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TL431 Working Principle: How Dose TL431 Work? [Datasheet]

  • Contents

I Description

TL431 is a 3-terminal adjustable shunt reference voltage source with good thermal stability. Also, TL431 is also called a voltage regulator or 3-terminal sampling integrated circuit. Its output voltage can be arbitrarily set to any value in the range from Vref (2.5V) to 36V with two resistors.

The typical dynamic impedance of TL431 is 0.2Ω. Because of its good performance and low price, it is widely used in various power circuits.

TL431 Adjustable Zener - How to Use it

Catalog

I Description

II TL431 Features

III TL431 Functional Block Diagram

IV TL431 Working Principle

V TL431 Application Notes

5.1 Reduce Output Noise

5.2 Current

5.3 Power Consumption

5.4 Selection of Sampling Resistors R1 and R2

5.5 Minimum Maintenance Current and Minimum Cathode Voltage

VI Conclusion

Component Datasheet

FAQ

Ordering & Quantity

II TL431 Features

Features of TL431 regulator are as follows:

  • Reference Voltage Tolerance at 25°C
    • 0.5% (B Grade)
    • 1% (A Grade)
    • 2% (Standard Grade)
  • Adjustable Output Voltage: Vref to 36 V
  • Operation From −40°C to 125°C
  • Typical Temperature Drift (TL43xB)
    • 6 mV (C Temp)
    • 14 mV (I Temp, Q Temp)
  • Low Output Noise
  • 0.2-Ω Typical Output Impedance
  • Sink-Current Capability: 1 mA to 100 mA

III TL431 Functional Block Diagram

The encapsulation form of TL431 is the same as that of plastic-encapsulated transistor 9013. As shown in Figure 1(a). Similar products also have the dual in-line shape shown in Figure 1(b). It has 3 pins:

  • Cathode
  • Anode
  • Reference Terminal

They are abbreviated as C, A, and R respectively, and the symbols in the circuit are shown in Figure 1(c).

tl431 pinout and simplified schematic

Figure 1. TL431 Pinout and Simplified Schematic

Figure 2 is a schematic diagram of the TL431 Functional Modules.

As can be seen from the figure, Vref is an internal 2.5V reference source, connected to the inverting input of the operational amplifier. It can be known from the characteristics of the op-amp that only when the voltage at the REF terminal (non-inverting terminal) is very close to Vref (2.5V), a stable, non-saturated current will pass through the transistor.

And with the slight change in the voltage of the REF terminal, the current through the transistor will change from 1 to 150mA (this figure is by no means the actual internal structure of the TL431, it is only used to analyze the function).

tl431 functional module

Figure 2. TL431 Functional Module

IV TL431 Working Principle

TL431 is equivalent to an adjustable Zener voltage regulator, and the output voltage is set by an external precision divider resistor. In the circuit shown in Figure 2, when the resistance values of R1 and R2 are determined, the two introduce feedback to the partial pressure of V0. If V0 increases, the feedback amount increases, and the Shunt of TL431 increases, which in turn leads to V0 decline.

Obviously, the negative feedback circuit of this depth must be stable when the voltage at the REF terminal is equal to the reference voltage, at this time V0=(1+R1/R2)Vref. Choosing different values of R1 and R2 can get any voltage output ranging from 2.5V to 36V. In particular, when R1=R2, V0=5V. It should be noted that the necessary conditions for the TL431 to work must be guaranteed when selecting the resistor, that is, the current through the cathode must be greater than 1mA.

tl431 equivalent circuit

Figure 3. Tl431 Equivalent Circuit

So the specific Working Principle of TL431 is:

When the input voltage increases, the output voltage increases and the output sampling increases.

At this time, the internal circuit is adjusted to increase the current flowing through itself. This also increases the current limit circuit. As a result, the voltage drop of the current limiting resistor increases. The output voltage is equal to the input voltage minus the current-limiting resistance and the increase in voltage drop cause the output voltage to decrease. So as to achieve voltage regulation.

V TL431 Application Notes

5.1 Reduce Output Noise

The use of TL431 is very similar to the use of Zener diode. When the Zener diode works in the circuit, it will produce an irregular periodic noise. This kind of irregular noise is called Zener noise. Although the level of Zener noise is not high, it is one of the important reasons that affect the output characteristics of the Zener diode. We can use capacitors in parallel to absorb the Zener noise of the second regulator. In this way, the output characteristics of the Zener diode can be improved.

