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LTC4278

4278fc

An input diode bridge must be rated above the maximum

current the PD application will encounter at the tempera-

ture 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

derating curve.

Asilicondiodebridgecanconsumeover4%oftheavailable

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

voltagedependencythatcanreducetheperceivedsignature

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 derated. The larger package may not be accept-

able 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

profile. Figure 7 shows how to reconnect the two diode

bridges. Consult the diode bridge vendors for the derating

curve when only one of four diodes is in operation.

Input Capacitor

The IEEE 802.3af/at standard includes an impedance

requirement in order to implement the AC disconnect

function. A 0.1µF capacitor (C14 in Figure 7) is used to

meet this AC impedance requirement.

Transient Voltage Suppressor

The LTC4278 specifies an absolute maximum voltage of

100V and is designed to tolerate brief overvoltage events.

However, the pins that interface to the outside world can

routinely see excessive peak voltages. To protect the

LTC4278, install a transient voltage suppressor (D3) be-

tween the input diode bridge and the LTC4278 as shown

in Figure 7.

APPLICATIONS INFORMATION

Figure 7. PD Front-End with Isolation Transformer, Diode Bridges,

Capacitors, and a Transient Voltage Suppressor (TVS)

RX–

RX+

TX–

TX+

RJ45

COILCRAFT

ETHI - 230LD

4278 F07

SMAJ58A

TVS

B1100

BR1

HD01

BR2

HD01

TO PHY

VPORTP

LTC4278

VPORTN VNEG

SPARE–

SPARE+

C14

0.1µF

100V

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