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TPS23756: PoE communication of TPS23756...

Peter Tseng
Genius 5780 points
Part Number: TPS23756
Other Parts Discussed in Thread: TPS23734EVM-094

Dear Sirs,

My customer design their PD product with TPS23756 and set as Class4.

If customer disabled some functions in their product and make total power consumption under 12W, then connected to 802.3af PSE.

Will it has issue to communicate with 802.3af PSE?

How many power will PD product get from PSE?

Thanks.

  • 0
    TI__Mastermind 21320 points

    Hello Peter, 

    Yes the TPS23756  can negotiate <12W. If a device is 802.3.at compliant, then it is also 802.3.af compliant. Same is true for 802.3.bt. They are backwards compatible. 

    The amount of power will depend on the Class resistor. But the resistor can be set to Class 0-3, whatever the customer needs. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Genius 5780 points

    Dear Michael,

    Just double confirmed, what you mean is, the 802.3af PSE can communicate with 802.3at device even set as Class4, right?

    And, the 802.3at device only can get Max 13W from 802.3af PSE, right?

    Thanks.

  • 0 in reply to Peter Tseng
    TI__Mastermind 21320 points

    Hello,

    No. If the class resistor is set to Class 4, a 802.3.af PSE will not recognize that resistance as a valid class. A 802.3.af PSE only knows Class 0-3. Unless the PSE has a default setting to send 13W, but I do not think this is 802.3.af standard. 

    The TPS23756 can be set to class 3 and therefore negotiate class 3 power, but not if the class is set to 4. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    TI__Mastermind 21320 points

    Hello, 

    I tested the TPS23734EVM-094 with a 802.3.af PSE to confirm the behavior. I used the PD5301 as the PSE. 

    The PSE and PD successfully negotiated Class 3 power (13W). The T2P (TPL) pin remained high, indicating the PD knew it was not a 802.3.at PSE. 

    So it appears some PSE's will default to send Class 3 power when a PD shows Class 4. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments