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TPS23881: Power supply procedure between PSE and PD

JH Shin
Guru 10880 points
Part Number: TPS23881
Other Parts Discussed in Thread: TPS2372,

Hello,

My customer is using the TPS23881 and TPS2372 to support the 802.3bt standard.

They think that the power supply between the PSE and the PD will be initiated via the signal below. Is it correct?

If you have a detailed description of how the above signals are exchanged between PSE and PD, please share.

Thank you.

JH

  • 0
    TI__Genius 15275 points

    Hi JH,

    Yes, this is called the handshake process between the PSE and PD. The PSE will send a detection signal to initiate the transaction between the PSE and PD. There will be two signals sent and the current/voltage measured between them. This will give a resistance value which should be around 25kohms. Once detection is completed, classification begins. During classification, the PSE is determining how much power the PD is requesting. This is done through 5 finger classification, with each finger representing a different power level. Once classification is done, this initiates startup and normal operation. 

    For more information on this process or PoE training series please visit the link below. 

    https://training.ti.com/poe-handshaking

    Very respectfully,

    Brandon Beader

  • 0 in reply to Brandon Beader
    Guru 10880 points

    Hi Brandon,

    Thanks for your kind reply.

    My customer has additional questions about the handshake process.

    Does Vport-Alt A and Vport-Alt B in the first question mean the Alternative-A and Alternative-B above?

    It is said that PoE worked normally when line 4 was disconnected in the field.

    Does PoE not work when a problem occurs in which line among lines 1 to 8?

    Thanks a lot.

    JH

  • 0 in reply to JH Shin
    TI__Genius 15275 points

    Hi JH,

    Yes you are correct, Vport-Alt A and Vport-Alt B refer to Alternative A (Pairs 1,2, and 3,6) and Alternative B(Pairs 4,5 and 7,8) respectively. 

    Yes depending on the load, even if a channel is disconnected it will still operate. In 4 pair there are 2 channels (Alt A is one channel and Alt B is the other channel. If the current falls below the DC disconnect threshold in one channel, this channel will power down. If it is a single signature class 4 and below PD, one channel will remain on to power the load. 

    Very respectfully, 

    Brandon Beader