TL431 Precision Shunt Regulator / Reference
The TL431 is a three-terminal precision adjustable shunt regulator / voltage reference. It often gets described as a “programmable zener,” which is a useful first mental picture, but internally it is an active analog IC rather than a simple zener junction.
Three terminals
| Terminal | Purpose |
|---|---|
| REF | Senses the resistor-divider voltage used to establish the operating point. |
| CATHODE | The controlled shunt-current terminal. |
| ANODE | The lower-potential return terminal. |
Why it is more precise than an ordinary zener
The device contains an internal reference and amplifier. A resistor divider can program the cathode voltage above its internal reference. The result is a sharp, controllable shunt-regulation characteristic useful in precision references, regulated supplies, battery chargers, and feedback networks.
A very common switching-power-supply role
In many isolated switch-mode power supplies, a TL431 on the low-voltage secondary side compares the output voltage with its reference. It can drive an optocoupler that sends an error signal back across the isolation barrier to the primary-side controller.
It still needs current
Like a zener shunt regulator, the TL431 works by sinking current. The surrounding resistor network must provide enough current for regulation without exceeding the device or resistor power ratings.
Manufacturer reference: TI TL431.