TL431 Regulator : Pin Configuration & Its Applications

In 1977, Texas Instruments introduced the TL431 regulator diode and its working is similar to the Zener diode. It is a 3-terminal bipolar transistor, which is equivalent to an ideal n-type transistor with no hysteresis loss. It is a shunt voltage regulator, which is adjustable. The equivalent series of TL431 regulators are ATL431, TL432, LM431, KS431, TS431, and 142EH19 regulators.

The function of these regulator circuits is similar except their size, precision, layout, operating currents, and speed characteristics are different. It is an 8-pin regulator available in the TO-92 package type. This article gives all details about the working of the TL431 regulator with simple circuits.

What is TL431 Regulator?

The TL431 Regulator is an adjustable precision type shunt voltage regulator IC. By changing the values of connected resistors, the output voltage can be programmed. Hence it is called a programmable shunt regulator. Almost the working of this IC is similar to the Zener diode except that the voltage rating is programmable.

It is also referred to as an adjustable Zener diode. It is mainly used in electrical circuits to provide positive voltage reference or negative voltage reference. Its cost is low and is commonly used as a reference voltage provider in isolated power supply circuits. The TL431 regulator chip is shown in the figure below.’

Pin Configuration of TL431 Regulator
Pin Configuration of TL431 Regulator

It is a 3-terminal shunt voltage regulator diode integrated circuit, which is available in a package type of To-92. The resistor divider connected to the reference terminal of the IC is adjusted to change the output voltage rating from 2.5 Volts to 36 Volts. The operating current ranges from 1 mA to 100 mA with a typical impedance value of 0.22 Ohms.

It provides stable performance on a wide operating temperature range. In various applications, it is used when replacement of Zener diodes is necessary because its working is almost similar to the Zener diode except that the output voltage is programmable and adjustable. It is widely used in over voltage and under voltage monitoring circuits, window voltage monitoring circuits, and power supplies as a reference voltage source.

TL431 Regulator Pin Configuration/Pin Diagram

The TL431 regulator is a 3-terminal shunt regulator with certain thermal stability over applicable temperature ranges of automotive, military, and commercial. The TL431 regulator pin configuration/pin diagram is shown in the figure below. The schematic symbol of the TL431 regulator diode is shown in the figure below.

Schematic Symbol of TL431
Schematic Symbol of TL431
  • Pin1 (Reference): It is used to set the voltage rating of the Zener diode
  • Pin2 (Anode): It is the anode terminal of the equivalent Zener diode
  • Pin3 (Cathode): It is the cathode terminal of the equivalent Zener diode.

TL431 Specifications

The TL431 specifications or the technical features are as follows,

  • It is a Zener diode, which is programmable
  • The output voltage ranges from 2.5 Volts to 36 Volts
  • The output voltage tolerance will be +-4%
  • The output current or sink current ranges from 1 mA to 100 mA
  • The output impedance will be 0.22 Ohms
  • The 3-pin TL431 is available in the To-92 package type. The 8-pin TL431 is available in SOIC and PDIP packages.
  • The operating temperature ranges from -40°C to 125°C
  • The output noise is low.
  • The typical temperature drift: 6 mV (C temp) and 14 mV (I Temp, Q temp)
  • At 25°C, the reference voltage tolerance will be 1% (A grade), 0.5% (B grade), 2% (Standard grade)
  • Its cost is very low.

Circuit Diagram/How to Use

The circuit diagram of the TL431 regulator integrated circuit is illustrated in the figure below.

Circuit Diagram of TL431 Regulator
Circuit Diagram of TL431 Regulator

From the above circuit diagram, we observe that the NPN transistor is biased from an op-amp with a 2.5 Volts precise reference voltage rating on the non-inverting terminal. The collector terminal of the transistor is connected to the cathode terminal of the TL431 IC and the emitter terminal of the transistor is connected to the anode terminal of the TL431 IC.

Now consider that the TL431 regulator IC works as a comparator. The one side of the comparator will have a 2.5 Volts precise voltage and the other side is set with the help of a reference terminal. This feature is mainly applicable in SMPS (switch mode power supplies) because the TL431 regulator IC can compare the obtained output voltage with the desired output voltage and controls the frequency of the switching by providing the feedback path. To isolate the high voltage part, the optocoupler is generally used with this circuit set-up.

Now let’s learn a few basic and simple circuits using the main electronic component TL431 regulator.

Precision Reference Voltage circuit

The circuit diagram of the precision reference voltage circuit using the TL431 regulator is shown below. It provides better temperature stability and a huge output current. To avoid self-excitation, the value of CL should be considered carefully while connecting to the capacitive loads.

Precision Voltage Reference Circuit
Precision Voltage Reference Circuit

Adjustable Regulated Power Supply

The adjustable range of output voltage Vo is between 2.6 Volts and 36 Volts.

The expression for output voltage Vo is written as,

Vo = Vref (1+R1/R2)

since Vref is set to 2.5 Volts

The voltage at which the circuit can withstand is related to (Vo-Vi). If there is a large voltage difference, then the power consumed by the resistor increases. The circuit diagram of the adjustable regulated power supply using this regulator diode is shown below.

Adjustable Regulated Power Supply
Adjustable Regulated Power Supply

Voltage Comparator

In this application, the voltage Vref = 2.5 Volts is used efficiently. The small internal resistance present in the TL431 regulator diode helps to track the input and output waveforms very easily and efficiently. The circuit diagram of the voltage comparator using the TL431 regulator diode is shown in the figure below.

Voltage Comparator Circuit Diagram
Voltage Comparator Circuit Diagram

Voltage Monitor

The transfer characteristics of the regulator are utilized to design a voltage monitor practically. At the upper and lower voltage limits, the LED power, upper and lower voltages will be (1+R1/R2) and (1+R3/R4) respectively. The below circuit diagram illustrates the voltage monitor using the TL431 regulator diode.

Voltage Monitor Circuit
Voltage Monitor Circuit

Overvoltage Protection Circuit

When the input voltage Vi exceeds a definite voltage, then the regulator starts triggering. At this moment, the thyristor is turned ON to generate a large instantaneous current. This helps to blow the fuse and protect the rear circuit. At the protection point, the voltage will be (1+R1/R2)Vref. The circuit diagram illustrating the overvoltage protection using TL431 is shown below.

Over Voltage Protection Circuit
Over Voltage Protection Circuit

Where to Use/Applications of TL431 Regulator

The applications of the TL431 regulator are listed below.

  • It is used in SMPS (Switch Mode Power Supplies)
  • It is used in circuits of isolated power supply.
  • Used in voltage comparators with integrated reference
  • Used in current regulation circuits.
  • Used in precision reference voltage source circuits
  • Used in regulated power supply circuits, which is adjustable
  • Used in circuits of overvoltage protection
  • Used in voltage monitors
  • Used in constant current source circuits
  • Used in controllable shunt characteristics
  • Used when replacement of Zener diodes is required.
  • Used in solar applications
  • Used in monitor and battery charger circuits

Alternative TL431 regulators are Zener Diode & the variants of TLV431, LM431 & TS431L1 regulators.

Thus, this is all about an overview of the TL431 regulator datasheet which includes – definition, specifications, pin configuration/pin diagram, circuit diagram/how to use, and where to use/TL431 regulator applications. Here is a question for you, “What are the advantages of TL431 Regulator? “