LTC4263CS datasheet(10/24 Pages) LINER

10 / 24 page

LTC4263

4263fe

APPLICATIONS INFORMATION

POE OVERVIEW

Over the years, twisted-pair Ethernet has become the most

commonly used method for local area networking. The

IEEE 802.3 group, the originator of the Ethernet standard,

has deﬁned an extension to the standard, IEEE 802.3af,

which allows DC power to be delivered simultaneously

over the same cable used for data communication. This

has enabled a whole new class of Ethernet devices, in-

cluding IP telephones, wireless access points, and PDA

charging stations which do not require additional AC

wiring or external power transformers, a.k.a. “wall warts.”

With about 13W of power available, small data devices

can be powered by their Ethernet connections, free from

AC wall outlets. Sophisticated detection and power mon-

itoring techniques prevent damage to legacy data-only

devices while still supplying power to newer, Ethernet-

powered devices over the twisted-pair cable.

The device that supplies power is called the Power Sourcing

Equipment (PSE). A device that draws power from the

wire is called a Powered Device (PD). A PSE is typically an

Ethernet switch, router, hub, or other network switching

equipment that is commonly found in the wiring closets

where cables converge. PDs can take many forms. Digital

IP telephones, wireless network access points, PDA or

notebook computer docking stations, cell phone chargers,

and HVAC thermostats are examples of devices that can

draw power from the network.

A PSE is required to provide a nominal 48V DC between

either the signal pairs or the spare pairs (but not both)

as shown in Figure 1. The power is applied as a voltage

between two of the pairs, typically by powering the cen-

ter taps of the isolation transformers used to couple the

differential data signals to the wire. Since Ethernet data

is transformer coupled at both ends and is sent differen-

tially, a voltage difference between the transmit pairs and

the receive pairs does not affect the data. A 10Base-T/

100Base-TX Ethernet connection only uses two of the four

pairs in the cable. The unused or spare pairs can option-

ally be powered directly, as shown in Figure 1, without

affecting the data. 1000Base-T uses all four pairs and

power must be connected to the transformer center taps

if compatibility with 1000Base-T is required.

The LTC4263 provides a complete PSE solution for de-

tection and powering of PD devices in an IEEE 802.3af

compliant system. The LTC4263 controls a single PSE

port that will detect, classify, and provide isolated 48V

power to a PD device connected to the port. The LTC4263

senses removal of a PD with IEEE 802.3af compliant AC

or DC methods and turns off 48V power when the PD is

disconnected. An internal control circuit takes care of

system conﬁguration and timing.

Figure 1. System Diagram

4263 F01

SMAJ58A

58V

0.1μF

SMAJ58A

58V

DATA PAIR

SPARE PAIR

–48V SUPPLY

–48V RETURN

CAT 5

20Ω MAX

ROUNDTRIP

0.05μF MAX

RJ45

1N4002

PSE

–48VIN

–48VOUT

RCLASS

LTC4267-BASED

PD/SWITCHER

GND

OUT

5mF ≤ CIN

≤ 300μF

–

VOUT

0.1μF

VSS

OUT

VDD48

VDD5

LTC4263

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