LTC4265

13

4265f

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

CONTACT INFORMATION

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

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.

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 specification

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 configuration extends the maximum

operating current while maintaining a smaller package

profiles. 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.