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LM3880-Q1: Abs Max of FALG pin

K.Hamamoto
Guru 12155 points
Part Number: LM3880-Q1

Hi 

I have a question about the pull-up voltage of FLAGx pin.

At the datasheet, the recommend voltage of FLAGx pin is -0.3V~VCC+0.3V. However the abs max is -0.3V~6.0V.

Is it possible the FLAGx pin is pulled up to 5V when the VCC is applied by another supply (e.g 3.3V)?

I refereed the following post. 

https://e2e.ti.com/support/power_management/svs_and_sequencers/f/212/t/551649

In this case, the voltage of FLAG pin does not exceed the abs max. However it is exceeded the recommend voltage. 

Regards,

Koji Hamamoto

  • 0
    TI__Guru 66170 points
    Hi Koji-san

    When Flags are pull-up to different VCC Rail, they actually follow closely to the spec "0.3V + VCC". So if you have 5V pull up on Flags but 3.3 for VCC, the FLAGs may close to 3.3V instead of 5V.

    It is always recommended to use the same VCC rail for Flags' Pull-up.

    What're the reasons to use different Vcc for Flags?

    Regards
    Yihe
  • 0 in reply to yihe
    Guru 12155 points
    Hi Yihe-san

    Thank you for your prompt reply.

    The reason is to reduce the current consumption of the VCC at standby.
    The pull-up supply for FLAG pin is not turned on at the standby . That is why the pull-up supply is different with Vcc.

    We need to talk with our customer about this. Then I will feedback to you.

    Regards,
    Koji Hamamoto
  • 0 in reply to K.Hamamoto
    TI__Guru 66170 points
    H Koji-san

    Maybe the follow method is possible to solve the issue:
    Adding a schottky diode in the VCC path may help. If Vcc = 3.3 and the FLAG pin is pulled up to 5V, the diode will prevent back-powering of the 3.3V rail and the FLAG output will be able to be pulled up to it's full value.

    Regards
    Yihe
  • 0 in reply to yihe
    Guru 12155 points
    Hi

    Thank you for your support.
    Do you mean the diode is added to VCC rail in series at VCC pin?

    If it is not, could you explain to me how can we add the diode?

    Regards,
    Koji Hamamoto