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TPS7A33: Feed-Forward Cap Question

David Hildebrand
Expert 4860 points
Part Number: TPS7A33

Forum:

I am supporting a customer who is using the TPS7A33 for his design.  He has reviewed the app note for the pros and cons of using a Feed-forward Capacitor (see attached document).  In the app notes it states “LDO shut-down, CFF may result in exceeding the negative absolute maximum value of the FB pin”. 

He would like to use the FF Cap because it gives optimal noise performance, but is this something he should be concerned with?

  • 0
    TI__Expert 4435 points
    Hi David,

    Cff forms a RC circuit with R1 (upper feedback resistor), Tau = R1*Cff. If you keep Tau small enough, there is no concern of pushing FB pin negative beyond abs max rating.

    If R1*Cff is large enough, the Cout for the regulator can discharge faster than Cff. The voltage on Cff can push Vfb negative as Vout approaches GND.

    To prevent FB pin going negative, add a Schottky diode clamp with anode to GND and cathode to FB pin. This will also provide a faster discharge path for Cff. Another way to prevent FB pin going negative is to keep Cff time constant (Cff*R1) less than Cout discharge time constant Cout*Rload.