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TPS56C20: Ringing on Start-Up (even on dev board)

c_oflynn
Prodigy 100 points
Part Number: TPS56C20

I was troubleshooting a design that uses the TPS56C20, and had ringing in some conditions (5-10kHz ringing on output - obvious as audible inductor whine). It seemed to be related to my test setup & large inductance leads. But I then noticed a ringing around the same frequency on turn-on (much shorter - but still present) after changing around the load setup.

I'm seeing ringing on the device during power-on. It's more noticeable at certain loads (often ~3-6A), but seems to always be present. Using the dev board "fresh from the box" I discovered it also happens there too (my next step in troubleshooting was getting a "known good" example) - see these photos:

On my own board - if I slow down the turn-on time by changing the SS capacitor, you still get it even with an insanely slow turn-on time Yes that's 200ms/div):

Going back to the dev board, here is my setup:

I get this ringing regardless of input voltage and for many current settings. The load is a DL3021. Is the test setup wrong somehow or is this an issue with the part?

One caveat is I *need* to use the programmable VCC-INT feature so don't want to use just an external feedback network. But I was seeing this problem with both the default internal & external feedback options. I didn't seem to find a lot of details in the datasheet on actual stability information besides the recommended values.

Appreciate any thoughts - whether official TI or anyone else who has seen something similar. I assumed it was just my quick prototype design issues until I recreated on the dev board!

Thanks,

  -Colin

  • 0
    Prodigy 100 points

    My two photos I tried to copy/paste in are here instead:

    Slow start-up ringing:

    Test setup:

  • 0 in reply to c_oflynn
    TI__Expert 7500 points

    Hi,

    I checked on EVM "fresh from box", but I couldn't see any issue.

    According to your test setup on EVM, some suggestions:

    1. Don't connect Eload, does the issue still have?

    2. What is the input voltage and input current limit?

    3. The output wire connect to Eload is too thin. Recommend to use thick wire to test with 3-6A loading.

    4. If you still have the issue, please help to capture the VIN, Vout, SW and Iout waveforms together. We need to check the waveforms for further analysis.

    Thanks,

    Lishuang

  • 0 in reply to Lishuang Zhao
    Prodigy 100 points

    Thanks for checking you aren't seeing that - I did a lot more investigation and finally wondered if my electronic load was the issue all along. Indeed just connecting another PSU to the load shows the same turn-on ringing when going through 0-->1V.

    I rigged a load for the dev-kit using a spool of wire that had enough resistance to give me ~5A and there wasn't any ringing issues at all (and this would be a rather inductive load). Will switch to some power resistors for rest of testing once they come in. Appreciate the confirmation!

  • 0 in reply to c_oflynn
    TI__Expert 5470 points

    Hi.

    Thanks for your double check with inductive load.

    Same as Lishuang's suspect, I think it seems to be related with current limit or the limited instant output power ability of power source. For the inductive R-L load, the load current at startup will be limited by the inductor and smaller than that of a resistive load.

    During the start-up, the input power needs to charge capacitor and providing the load power at same time. So it's possible that the insufficient power only happens during start-up but won't happen during steady state operation.

    Since you have clicked the resolved button, I'll close this threads now. And you can still reply or start a new thread if you have any more question.

    Thanks,

    Andrew

  • 0 in reply to c_oflynn
    TI__Expert 7500 points

    Hi,

    Glad to hear the issue is solved.

    Thanks,

    Lishuang