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TPS74701: PG status when no power on IN PIN

Lin_Charles
Expert 4260 points
Part Number: TPS74701
Other Parts Discussed in Thread: LMZ31707, TPS74401

Hi team,

It is nice to know you here. My customer wants to use the TPS74701 PG signal to control LMZ31707 enable pin. However, 3.3V comes first and then12V, and then 1.5V. And they need 0.9V power on first and then 1.2V. They are afraid that before 1.5V power on, TPS74401 PG is on high impedance status, which will lead to 1.2V power on first. Could you please help check what is PG status if no power on TPS74701? Thanks.

BR,

Charles Lin

  • 0
    TI__Genius 13085 points

    Hi Charles,

    Yes the PG signal will pull up to 3.3V if the TPS747 does not have an input voltage. Without an input voltage the internal circuitry cannot activate the pull-down FET used to keep the PG signal low. 

    The best thing to do is usually to pull PG up to Vout, since Vout will be low until Vin is present. I see though that Vout is 0.9V, if that is too low for the LMZ device to recognize as a logic high signal, then is it possible to use Vin instead? If PG is pulled up to Vin there is a narrow range of Vin voltages where the PG signal can float up when Vin is low(typically 0.6V -0.9V) however if that doesn't register as a logic high signal then the LMZ device would stay disabled. 

    If neither of those solutions are acceptable they may want to consider using a standalone supervisor.