A programmable cmos bandgap voltage reference circuit using current ...
A PROGRAMMABLE CMOS BANDGAP VOLTAGE REFERENCE CIRCUIT USING CURRENT CONVEYOR Qadeer Ahmad Khan', Debashis Dutta2 'Semiconductor Product Sector, Motorola India Limited, Gurgaon, INDIA,E-mail: [email protected] 2Departmentof Electronics, Ministry of Information Technobgy, New Delhi, INDIA,E-mail: [email protected] 2. CIRCUIT DESCRIPTION AND ,4NALYSIS
ABSTRACT This paper presents a new structure of a bandgap circuit which can be programmed to get any desired output voltage. Unlike conventional bandgap circuits, the proposed bandgap circuit uses MOS transistors operating in subthreshold region instead of bipolar transistors to generate PTAT and IPTAT currents. Use of the current conveyor makes the circuit capable of being operated at lower supply voltage. The circuit is capable of generating variable as well as multi voltage references with low dependency on process, voltage and temperature. The proposed architecture was designed in 0 . 1 3 ~CMOS technology with 1.5V power supply and simulations were carried out using SPICE. The total variation in.the output voltage is 0.67% over the temperature range of -40-125 deg C.
VDD
qMb jR
1. INTRODUCTION
Figure 1. The proposed Bandgap Voltage Reference Circuit
Bandgap voltage reference circuits designed so far [l-41 are based on op-amp. A conventional op-amp based bandgap voltage reference circuit generates the output of around 1.2V which makes the circuit unable to work at lower supply voltages. The second-generation current conveyors (CCII) [5-61 introduced by Sedra & Smith have been found as very effective building block for analog applications. Many Op-amp based circuits have been implemented using CCIIs, due to latter's inherent advantage of attaining larger signal bandwidth, greater linearity, capability of operating at lower supplies and low power consumption. In this paper we have proposed a new architecture of a bandgap circuit using CCII, which is capable of operating at lower supply voltages. Unlike the conventional bandgap circuits, the proposed bandgap circuit uses MOS transistors operating in subthreshold region instead of bipolar transistors to generate PTAT and IPTAT currents. The circuit is capable of generating variable as well as multi voltage references with low dependency on process, voltage and temperature. Due to this generic nature of proposed circuit, it can be used for various applications where different reference voltages are required without any change in the circuit and can be programmed digitally.
1t-
-..,[
Figure 2. Schematic of a low vclltage CMOS CCIIFigure. 1 shows the complete schematic of the proposed bandgap circuit, which consists of a CCII-, diode and connected nmos (Ma, Mb), resistors (R., R1, W), pmos (Ml, M2). Figure 2. shows the circuit of a low voltage CCII- derived from CCII+ proposed in [7] by adding a current mirror at the output.
[CECS-2003
0-7803-8163-7/03/$17.00 0 2003 IEEE
8
VDD i
The voltage at input Y of CCII is given by Vy = Vgs(Ma) Ml
According to property of CCII, Vx=Vy Hence Vx = Vgs(Ma) The current coming from the X terminal of CCII is given by: vgr(Mn) Ix = vEs(Mo)+ I d ( M b ) = Ix = -
+
R
R
vgs(Mo)
- vgs(Mb)
RI
(1)
R23
-.f
AV& +-=Iz RI Also Iz = Ix (from property of CCII) *Ix=-
Vgs(Ma)
R
Figure 3. The proposed Bandgap Structure for multi output
Hence from circuit
Id(M1) =Id(M2= Id(M3) = Iz
The values and number of resistances can be chosen according to the voltage levels and number of outputs required.
= Ix
Applying KCL at Vy, we get
Id(M1) =
~
R
3. PROGRAMMING OF THE PROPOSED CIRCUIT
-k Id(Ma)
We know Id(MI) = Ix (from eq.(2) Hence from eqs (1) and (3)
The bandgap output voltage can be programmed by scaling up or down the output current Ibg. This could easily be done either by changing the W/L of M3 in Figure 1. or can be programmed digitally by adding the binary weighted currents using a current DAC as shown in Figure 4 and 5 .
