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TPS7A84: Is PG valid when only the bias input is valid?

Per Magnusson
Intellectual 500 points
Part Number: TPS7A84

Will the PG output be valid (low) on a TPS7A84 when the bias rail is valid (above 3 V) but the input voltage is still low (close to 0 V)?

  • 0
    TI__Guru 50090 points

    HI Per Magnusson1,

    PG is not dependent on VBIAS. It is a function of the output voltage being higher than 89.3% of the target regulation value:

    VBIAS is only required if you wish to have VIN between 1.1 and 1.4V or if you want to reduce VDO slightly.

    Does this answer your question?

    Regards,

  • 0 in reply to JCHK
    Intellectual 500 points
    Hi John,

    No, that was not my question. I would like it to be the case that if BIAS is e.g. 3V and IN is e.g. 0 V, the PG output should be low (since clearly the output voltage cannot be good if there is no input voltage). The PG output is obviously not reliable when there is neither any input or bias voltage available, since the chip does not have any supply to drive the gate of the open drain MOSFET of the PG output.

    If I have a pullup on the PG output in this situation with no power to the chip, I would expect the PG to be pulled high. This is undesirable, but not much can be done about it. My question is whether it is enough to give the chip a bias voltage for the PG pin to go low when the output voltage is not good (since the input voltage is too low).

    I hope that made it clearer.

    Per
  • 0 in reply to Per Magnusson
    Intellectual 500 points
    It would be great if someone from TI could answer my question regarding whether PG is reliably pulled low when VBIAS is available to the device while VIN is not available (and the output voltage therefore cannot be good, so PG should be low).

    Per
  • 0 in reply to Per Magnusson
    TI__Guru 55940 points

    Hi Per,

    Yes when BIAS is high, the PG circuitry is operational even if Vin is not available.  As such, when Vout is low, PG is also logic low.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Intellectual 500 points

    Excellent. Many thanks Ryan!

    Per