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TPS548B22: abnormal vout drop during power off

Feng bin Su
Expert 7510 points
Part Number: TPS548B22

Hi team,

My customer have used TPS548B22RVFR to step down 12V to 1.8V. We found the output has some strange as attached waveform.

It has down to 0.86V when we turn off 12V Adapter. They would like to know any reason to induce this drop and will be back to 1.8 suddenly.

Please refer the schematic and waveform as below.

  • 0
    TI__Mastermind 35115 points

    Hi Feng,

    I think what is happening here is primarily related to the input voltage waveform. When the output drops, the input voltage has dropped down to ~4V tripping the VDD UVLO and turning off the TPS548B22. When the TPS548B22 its loading on the 12V adapter suddenly goes away causing the input voltage to spike back up. The VDD voltage goes back above its UVLO and the TPS548B22 turns back on again bringing the output voltage up to 1.8V.

    I suspect it is stopping at ~1V because at this point the load on the 1.8V rail turned off and went away. Maybe it is being controlled by the PG of the TPS548B22 here. This would explain why the output voltage ramps down more smoothly the second time the input voltage goes below the VDD UVLO.

    If this shutdown waveform is a concern, a resistor divider from the input rail to the EN_UVLO pin could be added to add more hysteresis and prevent the TPS548B22 from turning back on again. For example a 56k/15k divider would give a turn on of ~7.6V and turn off of ~6.2V.

    Anthony