What us TL431 IC? where is it used?

I came across the name of TL431 many times and while during this week I was learning negative feedback amplifier circuits I happened to encounter TL431 once again. I think it was during writing about Voltage Controlled Current Source with Op-Amp. The internal circuit of TL341 seems to resemble the circuit of voltage controlled current source using operational amplifier. So after reading what it is and where it can be used I wrote this note. One of the application is to build an adjustable shunt voltage regulator which is shown below.

But first what is TL431?

The TL431 is a versatile programmable shunt voltage reference IC. It's widely used in power supply and analog circuits for precise voltage regulation, feedback control, and reference generation.

🔧 Key Features of TL431:

Adjustable Output Voltage: Typically from 2.5V to 36V
Precision: Output voltage accuracy of ±0.5% (depending on the version)
Output Type: Shunt regulator — it behaves like a Zener diode with adjustable voltage
Sink Current Range: 1 mA to 100 mA

📘 Pinout (TO-92 / SOT-23 packages):

Reference (REF) – Used to set the output voltage
Anode (A) – Connected to ground or lower voltage
Cathode (K) – Connected to higher voltage (like output of a power supply)

📐 Voltage Regulation Formula:

The formula to calculate the output voltage (Vout) is:

V_{o u t} = V_{r e f} (1 + \frac{R 1}{R 2})

where:

Vout = Output voltage (cathode voltage w.r.t. ground)
Vref = Internal reference voltage (typically 2.495V, but check the datasheet for exact value)
R1 = Resistor from REF pin to ground
R2 = Resistor from REF pin to Vout

Key Notes:

The TL431 regulates the cathode voltage such that the REF pin is held at Vref (≈2.5V).
The minimum cathode current (Iₖₐ) must be maintained (usually 1mA) for proper regulation.
The maximum cathode-to-anode voltage (typically 36V) must not be exceeded.

Example Calculation:

If R1 = 10kΩ and R2 = 10kΩ, then:

V_{o u t} = 2.5 V (1 + \frac{10 k Ω}{10 k Ω}) = 2.5 V \times 2 = 5 V

The series resistance can be calculated using the formula below,

\( R_s =\frac{ V_{in}-V_{ref}}{I_{out}}=\frac{ V_{in}-2.5}{I_{out}} \)

Video demonstration

The following is circuit animation video of how the TL431 Adjustable Shunt Voltage Regulator works.

✅ Common Applications:

Switch Mode Power Supplies (SMPS)
- Used in the feedback loop to regulate output voltage
- Works with an optocoupler for isolated feedback
Precision Voltage Reference
- Acts as a more accurate alternative to Zener diodes
Battery Chargers
- Controls charging voltage with precise feedback
Overvoltage/Undervoltage Protection
- Detects voltage limits and triggers shutdown or alarm circuits
Adjustable Voltage Regulators
- Can work with external transistors for higher current