LTC4278 datasheet(21/40 Pages) LINER

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LTC4278

4278f

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.

That is, the auxiliary power source supplies the power even

if PoE power is already present. This conﬁguration also

provides a seamless transition from PoE to auxiliary power

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 recognizes a Type 2 PSE. Initializing PD operation

in 12.95W mode eliminates interoperability issue in case

a Type 2 PD connects to a Type 1 PSE. Once the PD rec-

ognizes 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 conditions

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

SWITCHING REGULATOR OVERVIEW

The LTC4278 includes a current mode converter designed

speciﬁcallyforuseinanisolatedﬂybacktopologyemploying

synchronous rectiﬁcation. The LTC4278 operation is similar

to traditional current mode switchers. The major difference

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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