Datasheet

LTC4257-1
13
42571fb
APPLICATIO S I FOR ATIO
WUUU
the overtemperature trip point. When the die cools down
below the trip point, classification current is enabled again.
If the PD is designed to operate at a high ambient tempera-
ture and with the maximum allowable supply (57V), there
will be a limit to the size load capacitor that can be charged
up before the LTC4257-1 reaches the overtemperature trip
point. Hitting the overtemperature trip point intermittently
does not harm the LTC4257-1, but it will delay completion
of capacitor charging. Capacitors up to 200µF can be
charged without a problem.
EXTERNAL INTERFACE AND COMPONENT SELECTION
Transformer
Nodes on an Ethernet network commonly interface to the
outside world via an isolation transformer (Figure 8). For
powered devices, the isolation transformer must include
a center tap on the media (cable) side. Proper termination
is required around the transformer to provide correct
impedance matching and to avoid radiated and conducted
emissions. Transformer vendors such as Pulse, Bel Fuse,
Tyco and others (Table 3) can provide assistance with
selection of an appropriate isolation transformer and
proper termination methods. These vendors have trans-
formers specifically designed for use in PD applications.
Table 3. Power over Ethernet Transformer Vendors
VENDOR CONTACT INFORMATION
Pulse Engineering 12220 World Trade Drive
San Diego, CA 92128
Tel: [phone]
FAX: [phone]
http://www.pulseeng.com/
Bel Fuse Inc. 206 Van Vorst Street
Jersey City, NJ 07302
Tel: [phone]
FAX: [phone]
http://www.belfuse.com/
Tyco Electronics 308 Constitution Drive
Menlo Park, CA 94025-1164
Tel: [phone]
FAX: [phone]
http://www.circuitprotection.com/
Diode Bridges
IEEE 802.3af allows power wiring in either of two configu-
rations on the TX/RX wires, plus power can be applied to
the PD via the spare wire pair in the RJ45 connector. The
PD is required to accept power in either polarity on both
the main and spare inputs; therefore, it is common to
install diode bridges on both inputs in order to accommo-
date the different wiring configurations. Figure 8 demon-
strates an implementation of these diode bridges. The
specification also mandates that the leakage back through
the unused bridge be less than 28µA when the PD is
powered with 57V.
Figure 8. PD Front End with Isolation Transformer, Diode Bridges and Capacitor
16
14
15
1
3
2
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
T1
PULSE H2019
42571 F08
1
7
8
5
4
11
9
10
6
8
7
D3
SMAJ58A
TVS
BR1
DF01SA
BR2
DF01SA
TO PHY
GND
8
54
LTC4257-1
C1
V
IN
V
OUT
V
OUT
SPARE
–
SPARE
+
C14
0.1µF
100V