Hi,
I’d like to use SCDIS mode to determine power allocated by PSE
I’m confused as to how I’m meant to determine between a 40W and 51W power allocation, for my application I need to know if I’ve got the full 51W available or not.
I note that if I don’t use SCDIS I seem to also have the same issue?
Thanks
Hello Aivaras,
Thanks for your question. I understand your source of confusion. Something important about PoE is that PD's have 8 classes, but PSE's have 4 types. Please look at the table below:
So as you can see, a Type 3 PSE can support either class 4 or class 5. But there is no such thing as a Class 4 PSE. If it is type 3, it can do up to 60W (51W at the PD). Does that make sense?
So from the PSE perspective, during the handshake, the class 4 & 5 are determined in finger 4:
So once we get to the fourth finger, as you mentioned, there is no distinction between Class 4 and Class 5. What TPH, TPL, BT (and SCDIS ) show is what Type of PSE (in this case type 3, 4pairs). My understanding of the PSE is that if it can supply up to 51W on that port for Class 5, it will. I do not think the PSE's demote from Class 5 to Class 4.
And all SCDIS does is determine if you want the code in serial or parallel.
If it is any help, Table 2 in the TPS23880 datasheet explains power demotion and how classes operate.
If this post answers your question, please indicate so by marking this thread as resolved. Thank you.
Regards,
Michael P.
Applications Engineer
Texas Instruments
