TL431 is a three-terminal adjustable precision voltage reference integrated circuit developed by Texas Instruments, which has good thermal stability. Its output voltage can be arbitrarily set to Vref (2.5V) with two resistors. Any value within the 36V range. At present, it is a kind of voltage regulator device widely used in the market. There is a bandgap reference internally, and its temperature stability and reference accuracy will affect the performance of the entire device, so having a high-performance reference has a significant impact on overall device performance.
1, temperature compensation reference source
Figure 1TL431 schematic
A relatively precise reference source (see Figure 1) is used in this circuit. Compared with the traditional reference circuit, the circuit adds nonlinear temperature compensation. The nonlinearity here includes exponential curvature compensation and second-order compensation. . As shown in Figure 2, it is a schematic diagram of the circuit with curvature compensation.
Figure 2 curvature compensation reference source
As shown in FIG. 2, the resistors R3 and R2 have the same voltage value, and the resistance value ratio relationship is: R3: R2 = 3: 1, the current through the two resistors is I3, I2, respectively, and the size ratio is 1 : 3. The current of resistor R1 is the sum of the currents flowing through resistors R3 and R2: I1 = IR + I2.
Its circuit reference source expression:
Vref=Vbe1+Vbe3+I1R1+I2R2(1)
According to the KVL equation, the current flowing through the resistor R3 in the circuit can be calculated, and because Ib = Ic / β, where β is the current gain of the transistor:
I3=βVTlnM/(R5+(β+1)R4)(2)
Where: M is the area ratio of the emitter regions of Q3 and Q4 in the circuit. According to I3, the currents I1 and I2 can be obtained. Finally, the expression of the reference voltage is:
In the above formula, both Vbe and β are variables related to temperature, and their expressions are as follows:
Where: α, γ are process-dependent but temperature-independent constants; Vg0 and Vbe silicon gap voltages and silicon base emitter voltages. Bringing equations (4) and (5) into equation (1) gives the expression of the reference voltage with respect to temperature:
Where: A, B are constant terms; K1, K2 can be obtained by resistance adjustment, as in the above formula, the reference voltage has three parts, a constant term, a term, a nonlinear term, and the linear term and the nonlinear term are:
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