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TPS54218: DC-DC soft-start rise time

Don Foong
Expert 4100 points
Part Number: TPS54218

Hi Team,

We have a few technical queries from one of our customer, on the TPS54218 DC-DC soft-start rise time. Hope you can help to advice, thanks.

In datasheet, it stated that the soft-start is recommended between 1ms to 10ms.

 

If there is a design that need to increase soft-start timing to 10.8ms ~ 13.2ms (due to in-rush current limit), what will be the effect aside to rise time? Below are some areas in thought

  • Rise time consistency
  • DC-DC stability
  • Switching frequency
  • Others (effects that I am not aware of)

What’s the reason behind the soft start recommendation range?

 

Looking forwards to your response. Thanks.

Thanks and regards,

Don Foong

  • 0
    TI__Guru**** 191445 points
    Don,

    That is just a nominal range. 10.8 or 13.2 (even 15) msec will not be an issue for you.
  • 0 in reply to JohnTucker
    TI__Expert 4100 points

    Hi John,

    Thanks for the help on this case. Customer has some findings hope your can help to clarify and advise for below questions. Thanks in advance.

    Customer has observed some discrepancy between calculation versus measurement. They are using 27nF (10% tolerance) SS capacitor, from  equation (4) they are expecting the soft-start rise time to be 10.8ms ~ 13.2ms. Below in yellow boxes are some of the measurements taken from moment it rise up to it first hitting steady state (3.3V), it is faster then the expected rise time. Attached is the schematic of the TPS54218 and waveform (3.3Vout, Vss and Vfb) for your reference.

    1. Can you help to advise the tolerance/range limit across temperature (-40C to 125C) for the following parameters, Ichg and Vss-ref ? Customer hope to re-calculate the expecting soft-start rise time with temperature variation.

    2. Customer found that if they use 10nF SS capacitor, the result is more consistent with the formula (4) in datasheet and the rise time is within the tolerance Is there a limit to maximum soft-start rise time ?

    3. What is the possible reason of the deviation of rise time from provided formula ?

  • 0 in reply to Don Foong
    TI__Guru**** 191445 points
    We are working this off line now directly with the customer.