Design of a high-precision low-supply voltage bandgap reference

The reference voltage source whose output does not change with temperature and power supply voltage is widely used in analog and hybrid integrated circuits, especially in high-precision occasions. The reference voltage source is the premise of the entire system design.
Bandgap reference voltage sources have the advantages of low temperature coefficient and high power supply voltage rejection ratio, as well as compatibility with standard CMOS process, so they have become a commonly used reference voltage source implementation method. The literature designed a traditional bandgap reference circuit with temperature compensation, but its power supply voltage and temperature coefficient are too high, and the output voltage is about 1.25 V, which is difficult to meet the low voltage requirements. The literature designed a low power supply voltage bandgap reference circuit, but the output reference voltage is too high. The literature proposed a solution and designed a low-voltage bandgap reference source, but the circuit structure is complicated.
Based on the analysis of several reference sources, this paper uses 0.25 μm CMOS process to design a bandgap reference source with low power supply voltage, low output voltage, high power supply voltage rejection ratio and high precision. Hspice simulation shows that the designed circuit has good performance.

1 Low-voltage bandgap reference circuit proposed by Banda
The principle of the traditional bandgap reference circuit is to add two voltages with opposite temperature coefficients with appropriate weights. The typical value of the reference voltage is about 1.2 V. Banda proposed a current summing bandgap reference circuit, which weights and sums the currents with positive and negative temperature coefficients, and then allows this current to flow through the resistor to generate a reference voltage that is independent of temperature, as shown in Figure 1. The sizes of M1, M2, and M3 tubes are the same, and the values ​​of R1 and R2 are equal. The output reference voltage is approximately

By adjusting the ratio of R2 and R3, the value of the bandgap reference voltage can be controlled to obtain a value lower than the traditional bandgap reference voltage.
The minimum power supply voltage of the bandgap reference circuit is subject to two restrictions
: (1) The magnitude of the output reference voltage limits the power supply voltage

. VSDati is the source-drain voltage of the CMOS tube operating in the saturation region.
(2) The input voltage of the op amp also limits the power supply voltage.
If the operational amplifier uses a PMOS differential input, the minimum power supply voltage

is 2 V. Under the restrictions of these two factors, the power supply voltage of the bandgap reference circuit is generally above 2 V. This paper designs a low power supply voltage, low output voltage, and high-precision bandgap reference circuit from two aspects.

2 Current summing bandgap reference circuit
The circuit shown in Figure 1 solves the problem of generating a low reference voltage source, but the requirements for the power supply voltage are still high. The following improvements are made in this paper: (1) In order to reduce the sensitivity of the output reference voltage to the power supply voltage, a common source and common gate structure is introduced, and its bias current is provided by a bias circuit that is independent of the power supply. (2) No specific amplifier is given in the literature, so an operational amplifier is designed as a supplement. (3) In order to ensure the normal operation of the PNP tube, the voltage at the op amp input terminals Va and Vb in Figure 1 is about 700 mV, which limits the further reduction of the power supply voltage. In this paper, the resistors R1 and R2 are divided into two series of resistors R11, R12 and R21, R22 respectively, and the voltage level of the op amp input terminals Va2 and Vb2 is about 250 mV through resistor voltage division, as shown in Figure 2. The feedback system composed of the operational amplifier and the MOS tube can force Va2=Vb2. The design of resistors R11=R21, R12=R22 can ensure that Va1 and Vb1 are also equal.

The sizes of M21, M22 and M23 tubes are equal, so that the currents of the three branches are equal.

Among them, R2=R21+R22.
The sum of I21 and I22 is mirrored to the output end through the current mirror, and the bandgap reference voltage can be derived.

It can be seen that the temperature coefficient of the output voltage can be made 0 by adjusting the size of resistors R0 and R2, and the bandgap reference voltage below 1.2 V can be obtained by adjusting the size of resistors R3 and R2.

3 Design of low voltage bandgap reference circuit
3.1 Operational amplifier
A two-stage operational amplifier is used, the first stage provides high gain and the second stage provides large swing. As shown in Figure 3. Since the input voltage of the operational amplifier is low, about 250 mV, the first stage operational amplifier uses a PMOS tube as a differential input. The load uses a diode-connected device, and in order to improve the suppression of the power supply voltage, a bias circuit that is independent of the power supply is used. The second stage operational amplifier uses a simple common source stage structure to provide the maximum output swing, and uses a current mirror as an active load to achieve a single-ended output.

The minimum operating voltage of the operational amplifier

is about 0.5 V when the threshold voltage of the PMOS tube is about 0.5 V, the input of the operational amplifier is 0.2-0.3 V, and 0.1 V of the PMOS tube overdrive voltage is reserved. It can be preliminarily inferred that the minimum power supply voltage of the operational amplifier is about 1 V.
3.2 Bias circuit
In order to improve the suppression of the power supply voltage change of the bandgap reference circuit, the bias circuit adopts a bias that is independent of the power supply. Figure 4 is the designed bias circuit. Connecting RS to M9 instead of connecting it between the source and ground of M9 according to the traditional connection method reduces the body effect of M9 to a certain extent. In addition, the influence of the voltage divider of RS on M9 is eliminated, so that the circuit can work at a lower power supply voltage. However, the resistance value of RS should be carefully determined after simulation to ensure that M9 works in the saturation region.

It can be calculated that the reference current (current of RS) is

Among them, the width-to-length ratio of M9 tube is K times that of M8 tube, and the width-to-length ratios of M7 and M6 are equal.
The circuit shown in Figure 4 has two equilibrium points, namely the zero point and the normal working point, and the startup circuit needs to be shown on the left side of Figure 4. Assuming that the bias circuit is in the off state, then M6~M9 are all turned off, point x is high level, M1 is turned off, and M4 is turned off. Therefore, although the gate of M2 is grounded, the leakage current is very small, and the drain-source voltage difference of M2 is very small, so the voltage at point y is high enough to turn on M5, and the voltage at point x gradually decreases, and the circuit starts. Then, M1 is turned on, followed by M3 and M4, and the voltage at point y gradually decreases, and finally M5 is turned off.

3.3 Design of some key parameters
At room temperature, the area ratio of the bipolar transistor is designed to be n=8, and the temperature coefficient of the voltage is calculated


To ensure that the temperature coefficient of formula (8) is 0, then, in order to reduce the input resistance ratio of the operational amplifier R21 and R22 is set to 1.5, to ensure that the output is about 0.9 V, the ratio of the resistors R3 and R2 is, after Hspice simulation, the designed circuit parameters are R21=120 kΩ, R22=80 kΩ, R0=20 kΩ, R3=150 kΩ.

4 Simulation results
Based on the UMC 0.25μm CMOS process, Hspice is used to simulate and verify the designed circuit. Figure 5 shows the output temperature characteristics when the power supply voltage is 1.5 V and the output reference voltage is 900 mV. In the temperature range of -40 to 120℃, the output reference voltage changes by 1.3 mV. The designed circuit has good temperature characteristics.

Figure 6 shows the variation of the output reference voltage with the power supply voltage. When the power supply voltage drops to 1 V, the output reference voltage drops rapidly. When the power supply voltage changes between 1.1 and 3.5 V, the output voltage changes by about 2 mV. The output voltage of the circuit changes little with the power supply voltage.
The main performance parameters of the designed reference voltage source are shown in Table 1.

5 Conclusion
Based on the traditional CMOS bandgap reference source, this paper adopts 0.25μm CMOS process and the principle of summing positive and negative temperature coefficient current to design a 0.9 V bandgap reference voltage source. The power supply voltage can be reduced to 1.1 V, the temperature coefficient is 8.1×10-6/℃, and the output of the reference voltage can be adjusted as needed. The simulation results show the correctness of the design. With the output buffer, the circuit can be integrated into the chip as its built-in reference source.