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TPS55288: 11 kHz Ringing from Vout During Low Output Current

Cory Van Beek
Prodigy 245 points
Part Number: TPS55288

Hello TI Support,

I'm using a TPS55288 for outputting USB-PD power from 5 to 20 V and up to 3 A.  The input is fixed at 14 V.  This has been working well for the most part, but we are having issues when running at 5 V and very low load (a few mA).  Under that condition, we have a large ripple on Vout, and since it's a 11 kHz, it creates an audible, high-pitch ringing sound.

I think that this is probably a result of instability in the loop compensation, but I'm not sure.  The datasheet mostly talks about instability when operating in boost mode, but we're in buck mode going down from 14 V to 5 V.  Are there best practices or recommended values for developing a compensation circuit that ranges from 5V to 20 V?  Here is my current compensation values:

The other thing that seems to impact output voltage ripple is output capacitance and ESR.  The Design Tool Excel sheet for TPS55288 recommends 81.3 µF of output capacitance for 50 mV of output ripple. We increased that to 123 µF on our design but are still seeing this issue.  We have tried using just ceramic capacitors as well as using a mix of ceramic and low ESR ( 3 mΩ) electrolytic capacitors on the output.

Here is our full schematic:

Any help is appreciated.

Best regards,

Cory

  • 0
    TI__Mastermind 27700 points

    Hi Cory,

    Thanks for reaching out on E2E.

    1. Can you capture the waveforms of Vin, Vout, SW and iL in one screenshot? I am think if the device works in PFM? How do you set the MODE pin? I don't see the resistor connected with MODE pin, it affect the operation modes at light load.

    2. For the schematic, recommend change R90 to 50kohm to set input average inductor current limit to 6.6A. Recommend change R88 to 25kohm.

    Regards,

    Bryce

  • 0 in reply to Bryce Xun
    Prodigy 245 points

    I'm currently running it in PFM mode and it looks like you're on to something there.  I see there's a note about running in FPWM mode:

    I'll try using FPWM and see if that get's rid of the noise and jitter.  I'll have some time to test it out early next week.

    Best regards,

    Cory

  • +1 in reply to Cory Van Beek
    TI__Mastermind 27700 points

    Hi Cory,

    Thanks for update. You can refer to 7.3.2 Operation Mode setting to set device in PWM mode.

    Regards,

    Bryce

  • 0 in reply to Bryce Xun
    Prodigy 245 points

    Hi Bruce,

    I got around to testing this, and it's much better in FPWM mode.  The audible tone goes away and the ripple decreased significantly.  Thank you for this recommendation and for the resistor swap recommendations as well.

    Best regards,

    Cory