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LTC4278

4278fc

APPLICATIONS INFORMATION

Table 1. LTC4278 Modes of Operation as a Function

of Input Voltage

VPORTP–VPORTN (V) LTC4278 MODES OF OPERATION

0V to 1.4V

Inactive (Reset After 1st Classification Event)

1.5V to 9.8V

(5.4V to 9.8V)

25k Signature Resistor Detection Before 1st

Classification Event (Mark, 11k Signature

Corrupt After 1st Classification Event)

12.5V to ON/OFF*

Classification Load Current Active

ON/OFF* to 60V

Inrush and Power Applied To PD Load

>71V

Overvoltage Lockout,

Classification and Hot Swap Are Disabled

*ON/OFF includes hysteresis. Rising input threshold, 37.2V Max.

Falling input threshold, 30V Min.

These modes satisfy the requirements defined in the

IEEE 802.3af/IEEE 802.3at specification.

INPUT DIODE BRIDGE

In the IEEE 802.3af/IEEE 802.3at standard, the modes of

operation reference the input voltage at the PD’s RJ45

connector. Since the PD must handle power received in

either polarity from either the data or the spare pair, input

diode bridges BR1 and BR2 are connected between the

RJ45 connector and the LTC4278 (Figure 2).

The input diode bridge introduces a voltage drop that

affects the range for each mode of operation. The

LTC4278 compensates for these voltage drops so that a

PD built with the LTC4278 meets the IEEE 802.3af/IEEE

802.3at-established voltage ranges. Note the Electrical

CharacteristicsarereferencedwithrespecttotheLTC4278

package pins.

DETECTION

During detection, the PSE looks for a 25k signature resis-

tor which identifies the device as a PD. The PSE will apply

two voltages in the range of 2.8V to 10V and measures

the corresponding currents. Figure 1 shows the detection

voltages V1 and V2 and the corresponding PD current.

The PSE calculates the signature resistance using the

ΔV/

ΔI measurement technique.

TheLTC4278presentsitsprecision,temperature-compen-

sated 25k resistor between the VPORTP and VPORTN pins,

alerting the PSE that a PD is present and requests power

to be applied. The LTC4278 signature resistor also com-

pensates for the additional series resistance introduced by

the input diode bridge. Thus a PD built with the LTC4278

conforms to the IEEE 802.3af/IEEE 802.3at specifications.

RX–

RX+

TX–

TX+

RJ45

POWERED

DEVICE

(PD)

INPUT

4278 F02

SPARE–

SPARE+

TO PHY

BR2

0.1µF

100V

BR1

LTC4278

VPORTN

VPORTP

Figure 2. PD Front End Using Diode Bridges on Main and Spare Inputs

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