A new application note from Maxim Integrated Products describes how to design a single-port power sourcing equipment (PSE) system to meet the IEEE802.3af signature, classification and power-management requirements. The single-port PSE in this example is designed using the MAX5922A IC. A copy of the application note, “Hot-Swap and Power Switching Circuits,” is available online at www.maxim-ic.com/appnotes.cfm/appnote_number/3507.
The MAX5922A circuit is configured as a midspan PSE and used as a single-port, midspan power injector. In this design, the J1 RJ-45 connector input receives data from an Ethernet router/switch and directly routes the data to the output. The circuit injects 48 V into the spare pairs (pins 4, 5, 7 and 8) of the J1 RJ-45 connector output. For an endpoint PSE application, the DCA pin may be connected to DGND and the output power to the signal pairs with PoE magnetics.
Power-over-Ethernet (PoE) technology enables switch/router and midspan systems, collectively named Power Sourcing Equipment (PSE), to deliver up to 15 W of power over the data cable. The power-receiving system is called a powered device (PD). To meet the IEEE802.3af standard, which governs how the PSE and PD interact, the PSE must detect and classify the PD correctly before powering it. Once the PD is powered, the PSE must sense the PD disconnection and power down the cable promptly. The PSE also must protect the cable and the PD from overcurrent and short-circuit conditions.