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How to calculate the soft start time of TPS54427 with large capacitive load

Di Wang
Expert 1740 points

Hi,

           The load requires a large input capacitance, so the capacitance is CD19 and CD20 in the below SCH. The total is 940uF. How to calculate the soft start time so I can choose the proper SS capacitor? I found if I set the SS capacitor as 10nF, sometimes, it didn't power up properly, but 20nF is fine.

           Thanks.

 

  • 0
    TI__Guru**** 191445 points

    Can you post waveforms of the good and bad start up?  I would need to see the SW node and inductor current as well as the SS and Vout voltages.  Your output filter is not within the recommended values for proper operation.  Can you tell me the ESR of CD19?  With that output filter, the IC may not work as intended.