In addition, the capacitor connected in parallel to the Zener diode can also absorb the ripple of the power supply, making the output voltage of the Zener diode more stable. Secondly, when the Zener diode is used in parallel with the capacitor, due to the charging effect of the capacitor, the settling time of the output voltage of the Zener diode will increase and the output voltage will rise slowly.

However, this is only the moment when the power is turned on.

tl431 regular ic

Figure 4. TL431 Shunt Regular

During normal operation, the output voltage of the Zener diode is completely stable. But when the TL431 is connected in parallel with the capacitor and the selected capacity value is not suitable, sometimes it not only does not play a good role. This will cause oscillation instead. Because, the current flowing through TL431 has a certain relationship with the capacity of the capacitor.

Experiments show that if a capacitor with a capacitance of 0.01~3μF is connected in parallel on the TL431, it is likely to cause the TL431 to oscillate. Therefore, when TL431 is used in parallel with a capacitor, the value of the capacitor in parallel with TL431 should be greater than 3μF or less than 0.01μF, which must be paid attention to. But when the output voltage is greater than 15V and IK is greater than 10mA, the occurrence of oscillation can be completely avoided. When we are in actual application, it is required to connect a 33uF/10V tantalum capacitor in parallel or a 47uF/16V electrolytic capacitor in parallel.

5.2 Current

For the current flowing through TL431:

  • The minimum current must be greater than 1mA, otherwise the voltage regulation performance will be lost.
  • The maximum current cannot exceed 100mA, otherwise the TL431 will be damaged.

Therefore, the choice of the current-limiting resistor is very important.

5.3 Power Consumption

For example, the common TO-92 package TL431 has a maximum power consumption of 0.7W. The actual consumption of TL431 in the circuit is:

P=Vo*I

  • Vo is the output voltage;
  • I is the current through TL431;

Therefore, TL431 can only output 140mA current when the output does not exceed 5V, and can only output 100mA current when the output voltage is 7V. This is because of the power consumption limitation. The conventional power consumption is 0.5~1.2W. When it is used under high temperature, high pressure, or high current conditions, attention should be paid to ventilation, heat dissipation, and safety.

5.4 Selection of Sampling Resistors R1 and R2

Do you know? The selection and placement of the sampling resistor can directly affect the voltage regulation accuracy and temperature characteristics. Therefore, the same type of precision resistors with small temperature coefficient, low noise and high-power margin must be selected.

According to the formula Vo = 2.5×(1 + R1 / R2), the maximum Vo is 36V, and the maximum ratio of R1 / R2 can be calculated as 13.4, that is, R1 is 13.4 times the maximum value of R2.

Because TL431 has higher open-loop gain and faster response speed, when the sampling point (the connection point of R1 and R2) is far away from the two poles, the circuit is prone to overshoot and self-excitation. So pay attention when using it.

5.5 Minimum Maintenance Current and Minimum Cathode Voltage

Because the internal reference Vref of TL431 is maintained by the cathode current and is lower than the voltage between electrodes. So we need to pay attention to:

  • After the output pole of TL431 is cut off, there must still be a cathode sustaining current greater than 0.2mA.
  • When the output pole is "saturated", the voltage between the poles is still at least 2.2V.

VI Conclusion

This blog summarizes the features, working principle and precautions of TL431. All in all, TL431 is a compact design, easy to use, reliable performance and cost-effective regulator benchmark. Therefore, it has a wide range of applications.


Component Datasheet

TL431 Datasheet


FAQ

  • What is the Use of TL431?

The TL431 is a "programmable precision reference" and is commonly used in switching power supplies, where it provides feedback indicating if the output voltage is too high or too low. By using a special circuit called a bandgap, the TL431 provides a stable voltage reference across a wide temperature range.

  • What is TL431 Transistor?

The TL431 is a Regulator Diode whose output voltage can be programmed by changing the value of resistors connected to it. It acts almost like a Zener diode except for that the voltage rating of this IC is programmable. It is commonly used to provide negative or positive voltage references.

  • How does a Shunt Regulator Work?

The shunt regulator or shunt voltage regulator is a form of voltage regulator where the regulating element shunts the current to ground. The shunt regulator operates by maintaining a constant voltage across its terminals and it takes up the surplus current to maintain the voltage across the load.

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