LTC4265 datasheet(6/20 Pages) LINER

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LTC4265

4265f

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.

The IEE802.3at standard establishes a higher power alloca-

tion for Power-over-Ethernet while maintaining backwards

compatibility with the existing IEEE802.3af systems. Power

Sourcing Equipments (PSE) and Powered Devices are

distinguished as Type-1 complying with the IEEE 802.3af

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 IEEE802.3at standard also establishes a new method of

acquiring power classiﬁcation from a PD and communicat-

ing 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 LTC4265 is speciﬁcally designed to support the front

end of a PD that must operate under the IEEE802.3at

standard. In particular, the LTC4265 provides the T2PSE

indicator bit which recognizes 2-event classiﬁcation. This

indicator bit may be used to alert the LTC4265 output load

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

resistor, classiﬁcation circuitry, inrush control, and ther-

mal shutdown, the LTC4265 is a complete PD Interface

solution capable of supporting in the next generation PD

applications.

MODES OF OPERATION

The LTC4265 has several modes of operation depending on

the input voltage applied between the GND and VIN pins.

Figure 1 presents an illustration of voltage and current

waveforms the LTC4265 may encounter with the various

modes of operation summarized in Table 1.

DETECTION V1

CLASSIFICATION

UVLO

POWER

BAD

UVLO

= RLOAD C1

PWRGD TRACKS

VIN

DETECTION V2

TIME

–10

TIME

–20

–30

–40

–50

INRUSH

POWER

BAD

PWRGD

TRACKS

GND

PWRGD

TRACKS

GND

POWER

BAD

POWER

BAD

TIME

POWER

GOOD

POWER

GOOD

DETECTION I1

CLASSIFICATION

DETECTION I2

LOAD, ILOAD

4265 F01

ICLASS DEPENDENT ON RCLASS SELECTION

INRUSH = 100mA

I1 =

V1 – 2 DIODE DROPS

25kΩ

ILOAD =

VIN

RLOAD

I2 =

V2 – 2 DIODE DROPS

25kΩ

GND

PSE

IIN

RLOAD

RCLASS

VOUT

GND

RCLASS

PWRGD

LTC4265

VOUT

VIN

IN DETECTION

RANGE

Figure 1. Output Voltage, PWRGD, PWRGD and

PD Current as a Function of Input Voltage

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