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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?.



  • Hello Diva,

    1. The PD controller receives energy from the PSE's Detection process, or effectively tries to present a valid Detection Signature while drawing as little current as possible. What you are looking for in the electrical characteristics is VSIG, which is the voltage range where the LT4294 presents a 25kOhm signature, and this defined as between 1.5V and 10V at the VPORT pin.

    The PD controller will provide IEEE 802.3bt-compliant behavior up to 60V, and the PD application should have polarity correcting and OR'ing diode bridges at the PD controller's input.

    Just a quick clarification, the PSE may use either current or voltage for its Detection measurement. Either way, it must perform at least two measurements and the voltage must be between 2.8V and 10V. For example, ADI PSEs first force two currents and measure two voltages, followed by forcing two voltages and measuring two currents.

    2. The easiest way for a PD designer to measure the detection signature is with two multimeters and a lab bench supply. You will act like a PSE by forcing a voltage, then measuring a current. For example, use the bench supply to provide 4V to the PD then measure the voltage and the current, that's the first dV/dI point. Then increase the voltage to 8V and measure the voltage and current for the second dV/dI point. You can now calculate the resistance by (V2 - V1) / (I2 - I1). The resistor shown on the top left of the block diagram is connected to VPORT, but it is disconnected outside of VSIG.