LTC4257IS8 Linear Technology, LTC4257IS8 Datasheet - Page 12

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

Specifications of LTC4257IS8

• 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
LTC4257
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
Pulse Engineering
Bel Fuse Inc.
Tyco Electronics
12
U
RJ45
Figure 8. PD Front End with Isolation Transformer, Diode Bridges and Capacitor
CONTACT INFORMATION
12220 World Trade Drive
San Diego, CA 92128
Tel: [phone]
FAX: [phone]
http://www.pulseeng.com/
206 Van Vorst Street
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/
U
1
2
3
6
4
5
7
8
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
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 accommodate the
different wiring configurations. Figure 8 demonstrates 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.
The IEEE standard includes an AC impedance requirement
in order to implement the AC disconnect function. Capaci-
tor C14 in Figure 8 is used to meet this AC impedance
requirement. A 0.1µF capacitor is recommended for this
application.
The LTC4257 has several different modes of operation
based on the voltage present between the V
pins. The forward voltage drop of the input diodes in a PD
design subtracts from the input voltage and will affect the
transition point between modes. When using the LTC4257,
it is necessary to pay close attention to this forward
voltage drop. Selection of oversized diodes will help keep
the PD thresholds from exceeding IEEE specifications.
The input diode bridge of a PD can consume 4% of the
avialable power in some applications. It may be desirable
to use Scottky diodes in order to reduce this power loss.
SMAJ58A
BR1
DF01SA
TVS
D3
4
V
IN
LTC4257
V
GND
OUT
8
5
4257 F08
C1
V
OUT
IN
and GND
4257fb