Datasheet

LTC4266A/LTC4266C
16
4266acfc
OPERATION
Figure 10. Power over Ethernet System Diagram
4266AC F10
S1B
S1B
SMAJ58A
0.22µF
100V
X7R
1µF
100V
X7R
Tx
Rx
Rx
Tx
SMAJ58A
58V
DATA PAIR
DATA PAIR
V
EE
SENSE GATE OUT
V
DD
INT
SCL
SDAIN
SDAOUT
0.25
SPARE PAIR
SPARE PAIR
1/4
LTC4266
DGND AGND
I
2
C
3.3V
INTERRUPT
–48V
CAT 5
20 MAX
ROUNDTRIP
0.05µF MAX
RJ45
4
5
4
5
1
2
1
2
3
6
3
6
7
8
7
8
RJ45
1N4002
=4
1N4002
=4
PSE PD
R
CLASS
–48V
IN
PWRGD
–48V
OUT
LTC4265
GND
DC/DC
CONVERTER
5µF ≤ C
IN
≤ 300µF
+
–
V
OUT
GND
0.1µF
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
common-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
more power that the PSE has available. For a 802.3af PSE,
the classification step is optional; if a PSE chooses not to
classify a PD, it must assume that the PD is a 13W (full
802.3af power) device.
New in 802.3at
The newer 802.3at standard supersedes 802.3af and brings
several new features:
• A PD may draw as much as 25.5W. Such PDs (and the
PSEs that support them) are known as Type 2. Older
13W 802.3af equipment is classified as Type 1. Type 1
PDs will work with all PSEs; Type 2 PDs may require
Type 2 PSEs to work properly. The LTC4266A/LTC4266C
is designed to work in both Type 1 and Type 2 PSE de-
signs, and also supports non-standard configurations
at higher power levels.
• The Classification protocol is expanded to allow Type 2
PSEs to detect Type 2 PDs, and to allow Type 2 PDs to
determine if they are connected to a Type 2 PSE. Two
versions of the new Classification protocol are avail-
able: an expanded version of the 802.3af Class Pulse
protocol, and an alternate method integrated with the
existing LLDP protocol (using the Ethernet data path).
The LTC4266A/LTC4266C fully supports the new Class
Pulse protocol and is also compatible with the LLDP
protocol (which is implemented in the data communica-
tions layer, not in the PoE circuitry).
• Fault protection current levels and timing are adjusted
to reduce peak power in the MOSFET during a fault;
this allows the new 25.5W power levels to be reached
using the same MOSFETs as older 13W designs.