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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Ensuring Proper Power-Up Behavior
There are strange interactions and complications during Power-up, involving UVLO-based instability, cable-resistance drops, negative-input impedance of switching converters, and so on, all of which make the entire area of PD-PSE design during Power-up rather complicated and misunderstood. The standard does seem ambiguous itself, if not misleading, at places on this particular aspect. So we do need to fall back on engineering judgment eventually.
As prospective PSE designers, we need to accept market reality too: Commercial PD’s are often rightly considered low-tech, and we should expect to find many out there with all sorts of strange behavior during the critical Power-up phase. We should therefore try to make ...

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