Idma) =Id(Mb) The output voltage Vbg is given by
Vbg=Iz.R2
M O
T
I
Substituting the value of Iz from eq (2), we get
R2
= -&(Ma)
Vfig
R
R2 +AV,
I
I
J-p
RI
Also,
V.
c
since Id(ua)=Id(lMb) so if W/L of h4b is m times of Ma
AV, = VT ln(m)
(8) Figure 4. Schematic for programming of the proposed bandgap reference
then Vbg
R2 R
+ R2
= -V g s ( M o ) -VTln(m)
R1
(9) -1
which is the final expression for the bandgap voltage reference. From above expression, Vgs(Ma) is multiplied with R2/R hence could be scaled down with the resistor ratio and we can achieve a variable bandgap voltage independent of PVTs. In order to take the multiple outputs, the output resistor R2 can be split in number of parts connected in series and multiple outputs can be tapped as shown in Figure 3.
1-1
1-8
Figure 5. A 3-bit current DAC for programming of Bandgap Reference
9
Figure 8. shows the response of the multi output bandgap reference shown in Figure 3. 'The output resistance R2 was split in four equal parts of 14.125K each to get 800mV, 600mV, 400mV and 200mV at outputs Vbgl, Vg2, Vbg3 and Vbg4 respectively.
If Ibg(Max) is the total current flowing into output resistor R2 when all the DAC units are ON, then the change in output voltage Vbg due to 1 LSB is given by:
4.
SIMULATIONRESULTS
The circuit was designed for Vbg=8OOmV in 0 . 1 3 ~ CMOS process at VDD=ISV. The simulations were carried out in SPICE. Following are the values of components used in the simulation: R1=15.4K, R2=56.5K, R=60K and m=5. The maximum variation with temp range from -40-125 DegC is 5mV which is 0.67% of the typical value. The total current consumption in the circuit is 80uA. Figure 6. shows the simulation waveform of Vbg with Temperature variation.
Figure 8. Response of the Multi output Bandgap Reference shown in Figure 3. Figure 9. shows the response of the programmable bandgap reference shown in Figure 4. The circuit was designed for 3-bits to get the 8 level of outputs. Vbg(max) has been set to 800mV, the change in output voltage due to 1 LSB is given by substituting Vbg(max)=800mV and N=3 in the equation no. 10, hence
800 23
Vbg(~s~) = -mV
= 100mV
The bandgap reference can be programmed with three control bits BO, B1 and B2 to get the output from lOOmV to 800mV with the step size of 100mV. Figure 6. Output response of the Proposed Bandgap Reference w.r.t. Temperature
Figure 7. Variation of Vgs And AVgs with Temperature 0 0
20"
4DU
+ L
BO"
nrnelsl
Figure 7. shows the variation of Vgs And AVgs of transistors Ma and Mb with temperature when operating in subthreshold regions.
Figure 9. Simulation waveform of the programmable Bandgap Reference shown in Figure 4. with 3-bit programming
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5. CONCLUSION A new approach for the design of programmable bandgap voltage reference based of current conveyors has been presented. The proposed circuit uses the MOS transistors operating in subthreshold region to generate PTAT and IPTAT currents and is capable of being operated at lower supply voltages. The bandgap reference output can be programmed digitally with the help of control bits to get any desired reference voltage. The proposed circuit was designed in 0 . 1 3 ~CMOS technology with 1.5V power supply and simulations were carried out using SPICE. The total variation in the output voltage is 0.67% over the temperature range of -40-125 deg C. 6. REFERENCES
[I] Bendali, A.; Savaria, Y. “Low-voltage bandgap reference with temperature compensation based on a threshold voltage technique”, IEEE International Symposium on Circuits and Systems, 2002. ISCAS 2002. , Volume: 3 2002 Page(s): 201 -204. [2] Ka Nang Leung; Mok, P.K.T., “A sub-1-V 15ppm//spl deg/C CMOS bandgap voltage reference without requiring low threshold voltage device”, IEEE Journal of Solid-state Circuits, Volume: 37 Issue: 4, April 2002 Page(s): 526 -530. [3] Ceng Jun; Chen Guican, “A CMOS bandgap reference circuit” 4th International Conference on ASIC, 2001,2001, Page(s): 271 -273. [4] Banba, H.; Shiga, H.; Umezawa, A.; Miyaba, T.; Tanzawa, T.; Atsumi, S.; Sakui, K., “A CMOS bandgap reference circuit with sub- 1-V operation”, IEEE Journal of Solid-state Circuits, Volume: 34 Issue: 5, May 1999, Page(s): 670 -674. [5] Wilson,B., “Recent developments in current conveyor and current-mode circuits”, IEE Proceeding, Vol. 137, Pt. G , No.2, April 1990, pp. 63-77. [6] Sedra, A., and Smith, K.C. “A second generation current conveyor and its applications”, IEEE Trans. on Circuit Theory, 1970, CT-17, pp. 132-134.
o
[7] Fem, G.E and Guemnni, N.C.: ‘Low-voltage low-power novel CCII topologies and applications’, The @ International Conference on Electronics, Circuits and Systems, 2001. ZCECS 2001. V01.2, pp. 1095-1098.
