I'm bringing up a POE PD based on LTC4269-2, and it doesn't feel like working. I would like to ask for the following information, that I think could be helpful for me:
1) What is the allowed current intensity for VNEG-VPORTN connection? The datasheet only vaguely mentions in "Absolute Maximum Ratings":
VNEG Pull-Up Current ..................................................1A
but it is not explained whether it is RMS current, or peak current, and whether it refers to a particular shape of waveform.
2) In what conditions PWRGD becomes low? The datasheet seems to only mention inrush current (at power on?) and excessive temperature. What other symptoms can be observed then?
In my case I got LTC4269-2 switching with limited duty cycle - max. 15%, but due to a mistake I had SD_VSEC only pulled high to VPORTP by 240k, and that resulted in more than 8V there. Taking into account the plots shown in the datasheet, the 15% could have been the "expected" limit, and the measured output voltage of ca. 3.5V agreed with the calculated value (power supply 48V, turns ratio of the transformer 2:1, duty cycle 15% - results in 3.6V).
I have populated a missing resistor, getting 240k/10k divider at SD_VSEC, closely resembling the divider shown in diagram of typical application in the datasheet. I expected to measure voltage of around 2V afterwards, but now I read almost zero at SD_VSEC, and I think that is result of PWRGD output pulling low. VREF outputs nothing, but VIN-VNEG is a bit below 15V, so the chip correctly connects VNEG to VPORTN.
I will appreciate any suggestions on how to debug or fix the problem.