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TPS23756 PoE+ priority with adapter recognition

Joe Rule
Intellectual 330 points
Other Parts Discussed in Thread: TPS23756

Hello,

     I am designing with the TPS23756 for the following use case:

* PoE+ priority
* PoE low power optional reduced functionality
* secondary supply on DC adapter input

     My challenge is understanding how to avoid "reduced functionality" when on secondary DC supply.  I am currently using an OR-ing configuration that allows for PoE prioritization over DC adapter input (output of DC diode onto VDD1 rail, APD currently pulled to ARTN).  Using the T2P with an optocoupler, this allows our system processor to recognize PoE+ vs. PoE and limit certain power-consuming functionality if in PoE mode (T2P high).  But it seems in this design config, when only DC secondary supply is available, T2P remains high, giving system processor the impression that the system is on low power PoE, and in turn, limiting functionality when it is not necessary.

Is there any solution to this?  We have a no-pop resistor in place between APD input and secondary DC supply input, but I think installing this will "correct" the T2P level, but result in DC supply taking priority over PoE.

Thanks for any help!

Joe

  • 0
    TI__Mastermind 40120 points

    Hi Joe,

    You're right that populating the APD resistor divider will pull T2P Low but prioritize the adapter. Since the circuit is internal to the part, one solution I can think of is to keep APD low and have extra logic circuitry (FET in parallel with T2P pin) to pull T2P Low when PoE .at PSE is connected or adapter.