A reference voltage source circuit in an engine high temperature difference environment

Engine control chips have been widely used in automobiles and are one of the core parts of automotive electronics. Engine control chips combine a large number of analog and digital circuit modules such as sensor interface circuits, ADCs, and controllers. For analog circuits, both too low and too high temperatures may cause chip failure. The reasons for circuit failure due to extreme temperature are usually: the bias current of the circuit changes too much with temperature, causing the circuit to deviate from the normal working state; the common mode voltage of the circuit output node drifts with the change of temperature, resulting in the next level circuit being unable to bias normally. In analog
circuit design, the bandgap reference voltage/current source is responsible for providing a stable bias reference current and voltage that does not change with temperature to the bias circuit, which is used to provide the circuit with a stable bias current and common mode voltage. Engine control chips are usually installed around the engine, and the extreme temperature of the engine compartment during long-distance driving may be as high as 125°C. In cold areas, the temperature of the engine compartment may be as low as -40°C in a cold car state. Under such a large temperature span, to ensure that the engine control chip can operate normally, the error between the reference current source and the reference voltage source must be controlled within a very small range, which puts higher requirements on the design of the bandgap reference module. The reference circuit must provide a constant output voltage/current signal in the range of -40℃~125℃. Therefore, it should have a lower temperature coefficient and a wider operating temperature range. In addition, since the working conditions of the engine often change due to driving conditions, and due to the voltage fluctuations caused by various electrical switches in the engine compartment, the power supply voltage for the engine control chip usually experiences severe ripple interference. This requires that the bandgap reference source in the chip should have a strong power supply rejection ratio. Based on the above two points, this paper proposes a bandgap reference voltage source circuit for automotive control chips to reduce the risk of chip failure caused by extreme temperature.
1 Design and analysis of bandgap reference voltage source circuit
The core principle of the bandgap reference is to generate a voltage/current with a first-order positive temperature coefficient, and add it to a voltage/current with a first-order negative temperature coefficient at a certain coefficient to achieve the effect of offsetting the temperature coefficient. The base and emitter voltage Vbe of a bipolar transistor can be regarded as a commonly used negative temperature coefficient source. Connecting a transistor in the form of a diode and taking the partial derivative of Vbe, the following conclusions can be obtained:

This paper proposes a second-order temperature compensation method based on resistors with different temperature coefficients. The circuit schematic is shown in Figure 6. Three polysilicon resistors (R6~R8) are added to the first-order circuit shown in Figure 2. The resistors have different temperature coefficients from the diffused resistors in the first-order circuit. The simulation results show that this method can achieve an output voltage error of 0.25 mV within a temperature range of 175℃.

The core circuit of the first-order bandgap reference voltage source is composed of transistors Q1~Q2, resistors R1~R8, transistors M1~M3, and error amplifier A1. Q3 and M6~M8 are used to generate a current IPTAT that changes in direct proportion to the temperature and provide it to the temperature sensor module in the engine control chip. R1, R3, and R4 are diffusion resistors with positive temperature coefficients. R6~R8 are polysilicon resistors with negative temperature coefficients, which play a role of high-order temperature compensation in the circuit. M5, M10, R5, and Q3 form the startup circuit. When the chip is powered on, M5 is turned on. When the circuit enters the normal working state, M5 is automatically cut off.

The first two terms in equation (5) are the same as equation (3), and the third term is a high-order compensation term. Since R6 and R2 have different temperature coefficients, R6/R2 is at least a first-order function of temperature. Since VT itself is a first-order function of temperature, the third term is at least a second-order function of temperature. By reasonably selecting the value of R6, the high-order temperature coefficient of Vbe can be offset to a large extent.
The output voltage variation curve after compensation by the method of this paper is shown in Figure 7. It can be seen from the figure that in the process of changing from -50℃ to 125℃, the output voltage changes by only 0.25 mV at most, achieving a significant compensation effect. In addition, due to the use of a current-type bandgap reference source structure, R1 and R3 are connected in parallel with the branch where the bipolar device is located, reducing the equivalent resistance of the branch, thereby weakening the influence of power supply voltage fluctuations on the node voltage and improving the power supply rejection ratio. Figure 8 shows the schematic diagram of the error amplifier A1, which uses a folded common source amplifier structure. The input transconductance stage is a bipolar NPN tube, which can reduce the influence of amplifier offset and noise. In addition, it should be noted that in the reference circuit with an error amplifier, the positive feedback loop and the negative feedback loop exist at the same time. As shown in Figure 6, the branch where M2, R2, and Q2 are located is negative feedback, while the branch where M1 and Q1 are located is positive feedback. In order to ensure the stability of the circuit, the system needs to be negative feedback as a whole, so the negative feedback coefficient should be greater than the positive feedback coefficient. In this design, the sum of the on-resistance 1/gm2 of R2 and Q2 is greater than the on-resistance 1/gm1 of Q1, so that the stability of the circuit is guaranteed.

This paper proposes a bandgap reference voltage source circuit for engine control chips based on the SMIC 0.18 μm MIXIC process. Based on the basic structure of the first-order current-type bandgap reference source, the circuit uses resistors with different temperature coefficients to perform simple and effective second-order temperature coefficient compensation. The reference voltage source has an output reference voltage error of less than 0.25 mV in the temperature range of -50℃~125℃, and the power supply rejection ratio can reach 99 dB at low frequency. The reference circuit has good temperature stability and anti-power supply interference capability, and has good application value in engine control chips.
References
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