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TPS23755: Option 1 input ORing without APD pin

Part Number: TPS23755
Other Parts Discussed in Thread: TPS23753A, TPS23753

The TPS23755 is lacking the APD pin of the TPS23753A for option 1 ORing of auxiliary power sources on the input side.

We have a 48 to 56VDC aux power source, which should have priority over PoE power supply. With the TPS23753 this was easily accomplished. How would we do this on the TPS23755 without the APD pin?

  • Hi Michael,

    Thanks for reaching out to us!

    For option 1 location, if you do nothing at the circuit, there should be 6 cases:

    1. No power -> adapter: work

    2. No power -> PSE: work

    3. PSE -> adapter (PSE has higher voltage): adapter cannot replace PSE, and it may cause adapter to PSE short circuit

    4. PSE -> adapter (PSE has lower voltage): adapter replaces the PSE to source the load, but no current limitation may cause adapter to PSE short circuit (if ideal diode bridge is used as the rectifier) and PD overcurrent protection

    5. adapter -> PSE (PSE has higher voltage): PSE failed detection, adapter higher priority

    6. adapter -> PSE (PSE has lower voltage): PSE failed detection, adapter higher priority

    Case #3 and #4 could cause troubles. I would recommend you to add the additional parts in red color. Pull DEN to Vss can turn off the internal hotswap between Vss and RTN. So you need to make Vgs high first to turn off the PSE, then set a RC time to make Vgs low again later. After Vgs is low, the adapter can provide the power and PSE will fail the detection.

    This is just a preliminary thought. Please also consider other side effects. 

    Best regards,

    Diang