2016 3 rd International Conference on Engineering echnology and Application (ICEA 2016) ISBN: 978-1-60595-383-0 A Low Power Bandgap Voltage Reference with Nonlinear Voltage Curvature Compensation Shuowei Jin*, Zhenni Li, Jingjiao Li & Aixia Wang College of Information Science and Engineering, Northeastern University, Shenyang, Liaoning, China ABSRAC: he reference voltage source is the main part both in the analog and mixed-signal integrated circuits. A good reference voltage source can provide a stable voltage for the circuit, so that the system can work stable accurately. his paper describes a low power bandgap voltage reference with second order compensated, which utilizes a Kujik bandgap reference as the first order curvature correction and adding second order non-linear voltage compensation. he presented circuit is implemented in a 0.18μm CMOS technology, and the voltage supply (VDD) is 1.8V. Piecewise curvature compensation is employed to reduce the temperature coefficient of the bandgap reference from 465ppm/ C to 5.5ppm/ C, with a temperature range 45~85 C. he circuit works under the condition of 27 C, the output of the reference voltage is 1.207 V. he total current consumption of the whole bandgap is 11.69μA, the total area of the layout is 0.10132mm 2 and the power supply rejection ratio is -63dB. Keywords: low power; bandgap voltage reference; second order compensated 1 INRODUCION During the last years, the research of mobile device has been widely used, and the growing demand for low power is pushed by the fact that the majority of recently proposed systems are battery-operated [1]. Bandgap reference voltage circuit is necessary for high precision processing systems, such as voltage regulators, analog to digital converters, digital to analog converters [2]. Aggressive technology scaling coupled with scaled power supply units is reduced area on the expense of lower dynamic range. Hence, to achieve a high dynamic range with the reduced supply voltage, the reference voltage needs to be very accurate [3,4]. Since 1971 when Widlar proposed the bandgap voltage reference [5], the references went through much of research and many new methods are proposed in the literature [6]. A first order bandgap voltage reference circuit is inadequate to provide high accuracy voltage, thus several high order curvature compensation techniques have been proposed [7]-[9]. A high order nonlinear compensation reference is achieved by using different-type resistors, with the cost of process dependency in [7]. In [8], a CA *Corresponding author: jinshuowei@ise.neu.edu.cn current is generated to compensate bandgap voltage for accuracy. While in [9] the high order nonlinear reference is canceled utilizing MOS transistor operating in weak inversion region. his paper presents a second order curvature correction voltage reference, which uses a Kujik bandgap voltage reference source circuit as the first order, by adding second-order curvature compensation circuit, in order to implement a bandgap reference voltage source with very low temperature coefficient. his article is based on CMOS tsmc0.18 standard process, starts with a short introduction about the Kujik voltage reference in Section 2, and briefly describes the temperature dependence of the base in Section 3. Second-order curvature compensation bandgap voltage reference source circuit is approached in Section 4, and shows the experimental results and performance comparison with other bandgap voltage references. Finally, Section 5 draws the conclusion. 2 KUJIK BANDGAP VOLAGE REFERENCE Kujik bandgap voltage reference sources can be applied to standard CMOS process, with low power 477
consumption, low temperature coefficient. Figure 1 presents the bandgap voltage reference implementation. 3.1 Nonlinear second order piecewise compensation circuit he traditional bandgap voltage reference compensates a first order temperature coefficient in the whole temperature range, and it utilizes the sum of VBE which has a negative temperature coefficient and VBE which has a Positive temperature coefficient. VBE is not a linear function of temperature, so the bandgap voltage can obtain accurate compensation only near the reference temperature 0, and the greater the error will be caused by the greater the temperature range. If the whole temperature range is divided into several segments and compensated respectively, the temperature coefficient will be reduced effectively. In this paper, the