LTC4265 LINER [Linear Technology], LTC4265 Datasheet - Page 13

• Manufacturer Part Number: LTC4265
• Description: IEEE 802.3at High Power PD Interface Controller with 2-Event Classifi cation Recognition
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Bel Fuse, Coilcraft, Halo, Pulse, and Tyco (Table 4) can
assist in selecting an appropriate isolation transformer
and proper termination methods.
Table 4. Power-over-Ethernet Transformer Vendors
VENDOR
Bel Fuse Inc.
Coilcraft Inc.
Halo Electronics
Pulse Engineering
Tyco Electronics
Input Diode Bridge
Figure 2 shows how two diode bridges are typically con-
nected in a PD application. One bridge is dedicated to the
data pair while the other bridge is dedicated to the spare
pair. The LTC4265 supports the use of either silicon or
Schottky input diode bridges. However, there are tradeoffs
in the choice of diode bridges.
CONTACT INFORMATION
206 Van Vorst Street
Jersey City, NJ 07302
Tel: [phone]
www.belfuse.com
1102 Silver Lake Road
Gary, IL 60013
Tel: [phone]
www.coilcraft.com
1861 Landings Drive
Mountain View, CA 94043
Tel: [phone]
www.haloelectronics.com
12220 World Trade Drive
San Diego, CA 92128
Tel: [phone]
www.pulseeng.com
308 Constitution Drive
Menlo Park, CA 94025-1164
Tel: [phone]
www.circuitprotection.com
An input diode bridge must exceed the maximum current
the PD application will encounter at the temperature the
PD will operate. Diode bridge vendors typically call out
the operating current at room temperature, but derate the
maximum current with increasing temperature. Consult
the diode bridge vendors for the operating current derat-
ing curve.
A silicon diode bridge can consume over 4% of the available
power in some PD applications. Using Schottky diodes can
help reduce the power loss with a lower forward voltage.
A Schottky bridge may not be suitable for some high
temperature PD application. The leakage current has a
voltage dependency that can reduce the perceived signature
resistance. In addition, the IEEE 802.3af/at specifi cation
mandates the leakage back-feeding through the unused
bridge cannot generate more than 2.8V across a 100k
resistor when a PD is powered with 57V.
Sharing Input Diode Bridges
At higher temperatures, a PD design may be forced to
consider larger bridges in a bigger package because the
maximum operating current for the input diode bridge is
drastically de-rated. The larger package may not be ac-
ceptable in some space-limited environments.
One solution to consider is to reconnect the diode bridges
so that only one of the four diodes conducts current in
each package. This confi guration extends the maximum
operating current while maintaining a smaller package
profi les. Figure 7 shows how to reconnect the two diode
bridges. Consult the diode bridge vendors for the de-rating
curve when only one of four diodes is in operation.
LTC4265
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