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LTC4278

4278fc

for PoE dominance. Furthermore, PD applications may

also opt for a seamless transition — that is, without power

disruption — between PoE and auxiliary power.

The most common auxiliary power option injects power at

VNEG. Figure 9 presents an example of this application. In

this example, the auxiliary port injects 48V onto the line via

diode D1. The components surrounding the SHDN pin are

selected so that the LTC4278 does not disconnect power

to the output until the auxiliary supply exceeds 36V.

This configuration is an auxiliary-dominant configuration.

Thatis,theauxiliarypowersourcesuppliesthepowereven

if PoE power is already present. This configuration also

providesaseamlesstransitionfromPoEtoauxiliarypower

when auxiliary power is applied, however, the removal of

auxiliary power to PoE power is not seamless.

Contact Linear Technology applications support for detail

information on implementing a custom auxiliary power

supply.

IEEE 802.3at SYSTEM POWER-UP REQUIREMENT

Under the IEEE 802.3at standard, a PD must operate under

12.95W in accordance with IEEE 802.3at standard until it

recognizesaType2PSE.InitializingPDoperationin12.95W

mode eliminates interoperability issue in case a Type 2

PD connects to a Type 1 PSE. Once the PD recognizes a

Type 2 PSE, the IEEE 802.3at standard requires the PD to

wait 80ms in 12.95W operation before 25.5W operation

can commence.

MAINTAIN POWER SIGNATURE

In an IEEE 802.3af/at system, the PSE uses the maintain

power signature (MPS) to determine if a PD continues to

require power. The MPS requires the PD to periodically

draw at least 10mA and also have an AC impedance less

than 26.25k in parallel with 0.05μF. If one of these condi-

tions is not met, the PSE may disconnect power to the PD.

SWITCHING REGULATOR OVERVIEW

The LTC4278 includes a current mode converter designed

specificallyforuseinanisolatedflybacktopologyemploying

synchronousrectification.TheLTC4278operationissimilar

totraditionalcurrentmodeswitchers.Themajordifference

is that output voltage feedback is derived via sensing the

output voltage through the transformer. This precludes

the need of an opto-isolator in isolated designs, thus

greatly improving dynamic response and reliability. The

LTC4278 has a unique feedback amplifier that samples a

transformer winding voltage during the flyback period and

uses that voltage to control output voltage. The internal

blocks are similar to many current mode controllers.

The differences lie in the feedback amplifier and load

APPLICATIONS INFORMATION

Figure 9. Auxiliary Power Dominant PD Interface Example

4278 F09

TVS

TO PHY

36V

100k

10k

BR1

–

BR2

–

0.1µF

100V

VPORTP

LTC4278

VPORTN

SHDN

VNEG

GND

–

ISOLATED

WALL

TRANSFORMER

RX–

RX+

TX–

TX+

RJ45

SPARE–

SPARE+

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