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ABSTRACT This paper presents a new structure of a bandgap circuit which can be programmed to get any desired output voltage. Unlike conventional bandgap circuits, the proposed bandgap circuit uses MOS transistors operating in subthreshold region instead of bipolar transistors to generate PTAT and IPTAT currents. Use of the current conveyor makes the circuit capable of being operated at lower supply voltage. The circuit is capable of generating variable as well as multi voltage references with low dependency on process, voltage and temperature. The proposed architecture was designed in 0 . 1 3 ~CMOS technology with 1.5V power supply and simulations were carried out using SPICE. The total variation in.the output voltage is 0.67% over the temperature range of -40-125 deg C.
VDD
qMb jR
1. INTRODUCTION
Figure 1. The proposed Bandgap Voltage Reference Circuit
Bandgap voltage reference circuits designed so far [l-41 are based on op-amp. A conventional op-amp based bandgap voltage reference circuit generates the output of around 1.2V which makes the circuit unable to work at lower supply voltages. The second-generation current conveyors (CCII) [5-61 introduced by Sedra & Smith have been found as very effective building block for analog applications. Many Op-amp based circuits have been implemented using CCIIs, due to latter's inherent advantage of attaining larger signal bandwidth, greater linearity, capability of operating at lower supplies and low power consumption. In this paper we have proposed a new architecture of a bandgap circuit using CCII, which is capable of operating at lower supply voltages. Unlike the conventional bandgap circuits, the proposed bandgap circuit uses MOS transistors operating in subthreshold region instead of bipolar transistors to generate PTAT and IPTAT currents. The circuit is capable of generating variable as well as multi voltage references with low dependency on process, voltage and temperature. Due to this generic nature of proposed circuit, it can be used for various applications where different reference voltages are required without any change in the circuit and can be programmed digitally.
1t-
-..,[
Figure 2. Schematic of a low vclltage CMOS CCIIFigure. 1 shows the complete schematic of the proposed bandgap circuit, which consists of a CCII-, diode and connected nmos (Ma, Mb), resistors (R., R1, W), pmos (Ml, M2). Figure 2. shows the circuit of a low voltage CCII- derived from CCII+ proposed in [7] by adding a current mirror at the output.
[CECS-2003
0-7803-8163-7/03/$17.00 0 2003 IEEE
8
VDD i
The voltage at input Y of CCII is given by Vy = Vgs(Ma) Ml
According to property of CCII, Vx=Vy Hence Vx = Vgs(Ma) The current coming from the X terminal of CCII is given by: vgr(Mn) Ix = vEs(Mo)+ I d ( M b ) = Ix = -
+
R
R
vgs(Mo)
- vgs(Mb)
RI
(1)
R23
-.f
AV& +-=Iz RI Also Iz = Ix (from property of CCII) *Ix=-
Vgs(Ma)
R
Figure 3. The proposed Bandgap Structure for multi output
Hence from circuit
Id(M1) =Id(M2= Id(M3) = Iz
The values and number of resistances can be chosen according to the voltage levels and number of outputs required.
= Ix
Applying KCL at Vy, we get
Id(M1) =
~
R
3. PROGRAMMING OF THE PROPOSED CIRCUIT
-k Id(Ma)
We know Id(MI) = Ix (from eq.(2) Hence from eqs (1) and (3)
The bandgap output voltage can be programmed by scaling up or down the output current Ibg. This could easily be done either by changing the W/L of M3 in Figure 1. or can be programmed digitally by adding the binary weighted currents using a current DAC as shown in Figure 4 and 5 .
Idma) =Id(Mb) The output voltage Vbg is given by
Vbg=Iz.R2
M O
T
I
Substituting the value of Iz from eq (2), we get
R2
= -&(Ma)
Vfig
R
R2 +AV,
I
I
J-p
RI
Also,
V.
c
since Id(ua)=Id(lMb) so if W/L of h4b is m times of Ma
AV, = VT ln(m)
(8) Figure 4. Schematic for programming of the proposed bandgap reference
then Vbg
R2 R
+ R2
= -V g s ( M o ) -VTln(m)
R1
(9) -1
which is the final expression for the bandgap voltage reference. From above expression, Vgs(Ma) is multiplied with R2/R hence could be scaled down with the resistor ratio and we can achieve a variable bandgap voltage independent of PVTs. In order to take the multiple outputs, the output resistor R2 can be split in number of parts connected in series and multiple outputs can be tapped as shown in Figure 3.
