LTC4263 datasheet(14/28 Pages) LINER

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LTC4274

4274fa

OPERATION

Figure 10. Power Over Ethernet System Diagram

Overview

Power over Ethernet, or PoE, is a standard protocol for

sending DC power over copper Ethernet data wiring.

The IEEE group that administers the 802.3 Ethernet data

standards added PoE powering capability in 2003. This

original PoE spec, known as 802.3af, allowed for 48V DC

power at up to 13W. This initial spec was widely popular,

but 13W was not adequate for some requirements. In

2009, the IEEE released a new standard, known as 802.3at

or PoE+, increasing the voltage and current requirements

to provide 25W of power.

The IEEE standard also defines PoE terminology. A device

that provides power to the network is known as a PSE, or

power sourcing equipment, while a device that draws power

from the network is known as a PD, or powered device.

PSEs come in two types: Endpoints (typically network

switches or routers), which provide data and power; and

Midspans, which provide power but pass through data.

Midspans are typically used to add PoE capability to existing

non-PoE networks. PDs are typically IP phones, wireless

access points, security cameras, and similar devices, but

could be nearly anything that runs from 25W or less and

includes an RJ45-style network connector.

The LTC4274 is a third-generation single PSE controller

in either an endpoint or midspan design. Virtually all nec-

essary circuitry is included to implement a IEEE 802.3at

compliant PSE design, requiring only an external power

MOSFET and sense resistor; these minimize power loss

compared to alternative designs with on-board MOSFETs

and increase system reliability in the event a single chan-

nel is damaged.

PoE Basics

Common Ethernet data connections consist of two or four

twisted pairs of copper wire (commonly known as CAT-5

cable), transformer-coupled at each end to avoid ground

loops. PoE systems take advantage of this coupling ar-

rangement by applying voltage between the center-taps

of the data transformers to transmit power from the PSE

to the PD without affecting data transmission. Figure 10

shows a high-level PoE system schematic.

To avoid damaging legacy data equipment that does not

expect to see DC voltage, the PoE spec defines a protocol

that determines when the PSE may apply and remove

power. Valid PDs are required to have a specific 25kΩ

common-mode resistance at their input. When such a PD

is connected to the cable, the PSE detects this signature

resistance and turns on the power. When the PD is later

disconnected, the PSE senses the open circuit and turns

power off. The PSE also turns off power in the event of a

current fault or short circuit.

When a PD is detected, the PSE optionally looks for a

classification signature that tells the PSE the maximum

power the PD will draw. The PSE can use this information

to allocate power among several ports, police the current

consumption of the PD, or to reject a PD that will draw

4274 F10

S1B

SMAJ58A

58V

0.22μF

100V

X7R

1μF

100V

X7R

SMAJ58A

58V

DATA PAIR

VEE

SENSE GATE OUT

VDD

INT

SCL

SDAIN

SDAOUT

0.25Ω

IRFM120A

SPARE PAIR

LTC4274

DGND

AGND

I2C

3.3V

INTERRUPT

–54V

CAT 5

RJ45

1N4002

PSE

RCLASS

VIN

PWRGD

VOUT

LTC4265

GND

DC/DC

CONVERTER

5μF ≤ CIN

≤ 300μF

–

VOUT

GND

0.1μF

100V

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