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TPS54231 Very Slow Start Up

Christopher Thunes
Prodigy 30 points
Other Parts Discussed in Thread: TPS54231

Hi,

I've been testing a design using the TPS54231 and have been experiencing *very* slow start up rates. With 12V input and 5V output, I'll see the output start at less than 1 volt, rise to 4.3V over 10 seconds or so, and continue rising albeit much more slowly. If I leave it running for 10 minutes or more the output has gotten as high as 4.8V, but never any closer to 5V. I have another almost identical design on the same board configured to output 6V (from the same 12V) and it has the same issues. I've tried with a number of different loads from no load up to 0.7A with the same result. Here is my schematic,

If anyone has any ideas or sees anything wrong with the schematic please let me know. Any help is appreciated and I can hopefully get some waveform captures later today if it would help.

Thanks,

Chris Thunes

  • 0
    TI__Mastermind 35185 points

    Hi Chris,

    After looking at the schematic I do not see any problems and have not experienced this before. If you could post waveforms including VOUT, PH, SS and COMP during the startup it will help. Was the testing done with a custom layout or on the EVM?

    Thanks,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 30 points

    Hi Anthony,

    Thank you for the quick response. I'll work on getting the waveforms captured. This is a custom layout which is part of a larger board.

    Thanks,
    Chris

  • 0 in reply to Christopher Thunes
    Intellectual 285 points

    Hi Anthony,

    Here are the waveforms you requested. I apologize if they're not great captures, I was using a borrowed o-scope I wasn't familiar with.

    Vout

    PH

    SS

    COMP

    Thanks,
    Chris

  • 0 in reply to Chris Thunes
    TI__Guru**** 191445 points

    Slow start time should be 4 msec.  Those waveforms all look way off.  SS should be a linear ramp from 0V to about 1.8 V and should ramp to 0.8 V in 4 msec.  Vouit should track that.  The COMP pin valley voltage is about 500 mV.  It looks too low.  What else is connected to the output of the converter?  Can you disconnect any external load and just use 5 ohm resistance?

  • 0 in reply to JohnTucker
    Prodigy 30 points

    I don't have a good way to isolate the 5V on the board, but the 6V supply doesn't have any load on it at the moment and exhibits the same issue, so I can get waveforms of it with an 8 ohm load tomorrow (this is the smallest sandbar resistor I have lying around).

    Thanks again for the help and quick responses.

    - Chris

  • 0 in reply to Christopher Thunes
    Intellectual 285 points

    Hi,

    Here are the new waveforms from the 6V regulator with an 8 ohm load.

    Vout

    PH

    SS

    COMP

    Vout, PH, and SS are on a 200 ms/div timescale while the COMP is 40 ms/div.

    Thanks,
    Chris

  • 0 in reply to Chris Thunes
    TI__Guru**** 191445 points

    Did you ever figure anything out?  The Vout waveform has generally the right shape, and you can see that the frequency fold back is working in Vout and PH.  I would expect that the initial rise not be so pronounced and that the slope of the remainder of the waveform be a little flatter, not 'sagging".  Can you check your components and workmanship?  Also how many out of how many total boards are showing this issue?  It may also be possible that you have an issue with your PCB.

  • 0 in reply to JohnTucker
    Intellectual 285 points

    Hi John,

    I haven't been able to figure out anything new. This board is one off (it's for my university's underwater robotics club), but there are two supplies using the TPS54231 on this board that are having the same issue. I've tried to verify the layout, workmanship and components to the best of my ability, but I'm not an electrical engineer by trade so I'm in somewhat unfamiliar waters with this sort of stuff. I tried to follow the layout guidelines in the datasheet as closely as possible, and I've double and triple checked the soldering work/part placement, so I feel like it might be a component selection error on my part. Here is a screen cap of the layout for the 5V rail,

    Everything is on the top layer except for the bootstrap capacitor C5. This is the parts list,

    C8, C9 http://search.digikey.com/us/en/products/TMK212BJ475KG-T/587-1782-1-ND/1212798
    C10, C13   http://search.digikey.com/us/en/products/CC0805KRX7R9BB103/311-1136-1-ND/303046
    C5 http://search.digikey.com/us/en/products/CC0805KRX7R9BB104/311-1140-1-ND/303050
    C11 http://search.digikey.com/us/en/products/08051C182KAT2A/478-5155-1-ND/1913245
    C12 http://search.digikey.com/us/en/products/500R15N470JV4T/709-1175-1-ND/1859507
    C7 http://search.digikey.com/us/en/products/C3216X5R1A336M/445-6005-1-ND/2444046
    D5 http://search.digikey.com/us/en/products/B240A-13-F/B240A-FDICT-ND/717810
    L1 http://search.digikey.com/us/en/products/7447715180/732-2193-1-ND/2175708
    R20 http://search.digikey.com/us/en/products/ERJ-6ENF1052V/P10.5KCCT-ND/119266
    R21 http://search.digikey.com/us/en/products/ERJ-6ENF2001V/P2.00KCCT-ND/119044
    R22 http://search.digikey.com/us/en/products/ERJ-6ENF2102V/P21.0KCCT-ND/119354

    If you have the time to take a look at the layout and parts selection I would really appreciate it, and I really appreciate the help thus far. I'm at a loss as to what might be going wrong, but hopefully it's a simple mistake in parts or layout on my part. Thanks again for the help.

    Best,
    Chris

  • 0 in reply to Chris Thunes
    Intellectual 285 points

    I still haven't been able to figure anything out with this. Does anyone have any ideas?

    - Chris