1-1
1-8
Figure 5. A 3-bit current DAC for programming of Bandgap Reference
9
Figure 8. shows the response of the multi output bandgap reference shown in Figure 3. 'The output resistance R2 was split in four equal parts of 14.125K each to get 800mV, 600mV, 400mV and 200mV at outputs Vbgl, Vg2, Vbg3 and Vbg4 respectively.
If Ibg(Max) is the total current flowing into output resistor R2 when all the DAC units are ON, then the change in output voltage Vbg due to 1 LSB is given by:
4.
SIMULATIONRESULTS
The circuit was designed for Vbg=8OOmV in 0 . 1 3 ~ CMOS process at VDD=ISV. The simulations were carried out in SPICE. Following are the values of components used in the simulation: R1=15.4K, R2=56.5K, R=60K and m=5. The maximum variation with temp range from -40-125 DegC is 5mV which is 0.67% of the typical value. The total current consumption in the circuit is 80uA. Figure 6. shows the simulation waveform of Vbg with Temperature variation.
Figure 8. Response of the Multi output Bandgap Reference shown in Figure 3. Figure 9. shows the response of the programmable bandgap reference shown in Figure 4. The circuit was designed for 3-bits to get the 8 level of outputs. Vbg(max) has been set to 800mV, the change in output voltage due to 1 LSB is given by substituting Vbg(max)=800mV and N=3 in the equation no. 10, hence
800 23
Vbg(~s~) = -mV
= 100mV
The bandgap reference can be programmed with three control bits BO, B1 and B2 to get the output from lOOmV to 800mV with the step size of 100mV. Figure 6. Output response of the Proposed Bandgap Reference w.r.t. Temperature
Figure 7. Variation of Vgs And AVgs with Temperature 0 0
20"
4DU
+ L
BO"
nrnelsl
Figure 7. shows the variation of Vgs And AVgs of transistors Ma and Mb with temperature when operating in subthreshold regions.
Figure 9. Simulation waveform of the programmable Bandgap Reference shown in Figure 4. with 3-bit programming
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5. CONCLUSION A new approach for the design of programmable bandgap voltage reference based of current conveyors has been presented. The proposed circuit uses the MOS transistors operating in subthreshold region to generate PTAT and IPTAT currents and is capable of being operated at lower supply voltages. The bandgap reference output can be programmed digitally with the help of control bits to get any desired reference voltage. The proposed circuit was designed in 0 . 1 3 ~CMOS technology with 1.5V power supply and simulations were carried out using SPICE. The total variation in the output voltage is 0.67% over the temperature range of -40-125 deg C. 6. REFERENCES
[I] Bendali, A.; Savaria, Y. “Low-voltage bandgap reference with temperature compensation based on a threshold voltage technique”, IEEE International Symposium on Circuits and Systems, 2002. ISCAS 2002. , Volume: 3 2002 Page(s): 201 -204. [2] Ka Nang Leung; Mok, P.K.T., “A sub-1-V 15ppm//spl deg/C CMOS bandgap voltage reference without requiring low threshold voltage device”, IEEE Journal of Solid-state Circuits, Volume: 37 Issue: 4, April 2002 Page(s): 526 -530. [3] Ceng Jun; Chen Guican, “A CMOS bandgap reference circuit” 4th International Conference on ASIC, 2001,2001, Page(s): 271 -273. [4] Banba, H.; Shiga, H.; Umezawa, A.; Miyaba, T.; Tanzawa, T.; Atsumi, S.; Sakui, K., “A CMOS bandgap reference circuit with sub- 1-V operation”, IEEE Journal of Solid-state Circuits, Volume: 34 Issue: 5, May 1999, Page(s): 670 -674. [5] Wilson,B., “Recent developments in current conveyor and current-mode circuits”, IEE Proceeding, Vol. 137, Pt. G , No.2, April 1990, pp. 63-77. [6] Sedra, A., and Smith, K.C. “A second generation current conveyor and its applications”, IEEE Trans. on Circuit Theory, 1970, CT-17, pp. 132-134.
o
[7] Fem, G.E and Guemnni, N.C.: ‘Low-voltage low-power novel CCII topologies and applications’, The @ International Conference on Electronics, Circuits and Systems, 2001. ZCECS 2001. V01.2, pp. 1095-1098.
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