control and logic circuitry. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping the output ripple below 10mV in a typical application. A minimum dropout voltage of 530mV is maintained when the input voltage drops below the programmed output voltage, such as during automotive cold crank. Current mode topology is used for fast transient response and good loop stability. A catch diode current limit provides protection against shorted outputs and overvoltage conditions, with thermal shutdown providing additional fault protection. An accurate programmable undervoltage lockout feature is available, producing a low shutdown current of 0.75µA. A power good flag signals when VOUT reaches 90% of the programmed output voltage. The LT3973 is available in small, thermally enhanced 10-lead MSOP and 3mm × 3mm DFN packages.
nn
APPLICATIONS Automotive Battery Regulation Power for Portable Products nn Industrial Supplies nn Gate Drive Bias nn
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
nn
TYPICAL APPLICATION
Efficiency 90
C3 0.47µF VIN
BOOST LT3973
OFF ON
C1 4.7µF
EN/UVLO PG RT
215k f = 600kHz
GND
L1 15µH
VOUT 5V 750mA
SW BD OUT
15pF
1M
FB
C2 22µF
1000
VIN = 12V
80
100
70
10
60
1
50
0.1
POWER LOSS (mW)
VIN 5.6V TO 42V
EFFICIENCY (%)
5V Step-Down Converter
316k 3973 TA01a
40 0.01
0.1
1 100 10 LOAD CURRENT (mA) 3973 TA01b
For more information www.linear.com/LT3973
0.01
3973fb
1
LT3973/LT3973-3.3/LT3973-5 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, EN/UVLO Voltage................................................42V BOOST Pin Voltage....................................................55V BOOST Pin Above SW Pin..........................................25V FB/VOUT, RT, PG Voltage..............................................6V BD Voltage.................................................................25V OUT Voltage...............................................................14V
Operating Junction Temperature Range (Note 2) LT3973E/LT3973E-X........................... –40°C to 125°C LT3973I/LT3973I-X............................. –40°C to 125°C LT3973H/LT3973H-X.......................... –40°C to 150°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) MSE Only........................................................... 300°C
PIN CONFIGURATION TOP VIEW *FB/VOUT
1
OUT
2
EN/UVLO
3
VIN
4
GND
5
TOP VIEW
10 RT 11 GND
1 2 3 4 5
*FB/VOUT OUT EN/UVLO VIN GND
9 PG 8 BD 7 BOOST 6 SW
11 GND
10 9 8 7 6
RT PG BD BOOST SW
MSE PACKAGE 10-LEAD PLASTIC MSOP
DD PACKAGE 10-LEAD (3mm × 3mm) PLASTIC DFN
θJA = 40°C/W, θJC = 5°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
θJA = 45°C/W, θJC = 10°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
* FB for LT3973, VOUT for LT3973-3.3, LT3973-5.
ORDER INFORMATION LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3973EDD#PBF
LT3973EDD#TRPBF
LGCH
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973IDD#PBF
LT3973IDD#TRPBF
LGCH
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973HDD#PBF
LT3973HDD#TRPBF
LGCH
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 150°C
LT3973EMSE#PBF
LT3973EMSE#TRPBF
LTFYS
10-Lead Plastic MSOP
–40°C to 125°C
LT3973IMSE#PBF
LT3973IMSE#TRPBF
LTFYS
10-Lead Plastic MSOP
–40°C to 125°C
LT3973HMSE#PBF
LT3973HMSE#TRPBF
LTFYS
10-Lead Plastic MSOP
–40°C to 150°C
LT3973EMSE-3.3#PBF
LT3973EMSE-3.3#TRPBF LTGGB
10-Lead Plastic MSOP
–40°C to 125°C
LT3973IMSE-3.3#PBF
LT3973IMSE-3.3#TRPBF
10-Lead Plastic MSOP
–40°C to 125°C
LT3973HMSE-3.3#PBF
LT3973HMSE-3.3#TRPBF LTGGB
10-Lead Plastic MSOP
–40°C to 150°C
LT3973EDD-3.3#PBF
LT3973EDD-3.3#TRPBF
LGGC
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973IDD-3.3#PBF
LT3973IDD-3.3#TRPBF
LGGC
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973HDD-3.3#PBF
LT3973HDD-3.3#TRPBF
LGGC
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 150°C
LT3973EMSE-5#PBF
LT3973EMSE-5#TRPBF
LTGGD
10-Lead Plastic MSOP
–40°C to 125°C
LT3973IMSE-5#PBF
LT3973IMSE-5#TRPBF
LTGGD
10-Lead Plastic MSOP
–40°C to 125°C
LT3973HMSE-5#PBF
LT3973HMSE-5#TRPBF
LTGGD
10-Lead Plastic MSOP
–40°C to 150°C
2
LTGGB
3973fb
For more information www.linear.com/LT3973
LT3973/LT3973-3.3/LT3973-5 ORDER INFORMATION LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3973EDD-5#PBF
LT3973EDD-5#TRPBF
LGGF
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973IDD-5#PBF
LT3973IDD-5#TRPBF
LGGF
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3973HDD-5#PBF
LT3973HDD-5#TRPBF
LGGF
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBD = 3.3V unless otherwise noted. (Note 2) PARAMETER
CONDITIONS
Minimum Input Voltage (Note 3) Quiescent Current from VIN
MIN l
VEN/UVLO Low VEN/UVLO High VEN/UVLO High, –40°C to 125°C VEN/UVLO High, –40°C to 150°C
4.2V < VIN < 40V
Switching Frequency
RT = 41.2k, VIN = 6V RT = 158k, VIN = 6V RT = 768k, VIN = 6V
Switch Current Limit
VIN = 5V, VFB = 0V
