Questions regarding the PoE PD controllers in general.

Hi Engineer Zone Team

I have a few questions regarding the PoE PD controllers in general although my application intends to use the LT4294.

Question 1. – Where does the PD controller receive power during the detection process.

In the PD controllers which have both PoE and AUX inputs and when the AUX input is not present the only source of power to the PD controller is through the VPORT pin as shown on first page of the LT4294 datasheet. But during the detection process the PSE will either use a voltage or current source to detect the signature resistance which is typically 25k. Assuming the PSE uses a voltage it is required by the standard to take two measurements which are at least separated by 1V. Furthermore, the voltage range used for detection by the PSE should be in the range 2.8V to 10V.  Now if the RJ45 connector at the PD has in built diodes then there will be two diode drops, say a total of 1V. This implies that the VPORT voltage now drops to a minimum of approximately 1.8V. Hence can I assume that the PD controller will function at this voltage. Is this the reason that there is no minimum voltage specified in the data sheet for LT4294 under the specification “VPORT Operating Input Voltage”?.

Question 2. – Can I measure the PD detection signature resistance of 25k with a multi-meter.

If the signature resistor is still present after detection, can I measure this resistance with a multi meter. Here I am assuming that the PD does not disconnect it to safe some power.

Is this the resistor shown connected to VPORT on the top left of the LT4294 block diagram?.