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TPS23753A: Power-on failure with specific ethernet switch (PSE)

Feng bin Su
Expert 8970 points
Part Number: TPS23753A


Hi team,

My customer feedback  will occur for the first time (white circle), but will power up for the third time (red frame), but will not occur with PoE injector and the PD is using our TPS23753A

The TPS23753A is Class 2

Switch is Class 7

We have a total system of about 5.12W (full load)

We suspect that it has triggered  PSE protection function during start-up. Therefore, we have asked customer to adjust class level of TPS23753A from class 2 to class 3 but it doesn't fix the issue.

Is there any protection technique at PD side  which will turn off the POE power? Or is there any suggestion that we can isolate the issue.

Please refer the zoom-in waveform of  vdd and current. Would you think that it is PSE protection to turn-off the POE  power?

Also refer the attached sifos test of TPS23753a and customer also asked about why do they fail at MPS test item. Thank for your help.

TPS23753a_PoE Test report.xlsm

  • 0
    TI__Expert 3700 points

    Hi, 

    Thank you for your question. 

    It looks like the MPS power is not being met during the first and second power-ons which is why we see the power cycling. 

    If you can turn on the load sooner so that it reaches the minimum power to meet MPS quicker I think that will help. 

    Alternatively, if you could add a dummy load of about 0.5W and I think this will help meet MPS requirements. 

    Let me know if you have any questions about this. Thanks! 

    Best Regards,

    Brett Colteaux 

  • 0 in reply to Brett Colteaux
    TI__Expert 8970 points

    Hi Brett,

    Thank for your help.

    The PoE test pass with dummy load at primary side POE_vdd.

    Because of small PCB dimension, customer would like to know that could the circuit add dummy load at secondary-side?

    If so, how to choose value/size of resistor?

    How many resistors dose the circuit need?

  • 0 in reply to Feng bin Su
    TI__Expert 3700 points

    Hi, 

    Thank you for your question. 

    One resistor can be placed on the secondary side from 5V_PoE to GND. 

    The current should be about 0.5W/5V = 100mA. The resistance should be 50 ohm (5V/100mA). I recommend a 1W rated resistor be used for thermal reasons. 

    Feel free to let me know if you have any questions about this. Thanks! 

    Best Regards,

    Brett Colteaux