LTC4264IDE datasheet(7/24 Pages) LINER

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LTC4264

4264f

APPLICATIONS INFORMATION

OVERVIEW

Power over Ethernet (PoE) continues to gain popularity as

more products are taking advantage of having DC power

and high speed data available from a single RJ45 connec-

tor. As PoE is becoming established in the marketplace,

Powered Device (PD) equipment vendors are running into

the 12.95W power limit established by the IEEE 802.3af

standard. To solve this problem and expand the application

of PoE, the LTC4264 breaks the power barrier by allowing

custom PoE applications to deliver up to 35W for power-

hungry PoE applications such as dual band and 802.11n

access points, RFID readers and PTZ security cameras.

The LTC4264 is designed to interface with custom Power

Sourcing Equipment (PSE) to deliver higher power levels

to the PD load. Off-the-shelf high power PSEs are available

today from a variety of vendors for use with the LTC4264

to allow quick implementation of a custom system. Alter-

nately, the system vendor can choose to build their own

high power PSE. Linear Technology provides complete

application information for high power PSE solutions

delivering up to 35W for 2-pair systems and as much as

70W when used in 4-pair systems.

One of the basic architectural decisions associated with

a high power PoE system is whether to deliver power

using four conductors (2-pair) or all eight conductors

(4-pair). Each method provides advantages and the

system vendor needs to decide which method best suits

their application.

2-pair power is used today in 802.3af systems (see

Figure 1). One pair of conductors is used to deliver the

current and a second pair is used for the return while two

conductor pairs are not powered. This architecture offers

the simplest implementation method but suffers from

higher cable loss than an equivalent 4-pair system.

4-pair power delivers current to the PD via two conductor

pairs in parallel (Figure 2). This lowers the cable resistance

but raises the issue of current balance between each con-

ductor pair. Differences in resistance of the transformer,

cable and connectors along with differences in diode bridge

forward voltage in the PD can cause an imbalance in the

currents ﬂowing through each pair. The 4-pair system in

Figure 2 solves this problem by using two independent

DC/DC converters in the PD. Using this architecture solves

the balancing issue and allows the PD to be driven by two

independent PSEs, for example an Endpoint PSE and a

Midspan PSE. Contact Linear Technology applications

support for detailed information on implementing 2-pair

and 4-pair PoE systems.

4264 F01

SMAJ58A

58V

0.1

µF

SMAJ58A

58V

DATA PAIR

VEE

SENSE GATE OUT

VDD

CMPD3003

10k

0.1

µF

100V

0.25

Ω

IRLR3410

S1B

SPARE PAIR

1/4

LTC4259A-1

DGND BYP

AGND

DETECT

3.3V

–54V

CAT 5

DF1501S

RJ45

PSE

RCLASS

VIN

PWRGD

VOUT

LTC4264

GND

DC/DC

CONVERTER

µF

MIN

–

VOUT

GND

0.47

µF

100V

0.1

µF

Figure 1. 2-Pair High Power PoE System Diagram

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