LTC4268-1
20
42681fc
EXTERNAL INTERFACE AND COMPONENT SELECTION
Transformer
Nodes on an Ethernet network commonly interface to
the outside world via an isolation transformer (Figure
9). For powered devices, the isolation transformer must
include a center tap on the media (cable) side. Proper
termination is required around the transformer to pro-
vide correct impedance matching and to avoid radiated
and conducted emissions. For high power applications
beyond IEEE 802.3af limits, the increased current levels
increase the current imbalance in the magnetics. This
imbalance reduces the perceived inductance and can
interfere with data transmission. Transformers specifi-
cally designed for high current applications are required.
Transformer vendors such as Bel Fuse, Coilcraft, Halo,
Pulse, and Tyco (Table 4) can provide assistance with
selection of an appropriate isolation transformer and
proper termination methods. These vendors have trans-
formers specifically designed for use in high power PD
applications.
Table 4. Power over Ethernet Transformer Vendors
V
PORT
MODE OF OPERATION
Bel Fuse Inc.
206 Van Vorst Street
Jersey City, NJ 07302
Tel: 201-432-0463
www.belfuse.com
Coilcraft Inc.
1102 Silver Lake Road
Gary, IL 60013
Tel: 847-639-6400
www.coilcraft.com
Halo Electronics
1861 Landings Drive
Mountain View, CA 94043
Tel: 650-903-3800
www.haloelectronics.com
Pulse Engineering
12220 World Trade Drive
San Diego, CA 92128
Tel: 858-674-8100
www.pulseeng.com
Tyco Electronics
308 Constitution Drive
Menlo Park, CA 94025-1164
Tel: 800-227-7040
www.circuitprotection.com
IEEE 802.3af allows power wiring in either of two configu-
rations on the TX/RX wires, and power can be applied to
the PD via the spare wire pair in the RJ45 connector. The
applicaTions inForMaTion
16
14
15
1
3
2
RX
6
RX
+
3
TX
2
TX
+
RJ45
T1
PULSE H2019
42681 F09
1
7
8
5
4
11
9
10
6
8
4
5
8
7
D3
SMAJ58A
TVS
BR1
HD01
10
BR2
HD01
TO PHY
V
PORTP
V
OUT
LTC4268-1
C1
V
PORTN
V
NEG
SPARE
SPARE
+
C14
0.1礔
100V
Figure 9. PD Front-End Isolation Transformer, Diode Bridges, Capacitors and TVS
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