Catch Schottky Current Limit
VIN = 5V
Switch VCESAT
ISW = 500mA
V
0.75 1.8
1.3 2.8 6 12
µA µA µA µA
1.21 1.21
1.225 1.235
V V
l
3.26 3.234
3.3 3.3
3.34 3.366
V V
l
4.94 4.9
5 5
5.06 5.1
V V
0.1
20
nA
0.0002
0.01
%/V
1.72 632 156
2.15 790 195
2.58 948 234
MHz kHz kHz
l
1.237
1.65
1.98
A
l
0.92
1.15
1.44
A
l
250
Switch Leakage Current
0.05 ISCH = 200mA, VIN = VBD = NC
550
Catch Schottky Reverse Leakage
VSW = 12V
0.05
Boost Schottky Forward Voltage
ISCH = 50mA, VIN = NC, VBOOST = 0V
820
Catch Schottky Forward Voltage
Boost Schottky Reverse Leakage
VREVERSE = 12V
Minimum Boost Voltage (Note 4)
VIN = 5V
BOOST Pin Current
ISW = 500mA, VBOOST = 15V
Dropout Comparator Threshold
(VIN - OUT) Falling, VIN = 5V
Dropout Comparator Hysteresis
UNITS
1.195 1.185
LT3973-5 Output Voltage
FB/Output Voltage Line Regulation
4.2
l
LT3973-3.3 Output Voltage
VFB = 1.21V
MAX
3.8
l l
LT3973 Feedback Voltage
LT3973 FB Pin Bias Current
TYP
l
l
400
mV 2
µA mV
2
µA mV
0.02
2
µA
1.4
1.8
V
10
13
mA
490
580
mV
40
mV
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3
LT3973/LT3973-3.3/LT3973-5 ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBD = 3.3V unless otherwise noted. (Note 2) EN/UVLO Pin Current
VEN/UVLO = 12V
EN/UVLO Voltage Threshold
EN/UVLO Falling, VIN ≥ 4.2V
l
EN/UVLO Voltage Threshold
EN/UVLO Rising, VIN ≥ 4.2V
l
1
30
nA
1.09
1.16
1.23
V
1.12
1.19
1.28
30
45
mV
10
13.5
%
1
µA
EN/UVLO Voltage Hysteresis PG Threshold Offset from Feedback Voltage
VFB Rising
6.5
PG Hysteresis as % of Output Voltage
0.8
PG Leakage
VPG = 3V
PG Sink Current
VPG = 0.4V
0.01 l
Minimum Switch On-Time Minimum Switch Off-Time (Note 5)
VIN = 10V
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3973E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization, and correlation with statistical process controls. The LT3973I is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT3973H is guaranteed over the full –40°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125°C. The junction temperature (TJ, in °C) is calculated from the ambient temperature (TA in °C) and power dissipation (PD, in Watts) according to the formula: TJ = TA + (PD • θJA) where θJA (in °C/W) is the package thermal impedance.
4
220
V
%
350
µA
70
ns
130
180
ns
Note 3: This is the minimum input voltage for operation with accurate FB reference voltage. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. Note 5: The LT3973 contains circuitry that extends the maximum duty cycle if there is sufficient voltage across the boost capacitor. See the Application Information section for more details. Note 6: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability or permanently damage the device.
3973fb
For more information www.linear.com/LT3973
LT3973/LT3973-3.3/LT3973-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Efficiency, VOUT = 3.3V
90
VIN = 12V
80
VIN = 12V
80
60 50
FRONT PAGE APPLICATION VOUT = 3.3V R1 = 1M R2 = 576k
30
0
0.1
0.2 0.3 0.4 0.5 LOAD CURRENT (A)
0.6
VIN = 24V
70
VIN = 36V
60 50
0.7
VIN = 24V
FRONT PAGE APPLICATION 0
0.1
0.2 0.3 0.4 0.5 LOAD CURRENT (A)
0.6
20 0.01
0.7
VIN = 36V
50
FRONT PAGE APPLICATION VOUT = 3.3V R1 = 1M R2 = 576k
30
3973 G01
0.1
1 10 100 LOAD CURRENT (mA)
3973 G02
Efficiency, VOUT = 5V
3973 G03
LT3973 Feedback Voltage
90
LT3973-3.3 Output Voltage
1.220
3.32
1.215
3.31
VIN = 24V
60
VIN = 36V
50
OUTPUT VOLTAGE (V)
70
FEEDBACK VOLTAGE (V)
VIN = 12V
80
1.210
1.205
1.200
40 30 0.01
FRONT PAGE APPLICATION 0.1
1 10 100 LOAD CURRENT (mA)
1.195 –50 –25
LT3973-5 Output Voltage
0
3.27 –50 –25
25 50 75 100 125 150 TEMPERATURE (°C)
5.02 SUPPLY CURRENT (µA)
4.98
4.96
3.0 2.5 2.0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G07
25 50 75 100 125 150 TEMPERATURE (°C)
No-Load Supply Current
FRONT PAGE APPLICATION VOUT = 3.3V R1 = 1M R2 = 576k LT3973-3.3
3.5
5.00
0
3973 G06
No-Load Supply Current 4.0
0
3.29
3973 G05
5.04
4.94 –50 –25
3.30
3.28
3973 G04
OUTPUT VOLTAGE (V)
60
40
40 30
VIN = 12V
70 EFFICIENCY (%)
VIN = 36V
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 24V
40
EFFICIENCY (%)
90
80
70
20
Efficiency, VOUT = 3.3V
Efficiency, VOUT = 5V
1.5
35 30 SUPPLY CURRENT (µA)
90
25
FRONT PAGE APPLICATION VIN = 12V VOUT = 3.3V R1 = 1M R2 = 576k
20 15 10 5
5
10
30 15 25 20 INPUT VOLTAGE (V)
35
40
3973 G08
0 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G09
3973fb
For more information www.linear.com/LT3973
5
LT3973/LT3973-3.3/LT3973-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Maximum Load Current
Maximum Load Current TYPICAL
1.2
LOAD CURRENT (A)
MINIMUM
1.0 0.8 0.6 0.4
LOAD CURRENT (A)
1.2
1.2
MINIMUM
0.9
0.6
0.3 FRONT PAGE APPLICATION VOUT = 3.3V
0.2 5
10
15
20 25 30 35 INPUT VOLTAGE (V)
40
0
45
5
10
15
20 25 30 35 INPUT VOLTAGE (V)
40
3973 G10
Load Regulation
0.4
Switch Current Limit
SWITCH CURRENT LIMIT (A)
LOAD REGULATION (%)
0.6
0.10 0.05 0 –0.05 –0.10
