I'm looking at hacking/building an ethernet shield to make use of a PoE switch I've got. I've been looking at a proper implementation of PoE (not just shoving current down a cat5 cable) and found the following IC, which seems to fit the bill:
It provides an open drain pin to signal when all is good - high impedance when ok, low if there's a problem. The idea (I believe) is to use this to control whether the DC/DC convertor is on and powering the device, or off due to some error. What would be the normal setup to use the open drain in this way?
Look at "Figure 4. Application circuit overview". Use a 100k pull-up resistor on the PG line. When the line goes LOW the power is good. If you have a DC/DC converter with an active-low Enable you should be able to just hook that up to PG.
Thanks for the response. I was under the impression that if "high impedance means that the power is good" was the case, then the pull up would be pulling the pin high to VDD if everything's ok, and connecting it to RTN (ground) if not. Did I misunderstand?
An enable pin on a DC/DC convertor would be exactly what's required, I just couldn't find any for the voltage range I was looking at (48v down to 5v). Wasn't even sure they existed!
then the pull up would be pulling the pin high to VDD if everything's ok, and connecting it to RTN (ground) if not.
That's right.
If the output is open drain then it will swing from 0v when the transistor is switched on to whatever voltage you pull it up to when it's not. The drawing shows it connected to VDD but if you connect the PU resistor to say 5v then that will be the high level presented to the DC/DC converter as long as they have a common GND.