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TPS74801: When can PG drive low during startup?

Part Number: TPS74801

Tool/software:

I apply VBIAS (5V) first, and then VIN much later. Is TPS74801 able to drive PG low with only VBIAS applied?

  • Hi George,

    If only VBIAS is applied and VIN is not present, there will be no VOUT. The PG output is high-impedance when VOUT is greater than VIT + VHYS. If VOUT drops below VIT or if VBIAS drops below 1.9 V, the open-drain output turns on and pulls the PG output low. So, in the absence of VOUT, PG pin will be low with only VBIAS applied.

    Best regards

    Ishaan

  • Just to confirm, only VBIAS is required by TPS74801 to drive the PG output low? That is, the internal PG open-drain transistor circuit can be solely powered from VBIAS (with no dependency on VIN)?

  • That is not what I meant. If VBIAS is present but VIN is absent, then VOUT will be 0V. As long as PG is connected to any 5.5-V or lower rail through an external pullup resistor, the comparator for the PG function will trigger and drive the FET to force PG low.

  • The circuit that drives the FET to force PG low is powered from which voltage?

  • Hi George. The circuit that drives the FET to force PG low is powered from VBIAS. So VIN is not needed to drive the PG FET Low