whole temperature range is divided into two parts, and the core compensation schematic is shown in Figure 2. Figure 1. Kujik bandgap reference source. he circuit can be implements in a standard 0.18μm CMOS technology. wo PMOS transistors have the same width to length ratio, and the emitter area of Q2 is 8 times of the emitter area of Q1, noted by n=8. hen we have the following equations: VA VBEQ0 VB VBEQ2 R0IQ2 (1) hen we have R0IQ2 VBEQ0 VBEQ2 VBE V lnn (2) Figure 2. he core compensation schematic. So, the reference voltage is VB, and given by V R I V V V B 0 Q2 BEQ2 BE BEQ2 V ln nv BEQ2 (3) where V=k/q is positive temperature coefficient, k is Boltzmann s constant, is the absolute temperature in Kelvin, q is the electronic charge. VB is the first order bandgap reference voltage Vref. 3 LOW POWER HIGH PRECISION BANDGAP VOLAGE REFERENCE A first order bandgap voltage reference only has a first order linear compensation, and its temperature coefficient is still not small enough. In order to obtain more stable output bandgap voltage reference, this paper presents a nonlinear second order correction technique by using resistances. Figure 3. he schematic of piecewise compensation. VBE is the piecewise function of temperature, can be written as VBE =VBE (R3+R4)+INLR4, the temperature coefficient of R3 and R4 is ignored here. he compensating current is made up of two parts, one is 478
the positive temperature coefficient current IPA, and the other is piecewise compensation current INL. INL is the piecewise function of temperature, it is 0 when the temperature is low and it is IPA-ICA when the temperature is high, where ICA is negative temperature coefficient current. INL can be made by the schematic shown in Figure 3. he current of MP0 is ICA, when the temperature is low we can have ICA>IPA. If MP1 is in the saturation region, then we can have IMp1=ICA and IMp1>IPA. But according to the kirchhoff s current law, the current of MP1 is lower than IPA, MP1 must be in linear region, the VDS of MP1 is small, MP2 and MP3 will cut off. When the temperature is high we can have ICA<IPA, MP1 is in the saturation region, and the current of MP1 is ICA. he VDS of MP1 is big, MP2 and MP3 turn on, and the current is the difference of ICA and IPA, the current of MP3 is INL. In conclusion, the current of MP3 is 0 when the temperature is low and is the difference of ICA and IPA when the temperature is high. 3.2 Nonlinear second order bandgap voltage reference he presented bandgap voltage reference is shown in Figure 4. he emitter area of Q1 is 8 times of the emitter area of Q0 and Q2, the voltage of A and B is the same, and the reference voltage can be changed by adjusting the value of the resistors. base-emitter voltage when the temperature is 0, η is a temperature constant depending on the technology, α is the order of the temperature dependence of the collector current. If α=0, the BJ current is independent of temperature. If α=1, the BJ current is proportional to absolute temperature. From (4), ln is a high order nonlinear term, the first order bandgap voltage reference compensates for term, while the second order implies the additional cancellation of the nonlinear term. In Figure 2, M14, R2, R4 and Q2 make up the second order compensation circuit. he current of Q2 has been compensated by the first order, so it is temperature independence, α=0, the base-emitter voltage is given by VBE2VG VG VBE0 V ln (5) 0 0 he current of Q0 is proportional to absolute temperature, α=1, the base-emitter voltage is VBE0 1 VG VG VBE0 1V ln 0 0 (6) From Figure 2, the voltage of R2 and R4 is the difference between the base-emitter voltage of Q0 and Q2 VNL VBE2VBE0 V ln (7) 0 he current of M12, M13 is given as VG VG VBE0 I IVBE INL IPA R1 R1 0 1 V V V ln ln lnn R1 0 R2 0 R3 (8) Figure 4. Nonlinear second order correction bandgap voltage reference he goal of the first order bandgap voltage reference is to cancel the temperature dependence of a base-emitter voltage (VBE) by summing with a PA voltage. But the relationship between VBE and temperature is not fully liner, we have the following expression: k VBE VBG VBG VBE0 ln (4) 0 q 0 Where VBG is the bandgap voltage, VBE0 is the From (8), the current is temperature-independent, and the summation of the terms which contain is 0. We can have V V V ln R G BE0 n (9) 3 R 2 0 1 V V ln ln (10) R R 2 0 1 0 Combining equations (9) and (10) leads to the following R R V V qr K ln 0 n G BE0 0 2 (11) 1 R2 1 From (11) and (12), the value of R1 and R2 can be set by a proper value of R0. (12) 479
