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I'm thinking of making a simple PCB with RJ45 connector delivering both data to the control MCU and power to the whole PCB using the PoE technology. I'd like to use a LM5071 as a power source and i.e. W5100 as a TCP/IP stack.

In all of the application notes and example projects for the LM5071, I can see all 4 wire pairs connected through diode rectifiers into the IC as a PoE+ and PoE-. I assume that this connection is there in order to support power distribution over both data and spare pairs.

Can I also connect all 8 wires to the W5100, even though I know that at least 4 of the wires will be used as a power source and will have high voltages? Or do I have to design the PCB for a fixed PoE switch configuration, i.e. data pairs will always contain data and will be connected to the W5100 and spare pair will always contain power and will be connected to the LM5071?

Also, how can I make this PCB 1Gb Ethernet ready (data and power sharing the same wires)? Will I need to use different IC for TCP/IP stack?

Thanks!

Tomáš 'Guns Blazing' Frček
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  • The W5100 only accepts four wires - the RX pair and the TX pair. Those are from the other (phy) side of the ethernet transformer and disconnected from DC present on the Ethernet cable. If I remember rightly, PoE uses the cable side of the transformer? Datasheet: https://www.wiznet.io/wp-content/uploads/wiznethome/Chip/W5100/Document/W5100_DS_V128E.pdf – pjc50 Sep 02 '20 at 09:25
  • Huh? The W5100 does 100Mbps Ethernet. It doesn't use 8 wires. – user253751 Sep 02 '20 at 10:46

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