Overview

The TL431 is a three-terminal adjustable precision shunt regulator IC originally designed by Texas Instruments and second-sourced by virtually every major semiconductor manufacturer. Acting as a programmable Zener diode with temperature stability across its operating temperature range, its output voltage can be set to any value between VREF (approximately 2.495 V) and 36 V using two external resistors.

With a low typical dynamic output impedance of 0.2 Ω and a sharp turn-on characteristic, the TL431 is an indispensable building block in power supply design. It is widely used as a voltage reference in precision analog circuits, an error amplifier in isolated switch-mode power supplies (SMPS) feedback loops (paired with an optocoupler), a voltage monitor, and a precision current source.

Quick reference

Pin configuration

Functional description

The TL431 contains an internal 2.495 V temperature-compensated voltage reference, an operational amplifier, and a high-current NPN transistor connected between Cathode and Anode.

When the voltage applied to the REF pin exceeds VREF (2.495 V), the internal op-amp turns on the output NPN transistor, causing current to flow from Cathode to Anode and pulling down VKA. In a closed-loop feedback network using a voltage divider ($R1$ and $R2$), the output voltage $V_{KA}$ is held stable at:

$V_{KA} = V_{\text{REF}} \times \left(1 + \frac{R1}{R2}\right) + I_{\text{REF}} \times R1$

Since $I_{\text{REF}}$ is typically around 2 µA, the second term is negligible for reasonable resistor values ($R1 < 100\,\text{k}\Omega$), simplifying the formula to:

$V_{KA} \approx 2.495\,\text{V} \times \left(1 + \frac{R1}{R2}\right)$

Absolute maximum ratings

Electrical characteristics

Typical application

Adjustable 5.0 V Voltage Reference Circuit

       +12V Unregulated Input
                 |
               [ R_LIMIT (470 ohm) ]
                 |
                 +--------------------> +5.0V Regulated Output (V_KA)
                 |
                 +----+
                 |    |
               [R1] (10k)
                 |    |
  Ref Pin (1) <--+    |
                 |    | (Cathode, Pin 3)
               [R2]  [ TL431 ]
              (10k)   | (Anode, Pin 2)
                 |    |
                GND  GND

Using $R1 = 10\,\text{k}\Omega$ and $R2 = 10\,\text{k}\Omega$:

$V_{KA} = 2.495\,\text{V} \times \left(1 + \frac{10\,\text{k}}{10\,\text{k}}\right) = 4.99\,\text{V} \approx 5.0\,\text{V}$

Common mistakes

• Cathode Current Below 1.0 mA: If IK drops below 1 mA, the internal reference and op-amp will not fully turn on, leading to poor regulation and out-of-spec output voltage. Ensure $R_{\text{LIMIT}}$ supplies at least 1 mA to 2 mA under maximum load.
• Unstable Capacitive Load (Oscillation): Connecting capacitive loads directly across Cathode and Anode can cause self-oscillation. Check the datasheet "Stability Boundary Conditions" chart; values below 1 nF or above 10 µF are generally stable, while 10 nF to 2.2 µF can cause instability depending on $V_{KA}$ and $I_K$.
• Ignoring $R1$ / $R2$ Current: Using extremely high resistor values ($R1 > 500\,\text{k}\Omega$) causes the 2 µA $I_{\text{REF}}$ input bias current to introduce significant setpoint error.

Notes & further reading

• SMPS Isolation: In switch-mode power supplies (e.g. flyback converters), the TL431 is placed on the secondary side driving an optocoupler LED to provide galvanically isolated voltage feedback to the primary PWM controller.
• Equivalent Parts: LM431, NCV431, AS431, AP431.