LTC4264
7
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 fl owing 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
0.1µF
Tx
Rx
Rx
Tx
SMAJ58A
58V
DATA PAIR
DATA PAIR
V
EE
SENSE GATE OUT
V
DD
CMPD3003
10k
1k
0.1µF
100V
0.25Ω
IRLR3410
S1B
SPARE PAIR
SPARE PAIR
1/4
LTC4259A-1
DGND
BYP AGND
DETECT
3.3V
–54V
CAT 5
DF1501S
DF1501S
RJ45
4
5
4
5
1
2
1
2
3
6
3
6
7
8
7
8
RJ45
PSE PD
R
CLASS
V
IN
PWRGD
V
OUT
LTC4264
GND
DC/DC
CONVERTER
5µF
MIN
+
–
V
OUT
GND
0.47µF
100V
0.1µF
Figure 1. 2-Pair High Power PoE System Diagram
