News and New Products

POE ICs offer increased power efficiency for PDs

By Margery Conner, Technical Editor -- EDN, 5/27/2009

Akros Silicon cannily plays the efficiency card—for both energy and space—with its new AS1854 POE (power-over-Ethernet) PD (powered-device) IC. Akros claims that the integration of parts into the 1854 PD-controller platform saves more than 75% in board space and 25% in BOM (bill-of-materials) costs. The chip enables higher power efficiency with its GreenEdge synchronous rectification, which the company claims can yield greater-than-8% improvement in efficiency in the 6 to 8W “sweet-spot” range. This range is typical for VOIP (voice-over-Internet Protocol) phones, currently the most common application for POE. GreenEdge digital-isolation technology also provides 2-kV on-chip isolation, eliminating the need for optoisolators that reduce system reliability and add cost and space. PDs require isolation between the power-management circuitry for the system devices, such as VOIP-phone electronics and display, and the primary side electronics and sensor-control circuits.

POE systems require “master” PSE (power-sourcing equipment) that provides power to networked equipment through an Ethernet cable. For an IEEE 802.3at draft version, the PSE must allocate as much as 25.5W per PD and 4.5W of cable loss. The AS1854’s PSE can determine the PD’s power usage and cable losses. In this way, you need not rely on a worst-case power budget and efficiently manage power. Budgetary pricing for the AS1854 is $4.69.

Texas Instruments has also introduced a family of three devices supporting POE PDs. The TPS2375X relies on external optoelectronic isolators rather than on-chip isolation. The TPS23754 supports 13 or 26W PD applications and provides dc/dc-converter topologies that can achieve more-than-90% power-conversion efficiency. The TPS23756 allows PDs to accept auxiliary input voltages as low as 12V, allowing the use of popular, and inexpensive 12V wall adapters. The TPS23757 supports less-than-13W systems that require the higher-efficiency dc/dc topologies. The TPS23754 is available now, and the TPS23756 will be available in the third quarter. Each comes in an HTSSOP-20 package for $1.90 (1000). The TPS23757, also available in third quarter, will sell for $1.65 (1000).