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TPS23734: Detecting power capability of PSE on the basis of TPL > TPH and BT

Part Number: TPS23734

Hi Team 

We are  using  TPS2373-4  in  our  application we are  getting    

TPH=1, TPL =0, BT =0 (which  indicate  25.5 W ) even POE  is  not plugged  in and  system is  running from DC .

Please  let me know  following  

1) What  should  be  default state of  TPH , TPL and  BT  when POE is not  plugged  in ?

2) What  could be  possible  reason that we are  getting  TPH=1 , TPL =0, BT =0  even PSE  is  not  plugged in .

3) How  we  can differentiate bet two  different wattage  having same  TPH , TPL and  BT values  for exp   62W and  71 W.

  • Hello Abhishek,

    You tagged the TPS23734 and used the TPS23734 in your E2E thread title. But you mentioned in your text that you are using the TPS2373-4 

    The TPS23734 and the TPS2373-4 are very different devices. TPS23734 does not have TPH, TPL and BT, so I am assuming you are using the TPS2373-4.

    Directly from the datasheet, "PSE allocated power outputs, binary coded. Open-drain, active-low outputs referenced to RTN. TPL becomes open and TPH pulls low if an auxiliary power adapter is detected via the APD input"

    ***Please note, active low outputs. 

    So if you are using a DC adapter voltage, TPL becomes open (ie 0 meaning the LED is off in the EVM) and TPH becomes low (ie it is active so it is a 1 meaning the LED will come on). So what you are describing is exactly as it should be in the datasheet. 

    Table 2 in the datasheet explains what the states are depending on the PSE Type and Class. They are active low, so not active would be high impedance. 

    You cannot tell the difference between different power levels because they fall within the same PSE Type. Below, i broke the table apart into the PSE Type:
    Red - Type 1 PSE not .bt

    Blue - Type 2 PSE not .bt

    First Green - Type 1 PSE power .bt

    Second Green - Type 2 PSE power .bt

    Third Green - Type 3 PSE power .bt

    Fourth Green - Type 4 PSE power .bt

    There is no way to tell 62W and 71W apart because both would have a 5-Finger PoE Classification handshake. Additionally, there is no reason to tell them apart because a Typ2 4 PSE must be able to supply 71W at the port, so if the PD requests 71W and it is granted, it will be 71W. If the PSE demotes the PD, it will be to 51W not 62W. 

    Please review our training videos for more explanation.