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LTC4278

4278fc

APPLICATIONS INFORMATION

OVERVIEW

Power over Ethernet (PoE) continues to gain popularity

as more products are taking advantage of having DC

power and high speed data available from a single RJ45

connector. As PoE continues to grow in the marketplace,

powered device (PD) equipment vendors are running into

the 12.95W power limit established by the IEEE 802.3af

standard.

TheIEE802.3atstandardestablishesahigherpoweralloca-

tion for Power over Ethernet while maintaining backwards

compatibility with the existing IEEE 802.3af systems.

Powersourcingequipment(PSE)andpowereddevicesare

distinguished as Type 1 complying with the IEEE 802.3af/

IEEE 802.3at power levels, or Type 2 complying with the

IEEE 802.3at power levels. The maximum available power

of a Type 2 PD is 25.5W.

The IEEE 802.3at standard also establishes a new method

of acquiring power classification from a PD and communi-

cating the presence of a Type 2 PSE. A Type 2 PSE has the

option of acquiring PD power classification by performing

2-event classification (layer 1) or by communicating with

the PD over the data line (layer 2). In turn, a Type 2 PD

must be able to recognize both layers of communications

and identify a Type 2 PSE.

The LTC4278 is specifically designed to support the front

end of a PD that must operate under the IEEE 802.3at

standard. In particular, the LTC4278 provides the T2P

indicator bit which recognizes 2-event classification. This

indicator bit may be used to alert the LTC4278 output load

that a Type 2 PSE is present. With an internal signature

resistor, classification circuitry, inrush control, and ther-

mal shutdown, the LTC4278 is a complete PD Interface

solution capable of supporting in the next generation PD

applications.

MODES OF OPERATION

The LTC4278 has several modes of operation depending

on the input voltage applied between the VPORTP and

VPORTN pins. Figure 1 presents an illustration of voltage

and current waveforms the LTC4278 may encounter with

the various modes of operation summarized in Table 1.

DETECTION V1

CLASSIFICATION

OFF

POWER

BAD

OFF

τ = RLOAD C1

PWRGD TRACKS

VPORTN

DETECTION V2

TIME

–10

TIME

–20

–30

–40

–50

INRUSH

POWER

BAD

PWRGD

TRACKS

VPORTP

PWRGD

TRACKS

VPORTP

POWER

BAD

POWER

BAD

TIME

POWER

GOOD

POWER

GOOD

DETECTION I1

CLASSIFICATION

DETECTION I2

LOAD, ILOAD

4278 F01

ICLASS DEPENDENT ON RCLASS SELECTION

INRUSH = 100mA

I1 = V1 – 2 DIODE DROPS

25kΩ

ILOAD =

VPORTP

RLOAD

I2 = V2 – 2 DIODE DROPS

25kΩ

VPORTP

PSE

IIN

RLOAD

RCLASS

PWRGD

LTC4278

VNEG

VPORTN

IN DETECTION

RANGE

Figure 1. VNEG, PWRGD, PWRGD and PD

Current as a Function of Input Voltage

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