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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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A Discrete PD Front-End for Testing PSEs
In Fig. 5.6 we have presented a popular circuit made of discrete components, one that has been used in almost similar forms across the industry for years in PoE testing and in finished products too. Note the LM317 is used as a programmable current source (any three-terminal regulator with a low-bias pin current can be used here). A known problem with this discrete PD front-end circuit is it does not have a very precise and predictable UVLO threshold, nor a very sharp “knee” either. But it does have almost no hysteresis and that could explain its wide popularity and usage. It corresponds to the insignificant hysteresis scenario discussed previously (see Fig. 5.4), and thereby, it enables the PSE current ...

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