LIMITED BY MAXIMUM JUNCTION TEMPERATURE; θJA = 45°C/W
FRONT PAGE APPLICATION VIN = 12V VOUT = 5V 0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G12
Switch Current Limit
2.0
0.20
–0.15 FRONT PAGE APPLICATION REFERENCED FROM VOUT AT 100mA LOAD –0.20 100 200 300 400 500 600 700 0
2.0
1.8 SWITCH PEAK CURRENT LIMIT
1.6 1.4 1.2
CATCH DIODE VALLEY CURRENT LIMIT 1.0 0.8
0
20
40 60 DUTY CYCLE (%)
LOAD CURRENT (mA) 3973 G13
SWITCH PEAK CURRENT LIMIT
1.6 1.4 1.2
CATCH DIODE VALLEY CURRENT LIMIT
1.0 0.8 –50 –25
100
80
1.8
0
25 50 75 100 125 150 TEMPERATURE (°C)
3973 G14
Switching Frequency
3973 G15
Switch VCESAT (ISW = 500mA) vs Temperature
Minimum Switch On-Time 150
2.4 2.2
350
LOAD CURRENT = 375mA
SWITCH ON-TIME (ns)
1.8 1.6 1.4 1.2 1.0 0.8 0.6
SWITCH VCESAT (mV)
125
2.0 FREQUENCY (MHz)
0.8
0 –50 –25
45
H-GRADE
3973 G11
0.25
0.15
LIMITED BY CURRENT LIMIT
1.0
0.2
FRONT PAGE APPLICATION VOUT = 5V
SWITCH CURRENT LIMIT (A)
LOAD CURRENT (A)
1.4
TYPICAL
1.4
0
Maximum Load Current
1.5
1.6
100 75 50
300
250
25
0.4 0.2 0 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G16
6
0 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G17
200 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G18
3973fb
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LT3973/LT3973-3.3/LT3973-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
BOOST PIN CURRENT (mA)
SWITCH VCESAT (mV)
500 400 300 200
200
400 600 800 1000 SWITCH CURRENT (mA)
5.0
20
4.5
15 10 5
100
0
25
INPUT VOLTAGE (V)
600
0
Minimum Input Voltage, VOUT = 3.3V
BOOST Pin Current
Switch VCESAT
0
1200
Minimum Input Voltage, VOUT = 5V
200
0
400
600
800 1000 SWITCH CURRENT (mA)
2.5
1200
0
100 200 300 400 500 600 700 LOAD CURRENT (mA) 3973 G21
Start-Up and Dropout Performance 9
FRONT PAGE APPLICATION VOUT = 5V
Minimum Input Voltage to Switch 4.0
FRONT PAGE APPLICATION
8 7
5.0
5
INPUT VOLTAGE (V)
TO START/TO RUN
3.5
VIN
6
VOLTAGE (V)
INPUT VOLTAGE (V)
TO START/TO RUN 3.5
3973 G20
6.0
5.5
4.0
3.0
3973 G19
6.5
FRONT PAGE APPLICATION VOUT = 3.3V
VOUT
4 3
4.5
2
4.0
0
3.0
2.5
1 0
100 200 300 400 500 600 700 LOAD CURRENT (mA)
2.0 –50 –25 TIME
0
25 50 75 100 125 150 TEMPERATURE (°C)
3973 G23
3973 G24
3973 G22
1.4
VFB Regulation Voltage
Boost Diode Forward Voltage
Catch Diode Forward Voltage
1.2
1.0
1.0
0.8
1.0
0.8
CATCH DIODE, VF (V)
BOOST DIODE VF (V)
VFB (V)
1.2 0.8 0.6 0.4 150°C 125°C 25°C –50°C
0.2 0.6 2.0
2.5
3.0 3.5 4.0 INPUT VOLTAGE (V)
4.5
5.0 3973 G25
0
0
50 100 150 BOOST DIODE CURRENT (mA)
200 3973 G26
0.6 0.4 150°C 125°C 25°C –50°C
0.2
0
0
600 200 400 800 1000 CATCH DIODE CURRENT (mA)
1200
3973 G27
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7
LT3973/LT3973-3.3/LT3973-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Catch Diode Leakage
Power Good Threshold
200 150
100
1.245
91
1.220
90
89
50 0 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C)
88 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C)
3973 G28
1.195
1.170
1.145 –50 –25
0
25 50 75 100 125 150 TEMPERATURE (°C) 3973 G30
Transient Load Response; Load Current is Stepped from 250mA to 500mA
VOUT 100mV/DIV
VOUT 100mV/DIV
IL 200mA/DIV
IL 200mA/DIV
50µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V
EN/UVLO RISING
3973 G29
Transient Load Response; Load Current is Stepped from 50mA (Burst Mode Operation) to 300mA
50µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V
3973 G31
Switching Waveforms, Burst Mode Operation
3973 G32
Switching Waveforms, Full Frequency Continuous Operation VSW 5V/DIV
VSW 5V/DIV IL 200mA/DIV
IL 200mA/DIV
VOUT 10mV/DIV
VOUT 5mV/DIV 5µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V ILOAD = 15mA f = 600kHz
8
EN/UVLO Threshold
92
THRESHOLD VOLTAGE (V)
VR = 12V
THRESHOLD (%)
CATCH DIODE LEAKAGE (µA)
250
3973 G33
1µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V ILOAD = 750mA f = 600kHz
3973 G34
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LT3973/LT3973-3.3/LT3973-5 PIN FUNCTIONS FB (Pin 1, LT3973 Only): The LT3973 regulates the FB pin to 1.21V. Connect the feedback resistor divider tap to this pin. VOUT (Pin 1, LT3973-3.3 and LT3973-5 Only): The LT3973-3.3 and LT3973-5 regulate the VOUT pin to 3.3V and 5V, respectively. This pin connects to the internal feedback divider that programs the fixed output voltage. OUT (Pin 2): The LT3973 regulates the VIN to VOUT voltage for dropout conditions. It will also pull current from this pin to charge the boost capacitor when needed. Connect this pin to the output. If programmed output is greater than 14V, tie this pin to GND. EN/UVLO (Pin 3): The part is in shutdown when this pin is low and active when this pin is high. The threshold voltage is 1.19V going up with 30mV of hysteresis. Tie to VIN if shutdown feature is not used. The EN/UVLO threshold is accurate only when VIN is above 4.2V. If VIN is lower than 4.2V, ground EN/UVLO to place the part in shutdown.
