Electronic – Why ground is positive on this part


I'm using a powered device controller (PD is what they call a device that receives power over Ethernet), and its pins and way of connecting things is really weird. This part, LTC4265, takes in power over Ethernet (POE), negotiates how much power POE can provide to it, and then outputs that power.

What I'm really curious about is why its GND pin is called "input voltage positive rail" and its Vout is "output voltage negative rail". If you look at the data sheet at http://cds.linear.com/docs/Datasheet/4265fa.pdf, page 2 will show a typical application circuit. Also, pin definitions are on page 5. Anyway, it shows Vout outputting negative voltage. Essentially, the positive POE voltage is connected to GND pin, which is then connected to ground, and the negative POE voltage is connected to Vout. Why do they do things this way?

Thanks in advance for the help!

Best Answer

Because in telecommunications (phone central offices), things run off of -48 V. So there are plenty of parts and equipment built that way.

This link explains why. It has to do with electroplating.