LTC4269-1 current spikes

Hello,

Let me confirm, you're testing LTC4269-1, DC1335B-C and you're seeing large input current spikes while testing with the auxiliary input? Your auxiliary supply is set to 44V and have a 1 Ohm series resistor to monitor current?

First, the converter is not seeing 44V. There is the current sense resistor voltage drop, about 0.6V, and the Schottky diode drop, another 0.7V. That will throw off your power calculations slightly. 

Second, IEEE 802.3at does not specify auxiliary mode current limits. The PD must not backfeed power onto the ethernet cable, but it can draw as much current as required from the auxiliary supply. It also does not specify auxiliary supply input capacitance, so you can add a bulk capacitor between the AUX+ and AUX- terminals. I would recommend this if you're using a 1 Ohm series current sense resistor.

How are you probing the sense resistor? Long leads to easily inject noise into this measurement.

Best Regards,

Eric

The reason for using AUX input and resistors for the load was to simplify the example. This way you could verify the results using your evaluation board.

To be more specific, the 44.1 V I measured initially was between GND and Aux+ using a handheld multi-meter at max load. When the current was flowing, the 0.6 V drop you mentioned would definitely be reflected in the scope measurement. That's why I measured it using a multi-meter but forgot to mention that.

The probe I am using the measure the sense resistor is a Tektronix P6245 probe connected to a Tektronix TDS 684B oscilloscope.

-------------------------------------

Here are scope shots without Aux input. I see the current spikes when I have a PSE (MAX5971A) connected to SPARE1, SPARE2 instead. This time the input voltage is 56 V and the load is not simply resistors. It's a custom PCB running from the 12.1 V PD output.

Since I am still seeing current spikes using a 802.3at PSE, could I increase the values on the Pi filter at the input of the transformer? L2 C5 on the DC1335B-C?

The reason for using AUX input and resistors for the load was to simplify the example. This way you could verify the results using your evaluation board.

To be more specific, the 44.1 V I measured initially was between GND and Aux+ using a handheld multi-meter at max load. When the current was flowing, the 0.6 V drop you mentioned would definitely be reflected in the scope measurement. That's why I measured it using a multi-meter but forgot to mention that.

The probe I am using the measure the sense resistor is a Tektronix P6245 probe connected to a Tektronix TDS 684B oscilloscope.

-------------------------------------

Here are scope shots without Aux input. I see the current spikes when I have a PSE (MAX5971A) connected to SPARE1, SPARE2 instead. This time the input voltage is 56 V and the load is not simply resistors. It's a custom PCB running from the 12.1 V PD output.

Since I am still seeing current spikes using a 802.3at PSE, could I increase the values on the Pi filter at the input of the transformer? L2 C5 on the DC1335B-C?