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TPS548D22: TPS548D22 start up waveform isn't monotonous

Frank Xiao
Genius 12055 points
Part Number: TPS548D22

Hi team,

we find the TPS548D22 start up waveform isn't monotonous. We want to check the reason. In my opinion, it's because the soft start and FB are not tracking well during the startup period. So increase the SS time should be helpful. Could you please help share some comments? Thanks.

  • 0
    TI__Mastermind 18510 points

    Looks like ss ramp up faster at the beginning, FB does not follow well, but that should not cause problem as my mind, I will forward expert for this part to review.

    Yuchang

  • 0
    TI__Mastermind 48200 points

     

    The TPS548D22 soft-starts in auto-skip mode, even when programmed for Forced Continuous Conduction.  The fact that the converter is still switching while the output voltage is decreasing indicates that there is some loading on the output through this transition.  It is possible that the converter reached some load compliance voltage where the loading characteristics changed, and the converter over-shot the soft-start ramp during its recovery, much like regulated output responding to a load transient.

    Is this from a customers board or our evaluation module?

    What was loading the output during the test?

    Most electronic loads, for example, have a load-compliance voltage, an input voltage to the load where they can begin drawing and regulating current.  Below that input voltage, they can't draw much current.  That causes the load profile to impose a transient on the supply voltage as it ramps up.

    If this is with no load, or with a resistive load, it could be a lag in the tracking of the rising reference voltage, followed by a small overshoot and recovery before tracking the reference, but the characteristics look more like a dynamic loading response.

  • 0 in reply to Peter James Miller
    TI__Genius 12055 points

    Hi Peter,

    The test is with the customer's board. No additional load when do the test, and the drop voltage point is only 90mv. So the load will have no change for the CPU is still off with that voltage. Thanks.

  • 0 in reply to Frank Xiao
    TI__Genius 14455 points

    Hi Frank,

         If the converter puts out a minimum on-time during the initial portion of the ramp up, this will cause the Vout to overshoot slightly before the on-time catches up. Can you try reducing the softstart time? This will put a higher demand on the on-time pulse and should either cause the drop voltage point to become lower than 90mV or go away completely.

    Regards,

    Gerold

  • 0 in reply to Gerold Dhanabalan - Online Design Tools
    TI__Genius 14455 points

    Hi Frank,

        Did you get a chance to look into this? 

    Regards,

    Gerold

  • 0 in reply to Gerold Dhanabalan - Online Design Tools
    TI__Genius 12055 points

    Hi Gerold,

    i have closed the case offline, sorry for forgetting the post. Thanks.