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Design of a high-performance bandgap voltage reference

Abstract: a bandgap voltage reference suitable for standard CMOS process is designed. A new second-order curvature compensation circuit is used to improve the temperature characteristics of the output voltage; High gain feedback loop is used to improve the power supply voltage suppression ability of the circuit. The results show that the temperature coefficient of the circuit is 3.3 ppm/℃, and the output changes only about 18 V in the power supply voltage range of 2.7 ~ 3.6 v

Keywords: CMOS; Bandgap voltage reference; Curvature compensation; Power rejection ratio

0 introduction

the reference voltage is integrated electricity and can be tested and analyzed according to GB, ISO, DIN, ASTM, JIS and other international standards; These loading racks can be equipped with various fixtures for testing standard samples and an important part of the extended circuit design, especially in high-precision voltage comparators, data acquisition systems, a/D and D/A converters, etc. the change of reference voltage with the fluctuation of temperature and power supply voltage will directly affect the performance of the whole system. Therefore, in high-precision applications, having a reference voltage with low temperature coefficient and high supply voltage suppression is the premise of the whole system design

because the traditional bandgap reference only compensates the first-order temperature of the transistor base emitter voltage, ignoring the influence of the curvature coefficient, the generated reference voltage and temperature still have great coherence, so the output voltage temperature characteristic is generally above 20 ppm/℃, which cannot meet the needs of high accuracy

based on the above requirements, a voltage reference suitable for high-precision applications is designed here. Based on the traditional bandgap reference, a new second-order curvature compensation method is proposed by using the exponential characteristics of MOS transistor current working in the sub threshold region. At the same time, in order to minimize the impact of power supply voltage fluctuations on the reference voltage, in addition to the cascode structure of the mirror phase current source of the bandgap circuit, a high gain feedback loop is added in the design. Here, the circuit principle is described in detail, and the problems that should be paid attention to in layout design are explained. Finally, the post simulation results are given

l circuit design

1.1 traditional bandgap reference analysis

usually, the bandgap reference voltage is obtained by adding PTAT voltage and CTAT voltage. Because the base emitter voltage VBE of the bipolar transistor has a negative temperature coefficient, and the difference between the base emitter voltages of the bipolar transistors with different areas biased at the same current has a positive temperature coefficient, when the two temperature coefficients are the same, add them together to obtain a reference voltage independent of temperature

the traditional bandgap circuit structure is shown in Figure 1, in which the emitter area of Q2 is m times that of Q1 and Q3, and the current flowing through Q1 ~ Q3 is equal. The operational amplifier works in the feedback state, and takes two points a and B as inputs to drive the current sources of Q1 and Q2, so that two points a and B are stabilized at approximately equal voltage

assume that the current flowing through Q1 is j, including:

because the first term in equation (5) has a negative temperature coefficient and the second term has a positive temperature coefficient, adjust the value of m to make the two terms have the same size and opposite direction temperature coefficient, so as to obtain a voltage independent of temperature. Ideally, the output voltage is independent of the power supply

however, the change of the base emitter voltage VBE of the transistor with temperature under the standard process is not purely linear, and because of the non ideality of the device, the output voltage will also be affected by the fluctuation of the power supply voltage. Among them, the change of the curve with temperature mainly depends on the characteristics of VBE itself, collector current and the offset voltage of the operational amplifier in the circuit. The characteristics of VBE itself have the most serious impact on the curvature. Therefore, in order to obtain a high-performance bandgap reference voltage, the curvature of the curve must be corrected. In this design, the high-order temperature characteristics of VBE are compensated, and the supply voltage suppression characteristics of bandgap circuit are optimized by quoting common source common gate and feedback circuit

1.2 High Performance Bandgap reference circuit

the complete circuit of this design is shown in Figure 2. M6 ~ M16 capacitor C and resistor R4 constitute an operational amplifier; M1 ~ M5 provide the required bias current for the amplifier; The basic band gap is composed of M13 ~ M18, Q1 ~ Q3, R1 and R2; M19, M20, R3 constitute a secondary curvature compensation circuit, M21 ~ M28 constitute a feedback amplification feedback circuit to suppress power fluctuations, and m29 ~ M31 complete the starting function of the circuit; Finally, the switching state of the circuit is realized by PWR

according to literature [2], the correction of quadratic curvature can be achieved by the resistance of different temperature coefficients, that is:

because R1 and R3 have different temperature coefficients, the ratio of the two is expanded by Taylor formula, including:

where: K1 is the temperature coefficient of R1, which is a positive value; K3 is the temperature coefficient of R3, which is negative. The greater the difference between the positive and negative temperature coefficients, the better the effect of curvature compensation

when the gate source voltage of the MOS transistor is close to the on voltage, the MOS transistor operates in the sub threshold region. At this time, the current flowing through the tube is exponentially related to the grid source voltage, and the current formula is as follows:

where: n is the sub threshold slope factor (1 N3); Id0 is a process related parameter. It can be seen from Figure 2 that since the current flowing through M19 is equal to that flowing through M15 and M17, there are:

sorted out by formula (4), formula (6) to formula (8):

due to m1/n 1, the difference between the temperature coefficients of R3 and R2 is amplified by the exponential relationship, which has a better compensation effect on the second-order temperature coefficient of vbe3, and this feature only needs one n-type MOS transistor to realize, which saves the occupied area of the resistance compared with the literature [3], It is very suitable for engineering use

1.3 improve the analysis of power suppression circuit and start-up circuit

in principle, the traditional bandgap circuit itself has good power suppression characteristics, and its output voltage is almost independent of the power supply voltage. However, most of the MOS transistors used in engineering at present are submicron devices, which inevitably produce secondary effects (mainly channel length modulation effect and bulk effect), affecting the current I flowing through MOS transistors. Therefore, in order to obtain an accurate reference voltage, additional circuits must be introduced to improve the power supply voltage suppression ability of the circuit

in this design, in addition to the cascode structure, M21 ~ M28 are added to suppress the power fluctuation, as shown in Figure 2. The core circuit voltage of the band gap is provided by v1. When the power supply voltage VDD rises, the V1 level will also rise. At the same time, M21 ~ M24 sense the potential difference between the two input nodes of the operational amplifier and further amplify it, raising the gate potential of M25. At the same time, the current flowing through M25 increases through the increase of M26 mirror phase current, reducing the equivalent output resistance of M25, and finally reducing the V1 level. Obviously, the higher the gain of the amplifier, the better the suppression of power fluctuations

since there are two bias points in the circuit, m29 ~ M31 starting circuit is added to ensure the normal operation of the circuit. When the power supply voltage is connected, the current of each branch may be zero, and the circuit is in an abnormal working state. At this time, the output voltage is also 0. Since the inverter of M30 and M31 groups, such as automobile front-end module, engine periphery, vehicle body, seat frame, battery bracket, power battery pack shell, etc., makes the grid potential of m29 high, m29 will turn on and inject current into the circuit to restore the normal working state of the circuit startup. At this time, the circuit output voltage is high, m29 grid potential becomes 0, and m29 is turned off, so the normal operation of the circuit will not be affected. PWR in the circuit mainly controls the switching state of the circuit. When PWR is connected to the high/low level, the circuit is in the off/on state

2 layout design

the final layout design is shown in Figure 3. In this design, the main problem that needs to be paid attention to in layout design is to ensure the matching and symmetry between devices. The layout of matched devices should be compact and ensure the consistency of the surrounding environment as much as possible, for example, the input differential pairs M8 and M9 of op amp, the same material resistors R1 and R2, etc. Because the offset of operational amplifier has a great impact on the performance of the circuit. The mismatch of resistance will also affect the temperature characteristics of the output voltage. In addition, it is also important to maintain the symmetry between the MOS tubes that make up the current mirror in this design. In order to suppress the channel length modulation effect, in this design, the channel length of MOS tube is twice the minimum length allowed by the process. Finally, to achieve the effect of inhibiting the unique gloss of nylon, a compromise relationship is taken between area and performance, and the area ratio of Q1 and Q2 is set as 8:1

3 post simulation results

the circuit design mainly adopts TSMC CMOS 0.18/M process, uses cadence spectrum for simulation, and uses calibre to complete the parameter extraction of layout

the change of post simulation output voltage with temperature is shown in Figure 4. It can be seen from the figure that within the temperature range of -40 ~ +120 ℃, the voltage changes only 0.39 MV, and the temperature coefficient is about 3.3 ppm/℃. The variation of the reference voltage with the power supply voltage is shown in Figure 5. The power supply voltage varies from 2.7 to 3.3 V, and the output reference voltage changes around 18 v

4 conclusion

a reference voltage source for high-precision applications is designed using 0.18 m standard CMOS process. A new second-order compensation method is used to improve the traditional band gap, and a feedback circuit is added to improve the power supply voltage suppression characteristics of the circuit. The results show that the temperature coefficient of the output voltage is only 3.3 ppm/℃. Within the fluctuation range of the power supply voltage of 2.7 ~ 3.3 V, the output voltage fluctuation is 18 v. moreover, only three devices are used in the second-order compensation part of the circuit, which saves the design area and is very suitable for practical engineering use, and has great practical value

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