GND (Pin 5, Exposed Pad Pin 11): Ground. The exposed pad must be soldered to the PCB. SW (Pin 6): The SW pin is the output of an internal power switch. Connect this pin to the inductor. BOOST (Pin 7): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. BD (Pin 8): This pin connects to the anode of the boost diode. This pin also supplies current to the LT3973’s internal regulator when BD is above 3.2V. PG (Pin 9): The PG pin is the open-drain output of an internal comparator. PG remains low until the FB pin is within 10% of the final regulation voltage. PG is valid when VIN is above 4.2V and EN/UVLO is high. RT (Pin 10): A resistor is tied between RT and ground to set the switching frequency.
VIN (Pin 4): The VIN pin supplies current to the LT3973’s internal circuitry and to the internal power switch. This pin must be locally bypassed.
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9
10
C1
VIN
RT
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–
+
GND
–
+
LT3973 ONLY
R2
1.09V
SHDN
FB R1
–
ERROR AMP
+
INTERNAL 1.21V REF
* LT3973-3.3: R1 = 12.72M, R2 = 7.39M LT3973-5: R1 = 15.23M, R2 = 4.88M
PG
RT
EN/UVLO
1.21V
VIN
Burst Mode DETECT
R1
LT3973-3.3 AND LT3973-5 ONLY*
R2
VC
VOUT
OSCILLATOR 200kHz TO 2.2MHz
SLOPE COMP
+
–
S
R Q
SWITCH LATCH
+
– + –
DCATCH
DBOOST
SW
BOOST
BD
OUT
C2
L1
C3
3973 BD
VOUT
LT3973/LT3973-3.3/LT3973-5
BLOCK DIAGRAM
3973fb
LT3973/LT3973-3.3/LT3973-5 OPERATION The LT3973 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, sets an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch-off when this current reaches a level determined by the voltage at VC (see Block Diagram). An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC node. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. Another comparator monitors the current flowing through the catch diode and reduces the operating frequency when the current exceeds the 1.15A bottom current limit. This foldback in frequency helps to control the output current in fault conditions such as a shorted output with high input voltage. Maximum deliverable current to the output is therefore limited by both switch current limit and catch diode current limit. An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the BD pin is connected to an external voltage higher than 3.2V, bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. If the EN/UVLO pin is low, the LT3973 is shut down and draws 0.75µA from the input. When the EN/UVLO pin exceeds 1.19V, the switching regulator will become active. Undervoltage lockout is programmable via this pin.
The switch driver operates from either VIN or from the BOOST pin. An external capacitor is used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. To further optimize efficiency, the LT3973 automatically switches to Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down reducing the input supply current to 1.8µA. If the input voltage decreases towards the programmed output voltage, the LT3973 will start to skip switch-off times and decrease the switching frequency to maintain output regulation up to a maximum duty cycle of approximately 97.5%. When the OUT pin is tied to VOUT, the LT3973 regulates the output such that it stays more than 530mV below VIN; this sets a minimum dropout voltage. This enforced minimum dropout voltage limits the duty cycle and keeps the boost capacitor charged during dropout conditions. Since sufficient boost voltage is maintained, the internal switch can fully saturate yielding good dropout performance. The LT3973 contains a power good comparator which trips when the FB pin is at 90% of its regulated value. The PG output is an open-drain transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT3973 is enabled and VIN is above 4.2V.
