LTC4267-1 PoE Current limitation

Hello,

ITH/RUN has two functions. First, implied with RUN allows the part to be disabled if the pin is externally pulled low. Second, ITH, is the output of the feedback error amplifier which is its compensation point and serves as the input to the current comparator.

The voltage at the ITH/RUN pin commands the pulse-width modulator formed by the oscillator, current comparator and RS latch. Specifically, the voltage at the ITH/RUN pin sets the current comparator’s trip threshold. The current comparator monitors the voltage across a current sense resistor in series with the source terminal of the external MOSFET. It turns off the MOSFET when the current comparator resets the latch or when 80% duty cycle is reached, whichever happens first. In this way, the peak current levels through the fl yback transformer’s primary and secondary are controlled by the ITH/RUN voltage. Since the ITH/RUN voltage is increased by the error amplifier whenever the output voltage is below nominal, and decreased whenever output voltage exceeds nominal, the voltage regulation loop is closed. In an isolated system, the opto-coupler serves to increase or decrease the ITH/RUN pin's voltage instead of the feedback error amplifier.

I suggest you use the DN361 values as your starting point. You may need to adjust your sense resistor or slope compensation, depending on your transformer and output current. Please note, the 13W for a Class 0 device is defined at the input. Including the the input diodes required by the IEEE standard, the real end-to-end efficiencies for non-synchronous flyback converter will be closer to 85%. This means 0.85 x 13W = 11W of useable power at the 5V rail, or only 2.2A instead of 2.6A.

Best Regards,

Eric

I have seen using simulations how the Current through the sense resistor (green) increases with VIth/RUN (red). It makes sense to me that the current through the primary FET would continue to increase with each cycle as the output voltage (pink) and Ith_run ramps up followed by a relatively constant duty when the output voltage is in regulation. 

How is the primary current and Vsense voltage actually related to that 0.7-1.9V range for the Ith_pin? Is there a value in the datasheet that relates the two measurements? Additionally, is the only recommended method for choosing the current sense resistor through the experimental methods described in the "Current Sense Resistor Consideration" section of the datasheet? 

Thank you again!