A novel design of bandgap voltage reference

Voltage reference is a crucial component in chip design, which directly affects the performance of the entire electronic product. High precision is one of the characteristics of the development of integrated circuits today. With the development of integrated circuits according to Moore's Law, people's requirements for circuit indicators are also increasing. Therefore, high-precision and high-performance reference sources are indispensable for integrated circuit chips. This paper designs a high-performance reference circuit with a small temperature coefficient. At the same time, it has low power consumption and high power supply voltage suppression characteristics in the power supply voltage range of 2.3 to 6.5V. It is suitable for various integrated circuit chips with high precision requirements and low power consumption.

1 Basic principle of reference operation

Figure 1 is a typical temperature-independent bandgap reference circuit architecture diagram. Its principle is to use the negative temperature coefficient of the transistor base-emitter voltage △VBE and the positive temperature coefficient of the difference between the base-emitter voltages of the two transistors △VBE to offset each other to generate a reference voltage with a zero temperature coefficient. As shown in Figure 1, Mp1 and Mp2 in the figure are LDMOS tubes, and most of the voltage drop of VDD falls on Mp1 and Mp2, so the circuit can withstand a higher power supply voltage. If the base current of the transistor is ignored, then we have

From equations (1) to (6), we can get

Where, N = IS1/IS2 is the ratio of the emitter area of ​​QN1 and QN2. The temperature coefficient of VBE2 is -1.5 mV/℃, and the temperature coefficient of VT is +0.086 mV/℃, so by selecting the appropriate N value and the ratio of R2/R1, we can get an output voltage with a zero temperature coefficient. In addition, by adjusting the ratio of R4 and R5, we can get the desired reference voltage without changing the adjusted zero temperature coefficient characteristics.

2 Novel bandgap reference circuit
As shown in Figure 2, the proposed reference voltage circuit is shown. The circuit consists of four parts: bias, operational amplifier, reference core and reference startup. The principle of the core circuit is as described above, and the operational amplifier and startup are described in detail below.

The operational amplifier of this circuit is shown in Figure 2. The main function of the operational amplifier is to ensure the accuracy of △VBE. However, the offset of the operational amplifier is a major source of error. Assuming that the offset voltage at the input terminal is VOS, after calculation, we can get

The key issue here is that the offset voltage is amplified by (1+R2/R3) times, which introduces an error in VREF. More importantly, VOS itself changes with temperature, which further increases the temperature coefficient of the output voltage. Therefore, the offset voltage should be minimized. There are many factors that cause offset, such as mismatch between resistors, mismatch between transistors, mismatch between threshold voltages of transistors at the input stage of the operational amplifier, and limited gain of the operational amplifier. Here, it is mainly improved by increasing the gain of the operational amplifier and precise layout design. As shown in Figure 2, the benchmark uses an operational amplifier with a multi-stage differential structure to increase its gain, increase the depth of negative feedback, and reduce offset. However, the increase in the number of operational amplifier stages will increase the power consumption of the circuit. Therefore, the bias current of the operational amplifier is designed to be a small amount that is independent of the power supply, so that it works at the edge of the saturation region, which also makes the circuit have a wider power supply voltage range.
PSR is an AC small signal parameter that characterizes the power supply rejection capability. It is defined as the ratio of the change in input voltage to the change in output reference voltage. In low-frequency conditions, the reference PSR is proportional to the gain of the op amp. Therefore, the greater the loop gain of the op amp, the stronger the suppression of the output VREF to the change in power supply VDD.
The startup part of the circuit consists of M25, M26, M27, M28, M29 and M30. Vb is generated by the bias part. EN is the enable signal, which is low level during normal operation. When EN is low and Vb reaches a certain level, M30 is turned on, and the M30 and M27 branches generate current, which increases the gate potential of M26 and M27. M26 then pulls down the gate potential of M29, and the M28 and M29 branches generate current, so that the reference part starts to work. The width-to-length ratio of M25 is designed to be much larger than the width-to-length ratio of M26, so that the gate potential of M28 is high after the reference works normally, and the M28 and M29 branches are turned off, and the startup part is separated from the reference.

3 Simulation results
The performance indicators of the designed bandgap reference circuit were simulated. Using the HSPICE tool, based on the Hynix 0.5μm CMOS process, the simulation condition is a full typical model at 25℃. From the DC characteristics of the reference in Figure 6, it can be seen that when the power supply voltage changes between 1.5 and 6V, the reference output still maintains good stability; Figure 7 is the temperature characteristic curve of the reference. When the temperature changes from -40 to 100℃, the change of the reference voltage is only 2.2 mV, and the temperature coefficient is 13.7×10-6/℃, showing the characteristics of low temperature drift; Figure 8 is the simulation curve of the reference loop stability, the loop gain of the reference is 110 dB, and the phase margin is 67°; Figure 9 is the simulation waveform of the power supply rejection characteristics of the reference, and the PSR is -117 dB at low frequency. The simulation results all meet the performance indicators.

4 Conclusion
This paper designs a high-precision, low-power bandgap reference circuit using CMOS technology. The operating range of the power supply voltage is 2.3 to 6.5 V. When the temperature changes from -40 to 100 °C, the temperature coefficient of the reference voltage is 13.2 × 10-6 / °C, and the power supply rejection capability at low frequency is -117 dB. The operating current is only 3 μA when the power supply voltage is 3.3 V. The simulation results show that the circuit has good characteristics.