Question about T2P pin of LT4295


My customer is considering to ues LT4295 for 802.3bt solution.

And for reviewing the spec. fo this device, they ask some questions about T2P pin.

Please refer below their questions and let me know your answers.

Q1) Where they should check the value of T2P response in below picture?

        A: at the T2P pin

        B: after photo coupler  without pull-up

        C: after photo coupler  with pull-up


       Q2) If they should measure after opto coupler, woul you let me know the reason why?



  • Hello Se-woong,

    Most PD applications are isolated and should measure the T2P signal after the opto-coupler, with a pull up resistor. This is Option C in your list.

    The T2P signal encodes the result of the physical layer classification and how much power the PSE has allocated to the PD. The PD’s Host Controller or main microprocessor should know this information. IEEE requires the ethernet cable’s conductors to be isolated from any external conductors. To be IEEE compliant, most PDs implement an isolated converter to separate the PoE ground domain from the digital ground domain. For an isolated PD based upon the LT4295, a single opto-coupler can be used to send the T2P signal across the isolation barrier. When the T2P Pin is High, current flows through the opto-coupler which pulls its collector Low. When the T2P Pin is Low, the pull-up resistor pulls the collector High. The PD’s Host Controller then measures Option C to determine allocated power. For a non-isolated PD, this information can be measured directly at the T2P pin, Option A.

    Best Regards,