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TPS2378: PowerPad Connection Issue

yifan wang
Prodigy 10 points

Part Number: TPS2378

Hi TI Engineers Hope this issue finds you well.  When I use the TPS2378 to design an IEEE802.3at PD device, I found that when I connect the PowerPad to RTN, the overall power consumption is nearly 1W higher than when the PowerPad is connected to VSS. Additionally, the dynamic response performance also becomes worse.  I have noticed that the datasheet of TPS2378 has said that the PowerPad must be connected to VSS, But I just want to know why this thermal pad will effect the electrical performance of this IC Thank you in advance for your answers

  • 0
    TI__Genius 9370 points

    Hi Yifan,

    Thank you for reaching out.

    If you connect PowerPad to RTN, the VSS and RTN is electrically connected. Thus the whole PoE PD part circuit structure is changed, and you are bypassing the PD controller.

    It might not only have impact on your dynamic response performance but also lead to failed PoE handshake.

    Please connect it to VSS as datasheet mentioned:

    Best,

    Zhining

  • 0 in reply to Zhining Zhang
    Prodigy 10 points

    Thank you Zhining.

    I noticed that section 7.3.9 in the datasheet states "PowerPAD is internally connected to VSS", but when tested with a multimeter in the case of a single IC, it indicates an open circuit between these two pins.  

    Does this mean that VSS and PowerPAD are internally connected only at a certain stage (detection stage or classification stage) before the handshake is completed by PoE, the internal MOS is turned on, and VSS and RTN start supplying power, rather than being connected all the time?

    Thank you again for your reply

    Sincerely yours,

    Yifan

  • +1 in reply to yifan wang
    TI__Genius 9370 points

    Hi Yifan,

    The internal structure or electrical connection of this part is not very straightforward to be descripted here.

    But they are definitely required to be connected on board.

    Otherwise it would not only has impact as you mentioned, it might also cause failed handshake which is not acceptable for most PoE applications.

    Best,

    Zhining

  • 0 in reply to Zhining Zhang
    Prodigy 10 points

    I understand it. Thank you!

  • 0 in reply to yifan wang
    TI__Genius 9370 points

    You are welcome Yifan.

    I'm closing this thread for now. Please feel free to reach out if you have more questions.

    Best,

    Zhining