TL431 Voltage Calculator

The TL431 (and compatible devices like TLV431, ATL431, KA431) is a 3-terminal programmable precision shunt regulator. It operates essentially like an adjustable Zener diode with an integrated temperature-compensated voltage reference (typically V_ref = 2.495V ≈ 2.5V) and an internal operational amplifier.

The output voltage V_out is programmed by establishing a resistor divider ratio across the Reference (REF), Cathode (C), and Anode (A) terminals:


Key Operating Parameters:
• Reference Voltage (V_ref): Nominally 2.495V (standard TL431) or 1.24V (low-voltage TLV431).
• Output Range: Adjustable from V_ref up to 36V.
• Minimum Cathode Current (I_K): Requires at least 1mA flowing from Cathode to Anode to maintain tight regulation.
• Reference Input Current (I_REF): Typically 2μA. To minimize error, ensure the voltage divider current through R₂ is significantly larger than I_REF (e.g. ≥ 100μA).

When designing high-precision power supply feedback loops (such as flyback converter optocoupler feedback circuits), matching standard E24 resistor values to obtain exact output voltages (like 5.00V, 12.00V, or 24.00V) can be challenging.

By adding a parallel resistor R₃ across the bottom resistor R₂, the effective bottom resistance becomes:


Substituting R_eff into the voltage equation gives:


This technique allows engineers to achieve sub-0.5% output voltage accuracy using two standard off-the-shelf E24 resistors in parallel instead of purchasing expensive non-standard precision parts.

Here is a handy formula summary for quick TL431 circuit design:


(Ensure I_K ≥ 1mA under minimum V_in and maximum load conditions).

What is the difference between TL431 and TLV431?

The standard TL431 has a reference voltage V_ref = 2.495V and operates up to 36V. The TLV431 is a low-voltage variant with V_ref = 1.24V, making it ideal for 1.8V, 2.5V, and 3.3V power rails.

Why does my TL431 circuit oscillate?

The TL431 has specific capacitive load stability boundaries. Connecting capacitive loads directly across Cathode and Anode without checking the TL431 stability boundary curve (often between 0.01μF and 2.2μF) can cause high-frequency oscillations. Use an ESR damping resistor or keep decoupling capacitance outside the unstable region.

Popular Target Voltages & Standard R1/R2 Pairs (V_ref = 2.5V):

• 3.3V Output: R₁ = 3.3kΩ, R₂ = 10.3kΩ (or R₁ = 1.6kΩ, R₂ = 5.1kΩ)
• 5.0V Output: R₁ = 10kΩ, R₂ = 10kΩ (R₁/R₂ = 1)
• 12.0V Output: R₁ = 38kΩ, R₂ = 10kΩ (or R₁ = 19kΩ, R₂ = 5.1kΩ)
• 24.0V Output: R₁ = 86kΩ, R₂ = 10kΩ

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