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TPS54318 Soft-Start time range

Hidetoshi Matsunami
Expert 2750 points
Other Parts Discussed in Thread: TPS54318

Hello, I'm Hidetoshi Matsunami.

I would like to ask the following question.

Is there anything wrong if SS time is out of recommend range?

Because TPS54318 rise time wave had some distortions.

Therefore, I tried to shorten the SS time and disappear some distortion.

Figure.1 is before change the SS time

Figure.2 is after change the SS time.

Best Regards,

Hidetoshi Matsunami

  • 0
    TI__Guru**** 191445 points

    I did not see any specific limitations to the SS time in teh datasheet.  In general, these circuits are designed for SS time from about 1 to 20 msec.  It is acceptable to go a little slower or faster.  WWith faster SS times, you have t be careful about current limit.  The dc current required during SS, not including the load current is I = Cout x Vout / tss.  So the charge current for the output capacitors is inversely proportional to the SS time.  For TPS54318 you may get some non linearities as you observe.  At first it is due to minimum on time, then there is frequency shift further up the slope in 3 places.  Decreasing the SS time allows you to transition thru those areas faster so they are less noticeable.

  • 0 in reply to JohnTucker
    Expert 2750 points

    Hi John,

    I appreciate your help.

    I proposed your advice to customer.

    Best Regards,

    Hidetoshi Matsunami