LTC4258 LINER [Linear Technology], LTC4258 Datasheet - Page 13

• Manufacturer Part Number: LTC4258
• Description: Quad IEEE 802.3af Power over Ethernet Controller with Integrated Detection
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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
INTERRUPT
I
–48V
2
GND
C
3.3V
V
INT
SCL
SDAIN
SDAOUT
DGND BYP
V
DD
EE
0.5
SENSE GATE OUT
LTC4259A
1/4
IRFM120A
0.1 F
100V
DETECT
U
AGND
10k
S1B
PSE
CMPD3003
U
0.1 F
1k
0.47 F
100V
X7R
W
SMAJ58A
58V
Rx
Tx
U
Figure 10. System Diagram
4
5
1
2
3
6
7
6
RJ45
ROUNDTRIP
0.05 F MAX
SPARE PAIR
DATA PAIR
DATA PAIR
SPARE PAIR
20 MAX
CAT 5
RJ45
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 10. 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 10, 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.
7
6
4
5
1
2
3
6
Rx
Tx
1N4002
1N4002
4
4
0.1 F
SMAJ58A
58V
R
–48V
PD
CLASS
LTC4257
IN
GND
–48V
5 F C
PWRGD
300 F
LTC4258
OUT
IN
CONVERTER
DC/DC
13
4258p
V
4258 F10
OUT
+
–