Im using the MP3924 for a single channel PoE PSE controller with a 30W limit set using a 30K resistor on the PMAX pin but I am not able to draw more than 24W before a PMAX limit is trigged. The PD device connected is detected as class 4 which should at least be capable of 25.5W but the port is powered off before it can reach that power. How does the MP3924 determine the power consumed used to trigger the PMAX limit?
It is just a current limit. So Class 4 threshold can be as low as 201mV on the current sense R ( 0.25ohm) which would be a current of 804mA. What is the tolerance on your current sense R? Is perhaps your layout poor? Maybe your wires are too skinny? How far is the load from the PSE how good is that connection.
Hi to answer your questions:
Rsense resistor used is 1% tolerance, 1206 package
Load is 3m away from PSE controller
Ethernet conductors are 24AWG
Rsense lines are connected in a kelvin connections and are 0.254mm width. Power traces are 1.5mm width on a 70um copper layer.
I’ve measured the voltage across Rsense to be 115.6mV before it cuts power to the PD. I’ve tested the DC rail supplying the MP3924 and its capable of 45W before failing. I should also note that the MP3924 is set to auto mode with no additional configuration via I2C.
I’ve tested changing the PMAX resistor to 39K and this has allowed me to get more than 25W at the PD but Im not keen on arbitrarily setting the PMAX value high without a good justification.
so that looks like the class 0-3 threshold, not the class 4. The setting isn’t arbitrary, a class 3 device is 15W so support expecting 25W out of a port that is set for 15 is likely not going to work
You could set for class 4 200mv and then use a higher sense resistor to back the actual threshold down. It is a bit goofy and could have larger system implications but it could achieve the goal you have outlined.