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11
LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to:
V R1= R2 OUT – 1 1.21
Reference designators refer to the Block Diagram. Note that choosing larger resistors will decrease the quiescent current of the application circuit. Setting the Switching Frequency The LT3973 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.2MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Table 1. Table 1. Switching Frequency vs RT Value SWITCHING FREQUENCY (MHz)
RT VALUE (kΩ)
0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
732 475 340 267 215 150 115 90.9 73.2 61.9 51.1 43.2 36.5
Operating Frequency Trade-Offs Selection of the operating frequency is a trade-off between efficiency, component size, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency, and narrower input voltage range at constant-frequency. The highest acceptable switching frequency (fSW(MAX)) for a given application can be calculated as follows:
fSW(MAX) =
12
VOUT + VD tON(MIN) ( VIN – VSW + VD )
where VIN is the typical input voltage, VOUT is the output voltage, VD is the integrated catch diode drop (~0.7V), and VSW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to accommodate high VIN/VOUT ratio. This is due to the limitation on the LT3973’s minimum on-time. The minimum on-time is a strong function of temperature. Use the minimum switch on-time curve (see Typical Performance Characteristics) to design for an application’s maximum temperature, while adding about 30% for part-to-part variation. The minimum duty cycle that can be achieved taking this on-time into account is: DCMIN = tON(MIN) • fSW where fSW is the switching frequency, and the tON(MIN) is the minimum switch on-time. A good choice of switching frequency should allow adequate input voltage range (see next two sections) and keep the inductor and capacitor values small. Minimum Input Voltage Range The minimum input voltage for regulation is determined by either the LT3973’s minimum operating voltage of 4.2V, its maximum duty cycle, or the enforced minimum dropout voltage. See the typical performance characteristics section for the minimum input voltage across load for outputs of 3.3V and 5V. The duty cycle is the fraction of time that the internal switch is on during a clock cycle. Unlike many fixed frequency regulators, the LT3973 can extend its duty cycle by remaining on for multiple clock cycles. The LT3973 will not switch off at the end of each clock cycle if there is sufficient voltage across the boost capacitor (C3 in the Block Diagram). Eventually, the voltage on the boost capacitor falls and requires refreshing. When this occurs, the switch will turn off, allowing the inductor current to recharge the boost capacitor. This places a limitation on the maximum duty cycle as follows: DCMAX = 1/(1+1/ βSW) where βSW is equal to the SW pin current divided by the BOOST pin current (see the Typical Performance Characteristics section), generally leading to a DCMAX of 3973fb
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LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION about 97.5%. This leads to a minimum input voltage of approximately: VIN(MIN1) =
VOUT + VD – VD + VSW DCMAX
Inductor Selection
where VOUT is the output voltage, VD is the catch diode drop (~0.7V), VSW is the internal switch drop (~0.5V at max load), and DCMAX is the maximum duty cycle. The final factor affecting the minimum input voltage is the minimum dropout voltage. When the OUT pin is tied to VOUT, the LT3973 regulates the output such that it stays more than 530mV below VIN. This enforced minimum dropout voltage is due to reasons that are covered in a later section. This places a limitation on the minimum input voltage as follows: VIN(MIN2) = VOUT + VDROPOUT(MIN) where VOUT is the output voltage and VDROPOUT(MIN) is the minimum dropout voltage (530mV). Combining these factors leads to the overall minimum input voltage: VIN(MIN) = max(VIN(MIN1), VIN(MIN2), 4.2V) Note that the LT3973 will begin switching at a lower input voltage (typically 3V) but will regulate to a lower FB voltage in this region of operation (see the Typical Performance Characteristics section). Maximum Input Voltage Range The highest allowed VIN during normal operation (VIN(OPMAX)) is limited by minimum duty cycle and can be calculated by the following equation: VIN(OP-MAX) =
be reduced below the programmed frequency to prevent damage to the part. The output voltage ripple and inductor current ripple may also be higher than in typical operation, however the output will still be in regulation.
VOUT + VD – VD + VSW fSW • tON(MIN)
where tON(MIN) is the minimum switch on time. However, the circuit will tolerate inputs up to the absolute maximum ratings of the VIN and BOOST pins, regardless of chosen switching frequency. During such transients where VIN is higher than VIN(OP-MAX), the switching frequency will
For a given input and output voltage, the inductor value and switching frequency will determine the ripple current. The ripple current increases with higher VIN or VOUT and decreases with higher inductance and faster switching frequency. A good starting point for selecting the inductor value is:
L = 1.5
VOUT + VD fSW
where VD is the voltage drop of the catch diode (~0.7V), L is in µH and fSW is in MHz. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions (start-up or short circuit) and high input voltage (>30V), the saturation current should be above 1.5A. To keep the efficiency high, the series resistance (DCR) should be less than 0.1Ω, and the core material should be intended for high frequency applications. Table 2 lists several inductor vendors. Table 2. Inductor Vendors VENDOR
URL
Coilcraft
www.coilcraft.com
Sumida
www.sumida.com
Toko
www.tokoam.com
Würth Elektronik
www.we-online.com
Coiltronics
www.cooperet.com
Murata
www.murata.com
This simple design guide will not always result in the optimum inductor selection for a given application. As a general rule, lower output voltages and higher switching frequency will require smaller inductor values. If the application requires less than 750mA load current, then a lesser inductor value may be acceptable. This allows use of a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. There are several graphs in the Typical Performance Characteristics section of this data 3973fb
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13
LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION
Input Capacitor Bypass the input of the LT3973 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 4.7µF ceramic capacitor is adequate to bypass the LT3973 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used (due to longer on-times). If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3973 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 4.7µF capacitor is capable of this task, but only if it is placed close to the LT3973 (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3973. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3973 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3973’s voltage rating. This situation is easily avoided (see the Hot Plugging Safely section). Output Capacitor and Output Ripple The output capacitor has two essential functions. It stores energy in order to satisfy transient loads and stabilize the LT3973’s control loop. Ceramic capacitors have very low
14
equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is:
COUT =
50 VOUT • fSW
where fSW is in MHz and COUT is the recommended output capacitance in μF. Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value capacitor if combined with a phase lead capacitor (typically 15pF) between the output and the feedback pin. A lower value of output capacitor can be used to save space and cost but transient performance will suffer. The second function is that the output capacitor, along with the inductor, filters the square wave generated by the LT3973 to produce the DC output. In this role it determines the output ripple, so low impedance (at the switching frequency) is important. The output ripple decreases with increasing output capacitance, down to approximately 1mV. See Figure 1. Note that a larger phase lead capacitor should be used with a large output capacitor. 16 WORST-CASE OUTPUT RIPPLE (mV)
sheet that show the maximum load current as a function of input voltage for several popular output voltages. Low inductance may result in discontinuous mode operation, which is acceptable but reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See Application Note 19.
