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TPS548D22: the TPS548D22 Vo has a voltage drop with no load startup(EVM, FCCM)

Frank Xiao
Genius 12055 points
Part Number: TPS548D22

Hi team,

My customer is using the TPS548D22 now. When we do the test with 12v input 5v output test with FCCM mode setting. We find the Vo has a voltage drop. What's the reason of the voltage drop here? Whether it's auto skip mode during startup period? Thanks.

  • 0
    TI__Mastermind 35115 points

    Hi Frank,

    Yes, this could be related to how the device is in SKIP mode during startup and this dip looks to occur at the transition to FCCM. Based on the title, it looks like this was tested on the EVM. Was the output filter modified to support a 5 V output?

    Anthony

  • 0 in reply to Anthony F
    TI__Genius 12055 points

    HI Anthony,

    they use the original Cout, the Cap will be derating during 5V. The test is with no load. Thanks.

  • 0 in reply to Frank Xiao
    TI__Mastermind 35115 points

    Hi Frank,

    Actually I was wondering about the inductance more than the output cap. If they are also using the original inductance on the EVM I think it will contribute to more undershoot in the transition to FCCM. With 5V output the inductance should ~5x what is used with 1 V output to get the same inductor ripple current. Instead of 1V across the inductor during the offtime, there is now 5V so it ramps down much quicker. In initial transition to FCCM I suspect the part may be sinking a significant amount of current due to the relatively small inductance and causing this undershoot.

  • 0 in reply to Anthony F
    TI__Genius 12055 points

    Hi Anthony,

    Sorry for making the mistakes. Update the inductor and Cout.  L=450nH, Cout= 3*100uF MLCC+47uF MLCC(6.3v), Thanks.

  • 0 in reply to Frank Xiao
    TI__Mastermind 35115 points

    Hi Frank,

    The ramp selection may also be contributing to a larger than usual undershoot in Vout during the transition to FCCM. For 5V output, the Rx3 option is the recommended starting point.

    If the behavior is not improved with the different ramp, could you help list all of the changes to the EVM or markup the schematic with the changes? It would also be good to get some waveforms zoomed in on the transition that include the SW node and inductor current if possible.

    Thanks,
    Anthony

  • 0 in reply to Anthony F
    TI__Genius 12055 points

    Hi Anthony,

    we will try to find the waveform with R*3. Could you please help explain the details reasons with current  test results? Because we cannot find the auto skip information in the startup period when we use the FCCM mode.  Thanks.

  • 0 in reply to Frank Xiao
    TI__Mastermind 35115 points

    Hi Frank,

    I don't have much to add outside of this potentially being a result of the SKIP to FCCM transition with the schematic tested. The relatively large ramp amplitude used in this test for a 5V output when using Rx1 is likely contributing to increased output voltage deviation during the transition. Larger ramp amplitude will result in slower transient response.

    Anthony

  • 0 in reply to Frank Xiao
    TI__Mastermind 48200 points

     

    The TPS548D22 will always operate in Auto-skip during soft-start, this prevents it from sinking current from a pre-biased output voltage.  Once Soft-start completes, the part will transition to FCCM or Auto-skip, as selected by the FSEL pin setting.

    When operating in Auto-skip mode with a very large internal ramp, the internal loop over-compensates for the average to ramp valley offset error, applying a larger correction factor that the part will need in FCCM operation.  As the part transitions to FCCM once soft-start is complete, it will generate a longer than normal off-time due to that offset compensation.  WIth a small inductor producing a high ripple current, the output capacitance is significantly discharged during that first long off-time pulse until the internal compensation circuit is able to compensate.

    You should see this significantly reduce, or even disappear by selecting the smaller 3xR ramp and selecting a higher inductance.

    If you require further reduction, we can look at adding a feedforward capacitor from VOS to RSP in order to boost the loop by reducing the feedback divider ratio at high frequency.