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WEBENCH® Tools/TPS54320: Phase margin + transient response

Pjs
Intellectual 890 points
Part Number: TPS54320

Tool/software: WEBENCH® Design Tools

Hi, 

I'm using Webench to simiulate a TPS54320 design. I've compensated the design to get a phase margin of around 60degrees.

I'm noticing a very very small blip in the transient response and wondered what causes it? I assume at the phase margin I have, that it's not an oscillatory bump?

Thanks

  • 0
    TI__Mastermind 35165 points
    Hi Pjs,

    This bump is likely not due to phase margin and is instead a large signal effect. I suspect either a PH pulse is being skipped or the LS reverse current limit is being hit. I suggest checking what the PH voltage and the inductor current looks like during this event.

    Best Regards,
    Anthony
  • 0 in reply to Anthony F
    Intellectual 890 points

    Hi Anthony,

    Many thanks for the quick reply. I took a quick look at the PH node and inductor current as you suggested. See below:

    It looks like the LS mosfet is rated significantly more that what's happening above and I'm not sure I can see a skipped pulse but would appreciate your take on this and if you have any further comments that would also be appreciated.

    Many thanks

  • 0 in reply to Pjs
    TI__Mastermind 35165 points

    Hi Pjs,

    Based on these waveforms it is not related to the LS FET reverse current limit or pulse-skipping. It might be because the two narrow PH pulses are at the minimum on-time of the TPS54320. If they are, it could result in the output overcharging slightly.

    I should say this response is not a concern to me as it shows there is enough phase margin for a stable design. The slight ring could be related to some mismatch in the steady-state AC model and the transient model used in WEBENCH. These are two different models. If you'd like me to keep looking into this to get some better understanding, can you share the schematic for this simulation?

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    TI__Mastermind 35165 points
    Hi Pjs,

    I'm thinking you do not need any further support here so I am going to close this post. If you'd like me to keep looking into this, feel free to reply back here.

    Best Regards,
    Anthony