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TPS23758: Weak points in design leading to failure

Matt Buckley
Prodigy 10 points
Part Number: TPS23758

Hello,

I have a design for the TPS23758 that is based on the reference designs, EVM and the schematic review guidelines provided by TI.  I've built about 25 boards with the design and have hundreds of hours of time on them, but I've now had about 5 boards that have failed with an apparent issue with the POE circuit.  I can power my boards up with a different voltage supply (VBUS) connected to the APD pin on the TPS23758, and the boards still work, suggesting it is not an issue with the load and could be with the POE circuit itself.

The full design pulls around 3.5 watts maximum (from the PSE), my POE output voltage is 5 VDC.

My question is: are there any common failure modes I missed accounting for in my design?  Or is there anything that sticks out as risky/inappropriate for this type of load?

I appreciate any help!

Best Regards, Matt

  • 0
    TI__Mastermind 20141 points

    Hi Matt,

    Thanks for reaching out to us. Looks like the issue happens before the PD interface controller.

    You may try to disconnect A1 and A3 from VSS. Please let me know if it works or not. 

    I will check your schematic in 2-5 business days.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 10 points

    Hi Diang,

    Thank you for the feedback, can you help me understand why this might be an issue?  I remember during the design phase that I had a number of questions about these pins because they are connected differently in different reference designs.  The datasheet says they are not connected internally and are for mechanical retention.

    Unfortunately it's pretty tricky for me to disconnect them because they are part of a polygon pour under the part.  I will see what I can do though.

    Thanks,

    Matt

  • 0 in reply to Matt Buckley
    TI__Mastermind 20141 points

    Hi Matt,

    Thanks for your reply.

    A 1-4 pins are not internally connected. I have a little concern if you connect most of them to VSS rather than their closest pin will cause some parasitic  capacitive coupling, which influence the PSE-PD detection.

    I will check your schematic in 1-4 business days and get back to you.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 20141 points

    Hi Matt,

    Thanks for your patience. I did not see other potential issues in your schematic besides the A1 and A3 pins.

    Another factor could cause handshake failure is the grounding connection: for example have you shorted VSS, PGND, or GND to earth?

    Best regards,

    Diang