I want to use the PM8805 as POE controller in a custom project for a 25W classified POE device. The PM8805 should generate the input power for 2 boards in this device.
However, when connecting only one board with approx. 30uF input capacitance, the power supply works without any problem. But when connecting the 2nd board with approx. 20uF additional capacitance, the power supply fails and the PM8805 is in a state, where it tries to switch on repeatedly and gets certainly hot. The POE LED on my switch is constantly lit in this situation.
My understanding of this behaviour is that it is caused by the overall inrush current out of the PM8805 into the 2 boards!?
To be able to develop an inrush current limiter without using the "real" boards, i bought a STEVAL-POE001V1 which i got on my table now.
Connecting the STEVAL-POE001V1 to my little POE-switch (TL-SG105PE) without any load makes the power-LED (D2 - blue) blinking with approx. 1s interval and so the POE-LED of my switch. (With a multimeter i can observe that 56V are switched on and off again on the output of the board)
I already change the resistors R14 (36.5Ohm --> 51Ohm, should result in a classification of 12.5W) and R15 (51Ohm --> open) but D2 is still blinking.
Following questions:
1. Is there any application note about PM8805 which can make me better understnding the behaviour of it.
2. Is there any closer description of the over-current protection of PM8805?
3. What can cause the described behaviour of the evaluation board?
I would appreciate any kind of help as i have to decide wheather to proceed with PM8805 or not. For my feeling, the POE controller with integrated FET-based rectifier bridge is a very smart idea!
Best regards,
Marco Schulz.
