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LM5145: LM5145 Pgood sink ability/pull down resistance when enable inactive(disabled)

Daniel Wang
Intellectual 1791 points
Part Number: LM5145

Hi WV Apps team,

A leading telecom company is using LM5145 in DC-DC step-down solution. Now they want to use its Pgood signal for enable signal. 

LM5145 PGood issue:

  • To use LM5145’s PGood signal to sequence/enable another DCDC converter.
  • LM5145’s PGood is currently pulled high by an earlier ready 3.3V BUS with 10k pull up resistor(earlier than LM5145 VIN active and EN active).
  • LM5145 Datasheet does not show Pgood sink capacity when VIN ready while Enable inactive. 
  • Therefore they are concerned that the PGood potential at this time may be high and mistakenly enable slave device while LM5145 output is not actually ready/good.

Here are the question:

  • What is the PGood potential or sink ability/pull down resistance if VIN is active but EN inactive(0V)? Considering pgood is pulled up by 10kohm to 3.3V.
  • Furthermore, what is the PGood potential or sink ability/pull down resistance if VIN and EN are both inactive(0V)? Considering pgood is pulled up by 10kohm to 3.3V.

Could you please kindly check and comment on above?

  • 0
    TI__Guru 66652 points

    Hi Daniel,

    VCC is not up when EN is low, so the PGOOD comparator and pulldown strength are likely not as effective. If VIN is at 0V, then there is no pulldown. The source for the PGOOD pullup should ideally be the output voltage. Then if VOUT is low when VIN and/or EN are low, there is no issue.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Intellectual 1791 points

    Hi Tim,

    Thanks a lot for your feedback. Understood that if VCC is not ready, then internal comparator may not work. The Pgood cannot be pulled down when VIN ready and En not ready.

    One last question, if VCC can be powered by an external aux power earlier than VIN ready. which means

    • VCC is powered by aux power
    • VIN is ready or not ready  but connected as below

    • EN not active

    at this point, will the Pgood comparator work as expected? 

  • 0 in reply to Daniel Wang
    TI__Guru 66652 points

    Hi Daniel,

    My understanding is that it will be active when VCC is supplied externally (as VIN will also be up), but please test this on the EVM to verify.

    Regards,

    Tim