This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TL431: Capacitor / Capacitance Value

Part Number: TL431

I am designing a series pass  Regulator using TL431.

Vin 15 V

Vout 3.3 V

Can I please have a clear insight about (1)HOW the capacitor induces stability in the circuit and(2) HOW to calculate the capacitance value ?

Also can I please get an explanation about(3) HOW does the Transistor work in the circuit?

I ll be highly obliged.

Thanks

 

  • Hi Rishab,

    1. The capacitor is added as a compensation for the closed-loop network.

    2. This capacitor is usually small (pF), creating a high frequency pole. This pole is used for stability in higher frequency operation, similar to a HF pole in an op-amp circuit. Since this is not always necessary, I believe the capacitor is optional and may not be needed in your application.

    3. Imagine 2 cases, one where the BJT is fully conducting and not conducting. When the transistor is not conducting (High Z), the TL431 would be open. But due to the bias resistor its not possible for the BJT to be not conducting and the TL431 to be in high Z. The opposite situation can be said the same, but in vice versa. 

    The transistor works by conducting just necessary amount to produce a 2.5Vref at the FB pin. If the BJT turned on too much, the FB voltage would be greater than 2.5 and the TL431 would regulate to counteract the high output voltage. This is called negative feedback.

    Hope this helps,

    Ethan