LTC4259A datasheet(13/32 Pages) LINER

13 / 32 page

LTC4259A

4259Af

APPLICATIO S I FOR ATIO

Figure 11. System Diagram

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, known as

802.3af, which allows DC power to be delivered simulta-

neously over the same cable used for data communica-

tion. This promises a whole new class of Ethernet devices,

including 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 monitor-

ing techniques prevent damage to legacy data-only de-

vices, while still supplying power to newer, Ethernet-

powered devices over the twisted-pair cable.

A device that supplies power is called 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 charg-

ers, 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 11. The power is applied as a voltage

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

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 differentially,

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 2 of the 4 pairs

in the cable. The unused or spare pairs can be powered

directly, as shown in Figure 11, without affecting the data.

However, 1000base-T uses all 4 pairs and power must be

connected to the transformer center taps if compatibility

with 1000base-T is required.

way, the condition causing the LTC4259A to pull the INT

pin down must be removed before the LTC4259A will be

able to pull INT down again. This can be done by reading

and clearing the event registers or by writing a 1 into bit 7

of this register. Setting bit 7 releases the Interrupt pin,

clears all the Event registers and clears all the bits in the

Interrupt register.

4259A F11

SMAJ58A

58V

0.1

µF

SMAJ58A

58V

DATA PAIR

VEE

SENSE GATE OUT

VDD

INT

SCL

SDAIN

SDAOUT

CMPD3003

10k

0.1

µF

100V

0.5

Ω

IRFM120A

S1B

SPARE PAIR

1/4

LTC4259A

DGND BYP

AGND

DETECT

I2C

3.3V

INTERRUPT

–48V

CAT 5

Ω MAX

ROUNDTRIP

0.05

µF MAX

RJ45

1N4002

×4

1N4002

×4

PSE

RCLASS

–48VIN

PWRGD

–48VOUT

LTC4257

GND

DC/DC

CONVERTER

µF ≤ CIN

≤ 300µF

–

VOUT

GND

0.47

µF

100V

X7R

0.1

µF

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