TL431 Calculator – Output Voltage & Cathode Resistor Window
Answer
VKA = Vref × (1 + R1/R2) + Iref × R1, with Vref = 2.495 V typical (2.440–2.550 V for the 2% grade). Size the series resistor so at least 1 mA flows into the cathode at minimum input and full load: Rs ≤ (VIN,min − VKA) / (IL,max + 1 mA + I_divider). Source: TI SLVS543S, p.6 and the detailed design procedure.
Calculator
How it works
The TL431 sinks whatever cathode current it needs to hold REF at Vref. R1 and R2 scale that reference up to VKA, and the REF input current Iref flowing through R1 adds a small offset. The part only regulates while cathode current is above Imin. This calculator always uses the 1 mA maximum, not the 0.4 mA typical, so a unit near the Imin limit still regulates at the lowest input and the highest load.
At the highest input and the lowest load, the current Rs delivers ends up in the TL431. That sets the worst-case dissipation: P = VKA × IKA and ΔT = P × 206 °C/W for the DBZ SOT-23. Rs power is (VIN,max − VKA)² / Rs. Use at least twice that rating.
Part numbers
By grade: TL431CDBZR (2%), TL431ACDBZR (1%), TL431BCDBZR (0.5%), plus I and Q temperature versions. Confirm the orderable on TI’s product page. Information only.
Disclaimer
Calculations use TI datasheet values as read on 26 Oct 2026 and ideal resistors unless stated otherwise. Resistor tolerance, temperature drift and layout add error. Check the current datasheet and test before production.
Updated: 2026-10-26 · Jasper
Parameters used on this page
| Parameter | Value | Source |
|---|---|---|
| Vref, 2% grade | 2440 / 2495 / 2550 mV | SLVS543S p.6 |
| Vref, A / B grade | A: 2470 / 2495 / 2520 mV; B: 2483 / 2495 / 2507 mV at 25 °C | Electrical Characteristics, A grade pp.9–11, B grade pp.12–14 |
| Iref | 2 µA typ / 4 µA max | p.6 |
| Imin | 0.4 mA typ / 1 mA max (A/B tables list 0.6–0.7 mA max; this page uses 1 mA) | p.6; pp.9–14 |
| Cathode current range | 1–100 mA | p.5 |
| Maximum VKA | 36 V | p.5 |
| RθJA, DBZ | 206 °C/W | p.5 |
| DBZ pinout | TL431: 1 cathode, 2 REF, 3 anode. TL432 swaps pins 1 and 2. | p.4 |
Worked examples
For a 5 V reference with R2 = 10 kΩ, exact R1 is 9.96 kΩ and 10 kΩ gives 5.010 V. The 2% band is about 4.90–5.12 V, A is about 4.96–5.06 V, and B is about 4.986–5.034 V. With VIN 9–12 V, load 0–20 mA and a 0.25 mA divider, Rs max is 187.8 Ω, so 180 Ω still leaves 1.92 mA in the cathode at 9 V and full load. At 12 V with no load, IKA is about 38.6 mA and the SOT-23 rises about 40 °C. Rs dissipates 0.27 W, so use at least a 0.5 W resistor. Designing to the 0.4 mA typical Imin instead of 1 mA pushes Rs to about 193 Ω and can drop out of regulation.
Pitfalls
- Sizing Rs only at full load and missing the TL431 dissipation at no load (the 5 V example is about 193 mW in a SOT-23).
- Letting IKA fall below 1 mA. Regulation is not guaranteed there.
- Mixing TL431 and TL432 pinouts on SOT-23. See the marking-code page.
- Ignoring SOT-23 heat. At 206 °C/W, a few tens of milliamps already means a noticeable temperature rise.
- Using a large R1. At R1 = 100 kΩ, Iref max of 4 µA shifts the output by 0.4 V.
FAQ
How do I set a TL431 to 5 V?
R1 = R2 = 10 kΩ gives 5.010 V typical (2.495 V × 2 + 2 µA × 10 kΩ).
What is the minimum cathode current of the TL431?
Design for 1 mA, the datasheet maximum of Imin. 0.4 mA is only the typical value.
How do I choose the TL431 series resistor?
Rs ≤ (VIN,min − VKA) / (IL,max + 1 mA + I_divider), then check TL431 power at VIN,max with no load.
Why is my TL431 output unstable or wrong?
Common causes are IKA below 1 mA, a TL432 pinout swap, or a large R1 with Iref error.
What is the maximum voltage of the TL431?
VKA 36 V.
Related
Sources
- Texas Instruments TL431/TL432 SLVS543S: https://www.ti.com/lit/ds/symlink/tl431.pdf (retrieved 2026-10-26).
- Updated: 2026-10-26. Jasper.