National Semiconductor LM5072 Manual

LM5072 5V out 25W IEEE

802.3at Compliant POE+ PD

Power Eval_Board

National Semiconductor

RD-180

PowerWise Design Center

November 16, 2009

1.0 Design Specifications

Inputs

Output #1

VinMin=38V

Vout1=5V

VinMax=60V

Iout1=5A

2.0 Design Description

Eval Board Description

 This LM5072 based 5V out 25W POE+ PD power supply is

fully compliant with IEEE 802.3at standard. This eval board

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 de-

tection 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 eval

board is high enough to keep input current under 800mA at

38Vin full load. The input under voltage lock has 7V hystere-

sis, with 38V rise and 31V fall. This eval board also has output

short circuit protection.

  This eval board 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.

LM5072

  The LM5072 Powered Device (PD) interface and Pulse-

Width-Modulation (PWM) controller provides a complete pow-

er solution for the PD connecting into Power over Ethernet

(PoE) networks. This controller integrates all functions nec-

essary to implement both a PD powered interface and DC-DC

converter with a minimum number of external components.

The low RDS(ON) PD interface hot swap MOSFET and pro-

grammable DC current limit makes LM5072 suitable for both

IEEE 802.3af and IEEE 802.3at compliant PD devices. The

100V maximum voltage rating simplifies selection of the tran-

sient voltage suppressor that protects the PD from network

transients. The LM5072 includes an easy to use PWM con-

troller. The PWM control scheme is based on peak current

mode control, which provides inherent advantages of line

feed forward, cycle by cycle current limit and simplifies feed-

back loop compensation.

IEEE 802.3af and IEEE 802.3at are the PoE specifications

this design is compliant with.

3.0 Features

■

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

4.0 Schematic

© 2009 National Semiconductor Corporation

www.national.com

LM5072 5V out 25W IEEE 802.3at Compliant POE+ PD Power Eval_Board

RD-180