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TPS23751: When APD is 0V, the circuit was started up with external power supply(no POE)

Shinichi Inoue
Guru 21566 points
Part Number: TPS23751

Dear Specialists,

My customer is evaluating TPS23751 and has a question.

I would be grateful if you could advise.

---

According to the data sheet
It is stated that when operating TPS23751 with external power supply (adapter) instead of POE, it is necessary to apply 1.5V or more between APD and RTN.

However, when I connected the APD to GND and checked using an external power supply (not POE), the circuit started up and 24V was output.

Isn't it an absolute requirement to apply 1.5V or more between APD and RTN?

Could you please see attached schematic.

・ POE LAN cable is not connected
・ APD and RTN / ARTN are directly connected(R36 in the actual schematic was shorted)
・ VSS is in the OPEN state
・ External DC power supply(no POE) is connected to VDD and RTN and ARTN.

Then, gradually raise the voltage of the DC power supply from 0V.
The circuit started at about 13 to 20V.

(1) Is this phenomenon normal?

(2) What is the cause. Is there any problem in the schematic. 

(3) Could you please confirm with TPS23751EVM, whether it will work the same.

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • +1
    TI__Mastermind 21320 points

    Hey Shinichi, 

    Yes this behavior is normal. Often we do this in the lab to test the DCDC without the PoE. 

    The APD pin is there to give the adapter priority. It will disconnect the internal pass MOSFET that connects VSS and RTN. It would be bad if the PSE was applying 54V to VDD_VSS (and VSS & RTN were connected), and then directly apply 48V to VDD_RTN. 

    If you are not using PoE at all, APD is not needed. 

    We have a paper on all the ways to use PoE OR and Adapter, and how to set priority for either: https://www.ti.com/lit/an/slva306a/slva306a.pdf?ts=1626357504621 

    I will comment, APD needs to be connected to the adapter OR to RTN. It cannot be left floating. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0
    Guru 21566 points

    Hi Michael,

    Thank you for your reply.

    I understand it is a normal operation.

    I thought I needed to control the APD when using an external DC / DC.

    I'll share your comment with the customer.

    When they have an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi