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TPS23753A: TPS23753A – Question about 802.3 bt don’t support Type-1 Class-0

Part Number: TPS23753A

We use TPS23753A solution and setting to Class-0 (Rcls = 1.27K ohms). We have learned that newer 802.3bt PSEs don’t officially recognize Class-0. May I know that is correct or not? If so, do you have any suggestion?

Our application is IP-Camer for indoor or outdoor. Sometime the power will down to around 1W. If we change to Class-3, may I know it has any impact? Thanks

BR, Gary

  • hello Gary, 

    Please see the above graphic. Class 0 stands for the Legacy class the predates the IEEE802.3.af standard (2002). Class 3 and Class 0 are the same amount of power. Switch class resistor to class 3

    The power per class is the maximum power available to the PD -- this is how much the PSE allocates to the port the PD is connected to. The PD can draw as little power as needed after negotiation, as long as it meets MPS. 

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

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • Hi Michael P,

    Above table show the Class0 has min power 0.44W and Class3 has 6.49W for the design constrain. It’s over MPS. Our application is IP-Camer for indoor or outdoor. Sometime the power will down to around 1W. If we switch to Class3, the input power is low than 6.49W. May I know it has any impact? Thanks

    BR, Gary

  • Gary, 

    Unfortunately this table is confusing in the way it conveys information. It was written a long time ago. 

    "Power AT PD PI" means what is the maximum load the PD will draw? If the PD drew 8W, then it would fall between the Class 3 minimum of 6.49W and class 3 maximum 12.95W. So it would be a class 3 device.

    If the PD drew 4W, then it would fall between the Class 2 minimum 3.84W and Class 2 maximum 6.49W. So it would be a class 2 device. 

    Once the power is negotiated with the PSE, then the PD can draw up to the Class Maximum and go as low as MPS. 

    Does that make sense?

    If this does not clarify, I suggest getting a PD and PSE EVM to understand how power allocation and power draw work together. 

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

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments