RD-180 - LM5072 5V out 25W IEEE 802.3at Compliant POE+ PD Power Reference Design

Design Description

This LM5072 based 5V out 25W POE+ PD power supply reference design was prototyped to be fully compliant with IEEE 802.3at standard. The design converts 48 Vdc to 5 Vdc. It can deliver up to 25W power as specified by the IEEE 802.3at standard. It has a 2 event detection circuit which can identify the PSE as either Type one or Type two. ( See National Semiconductor Application note `Fully Compliant IEEE 802.3at Interface Using the LM5072/LM5073` for details ) The 2 event detection circuit is located on the bottom layer of the PC board. In those applications where this 2 event detection feature is not required, the user only needs to populate top layer components.

The input current limit is 800 mA. The efficiency of this design is high enough to keep input current under 800mA at 38Vin full load. The input under voltage lock has 7V hysteresis, with 38V rise and 31V fall. This design also has output short circuit protection.

This design can also be powered by a 24V AUX power supply. In this configuration, the AUX input port bypasses the LM5072 internal hot swap MOSFET and allows input current to be greater than the 800 mA limit.

•	RD-180 Reference Design
•	CAD Design Files (source schematic, board layout and gerber files)

Additional Product Resources

Key Features of the LM5072 5V out 25W IEEE 802.3at Compliant POE+ PD Power Design

• DC/DC converter efficiency of 85% at 48V in 25W load
• With added two event detection circuit, this design is fully compliant with IEEE 802.3at
• Integrated POE+ RJ45 magetics connector provides Ethernet isolation
• Low ripple voltage <20 mV, tight dynamic load regulation <75 mV
• Inrush current limit, 800 mA input over current protection
• Output short circuit protection, soft start, input under voltage protection
• 24V AUX input port with Oring diode
• Uses standard FA2898-AL transformer from Coilcraft