# TL431 output wrong

The purpose of the circuit below is to get a V_cathode of 2.5V (Vref) when VDC is higher than 10.4V approximately. The problem is that when VDC>10.4V the voltage checked in V_cathode is 1.9V instead of 2.5V.

3V3_UC_PIN comes from a microcontroller pin that "enables" this circuit. It this case, it is always enabled (+3V3).

I tried with a lower R1 (33 ohms) as well but the result is the same.

What could be the problem here?

• That circuit won't do what you expect it to. Commented Mar 17, 2022 at 9:35
• You are running the device open-loop. Read the datasheet to find the correct closed-loop configuration. Commented Mar 17, 2022 at 9:54
• "when VDC is higher than 10.4V approximately" Could you explain this in more detail? Commented Mar 17, 2022 at 10:46
• When run open-loop like this, the on-state anode-cathode voltage will always be around 2V. If you just want a 2.5V reference you could connect the top of R2 to the cathode pin, and change the resistor values; to include your 10.4V check, you'd need more circuitry. Commented Mar 17, 2022 at 10:48
• Where is V_cathode going to? If back to the microcontroller, couldn't you use an analogue input to measure the voltage across the potential divider? Commented Mar 17, 2022 at 11:24

$$\V_{T+} = 2.5 \times \left( 1 + \frac{R_1}{R_3} \right)\$$
$$\V_{T–} = 2.5 \times \left( 1 + R_1 \times \frac{R_2}{R_1 + R_2} \times \frac{1}{R_3} \right)\$$