TL431 shunt reference: pinout, the output formula and the series resistor

The letter after 431 is the grade: no letter is 2 %, A is 1 %, B is 0.5 %. TI then adds the temperature range (C for 0 to 70 °C, I for −40 to 85 °C, Q for −40 to 125 °C) and the package, so a TL431CLP is the 2 % part for 0 to 70 °C in TO-92 (LP), printed "TL431C" on the body. In SOT-23 the body is too small for the name: Diodes prints AA for the TL431ASA and AB for the TL431BSA.

The TL432 is the same chip with a different pinout in the small packages. In SOT-23 a TL431 is cathode 1, REF 2, anode 3; a TL432 is REF 1, cathode 2, anode 3. Both makers read for this page draw it that way. Check the code on the body before you fit one in place of the other.

In SOIC-8 the TL431 is cathode on pin 1, REF on pin 8, and pins 2, 3, 6 and 7 all anode (TI p.4). Join every anode pin to ground.

The two sheets agree on the working range: the cathode from Vref to 36 V and up to 100 mA through it. They differ on the absolute maximum, 37 V for TI and 40 V for Diodes. Design to 36 V and treat 37 V as the edge.

The package limits the current long before 100 mA does. The TO-92 runs at 140 °C/W from junction to air (TI), the SOT-23 at 380 °C/W (Diodes). At 5 V and 20 mA the part already dissipates 100 mW, 38 °C of rise in a SOT-23.

All three grades share the same typical figure, 2.495 V. What you pay for is the window. The plain TL431 may sit anywhere from 2.440 V to 2.550 V; the B grade from 2.483 V to 2.507 V. Diodes gives the B grade's floor as 2.482 V, one millivolt below TI's.

Two other lines decide whether a circuit works at all. Imin, the least cathode current that keeps the part in regulation, is up to 1 mA for the plain TL431 and 0.6 mA for the A and B grades. Below it the output sags and the formula no longer holds. Iref, the current into the REF pin, is up to 4 µA, and it flows through R1, so it adds to the output.

• The error amplifier in a phone charger or adapter: it drives the LED of an optocoupler such as the PC817, so the mains side can see the output.
• A precise 2.495 V reference for an ADC or a comparator, with REF tied to the cathode.
• A better Zener: any voltage from 2.495 V to 36 V, set by two resistors, with a sharp knee. The Zener regulator lesson shows what it replaces.
• A crowbar or undervoltage detector, using the REF pin as a threshold.

It is a shunt part: it wastes whatever current the load does not take. For more than a few tens of milliamps use a series regulator such as the LM317, or put a transistor after the TL431.

The job: 5.01 V for a small load drawing about 5 mA (modelled as 1 kΩ), from a 12 V adapter that may read anywhere from 10.8 V to 13.2 V. The part is the plain TL431 in TO-92, so the design uses its 1 mA Imin. R1 and R2 are 10 kΩ each, the value TI uses in its own design example.

TI's test circuit (Figure 7-2, p.19) writes the Iref term as + Iref × R1, which is right: the REF current is drawn through R1 from the cathode, so it raises the output. The same sheet's design section (p.26) prints it with a minus sign. Use the plus.

Tie REF to the cathode and leave out R1 and R2: the cathode sits at Vref, 2.495 V typical. From a 5 V rail, a 1.2 kΩ resistor passes 2.09 mA, twice the plain TL431's Imin, which leaves room to draw about 1 mA from the reference before the part drops out. A load that needs more takes a smaller resistor, sized the same way as in step 3.

• Size the series resistor at the lowest input and the heaviest load, then check the part's current at the highest input with no load.
• Keep R1 at 10 kΩ or below unless you have worked out Iref × R1. At 100 kΩ it can add up to 0.4 V.
• In an opto-feedback loop, put a resistor across the optocoupler's LED so the TL431 still gets its 1 mA when the LED is nearly off.
• Adding a capacitor from cathode to anode can make it oscillate. TI marks the stable and unstable regions in its stability charts (Figures 6-16 and 6-18); check your current and capacitance there first.
• Need a tighter rail without hand-picking parts? Buy the B grade and 0.1 % resistors instead of trimming.

The A and B grades are always a safe swap for the plain TL431. The TL432 is not, unless the board is TO-92 or SOIC: in the small packages its REF and cathode are swapped.

1. Wire it as a 2.5 V reference: REF to cathode, anode to 0 V, and 1.2 kΩ from a 5 V supply to the cathode.
2. Measure the cathode: a plain TL431 should read 2.440 to 2.550 V, an A grade 2.470 to 2.520 V.
3. Raise the supply to 9 V. The cathode should barely move; the extra current all goes through the part.
4. If it reads far from 2.5 V, check REF-A-K against the flat face before you suspect the part.

The common mistakes: reading the TO-92 pins like a transistor and swapping REF and cathode, a series resistor too large to leave Imin, a TL432 fitted on a TL431 SOT-23 footprint, and a decoupling capacitor across the part that makes it oscillate.

Use the TL431A for anything new: it holds 1 % and it works from 0.6 mA. Keep R2 near 10 kΩ, size the series resistor for the lowest input with 1 mA to spare, and check the part's current at the highest input with no load. For more than about 50 mA, use it to control a transistor or switch to a series regulator.

The TL431 is sold loose in TO-92 and SOT-23. Ask for the A grade by name, and read the TO-92 body before you rely on the 1 % figure: TI marks its A grade 431AC, and a body marked TL431C is the 2 % part. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes. The voltage divider lesson covers the resistor pair in depth.