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TPS54326 output discharge function fail.

George Chen
Intellectual 2175 points
Other Parts Discussed in Thread: TPS54326

Team,

My customer found TPS54326 output discharge function is failed.

Please refer to below scopeshot.

Input is 12V, output 1.25V

While we pull down the enable signal, 1.25V output is drop.

But after 3 seconds, we still see the output is with 200mV.

It looks like the discharge function is failed.

What could be the possible reason to make this happened?

TPS54326.pdf

  • 0
    TI__Guru**** 191445 points
    The output discharge FET is only 50 ohm. Only 4 mA current is required to get a residual output voltage of 200mV. You may need to allow more time for complete discharge. If there is any leakage path to another voltage rail on the output, there may be enough current available to maintain some amount of voltage indefinitely.
  • 0 in reply to JohnTucker
    TI__Intellectual 2175 points
    John,

    The output capacitance is under 100uF.
    The discharge resistance is 50ohm, it should be easily to fully discharge the 200mV even 1.25V in 3 seconds.
    The time RC time constant is 5m second, 3 seconds is 600 times already.
    I will check if any leakage path.
  • 0 in reply to JohnTucker
    TI__Intellectual 2175 points
    John,
    After checking with customer's board, we do believe that TPS54326 is with some problem for the output discharge.
    You said that the output discharge resistance is about 50ohm, but still cannot fully discharge the 1.25V output which is with <100uF capacitance in 3 seconds. My customer can fully discharge the 1.25V output in 200ms by adding only 200ohm resistor.
    Could you please also check this at EVM?
    The case is quite urgent, could you please check with designer to see if TPS54326 can really fully discharge the output voltage or not?
    Or is there something we missed to make the output discharge function work well?
    Thank you.
  • 0 in reply to George Chen
    TI__Guru**** 191445 points

    I no longer support TPS54326, but I sometimes answer forum posts for parts I know well anyway.  I do not have any EVMs on hand as I no longer support those products.  You can try contacting Daniel Jing or Eric Guo directly for support.

  • 0 in reply to JohnTucker
    TI__Intellectual 2590 points
    Hi George,

    we have tested in lab, the discharge time is about 20ms in our EVM, will send you email for detial deacription,
    please help close this issue in E2E.
    thanks!