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TPS54394: The abnormal softstart waveform analysis of Vout.

Gordon Xu
Prodigy 440 points
Part Number: TPS54394

Hi expert,

My customer found the issue about softstart waveform, which has unsmooth ramp up scenario.

I also find the scenario in TPS54394 datasheet Fig. 11 as bleow red square frame:

My custmoer meet the issue in -25℃, 25℃, but issue disappered in 65℃. So could you help explain below question,

1. Why does the issue happen in -25℃, 25℃? Please help expian the detailed reason.

2.Why does the issue disapper in 65℃? Please help expian the detailed reason.

Thanks!

BR,

Gordon

  • 0
    TI__Expert 8100 points

    Hi Gordon,

    My colleague will give you response by tomorrow!

    BRs

    Zixu

  • 0 in reply to Zixu Wang
    TI__Prodigy 440 points

    Hi Zixu,

    Waiting for the response ASAP. Thanks!

    BR,

    Gordon

  • 0
    TI__Mastermind 24690 points

    Hi Gordon,

    Did you try on an EVM or a custom board? Can you please share your cold/room temp and high temp waveforms? And also the schematic and BOM used for this. I need to check with design team on this. 

    Thanks,

    Amod 

  • 0 in reply to Amod Vaze
    TI__Prodigy 440 points

    I find the scenario in TPS54394 datasheet Fig. 11 as bleow red square frame. So I think it is similar reason with my cusotomer. Please help expian the detailed reason about the red square frame waveform.

    BR,

    Gordon

  • 0 in reply to Gordon Xu
    TI__Expert 8100 points

    Hi Gordon,

    My colleague will give you reply by next Monday!

    BRs

    Zixu

  • 0 in reply to Gordon Xu
    TI__Mastermind 24690 points

    Gordon,

    I contacted design but the device is quite old and they cannot simulate it. However here is an explanation from design. 

    This step is natural for skip mode converter. At the beginning, buck converter is switching with 1st high side on. There always some energy dump to output cap. And there is no discharge path from Vout to ground. Not from low side FET because it’s skip mode. Not from loading because the voltage is too low. A voltage bump is there. At next or following cycle, VFB is higher than internal reference in soft start. It will wait until VFB is less than internal reference and start to switching again. That’s why there is a plateau.

    This should happen at all temperatures. However, some possibilities why it may have gone away with temperatures:

    1. The zero crossing threshold might change to negative with temperature above 65C. it makes the 1st high side on dump some energy to Cout but also discharged from Low side FET to ground. The net charge on Cout is 0.
    2. Some loading like leakage could be enable with such a low Vout with temperature above 65C

    If you have any more questions, please email me.

    Thanks,
    Amod