FRONT PAGE APPLICATION
14 12 10 8 6
VIN = 24V VIN = 12V
4 2 0
0
20
60 40 COUT (µF)
80
100 3973 F01
Figure 1. Worst-Case Output Ripple Across Full Load Range
When choosing a capacitor, look carefully through the data sheet to find out what the actual capacitance is under operating conditions (applied voltage and temperature). A physically larger capacitor or one with a higher voltage rating may be required. Table 3 lists several capacitor vendors. 3973fb
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LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION MANUFACTURER
WEBSITE
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3973 due to their piezoelectric nature. When in Burst Mode operation, the LT3973’s switching frequency depends on the load current, and at very light loads the LT3973 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3973 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3973. As previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3973 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3973’s rating. This situation is easily avoided (see the Hot Plugging Safely section). Low Ripple Burst Mode Operation To enhance efficiency at light loads, the LT3973 operates in low ripple Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode operation, the LT3973 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor. Because the LT3973 delivers power to the output with single, low current pulses, the output ripple is kept below 10mV for a typical application. See Figure 2. As the load current decreases towards a no load condition, the percentage of time that the LT3973 operates in sleep mode increases and the average input current is
greatly reduced resulting in high efficiency even at very low loads. Note that during Burst Mode operation, the switching frequency will be lower than the programmed switching frequency. See Figure 3. At higher output loads (above 90mA for the front page application) the LT3973 will be running at the frequency programmed by the RT resistor, and will be operating in standard PWM mode. The transition between PWM and low ripple Burst Mode is seamless, and will not disturb the output voltage. VSW 5V/DIV IL 200mA/DIV VOUT 10mV/DIV 3973 F02
5µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V ILOAD = 15mA f = 600kHz
Figure 2. Burst Mode Operation 700
SWITCHING FREQUENCY (kHz)
Table 3. Recommended Ceramic Capacitor Vendors
FRONT PAGE APPLICATION
600 500 400 300 200 100 0
0
100
200 300 400 500 600 LOAD CURRENT (mA)
700 3973 F03
Figure 3. Switching Frequency in Burst Mode Operation
BOOST and BD Pin Considerations Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.47µF capacitor will work well. Figure 4 shows two ways to arrange the boost circuit. The BOOST pin must be more 3973fb
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15
LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION 5.0
FRONT PAGE APPLICATION VOUT = 3.3V
4.5 INPUT VOLTAGE (V)
than 1.9V above the SW pin for best efficiency. For outputs of 2.2V and above, the standard circuit (Figure 4a) is best. For outputs between 2.2V and 2.5V, use a 1µF boost capacitor. For output voltages below 2.2V, the boost diode can be tied to the input (Figure 4b), or to another external supply greater than 2.2V. However, the circuit in Figure 4a is more efficient because the BOOST pin current and BD pin quiescent current come from a lower voltage source. You must also be sure that the maximum voltage ratings of the BOOST and BD pins are not exceeded.
4.0 TO START/TO RUN 3.5
3.0
2.5
VOUT
0
100 200 300 400 500 600 700 LOAD CURRENT (mA)
BD VIN
VIN
BOOST 6.5
C3
LT3973
FRONT PAGE APPLICATION VOUT = 5V
SW GND INPUT VOLTAGE (V)
6.0
(4a) For VOUT ≥ 2.2V
BD VIN
VIN
5.5
TO START/TO RUN
5.0
4.5
BOOST C3
LT3973 SW
VOUT
GND
4.0
0
100 200 300 400 500 600 700 LOAD CURRENT (mA) 3973 F05
3973 F04
(4b) For VOUT < 2.2V; VIN < 25V
Figure 4. Two Circuits for Generating the Boost Voltage
Figure 5. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
Minimum Dropout Voltage The LT3973 monitors the boost capacitor for sufficient voltage such that the switch is allowed to fully saturate. During start-up conditions when the boost capacitor may not be fully charged, the switch will operate with about 1V of drop, and an internal current source will begin to pull 70mA (typical) from the OUT pin which is typically connected to VOUT. This current forces the LT3973 to switch more often and with more inductor current, which recharges the boost capacitor. When the boost capacitor is sufficiently charged, the current source turns off, and the part may enter Burst Mode. See Figure 5 for minimum input voltage for outputs of 3.3V and 5V.
16
When the OUT pin is tied to VOUT, the LT3973 regulates the output such that: VIN – VOUT > VDROPOUT(MIN) where VDROPOUT(MIN) is 530mV. This enforced minimum dropout voltage keeps the boost capacitor charged regardless of load during dropout conditions. The LT3973 achieves this by limiting the duty cycle and forcing the switch to turn off regularly to charge the boost capacitor. Since sufficient voltage across the boost capacitor is maintained, the switch is allowed to fully saturate and the internal switch drop stays low for good dropout performance. Figure 6 shows the overall VIN to VOUT performance during start-up and dropout conditions. 3973fb
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LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION During dropout conditions when the output is below regulation, the output ripple may increase. At very high loads, this ripple can increase to approximately 200mV for the front page application. If lower output ripple is desired during such conditions, a larger output capacitor can be used. In order to not exceed the maximum voltage rating, tie the OUT pin to GND for programmed outputs greater than 14V. Note that this will result in degraded start-up and dropout performance. 9
VUVLO =
R3 + R4 • 1.19V R4
where switching should not start until VIN is above VUVLO. Note that due to the comparator’s hysteresis, switching will not stop until the input falls slightly below VUVLO. Undervoltage lockout is functional only when VUVLO is greater than 5.5V.
VIN
FRONT PAGE APPLICATION
8
VOLTAGE (V)
7 6 5 4
LT3973
VIN R3
1.19V EN/UVLO
VIN
+ –
SHDN
R4 VOUT
3973 F07
Figure 7. Undervoltage Lockout
3 2 1 0
D4 TIME
3973 F06
VIN
Enable and Undervoltage Lockout
Figure 7 shows how to add undervoltage lockout (UVLO) to the LT3973. Typically, UVLO is used in situations where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. UVLO prevents the regulator from operating at source voltages where the problems might occur. The UVLO threshold can be adjusted by setting the values R3 and R4 such that they satisfy the following equation:
BOOST LT3973
Figure 6. VIN to VOUT Performance
The LT3973 is in shutdown when the EN/UVLO pin is low and active when the pin is high. The rising threshold of the EN/UVLO comparator is 1.19V, with a 30mV hysteresis. This threshold is accurate when VIN is above 4.2V. If VIN is lower than 4.2V, tie EN/UVLO pin to GND to place the part in shutdown.
