High efficiency PoE++ PD controller offers up to 90-W delivered power

August 27, 2012 // By Paul Buckley
Linear Technology Corporation has introduced the LT4275, LTPoE++, PoE+ and PoE-compliant powered device (PD) interface controller for applications requiring up to 90 W of delivered power. 

PoE+ limits the maximum PD power delivery to 25.5 W, which is insufficient to power today’s new class of power-hungry applications, such as picocells, base stations, signage and heated outdoor cameras.  Linear Technology’s LTPoE++ Standard addresses this market by expanding the power budget to four different power levels (38.7 W, 52.7 W, 70 W and 90 W), enabling complete high power LTPoE++ systems.  The LTPoE++ standard employs a classification scheme that simply enables LTPoE++ power sourcing equipment (PSE) controllers and LTPoE++ PD controllers to reliably communicate with one another while maintaining interoperability with IEEE standard equipment.  The LT4275A (LTPoE++), LT4275B (PoE+) and LT4275C (PoE) efficiently deliver power to PD loads using just one IC.

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 novel approach allows users to size the MOSFET to their application’s specific heating and efficiency requirements, enabling the use of low RDS(ON) 30 mOhm MOSFETs if necessary.  

The LT4275 recognizes a PSE as either Type 1 hardware complying with the IEEE 802.3af 13 W power level, Type 2 hardware, complying with IEEE 802.3at 25.5 W power level or LTPoE++ hardware complying with 38.7 W to 90 W power levels, and passes power accordingly.  For efficient power allocation, PD users can configure a classification that represents the PD power usage.  A 100 V abs max rated input voltage means the LT4275 easily survives and protects PDs from the most common Ethernet line surges.  A programmable auxiliary power pin with signature corrupt provides support down to 9 V.  The LT4275 also includes a power good output, onboard signature resistor, undervoltage lockout and comprehensive thermal protection.

Availability and Pricing

The LT4275 is offered in industrial and automotive grades, supporting operating temperature ranges from -40 to 85°C and -40 to 125°C, respectively, and is available in