PoE+ and PoE-compliant powered device (PD) interface controllers have been unveiled by Linear Technology Corporation for applications requiring up to 90W of 
delivered power. PoE+ limits the maximum PD power delivery to 25.5W, which is insufficient to power today’s new class of power-hungry applications, such as picocells, base stations, signage and heated outdoor cameras.
The company’s LTPoE++ standard has been devised to cater to this market by expanding the power budget to four different power levels (38.7W, 52.7W, 70W and 90W), making complete high power LTPoE++ systems more accessible.
The LTPoE++ standard employs a classification scheme that enables LTPoE++ power sourcing equipment (PSE) controllers and LTPoE++ PD controllers to communicate with one another while maintaining interoperability with IEEE standard equipment.
The LT4275A (LTPoE++), LT4275B (PoE+) and LT4275C (PoE) deliver power to PD loads using just one IC.
Where traditional PD controllers integrate the power MOSFET, this device controls an external MOSFET to significantly reduce overall PD heat dissipation and make the most of power efficiency, especially important at higher power levels.
This is an interesting approach that allows users to size the MOSFET to their application’s specific heating and efficiency requirements, enabling the use of low RDS(ON) 30mOhm MOSFETs if necessary.
The LT4275 recognises a PSE as either Type 1 hardware complying with the IEEE 802.3af 13W power level, Type 2 hardware, complying with IEEE 802.3at 25.5W power level or LTPoE++ hardware complying with 38.7W to 90W power levels, and passes power accordingly. For efficient power allocation, PD users can configure a classification that represents the PD power usage.
A 100V 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 9V. This device also includes a power good output, onboard signature resistor, undervoltage lockout and comprehensive thermal protection.
This solution is offered in industrial and automotive grades, supporting operating temperature ranges from -40°C to 85°C and -40°C to 125°C, respectively,
Linear Technology
