I am looking into using the LT4293 for use within a design; however, I am confused about two issues.Looking at Table 6 in the LT4293's datasheet (LT4293 Interoperability), it shows that 802.3 lower class PSE devices will agree upon demotion for higher class PD devices and still supply power, yet lower class LTPoE++ PSE devices will deny power to higher class PD devices.
My first question is if I try to power a 90W LTPoE++ PD from a 38.7W LTPoE++ PSE, LTPoE power will obviously be denied, but will they mutually agree upon a Type 2 / Class 4 / 25.5W power connection or will the PD simply remain off with no power?
My second question is is there any way to have the LT4293 "demote" mismatched LTPoE++ combinations in a way similar to how 802.3 does? My goal for this project is to have the device be able to run off of any 802.3 or LTPoE++ PSE.
High Power IEEE 802.3bt and LTPoE++ PDs change their Class Signature on the third Class Event. These PDs count the number of Class Events to determine their allocated power. LTPoE++ PSEs only provide three classification events, and LTPoE++ PDs interpret three events as full power. IEEE 802.3bt PDs treat two class events and three class events as the same 25.5W power allocation. This allows IEEE 802.3bt PSEs to identify PDs without allocating more power. They may then provide more events to allocate more power.
For your question; a LTPoE++ 38.7W PSE provides three class events to all LTPoE++ PDs, because the third event lets the PSE to know the PD’s requested power. 38.7W LTPoE++ PSEs then deny power to PDs requesting more than 40W. The PD simply remains unpowered, otherwise the PD would interpret three class events as its requested power.
Demotion is not implemented in most LTPoE++ PSEs. It was assumed that high-power PD applications do not have a useful function in a power limited state. Your LTPoE++ 90W PD should be used with IEEE 802.bt or LTPoE++ 90W PSEs. Perhaps you could implement a “Legacy” switch that changes the PD from LTPoE++ 90W to LTPoE++ 38.7W, which would not be denied by any LTPoE++ PSE.