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TPS54527: Large ripple and unstable current

Hirokazu Takahashi
Expert 8420 points
Part Number: TPS54527


Dear Support team,

Could you please review below schematic and feedback to improve performance?

TPS54527.pptx

Best Regards,

Hirokazu Takahashi

  • 0
    TI__Mastermind 27260 points

    Takahashi-san,

    Using the 2.2uH inductor does look better so you can continue using that. With this high a VIN and low VOUT (14V-17V-> 0.85V), the device is most likely running with min on-time pulses. It should increase the off-time to be able to regulate the VOUT but this will reduce the switching frequency from the nominal 650kHz expected. 

    Here is what you can try:

    • You can only measure the VOUT ripple and not measure any other waveform. Also, you can use the 20MHz BW limit option on scope. From current measurements, it looks like HF noise is coupled into the output ripple measurements. A lot of this HF noise will also be removed with your FB. Your current measurements are with removal of the FB.
    • While measuring output ripple, if you are not already doing this: you can use the tip and barrel technique to ensure you are getting clean measurements.
    • The existing ripple at FSW looks to be within 51mV required spec. You can also try adding another 22uF output cap in parallel to existing caps to reduce the fundamental component of the ripple.

    Thanks,

    Amod

  • 0 in reply to Amod Vaze
    TI__Expert 8420 points

    Hello Amod-san

    Thanks for your support.
    We found fSW vibration. I attached waveform below.
    Average : 512kHz, frequency range 382~602kHz
    Since I think the fSW will be constant, does such a vibration mean any functional issue?

    Best Regards,
    Hirokazu Takahashi

  • 0 in reply to Hirokazu Takahashi
    TI__Expert 7500 points

    Hi,

    Amod will feedback to you on Monday US time.

    Thanks,

    Lishuang

  • 0 in reply to Hirokazu Takahashi
    TI__Mastermind 27260 points

    Takahashi-san,

    Yes I assume this scope shot is taken in persistence mode and you are observing the jitter inherent to the control scheme of TPS54527. So, it is normal to have frequency variation.

    In general, the DCAP2 control scheme has off-time jitter because on-time is modulated and off-time is variable. It is possibly a bit more pronounced in your case with the 0.85V Vout and high 15+V Vin as you are likely generating min on-time pulses and the off-time has to increase to ensure regulation. Here is a good read on jitter in DCAP or COT control schemes vs the regular voltage/current mode control: https://www.ti.com/lit/an/slua747a/slua747a.pdf 

    Thanks,

    Amod