ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 13

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
OVERVIEW
Power over Ethernet (PoE) continues to gain popularity
as an increasing number of products are taking advantage
of having DC power and high speed data available from a
single RJ45 connector. 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 LTC4268-1 breaks
the power barrier by allowing custom PoE applications
to deliver up to 35W for power hungry PoE applications
such as dual band access points, RFID readers and PTZ
security cameras.
The LTC4268-1 is designed to be a complete solution
for PD applications with power requirements up to 35W.
The LTC4268-1 interfaces with custom Power Sourcing
Equipment (PSE) using a high effi ciency fl yback topol-
ogy for maximum power delivery without the need for
optoisolator feedback. Off-the-shelf high power PSEs are
available today from a variety of vendors for use with the
LTC4268-1 to allow quick implementation of a custom
system. Alternately, 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.
–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
1k
0.1μF
0.47μF
100V
Figure 1A. 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
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 1A). 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 1B). This lowers the cable resis-
tance but raises the issue of current balance between each
conductor 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 1B 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
LTC4268-1
LTC4268-1
TYP APP
13
42681fa
42681 F01a
V
OUT
+
–