AND8247

In addition to the signature and classification circuitry, the PD must also include circuitry to limit the inrush current from the PSE to 400 mA when the input voltage is applied, and to prevent any quiescent currents or impedances caused by the DC−to−DC converter to be ignored during the signature and classification processes.

converters at the load end of the cables which transform the 48 V to logic levels such as 5.0 Vdc or 3.3 Vdc or both, to power the communications equipment. The PDs should be able to operate with a maximum average input power of 12.95 W, and should be able to tolerate an input voltage range of 36 to 57 Vdc. In addition, a certain "protocol" is required in which the PD is detected (Signature Mode) and then classified (Classification Mode) according to its maximum power level.

A solution to one aspect of Power Over Ethernet (POE) is presented here utilizing the ON Semiconductor NCP1031 series of monolithic, high voltage switching regulators with internal MOSFET. The AND8247 application note provides details for constructing an inexpensive, high efficiency, 6.5 W, 5.0 V DC output power supply along with the associated input circuitry for responding to POE detection and classification protocol. ON Semiconductor also can provide a demonstration PC board with this circuitry upon request.