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BQ27421-G1: SHUTDOWN -> POR timing

Ben Genereaux
Intellectual 1040 points
Part Number: BQ27421-G1

Hi team,

The BQ27421 TRM gives us this guidance for waking the part out of shutdown:

We have GPOUT pulled high via an external 10k pullup as recommended by the data sheet.

The documentation doesn't specify how long GPOUT should be driven low for before letting the pullup bring GPOUT high. Empirically, we can reliably wake the BQ27421 up with our initial guess of 200us, to match the SHUTDOWN->POR timing, but do you have any advice or detailed information about this.

Thanks,
Ben

  • 0
    TI__Guru 57697 points

    Hello Ben,

    As a general rule, shutdown mode is used for long term storage, so the time to enter shutdown mode is not defined. The GPOUT pin must not be high when the shutdown command is sent.

  • 0 in reply to Shirish
    TI__Intellectual 1040 points

    Hi Shirish,

    What about when waking from shutdown mode, when a user first unboxes the end product? The datasheet says we have to assert GPOUT for >= 200us to wake the part. If we have to drive GPOUT high for 200us, we need to start with GPOUT driven low- that way the BQ27421 can observe the low-to-high voltage transition. 

    How long we need to drive GPOUT low first, before driving it high, to guarantee the transition from SHUTDOWN to POR is taken.

    Thanks,
    Ben

  • 0 in reply to Ben Genereaux
    TI__Guru 57697 points

    Hello Ben,

    If in shutdown mode, then the bq27421 already has power. Just toggling the pin high is enough in this case because it will have been low during shutdown. If bq27421 was just powered on, then it is not in shutdown mode as RAM will reinitialize