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TPS7A30: Cout calculation

liping pan
Intellectual 460 points
Part Number: TPS7A30

1. what's the purpose of calculation Cout max? if Cout capacitor is bigger than this value, what's the impact?

2. where does the 140us comes from?

  • +1
    TI__Mastermind 33285 points

    Hello,

    There are two conditions that determine the startup time.  (1) the current limit circuit, and (2) the soft start capacitor.  If the soft start capacitor is set in such a way that turn on is very fast and current limit is engaged during turn on, then the actual turn on time will be slower than what Css would indicate because the current limit circuit is tripped.  Cmax is providing a mathematical approximation of the maximum Cout capacitance to prevent the current limit circuit from being tripped.  Set Cout(max) to cause the current limit to trip 2 decades below the turn on time from Css.  This way Css dominates and the current limit will not be operating during startup.

    2. 140us is 2 decades below the soft start time (14ms requirement in this example).

    Thanks,
    Stephen

  • 0 in reply to Stephen Ziel
    Intellectual 460 points

    So the Cmax only impacts start up time, it's not related any power supply performance.

    I used 10uF for 5V output, it's higher than 140us*220ms/5V = 6uF.

    If I don't care the start up time changes to about 30ms, everything should be fine, right?

  • +1 in reply to liping pan
    TI__Mastermind 33285 points

    Yes, I agree with your assessment.  You should be fine.

    Thanks,

    Stephen