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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Detection Signature Resistor Disengagement
At this point, it is a good idea to take a step back at a very basic issue: the presence of the 25-k detection resistor. In almost all cases discussed so far, and also in related literature, this resistor is shown as a stand-alone component just past the bridge on the PD-chip-side. It is always present. So when the voltage ramps up into the classification range (~18 V), it continues to draw current, of about 18/25 k = 0.7 mA. Not a big deal, and though it adds some current to the constant class-current-sink, its effect is less than 1 mA. When the port is fully powered up, it continues to draw about 50 V/25 k = 2 mA. In terms of wattage this is 50 V × 2 mA = 100 mW. Not entirely negligible. Keep in mind ...

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