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TPSM84424: Vout is present before EN

JDJ
Expert 4486 points
Part Number: TPSM84424
Other Parts Discussed in Thread: TPSM84A21

Hello, 

We are experiencing problems with the TPSM84424's output voltage is on before the rising edge of EN pin. 

Can you please advise on next steps for debugging? 

Thanks!

  • 0
    TI__Mastermind 35165 points

    Hi Jack,

    Is the EN voltage being probed right at the pin of the module? Are they controlling the EN pin with some other device which causes the EN pin to float during start up?

    Assuming they are not probing right at the EN pin of the module, the EN pin could be floating initially causing the device to turn on. This device has an internal current source on the EN pin turning it on if the EN pin is floating.

  • 0 in reply to Anthony F
    TI__Expert 4486 points

    It is probed right at the pin of the module. We have a CPLD driven the EN pin with an external 1kohm pull down, to ensure it is not floating. 

  • 0 in reply to JDJ
    TI__Mastermind 35165 points

    Thanks for adding in the SW pins. This may help provide a clue. The TPSM84424 isn't switching when the 5V comes up early so it doesn't look like the output coming up is due to the module turning one. If the TPSM84424 was pulling up the 5V, we would see continuous switching at the SW pin like what we see after EN goes high. This makes me wonder if there is some leakage path instead. Two possibilities I can think of:

    1. Is this happening on multiple boards or only one board? If it is only one board, it makes me suspect there might be an assembly issue causing the leakage path.
    2. Can they check to make sure there isn't some other path on the board to the 5V output?
  • 0 in reply to Anthony F
    TI__Expert 4486 points

    This is happening on all boards. We have spiked out only one possible leakage from 12V to 5V in our design. This being on the TPSM84A21MOJR Vin1 and Vin2 (12V tied together) to VG (connected to 5V). I will send a direct email of schematic for reference. Can you verify if that's possible? 

  • +1 in reply to JDJ
    TI__Mastermind 35165 points

    Hi Jack,

    This is most likely it. The TPSM84A21 VG+ is coming up and pre-charging the 5V output. is the TPSM84A21 EN pin floating causing it to turn on as soon as EN comes up?

    I am guessing the TPSM84A21 is turning on then back off due to how the 5V comes up for only around 300ms then starts decaying back to zero.