LTC4264IDE datasheet(16/24 Pages) LINER

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LTC4264

4264f

Diode Bridge

IEEE 802.3af allows power wiring in either of two conﬁgu-

rations on the TX/RX wires, and 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 data and spare inputs; therefore it is common to install

diode bridges on both inputs in order to accommodate

the different wiring conﬁgurations. Figure 9 demonstrates

an implementation of the diode bridges. The IEEE 802.3af

speciﬁcation also mandates that the leakage back through

the unused bridge be less than 28µA when the PD is

powered with 57V.

The PD may be conﬁgured to handle 2-pair or 4-pair

power delivery over the Ethernet cable. In a 2-pair power

delivery system, one of the two pairs is delivering power

to the PD—either the main pair or the spare pair, but not

both. In a 4-pair system, both the main and spare pairs

deliver power to the PD simultaneously (see Figures 1

and 2). In either case, a diode bridge is needed on the

front end to accept power in either polarity. Contact LTC

applications for more information about implementing a

4-pair PoE system.

The IEEE standard includes an AC impedance requirement

in order to implement the AC disconnect function. Capaci-

tor C14 in Figure 9 is used to meet this AC impedance

requirement. A 0.1µF capacitor is recommended for this

application.

The LTC4264 has several different modes of operation

based on the voltage present between the VIN and GND

pins. The forward voltage drop of the input diodes in a

PD design subtracts from the input voltage and will af-

fect the transition point between modes. When using the

LTC4264, 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 802.3af

speciﬁcations.

The input diode bridge of a PD can consume over 4% of

the available power in some applications. Schottky diodes

can be used in order to reduce power loss. The LTC4264 is

designed to work with both standard and Schottky diode

APPLICATIONS INFORMATION

bridges while maintaining proper threshold points for IEEE

802.3af compliance.

Figure 4 shows how two diode bridges are typically con-

nected in a PD application. One bridge is dedicated to the

data pair while the second bridge is dedicated to the spare

pair. For high power applications, a diode bridge typically

used in an IEEE 802.3af system cannot handle the higher

currents because the operating current is derated at the

upper temperature range. To solve this problem, the PD

application can utilize larger diode bridges, use discrete

diodes or consider the following alternative option.

Realizing that the two diode bridges do not need to be

exclusive to the data and spare pairs, the bridges may

be reconnected so that current is shared between them.

The new conﬁguration extends the maximum operating

current while maintaining the smaller package proﬁles.

Figure 9 shows an example of how the two diode bridges

may be reconnected. Consult the diode bridge vendors for

operating current derating curves when only one of four

diodes is in operation.

Auxiliary Power Source

In some applications, it may be necessary to power the PD

from an auxiliary power source such as a wall adapter. The

auxiliary power can be injected into the PD at several loca-

tions and various trade-offs exist. Figure 10 demonstrates

four methods of connecting external power to a PD.

Option 1 in Figure 10 inserts power before the LTC4264

interface controller. In this conﬁguration, it is necessary

for the wall adapter to exceed the LTC4264 UVLO turn-

on requirement. This option provides input current limit

for the adapter, provides a valid power good signal and

simpliﬁes power priority issues. As long as the adapter

applies power to the PD before the PSE, it will take priority

and the PSE will not power up the PD because the external

power source will corrupt the 25k signature. If the PSE

is already powering the PD, the adapter power will be in

parallel with the PSE. In this case, priority will be given to

the higher supply voltage. If the adapter voltage is higher,

the PSE should remove the port voltage since no current

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