Calculator KB-18
TL431 Calculator
An accurate voltage reference: costs next to nothing and is found in almost every power supply.
Result
In brief: Calculate the TL431 as an adjustable shunt regulator: divider resistors for voltages from 2.5 to 36 V, ballast resistor, its power dissipation, and cathode current. Example: Upper resistor R1: 10 kΩ.
Calculation example
| Desired voltage | 5 V |
|---|---|
| Lower resistor R2 | 10000 Ω |
| Supply voltage | 12 V |
| Load current | 10 mA |
| Current through the TL431 | 5 mA |
| Upper resistor R1 | 10 kΩ |
|---|---|
| Exact R1 calculation | 10.04 kΩ |
| Resulting voltage | 4.99 V |
| Ballast resistor Rk | 470 Ω |
| Power dissipation in Rk | 104.6 mW |
| Actual current through the TL431 | 4.915 mA |
How it is calculated
Rk = (Ui − Uo) / (IL + Ik)
The TL431 conducts just enough to maintain exactly 2.495 V at the REF input. So the divider sets the cathode voltage, while the ballast resistor must supply current to both the load and the IC.
What to look for
- The cathode current must not drop below 1 mA, or regulation is lost.
- Some capacitances at the output can make the TL431 unstable: check the stability graph in the datasheet.
Frequently asked questions
What output voltage does a TL431 set?
Uout = 2.495 · (1 + R1 / R2). For 5 V with R2 = 10 kΩ, R1 should be about 10 kΩ, giving an output of approximately 4.99 V.
What resistor should you use on the TL431 cathode?
Rk = (Uin − Uout) / (Iload + Ik). Keep the cathode current between 1 and 100 mA, and make sure it stays above the minimum even at the lowest input voltage.
Where is the TL431 used?
In the feedback loop of switching power supplies together with an optocoupler, as a voltage reference, and as a simple shunt regulator. In a TO-92 package it dissipates only fractions of a watt, so a transistor is added for higher currents.
