LTC4263
10
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 de ned 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 con guration and timing.
Figure 1. System Diagram
4263 F01
SMAJ58A
58V
0.1糉
0.1糉
Tx
Rx
Rx
Tx
SMAJ58A
58V
DATA PAIR
DATA PAIR
SPARE PAIR
SPARE PAIR
48V SUPPLY
48V RETURN
CAT 5
20?MAX
ROUNDTRIP
0.05糉 MAX
RJ45
4
5
4
5
1
2
1
2
3
6
3
6
7
6
7
6
RJ45
1N4002
4
1N4002
4
PSE
PD
48V
IN
48V
OUT
R
CLASS
LTC4267-BASED
PD/SWITCHER
GND
OUT
5mF d C
IN
d 300糉
+
V
OUT
0.1糉
0.1糉
V
SS
OUT
V
DD48
V
DD5
LTC4263
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