PM8805TR output voltage

[Sappherien]

Hello everyone 

I'm working on a student project with an STM35H563RIT6 and a Power over Ethernet (PoE) supply. I'm currently validating my design, but, reading the datasheet of the PM8805TR to do PoE, I just see the output current (max 2A) but no voltage (except VOB everywhere in the datasheet without any really data on it).

Did I passed over it without seeing the information ? Or are there any calculation to identify the output voltage ? 

Thanks in advance for your answer

Kind regards

Sappherien

[Peter BENSCH · Accepted solution]

Welcome @Sappherien, to the community!

First of all, the PM8805 is a PoE PD interface, i.e. intended for powered devices. Then please take another look at the specifications of the PoE standards, e.g. on Wikipedia. There you will find the voltage ranges 42.5V...57V (Type 3) and 41.1V...57V (Type 4) for IEEE 802.3bt.

If you then look at the PM8805 data sheet, you will see that the PoE voltage is switched from pins IN12, IN36, IN45 and IN78 to pin VOB, from where another MOSFET switches the voltage via Rsense to pin VOUT. In this respect, the PM8805 provides the PoE voltage (reduced by a few losses) at its output.

If you need a special voltage for your PoE PD systems, you can obtain it from VOUT with a voltage regulator, e.g. with an SMPS (e.g. a buck regulator).

Does this answer your question?

Regards
/Peter

[Sappherien]

Hello Peter, 

Sorry for the delay, and thanks for the welcomming ! 

Okay thank you, it is more clear to me. Does it means that the output voltage = input voltage ? Or I misunderstand ? 

[Peter BENSCH]

Yes, as mentioned the output voltage will be almost the same as the input voltage.