How Shunt Regulator TL431 Works, Datasheet, Application

In this post we learn how a shunt regulator IC works typically in SMPS circuits. We take the example of the popular TL431 device and try to understand its use in electronic circuits through a few of its application notes.

Electrical Specifications

Technically the device TL431 is called a programmable shunt regulator, in simple words it may be understood as an adjustable zener diode.

Let's learn more about its specifications and application notes.

The TL431 is attributed with the following main features:

Output voltage settable or programmable up to 36 volts.

Output Impedance low dynamic, around 0.2 Ohm.

Sink current handling capacity up to maximum 100mA

Unlike normal zeners, noise generation is negligible.

Switching response lightning fast.

How the IC TL431 works?

The TL431 is a three pin transistor like (such as BC547) adjustable or programmable voltage regulator.
The output voltage can be dimensioned using just two resistors across the specified pin outs of the device.

The diagram below shows the internal block diagram of the device and also the pin out designations.

The following diagram indicates the pin outs of the actual device. Let's see how this device can be configured into practical circuits.

Circuit Examples using TL431

The circuit below shows how the above device TL431 can be used as a typical shunt regulator.

The above figure shows how with the help of just a couple of resistors the TL431 can be wired up as a shunt regulator for generating outputs between 2.5v to 36v. R1 is a variable resistor which is used for adjusting the output voltage.

Calculating Shunt Regulator Resistors

The following formula holds good for acquiring the values of the various components used for fixing the shunt voltage.

Vo = (1 + R1/R2)Vref

In case a 78XX needs to be  used in conjunction with the device, the following circuit can be used:

The ground of the TL431 cathode  is connected with the ground pin of the 78XX. The output from the 78XX IC is connected with the potential divider network which determines the output voltage.

The parts can be identified through the formula shown in the diagram.

The above configurations are restricted to a max 100 mA current at the output. For getting higher current a transistor buffer may be used, as shown in the following circuit.

In the above diagram most of the parts placement is similar to the first shunt regulator design, except that here the cathode is provided with a resistor to positive and the point also becomes the base trigger of the connected buffer transistor.

The output current will depend on the magnitude of current the transistor is able to sink.

Application Areas of the IC TL431

Although the above configurations can be used in any place where precision voltage setting and references may be required, it's extensively used in SMPS circuits nowadays for generating precise reference voltage for the connected opto coupler, which in turn prompts the input mosfet of the SMPS to regulate the output voltage precisely to the desired levels.

For more info please go to https://www.fairchildsemi.com/ds/TL/TL431A.pdf