LTC4266 LINER [Linear Technology], LTC4266 Datasheet - Page 14

• Manufacturer Part Number: LTC4266
• Description: Quad IEEE 802.3at Power over Ethernet Controller
• Manufacturer: LINER [Linear Technology]

LTC4266
OPERATION
Overview
Power over Ethernet, or PoE, is a standard protocol for
sending DC power over copper Ethernet data wiring.
The IEEE group that administers the 802.3 Ethernet data
standards added PoE powering capability in 2003. This
original PoE spec, known as 802.3af, allowed for 48V DC
power at up to 13W. This initial spec was widely popular,
but 13W was not adequate for some requirements. In
2009, the IEEE released a new standard, known as 802.3at
or PoE + , increasing the voltage and current requirements
to provide 25W of power.
The IEEE standard also defines PoE terminology. A device
that provides power to the network is known as a PSE, or
power sourcing equipment, while a device that draws power
from the network is known as a PD, or powered device.
PSEs come in two types: Endpoints (typically network
switches or routers), which provide data and power; and
Midspans, which provide power but pass through data.
Midspans are typically used to add PoE capability to existing
non-PoE networks. PDs are typically IP phones, wireless
access points, security cameras, and similar devices, but
could be nearly anything that runs from 25W or less and
includes an RJ45-style network connector.
The LTC4266 is a third-generation quad PSE controller
that implements four PSE ports in either an endpoint or
midspan design. Virtually all necessary circuitry is included
to implement a IEEE 802.3at compliant PSE design, requir-
ing only an external power MOSFET and sense resistor per
14
SMAJ58A
–48V
GND
INTERRUPT
1µF
100V
X7R
I
2
C
3.3V
V
INT
SCL
SDAIN
SDAOUT
DGND
V
DD
EE
0.25
SENSE GATE OUT
LTC4266
S1B
1/4
IRFM120A
AGND
PSE
0.22µF
100V
X7R
Figure 10. Power Over Ethernet System Diagram
S1B
Rx
Tx
4
5
1
2
3
6
7
8
RJ45
ROUNDTRIP
0.05µF MAX
SPARE PAIR
DATA PAIR
DATA PAIR
SPARE PAIR
20 MAX
CAT 5
channel; these minimize power loss compared to alterna-
tive designs with on-board MOSFETs and increase system
reliability in the event a single channel is damaged.
PoE Basics
Common Ethernet data connections consist of two or four
twisted pairs of copper wire (commonly known as CAT-5
cable), transformer-coupled at each end to avoid ground
loops. PoE systems take advantage of this coupling ar-
rangement by applying voltage between the center-taps
of the data transformers to transmit power from the PSE
to the PD without affecting data transmission. Figure 10
shows a high-level PoE system schematic.
To avoid damaging legacy data equipment that does not
expect to see DC voltage, the PoE spec defines a protocol
that determines when the PSE may apply and remove
power. Valid PDs are required to have a specific 25k com-
mon mode resistance at their input. When such a PD is
connected to the cable, the PSE detects this signature
resistance and turns on the power. When the PD is later
disconnected, the PSE senses the open circuit and turns
power off. The PSE also turns off power in the event of a
current fault or short circuit.
When a PD is detected, the PSE optionally looks for a
classification signature that tells the PSE the maximum
power the PD will draw. The PSE can use this information
to allocate power among several ports, police the current
consumption of the PD, or to reject a PD that will draw
RJ45
7
8
4
5
1
2
3
6
Rx
Tx
1N4002
1N4002
×4
×4
0.1µF
SMAJ58A
58V
R
–48V
PD
CLASS
LTC4265
IN
GND
–48V
PWRGD
5µF ≤ C
≤ 300µF
OUT
IN
CONVERTER
DC/DC
4266fb
V
4266 F10
OUT
+
–