Develop PoE with Si3402
I had bad luck designing PoE despite a lot of effort. The problem is that there is a lot of standards for PoE (2 main), using different wires on the cable, and that from my experience a lot of product doesn't even follow the standard or are out of spec. 

You should read first the PoE standard and then check which standard this chip is using.

PoE has a fairly complex startup and timing limitation in power draw and you can't just hookup a standard power supply, you need a proper PoE supply, with the standard that fits your chip.

You can see the power-up sequence on Figure 5 of the datasheet.