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TPS23756: Short RTN and VSS

chentsu chung
Mastermind 6180 points
Part Number: TPS23756

Hi, 

I know that connect RTN to VSS is not recommend because this will lead to operation abnormal or damaged.

Here are the questions:

1. If in order to use PWM part of TPS23756, why I can't just short RTN to VSS? Is there any connection between PoE part and PWM part?

2. If the connection result in IC abnormal, then which step goes wrong in start-up sequence?

3. While use APD function for external adapter, then the PoE part should totally shutdown, then why connect RTN to VSS is also result in operation abnormal?

I know short RTN to VSS is not allow, but I'm curious about the operating principle, thank you very much.

  • 0
    TI__Genius 9230 points

    Hi Chentsu,

    1. The pass FET between RTN & VSS provides protection from over current & provides a inrush current limit for charging the bulk capacitor. Without the pass FET you will need to find another means of current limiting during inrush.

    2. If you short RTN to VSS, you will not be able to successfully detect. The current that would typically go through DEN pin will end up going into the bulk capacitor referenced to RTN. The purpose of the pass FET is to allow for signalling to occur on VDD-RTN before allowing current to flow into the primary side of the power stage (referenced to RTN).

    3. Using APD will force the RTN-VSS pass FET off, blocking the PoE input. This is normal operation which allows for the adapter to take priority.

    If you short RTN-VSS before PSE input, you will not be able to detect successfully & the PSE will not provide power. If you short RTN-VSS after PSE startup, then apply the adapter, the PoE input will block the adapter ORing diode if the PSE input voltage is higher than the adapter input voltage.


    Thanks,
    Tom