ltc4264cde-trpbf Linear Technology Corporation, ltc4264cde-trpbf Datasheet - Page 7

• Manufacturer Part Number: ltc4264cde-trpbf
• Description: High Power Pd Interface Controller With 750ma Current Limit
• Manufacturer: Linear Technology Corporation

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
–54V
GND
3.3V
V
DGND BYP
V
DD
EE
0.25Ω
LTC4259A-1
SENSE GATE OUT
1/4
IRLR3410
0.1µF
100V
DETECT
AGND
10k
S1B
CMPD3003
PSE
0.1µF
1k
0.47µF
100V
Figure 1. 2-Pair High Power PoE System Diagram
SMAJ58A
58V
Rx
Tx
4
5
1
2
3
6
7
8
RJ45
DATA PAIR
DATA PAIR
SPARE PAIR
SPARE PAIR
CAT 5
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.
RJ45
7
8
4
5
1
2
3
6
Rx
Tx
DF1501S
DF1501S
0.1µF
PD
SMAJ58A
58V
R
V
IN
CLASS
LTC4264
GND
PWRGD
LTC4264
V
MIN
5µF
OUT
CONVERTER
DC/DC
7
V
4264f
4264 F01
OUT
+
–