Power beyond PoE+

Date
09/12/2012

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As standards-based PoE (power over Ethernet) nears its 10th anniversary, several important applications are outstripping both the original 802.3af and 802.3at power levels. Early applications for PoE, such as enterprise telephony, could be satisfied with 802.3af's 13-W limit for PD (powered-device) input power. As applications for remote data nodes grew, so did power demands, resulting in PoE+, which can deliver 25 W per node as codified by 802.3at in 2009. In keeping with the old adage, nothing succeeds like success (or is it give em a foot and they'll take a yard), applications have again grown with ever more use for ever more power. The 802.3 standards committee, however, has yet to agree on a second extension or the classification method that will allow the PSE (power sourcing equipment) to detect the power levels appropriate for particular PDs. Alas, the world does not stand still, not even for IEEE standards committees. So product designers seeking to power their circuits through Ethernet data cables are looking to power IC makers with proprietary devices—the market demand that Linear Technology responded to with its LTPoE++ line of devices. LTPoE++ supports PoE and PoE+ standard power levels and four more that go beyond the standards' limits: 38.7 W, 52.7 W, 70 W, and 90 W. Among applications that benefit from the higher power levels are heated security cameras, picocells, POS (point-of-sale) devices, digital signage, and medical monitoring devices—any network client application that needn't be located near mains-power access. The new LT4275, for example, is a PD controller compliant with PoE, PoE+, and LTPoE++. Unlike traditional PD controllers that integrate the power MOSFET, the LT4275 controls an external MOSFET to drastically reduce overall PD heat dissipation and maximize power efficiency, especially important at higher power levels. This approach allows product designers to size the MOSFET to their application's specific heating and efficiency requirements. The LT4275 pairs autonomously with Linear Technology's PSE controllers without reliance on the LLDP (link-layer discovery protocol), which can require extensions to standard Ethernet stacks and represent a significant software development effort. Instead, one external resistor sets the PD controller's classification code to PoE and PoE+ power levels and a second external resistor identifies the LTPoE++ power level to the PSE during startup. The PD controller tolerates line surges to 100 V, protecting PDs from common line surges. It's available in industrial (-40 to 85 °C) and automotive (-40 to 125 °C) grades and includes on-chip over-temperature protection. The LT4275 is available in 10-lead MSOP and 3 x 3 mm DFN packages with prices starting at $1.45 each in 1000-piece quantities. Linear Technology

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