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TPS2378: Priority of Auxillary power over PoE

Electro_crazy
Intellectual 525 points
Part Number: TPS2378

Hello,

In the datasheet of TPS2378 it is mentioned that the Auxillary power supply (APD) has priority over PoE but the circuit which I have designed using this IC, the first connected power supply is used as active power. Ex.: If i connect PoE first then power is supplied from PoE even if I connect external power later.

I have attached my schematic. Could you please let me know the reason for this issue.

Thanks

  • 0
    TI__Genius 9230 points
    Hi Jain,

    I'm not seeing anything unusual about your schematic. Could you confirm that your APD pin is rising above 1.5V when you connect your adapter? Your voltage divider ratio looks fine, but there may be an assembly error.

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    Intellectual 525 points

    Hello,

    I have connected an external DC adapter of 9V but still APD does not work when PoE is connected. I have almost 10 PCBs and it does not work in any PCB so there couldn't be any assembly error.

    I have also another problem. I have now connected a load resistor (RL) of 120 Ohm at the output of the buck converter which has an output of 3.3V as shown in the attached schematic.

    So in this case 3.3V / 120 Ohm = 27.5mA , the PoE works fine.

    But when I connect RL = 47 Ohm which means 70mA at load, then the voltage at the output of buck converter is only 1.6V and the PoE chip (TPS23789) becomes hot.

    I guess that somehow the PoE chip is not supplying enough current from PoE when I connect RL = 47 Ohm which makes it hot.

    Could you please check the schematic if there is any changes required in the circuit.

    Do I have to connect CDB to some other pin?

    Thanks

  • 0 in reply to Electro_crazy
    TI__Genius 9230 points
    Hi Jain,

    The voltage divider for APD is sized for a 48V adapter (hits rising APD UVLO at 37V). Change your APD voltage divider to meet the rising APD UVLO (pg. 5 of TPS2378 datasheet) at your intended adapter voltage.

    Have you passed inrush? The CDB pin is used to keep the buck controller disabled so that inrush (charging the input bulk cap C212) can be completed. See section 7.3.2 for notes on how to implement this.

    Regards,
    Tom Amlee
  • 0 in reply to Thomas Amlee
    Intellectual 525 points
    Hi,

    The priority of the Auxillary power works now.

    Also I have connected a capacitor at the Soft start pin of the buck converter and PoE now works fine.

    Thanks for the quick support for the issue and I will mark as 'Resolved my issue' after doing few more tests to check if everything works properly.
  • 0 in reply to Electro_crazy
    TI__Genius 9230 points
    Hello Jain,

    Congrats on getting to resolution! Once you have finished your tests, please do click the green "resolved" button in order to close out this thread.

    Regards,
    Tom Amlee
    PoE PD Apps Engineer