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PMP20859: Questions

Part Number: PMP20859
Other Parts Discussed in Thread: TPS3808, TPS2372, TPS2373

Hi Experts,

I have a few questions regarding this design:

  1. Why is there a need for the UVLO of the converter to be pulled low by Q9? I thought that unless the inrush phase is finished, PG stays low and disconnects PWR_GND from RTN1. Shouldn’t the converter be off even without pulling UVLO low?
  2. What is the reason for having Q5, Q9, Q10, and Q12? Can one not go straight to Q9?
  3. I assume that D9 and D10 are used for protection. What situation would potentially cause them to conduct? Perhaps when Q11 turns on and there is a difference in voltage between DC and POE?
  4. What are D14 and D16 for? For D16 case, isn't Q11 already blocking current flow during classification?
  5. What is the TPS3808 used for?

Thank you,


  • Hi Sith,

    1.  Under most conditions PWR_GND is disconnected from RTNx by the low side FET and diode.  During testing some marginal conditions were found that required the PG connection through Q9.

    2.  The PG signals from the two TPS2372 PD controllers are on different returns (RTN1 and RTN2).  These components are needed to level shift the two PG signals to PWR_GND.

    3.  D9 and D10 discharge the gates of the FETs at power down.  Again during testing we found some marginal conditions, such as rapidly applying/disconnecting the power, where the gates did not discharge completely and we were not able to achieve seamless transition.

    4.  If there is a pre-bias between VDD/PWR_GND (such as when AUX or the second PD is powering the circuit), the body diode of Q11 (Q16) and the TPS2372 hot swap will allow a bias between VDD/VSS_x.  This would disrupt the detection and not allow the PSE to apply power.  D16 (D26) blocks the pre-bias voltage from appearing across VDD/VSS_x.  I don't recall the reason for D14 (D22).  I'm sure they were added as a result of testing.

    5.  This converter operates with AUX priority.  To achieve seamless transition, the AUX must be in regulation range before Q11 (Q16) are turned off, disconnecting the PoE power.  The TPS3808 monitors AUX and only turns off Q11 (Q16) when AUX is above 40V.

    Thanks, David

  • Hi David,

    Thank you for your answers. I have a follow up question for 2.

    I am trying to adapt this design for the TPS2373 and will only have a single POE input. Does this mean that PG can be connected straight to Q9?



  • Sith,

    I have only tested the design with one channel removed, but with all the level shift circuits.  It might be possible to make a direct connection.  I would keep the circuit as is and just remove one PoE channel.  You can make an option for the direct connection and see if you get seamless transition between the PoE and AUX.