LTC4257IS8-1 Linear Technology, LTC4257IS8-1 Datasheet - Page 13

• Manufacturer Part Number: LTC4257IS8-1
• Description: IC CONTROLLER POE INTERFAC 8SOIC
• Manufacturer: Linear Technology
• Type: Power over Ethernet Switch (PoE)r

Specifications of LTC4257IS8-1

• Applications: IP Phones, Power over LAN, Network Routers and Switches
• Internal Switch(s): Yes
• Current Limit: 350mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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.
U
RJ45
1
2
3
6
4
5
7
8
Figure 8. PD Front End with Isolation Transformer, Diode Bridges and Capacitor
U
TX
TX
RX
RX
+
–
+
–
SPARE
SPARE
+
–
W
PULSE H2019
16
15
14
11
10
9
T1
1
2
3
6
7
8
BR2
DF01SA
U
TO PHY
C14
0.1µF
100V
Table 3. Power over Ethernet Transformer Vendors
VENDOR
Pulse Engineering
Bel Fuse Inc.
Tyco Electronics
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.
SMAJ58A
BR1
DF01SA
TVS
D3
4
V
IN
LTC4257-1
V
GND
OUT
CONTACT INFORMATION
12220 World Trade Drive
San Diego, CA 92128
Tel: [phone]
FAX: [phone]
http://www.pulseeng.com/
Jersey City, NJ 07302
Tel: [phone]
FAX: [phone]
http://www.belfuse.com/
308 Constitution Drive
Menlo Park, CA 94025-1164
Tel: [phone]
FAX: [phone]
http://www.circuitprotection.com/
206 Van Vorst Street
8
5
42571 F08
C1
V
OUT
LTC4257-1
13
42571fb