Design method of bandgap reference voltage source circuit under low voltage supply

This paper adopts a low voltage bandgap reference structure. It is completed under TSMC 0.13μm CMOS process conditions, including the design of core circuit, operational amplifier, bias and startup circuit, and the circuit is simulated and verified using Cadence Spectre.

The reference voltage is an indispensable parameter in the design of mixed analog and digital circuits, and the bandgap reference voltage source is the most widely used solution to generate this voltage. With the large number of handheld devices in use today, low-power design has become a major trend in circuit design. With the decrease in CMOS process size, the power consumption and area of ​​digital circuits will decrease significantly, but the decrease in power supply voltage poses new challenges to the design of analog circuits. The traditional bandgap reference voltage source structure no longer meets the requirements of power supply voltage, so a new low-voltage design solution has emerged.

1 Working Principle of Traditional Bandgap Voltage Reference

The working principle of the traditional bandgap reference voltage source is to generate a DC voltage with a zero temperature coefficient by offsetting two temperature coefficients. Figure 1 shows the structure of the core part of the traditional bandgap reference voltage source. The area of ​​the bipolar transistor Q2 is n times that of Q1.

Assuming that the gain of the operational amplifier is high enough, the voltage levels at its input terminals are approximately equal when the circuit offset is ignored, then:

VBE1=VBE2+IR1 (1)

Among them, VBE has a negative temperature coefficient and VT has a positive temperature coefficient. In this way, by adjusting n and R2/R1, Vref can get a value of zero temperature coefficient. Generally, at room temperature, there is:

However, under the 0.13μm CMOS process, the supply voltage of the low-voltage MOS tube is around 1.2 V, so the traditional bandgap reference voltage source structure is no longer applicable.

2 Working Principle of Low Power Bandgap Reference Voltage Source

The core idea of ​​the bandgap reference voltage source under low power supply voltage is the same as that of the bandgap reference of traditional structure, which also uses the characteristics of process parameters changing with temperature to generate voltages with positive and negative temperature coefficients, thereby achieving the purpose of zero temperature coefficient. Figure 2 shows the core circuit of the bandgap reference voltage source under low voltage, including the reference voltage generation part and the startup circuit part.

2.1 Bandgap reference circuit

Since the input levels of the amplifier are approximately equal, the following equation can be obtained from the current mirror principle:

Thus, by appropriately selecting the values ​​of R2/R1, R2/R3 and n, a reference level can be obtained at a low power supply voltage.

Based on the design considerations of the layout, n can be selected as 8, which can better achieve the matching of the transistors and reduce the error. The current source uses a common source and common deletion structure, which can improve the accuracy of current copying and reduce the influence of the power supply voltage on Vref, and is beneficial to PSRR to a certain extent.

Although the absolute value of the resistor in the CMOS process will have deviations, the resistor ratio is used here, so the ratio should be as accurate as possible. The specific method is to represent R1, R2, and R3 with unit resistors in parallel and series. When designing the layout, these resistors should be placed together as much as possible, and dummy resistors should be added around them to minimize the impact of process deviation on the resistor ratio.

2.2 Startup Circuit

Before the circuit is turned on, Pup can be set to 0, switch M1 is turned off, the inverter input is at a high level, and switch M2 is not turned on; when the signal Pup is set to 1, switch M1 is turned on, the voltage at the inverter input is pulled down, switch M2 is turned on, the voltage at point P is pulled down, the bandgap reference circuit part starts to work, and M3 is turned on accordingly; thereafter, as M3 starts to work, the current flowing through the resistor Rstup raises the potential at the inverter input, and when it exceeds the reverse voltage of the inverter. The output is low potential, switch M2 is turned off, and the startup circuit ends. The selection of M3 and Rstup is a noteworthy part of the startup circuit. The voltage value obtained by multiplying the current from the M3 mirror image and the resistance value of Rstup must be sufficient to make the inverter output a low voltage and turn off switch M2 before the voltage at point P stabilizes.

3 Simulation Analysis

FIG3 is a curve showing the variation of the reference voltage amplitude with temperature. It can be seen that from -30 to 100°C, Vref fluctuates within 3 mV and the error range is within 5%.

The PSRR simulation results of this design are shown in Figure 4. As can be seen from Figure 4, at low frequencies, its PSRR is about -81 dB.

Figure 5 is the power supply voltage scan simulation result of this design. It can be seen from the figure that when the power supply voltage is between 1 and 1.8 V, the reference circuit can stably output a voltage reference value of about 600 mV.

4 Conclusion

This paper presents a design method for a bandgap reference voltage source circuit when the power supply is low. This circuit improves the traditional bandgap reference circuit so that the output reference voltage can still meet the zero temperature coefficient at 600 mV. This design is based on TSMC 0.13 μm C-MOS process. Through simulation, the results show that the temperature coefficient of the circuit in the range of -30 to 100°C is 12×10-6°C, and the PSRR at low frequency is about -81 dB. In the power supply range of 1 to 1.8 V, the circuit can work normally, and the output voltage is about 600 mV.