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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Detection Signature Resistor beyond Detection
The first use of this resistor is in ultra-low-cost PDs. Suppose there are no classification-current-sinks present inside a PD, but there is a 25-k detection resistor across the port always. If so, in the class-event range, the minimum current the detection resistor will draw is about 15 V/25 k = 0.6 mA. This will be naturally interpreted as Class 0 by any PSE. Therefore, power-up will occur, whether optimal or not.
Let us do some more accurate estimates on the dashed discharge curves in Fig. 4.7 for example. We ask how long does it take for a maximum of 670 nF port capacitance (the “must accept” range, see bottom of Fig. 3.5) to discharge from 14.5 V (the max value of VMARK_TH) down to 10 V (the ...

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