A design scheme of linear precision voltage regulated power supply based on TL431

introduction

TL431 is a three-terminal adjustable precision voltage reference integrated chip with good thermal stability. It has the characteristics of small size, low price and excellent performance. Its output voltage can be set to any value from the reference voltage (2.5V) to 36V with two resistors. The typical dynamic impedance is only 0.2Ω, the voltage reference error is ±0.4%, the load current capacity is from 1.0mA to 100mA, the temperature drift is low, the output noise voltage is low, etc. Based on the above characteristics, it can not only be used in constant current source circuits, voltage comparator circuits, voltage monitor circuits, overvoltage protection circuits and other circuits, but also widely used in linear regulated power supplies , switching regulated power supplies and other DC regulated power supply circuits. This article introduces the parallel and series types of TL431 in linear regulated power supplies in detail.

Internal structure and function of TL431

1. Symbol of TL431

The symbol of the device is shown in Figure 1. The three pins are: cathode (CATHODE), anode (ANODE) ​​and reference terminal (REF), and the reference voltage is 2.5V.

2. Internal circuit diagram of TL431

As can be seen from the internal circuit diagram Figure 2, it consists of a multi-pole amplifier circuit, a bias circuit, a compensation and protection circuit, in which transistor V1 constitutes the input pole, V3, V4, and V5 constitute the voltage reference, the mirror constant current source composed of V7 and V8 and V6 and V9 constitute a differential amplifier as an intermediate stage, V10 and V11 form a composite tube to constitute the output, and some other resistors, capacitors, and diodes play the role of bias, compensation, and protection respectively. In principle, it is a single-ended input and single-ended output DC amplifier.

However, its equivalent functional diagram is shown in Figure 3, which consists of a 2.5V precision reference voltage source, a voltage comparator and an output switch tube. The output voltage of the reference terminal is compared with the precision reference voltage source Vref. When the reference terminal voltage exceeds 2.5V, TL431 is turned on immediately. Because the R terminal control voltage error is ±1%, the reference terminal can accurately control the conduction and cutoff of TL431.

Parallel voltage regulator circuit design

1. Basic parallel voltage regulation circuit principle

TL431 contains a 2.5V reference voltage inside, so when output feedback is introduced at the Vref end, the device can control the output voltage through a wide range of shunting from the cathode to the anode. As shown in the circuit in Figure 4, when the resistance values ​​of R1 and R2 are determined, the two introduce feedback to the voltage division of VO. If it increases, the feedback amount increases, and the shunting of TL431 also increases, which in turn causes VO to drop. Obviously, this deep negative feedback circuit must be stable at a reference voltage, at which time VO=(1+R1/R2)Vref. By selecting different values ​​of R1 and R2, any voltage output in the range of 2.5V to 36V can be obtained. In particular, when R1=R2, VO=5V. If R1 is short-circuited and R2 is open-circuited, that is, the reference end is short-circuited with the cathode end, then the output voltage VO=2.5V (reference end voltage) is obtained, which is most suitable for use as a reference voltage source in digital voltmeters and analog-to-digital converters or other circuits.

It should be noted that when selecting the resistor R, the necessary conditions for the operation of TL431 must be guaranteed, that is, the current through the cathode must be greater than 1mA, and R1 and R2 must be selected as similar resistors with an accuracy of 1% to ensure the long-term stability of the reference power supply.

2. Parallel current expansion and voltage stabilization circuit

In many application circuits, FIG5 is actually a circuit slightly improved on the basis of FIG4, using transistors to expand the current to form a large current reference voltage source, and the transistors in FIG5 can be selected with transistors of different powers according to the size of the load current. At this time, the power of the current limiting resistor R should also be increased accordingly.

The parallel voltage regulator circuit is suitable for various sensors, transmitters, and various A/D converters of instruments. Figure 6 shows its application in the thermocouple transmitter.

Series current expansion voltage stabilization circuit

In addition to being used in parallel voltage regulation circuits, the power consumption of the current limiting resistor R in Figures 4 and 5 will change with the input voltage. A high-power current limiting resistor is required, otherwise it will be damaged due to excessive current.

The above problem is solved by adopting the connection method of series connection, as shown in Figure 7. A part of the current flowing through the current limiting resistor R is shunted to two NPN transistors V1 and V2, and the NPN type Darlington tube is formed through the same polarity connection of NPN+NPN. The amplification factor is the product of the amplification factors of V1 and V2. In this way, the collector not only shunts most of the current borne by the resistor R in Figure 5, solving the problem of the required high-power resistor, but also because the current amplification ability of the Darlington tube is enhanced, the load capacity of TL431 is enhanced, and a high-current load can be connected without a high-power transistor.

In practical applications, when the load current is 50mA, the current flowing through R is only 1mA, and the current flowing through the Darlington collector is 49mA. The resistor R can be a small-power surface mount resistor to meet the needs and realize the constant voltage function. Moreover, under wide power input, the output change is small. As shown in Table 1, the test conditions are R1=2.5KΩ, R2=3.5KΩ, R=3KΩ, the load current is 50mA, TL431 uses SOT-89 package, model TL431QPK, and only changes 1mV under wide voltage power supply. If used in a parallel circuit, it can replace the circuit shown in Figure 5 for better results.

The circuit shown in Figure 6 has good thermal stability. Since the temperature drift of TL431 is low, 6mV from 0℃ to 40℃, 14mV from -40℃ to 85℃ and -40℃ to 125℃, the temperature characteristics of the output voltage are much better than those of ordinary constant voltage sources. If R1 and R2 use resistors with low temperature drift coefficients, the overall temperature drift of the constant voltage circuit is also very small, and no additional temperature compensation circuit is required in the application. In actual applications, R1=2.5KΩ, R2=3.5KΩ, 3 chips are randomly selected from 50 chips of model TL431QPK, SOT-89 package, and the circuit shown in Figure 6 is used as the voltage stabilization circuit of the differential transformer displacement sensor. The temperature range is -40℃~125℃. The temperature drift measurement is shown in Table 2, with only a few to more than ten millivolts, which meets the given indicators.

It can be seen that the series circuit improves the stability accuracy, reduces the power consumption of TL431, and reduces the probability of TL431 being damaged due to overheating. The above circuit can not only be used as an industrial-grade sensor voltage stabilization reference, but also can be a precision voltage stabilization power supply reference for military-grade products with a wide temperature range and low temperature drift. It is a practical precision linear voltage stabilization circuit.

Conclusion

Based on the internal structure and characteristics of the TL431 three-terminal adjustable precision, this paper describes the basic composition and performance of the parallel voltage regulator circuit and the series voltage regulator circuit. A design scheme for a linear precision voltage regulator of TL431 is proposed . After a large number of experiments and long-term applications, it is proved that the adjustable precision voltage regulator based on TL431 designed in this scheme has achieved the functions of high power, wide voltage, and low temperature drift, with high cost performance and strong performance.