Figure 5. he whole circuit of the bandgap voltage reference. Figure 6. Layout of the presented reference. 3.3 he whole circuit of the bandgap voltage reference he whole circuit of the bandgap voltage reference is shown in Figure 5, which is made up of: bias circuit, OA, bandgap circuit and start-up circuit. he OA is shown in the middle of Figure 5. It is a folded cascode amplifier, for it has high voltage gain and good power supply ripple rejection. Using common source common grid, which has excellent shielding characteristics, can avoid the change of the output node to influence the input MOS. Meanwhile it has a high output impedance, and reduces the impact on current by the output voltage. Due to the 1 / f noise is inversely proportional to the area of the MOS pipe, the input PMOS is large, and it can reduce the power supply ripple rejection. he bias circuit is shown in the left of Figure 5. Appropriate bias voltage and current is provided by the left four MOS, and the bias voltage and current is low pertinence to the power supply. For determining the current, R0 in the source of M0 is set. Meanwhile the source and the substrate of the MOS is connected to eliminate the effect of the body. o prevent the bandgap reference from working on the zero working point, a start-up circuit formed by M1, M2, and M3 is necessary, which is shown in the right of Figure 6. If the bandgap reference works on the zero working point, the current is 0 of the whole circuit, Vref is 0, M3 cuts off. he gate voltage of M2 is Vdd+Vthp, which drives M2 on, the output of OA is then pulled down, the current of the bandgap reference no longer is 0, the circuit works up to the normal working point. Choosing proper size of M1 and M3, the gate voltage of M2 is lower than Vthn, then M2 cuts off, meanwhile the current of M3 is infinitesimally small. he start-up circuit does not affect the bandgap reference on normal working point. he layout of the presented reference is shown in Figure 6, and the total area is 0.10132mm 2. 4 SIMULAION RESULS his second order nonlinear compensated bandgap voltage reference will be fabricated in 0.18μm CMOS technology and the simulation tool that we used is Cadence. he supply voltage is 1.8V. he total current consumption of the whole bandgap is 11.69μA. he simulation result for the first order reference output voltage versus temperature is shown in Figure 7. he maximum output voltage is 1.170073V, the minimum voltage is 1.16927V and the output voltage is 1.16927V when the temperature is 27 C. he temperature coefficient is only 465ppm/ C. he simulation result for the second order nonlinear compensated reference output voltage versus temper- 480
ature is shown in Figure 8. he maximum output voltage is 1.207003V, the minimum voltage is 1.20617V and the temperature coefficient is only 5.5ppm/ C. current consumption and PSRR is good, the temperature coefficient is normal, and the active area is a little big. Figure 7. Simulation results for the first order reference output voltage versus temperature. Figure 9. Simulation result for PSRR. Figure 8. Simulation results for the second order nonlinear compensated reference output voltage versus temperature. he simulation result for PSRR (Power Supply Rejection Ratio) is shown in Figure 9. he PSRR is still -63dB at 1MHz. he simulation results for process corners are shown in Figure 10, and the process corner is the best. able 1 shows the comparison with other results. From the table, the reference voltage is highest, the Figure 10. Simulation results for process corners. 5 CONCLUSION A second order nonlinear compensated bandgap voltage reference is presented. he presented reference achieved a temperature coefficient of 5.5ppm/ C from -45 C to 85 C, and PSRR -63dB at 1MHz. he output value is from 1.20617V to 1.207003V. he total current consumption of the whole bandgap is 11.69μA. he total area of the layout is 0.10132mm 2. able 1. Comparison with other published results. Parameter Ref.[10] Ref.[11] Ref.[12] Presented reference Year reported 2012 2015 2014 2016 Supply voltage(v) 1.8 1.2 3 1.8 Current consumption(µa) 9.74 120 186 10.69 Reference voltage(mv) 500 735 1.158 1207 PSRR -58-30 -70-63 emperature coefficient(ppm/ C) 7.5 4.2 2.36 5.5 Active area(mm 2 ) 0.075 0.063 NA 0.101 echnology (µm CMOS) 0.35 0.13 0.35 0.18 481
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