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TPS544B20 PGOOD Sequencing to LMZ31704 INH/UVLO

Other Parts Discussed in Thread: TPS544B20, LMZ31704


I'm using a combination of TPS544B20 and LMZ31704 DC/DCs for low voltage power generation on my card.  I'm using the PGOOD output of the regulators to sequence the power-on.  I have the PGOOD output of a TPS544B20 tied directly to the INH/UVLO input of an LMZ31704.  The TPS544B20 is achieving the desired output and appears to be releasing PGOOD but PGOOD is only rising to approximately 600mV.  This isn't enough to enable the downstream LMZ31704.  

What is preventing the line from pulling up?  If TPS544B20 is in an open state, the LMZ31740 should have no problem pulling the line above the ON threshold.  I've polled the TPS544B20 via PMBus and all fault status registers read 0.

Note:  We've had no problems with LMZ31704 PGOOD tied directly to downstream LMZ31704 INH/UVLO.

  • It sounds like the PGOOD FET may be loading the INH pin of the LMZ31704. Typically, when INH is allowed to float, the voltage on this pin is 2.8V. I would like to see your schematic if possible. You can contact me directly at Thanks, Jason