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TPS23754: TPS23754 no output when use PSE TPS23861

Shiheng Huang
Prodigy 155 points
Part Number: TPS23754

Dear all:

  TPS23754 no output when use PSE TPS23861 power supply, output normal use 48V power supply, the follow is the sch, please help check , thank you!

pd.pdf

  • 0
    TI__Mastermind 41760 points

    Hi User,

    It looks like your VSS and RTN is shorted together on your schematic, so the system is probably failing detection. VSS and RTN should be separate grounds. VSS relates to the PD negotiation side with the PSE while the RTN ground relates to the load side.

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    Prodigy 155 points

    Dear Darwin：

    Why does VSS and RTN not separate from the grounds cause detection failure?And I want to know what will cause the detection to fail?Thanks!

  • 0 in reply to Shiheng Huang
    TI__Mastermind 41760 points

    Hi Huang,

    There are two things that would cause detection to fail if you short VSS and RTN:

    1. The effective input capacitance will be dominated by the bulk cap. The max input cap is 0.12uF per the standard and bulk cap can be as high as 47uF.

    2. The impedance of the RTN components will be lower than the 25k resistance required for detection. The detection current will be larger (outside of PSE acceptable range) so PSE will not turn ON.

      

  • 0 in reply to Darwin Fernandez
    Prodigy 155 points

    Dear Darwin：

    Thank you very much for your answer.I want to know more about the working process of this chip.Can you tell me that in addition, is there another important reason that will cause the detection to fail?Thanks!

  • 0 in reply to Shiheng Huang
    TI__Mastermind 41760 points

    Hi Shiheng,

    Sure no problem, all our PoE PD ICs follow the IEEE802.3 standards. We have training videos that goes over key phases in PoE (from plugin to normal operation to plug out of the RJ45) in the link below.

    Other than this, what also caught my eye in your schematic is D7 should be unidirectional.

    T2P functionality should be used like described in the datasheet and the FAQ below.