BD VIN
EN/UVLO
SW
GND
FB
VOUT
+
BACKUP
3973 F08
Figure 8. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects the Circuit from a Reversed Input. The LT3973 Runs Only When the Input Is Present
Shorted and Reversed Input Protection If the inductor is chosen so that it won’t saturate excessively, a LT3973 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT3973 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode ORed with the LT3973’s output. If the VIN pin is allowed to float and the EN/UVLO pin is held high (either by a logic signal or because it is tied to VIN), then the LT3973’s internal circuitry will pull its quiescent current through its SW pin. This is fine if the system can tolerate 3973fb
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17
LT3973/LT3973-3.3/LT3973-5 APPLICATIONS INFORMATION a few µA in this state. If the EN/UVLO pin is grounded, the SW pin current will drop to 0.75µA. However, if the VIN pin is grounded while the output is held high, regardless of EN/ UVLO, parasitic diodes inside the LT3973 can pull current from the output through the SW pin and the VIN pin. Figure 8 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input.
GND GND 1
10
2
9
EN/UVLO
3
8
VIN
4
7
5
6
PG
PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 9 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3973’s VIN and SW pins, the internal catch diode and the input capacitor. The loop formed by these components should be as small as possible. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components. The SW and BOOST nodes should be as small as possible. Finally, keep the FB nodes small so that the ground traces will shield them from the SW and BOOST nodes. The exposed pad on the bottom must be soldered to ground so that the pad acts as a heat sink. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT3973 to additional ground planes within the circuit board and on the bottom side. Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3973 circuits. However, these capacitors can cause problems if the LT3973 is plugged into a live supply. The low loss ceramic capacitor, combined with stray inductance in series with the power source, forms an under damped tank circuit, and the voltage at the VIN pin of the LT3973 can ring to twice the nominal input voltage, possibly exceeding the LT3973’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3973 into an energized supply, the input network should be designed to prevent this overshoot. See Application Note 88 for a complete discussion.
18
VOUT
GND VIAS TO LOCAL GROUND PLANE VIAS TO VOUT
3973 F09
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
High Temperature Considerations For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT3973. The exposed pad on the bottom must be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the LT3973. Placing additional vias can reduce thermal resistance further. The maximum load current should be derated as the ambient temperature approaches the maximum junction rating. Power dissipation within the LT3973 can be estimated by calculating the total power loss from an efficiency measurement and subtracting inductor loss. The die temperature is calculated by multiplying the LT3973 power dissipation by the thermal resistance from junction to ambient. Finally, be aware that at high ambient temperatures the internal Schottky diode will have significant leakage current (see the Typical Performance Characteristics section) increasing the quiescent current of the LT3973 converter. Other Linear Technology Publications Application Notes 19, 35 and 44 contain more detailed descriptions and design information for buck regulators and other switching regulators. The LT1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note 100 shows how to generate a bipolar output supply using a buck regulator. 3973fb
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LT3973/LT3973-3.3/LT3973-5 TYPICAL APPLICATIONS 3.3V Step-Down Converter VIN 4.2V TO 42V
C3 0.47µF VIN
BOOST LT3973
OFF ON
C1 4.7µF
5V Step-Down Converter VIN 5.6V TO 42V
EN/UVLO PG RT
BD OUT
15pF
1M
C2 22µF
FB
OFF ON
C1 4.7µF
RT
BOOST
LT3973-3.3 OFF ON
C1 4.7µF
EN/UVLO PG
GND
215k
VOUT 3.3V 750mA
BD
OFF ON
BOOST
OFF ON
C1 4.7µF
EN/UVLO PG RT
215k
215k
R1 1M
22pF
R2 931k
C2 47µF
OFF ON
C1 4.7µF
215k
C1 4.7µF
EN/ULVO PG RT
215k f = 600kHz
GND
FB
BOOST
GND
C3 0.47µF L1 10µH
22pF
FB
VOUT 1.8V 750mA R1 487k R2 1M
C2 47µF
3973 TA07
5V, 2MHz Step-Down Converter VIN 5.6V TO 28V TRANSIENTS TO 42V
C3 0.47µF L1 22µH
15pF
R1 1M R2 113k
C2 22µF
C3 0.47µF VIN
BOOST LT3973
VOUT 12V 750mA
SW BD OUT
3973 TA05
f = 600kHz
12V Step-Down Converter
OFF ON
C2 22µF
VOUT
EN/UVLO SW BD OUT PG RT
3973 TA06
BOOST
BD
LT3973
VOUT 2.5V 750mA
FB
LT3973
GND
VIN
L1 10µH
f = 600kHz
VIN
VOUT 5V 750mA
SW
VIN 4.2V TO 25V
BD
VIN 12.6V TO 42V
L1 15µH
1.8V Step-Down Converter
SW
GND
C3 0.47µF
f = 600kHz
C3 1µF
OUT
EN/UVLO PG RT
2.5V Step-Down Converter
LT3973
C2 22µF
R2 316k
OUT
C1 4.7µF
C2 22µF
VOUT
BOOST
LT3973-5
3973 TA04
VIN
FB
R1 1M
3973 TA03
VIN
L1 15µH
SW
VIN 4.2V TO 42V
15pF
5V Step-Down Converter
C3 0.47µF
f = 600kHz
BD
VIN 5.6V TO 42V
OUT RT
VOUT 5V 750mA
f = 600kHz
3.3V Step-Down Converter
VIN
L1 15µH
SW
GND
215k
3973 TA02
VIN 4.2V TO 42V
EN/UVLO PG
OUT
576k
f = 600kHz
BOOST LT3973
VOUT 3.3V 750mA
SW
GND
215k
VIN
L1 15µH
C3 0.47µF
OFF ON
C1 2.2µF
3973 TA08
EN/UVLO PG RT
43.2k f = 2MHz
GND
L1 10µH
VOUT 5V 750mA
SW BD OUT FB
10pF
R1 1M R2 316k
C2 10µF
3973 TA09
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19
LT3973/LT3973-3.3/LT3973-5 TYPICAL APPLICATIONS 5V Step-Down Converter with Undervoltage Lockout VIN 6V TO 42V
kΩ
+
0.47µF
–
VIN
3.9M
BOOST LT3973
976k
EN/UVLO PG
4.7µF
RT
15µH
VOUT 5V 750mA
SW BD 15pF
215k
FB GND OUT
22µF 316k 3973 TA10a
f = 600kHz
Input Current During Start-Up
Start-Up from High Impedance Input Source
60
UVLO PROGRAMMED TO 6V
INPUT CURRENT (mA)
50 40
INPUT CURRENT DROPOUT CONDITIONS
30
FRONT PAGE APPLICATION
10 0
20
VIN 5V/DIV VOUT 2V/DIV
FRONT PAGE APPLICATION WITH UVLO PROGRAMMED TO 6V
20
–10
1M
0
2
6 8 4 INPUT VOLTAGE (V)
10
12
5ms/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V 1k INPUT SOURCE RESISTANCE 2.5mA LOAD
3973 TA10c
3973 TA10b
3973fb
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LT3973/LT3973-3.3/LT3973-5 PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DD Package DD Package Plastic DFN (3mm × 3mm) 10-Lead10-Lead Plastic DFN (3mm × 3mm)
(Reference DWG # 05-08-1699 (Reference LTC DWGLTC # 05-08-1699 Rev C) Rev C)
0.70 ±0.05
3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ±0.05
0.50 BSC 2.38 ±0.05 (2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 ±0.10 (4 SIDES)
R = 0.125 TYP 6
0.40 ±0.10 10
1.65 ±0.10 (2 SIDES)
PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER
PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF
5
0.75 ±0.05
0.00 – 0.05
1
(DD) DFN REV C 0310
0.25 ±0.05 0.50 BSC
2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD
NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
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21
LT3973/LT3973-3.3/LT3973-5 PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev I)
BOTTOM VIEW OF EXPOSED PAD OPTION
1.88 ±0.102 (.074 ±.004)
5.10 (.201) MIN
1
0.889 ±0.127 (.035 ±.005)
1.68 ±0.102 (.066 ±.004)
0.05 REF
10
0.305 ± 0.038 (.0120 ±.0015) TYP RECOMMENDED SOLDER PAD LAYOUT
3.00 ±0.102 (.118 ±.004) (NOTE 3)
DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY NO MEASUREMENT PURPOSE
10 9 8 7 6
DETAIL “A” 0° – 6° TYP
1 2 3 4 5
GAUGE PLANE
0.53 ±0.152 (.021 ±.006) DETAIL “A”
0.18 (.007)
SEATING PLANE
1.10 (.043) MAX
0.17 – 0.27 (.007 – .011) TYP
0.50 (.0197) NOTE: BSC 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL NOT EXCEED 0.254mm (.010") PER SIDE.
22
0.497 ±0.076 (.0196 ±.003) REF
3.00 ±0.102 (.118 ±.004) (NOTE 4)
4.90 ±0.152 (.193 ±.006) 0.254 (.010)
0.29 REF
1.68 (.066)
3.20 – 3.45 (.126 – .136)
0.50 (.0197) BSC
1.88 (.074)
0.86 (.034) REF
0.1016 ±0.0508 (.004 ±.002) MSOP (MSE) 0213 REV I
3973fb
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LT3973/LT3973-3.3/LT3973-5 REVISION HISTORY REV
DATE
DESCRIPTION
A
4/12
Title and Features modified to include fixed output versions.
PAGE NUMBER 1
Absolute Maximum Ratings, Pin Configuration, and Order Information sections modified to include fixed output versions.
2
Electrical Characteristics table modified to include fixed output versions.
3
Graphs modified to include fixed output versions. Pin Functions and Block Diagram modified to include fixed output versions. B
3/15
5 9, 10
Applications for fixed output versions added.
19
Clarified package designator from MS to MSE – All
2
Clarified Electrical Characteristics
3
3973fb
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. For more information www.linear.com/LT3973
23
LT3973/LT3973-3.3/LT3973-5 TYPICAL APPLICATION 1.21V Step-Down Converter VIN 4.2V TO 25V VIN
BOOST LT3973
OFF ON C1 4.7µF
340k
C3 0.47µF L1 10µH
EN/UVLO SW BD FB PG OUT RT GND
f = 400kHz
VOUT 1.21V 750mA C2 47µF
3973 TA10
RELATED PARTS PART NUMBER
DESCRIPTION
COMMENTS
LT3970/LT39703.3/LT3970-5
40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5µA
VIN = 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD < 1µA, 3mm × 2mm DFN-10, MSOP-10
LT3990
62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5µA
VIN = 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD < 1µA, 3mm × 3mm DFN-16, MSOP-16E
LT3971
38V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.8µA
VIN = 4.3V to 38V, VOUT(MIN) = 1.2V, IQ = 2.8µA, ISD < 1µA, 3mm × 3mm DFN-10, MSOPE-10
LT3991
55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.8µA
VIN = 4.3V to 55V, VOUT(MIN) = 1.2V, IQ = 2.8µA, ISD < 1µA, 3mm × 3mm DFN-10, MSOPE-10
LT3682
36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter
VIN = 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD < 1µA, 3mm × 3mm DFN-12
24 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LT3973 (408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT3973
3973fb LT 0315 REV B • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2011
