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LTC4265

4265f

APPLICATIONS INFORMATION

These modes satisfy the requirements deﬁned in the IEEE

802.3af/at speciﬁcation.

Table 1. LTC4265 Modes of Operation as a Function

of Input Voltage

GND (V)

LTC4265 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/UVLO* Classiﬁcation Load Current Active

ON/UVLO* to 60V

Inrush and Power Applied To PD Load

>71V

Overvoltage Lockout, 4265 Operations are Disabled

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

Falling input threshold, 30V Min.

INPUT DIODE BRIDGE

In the IEEE 802.3af/at 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 LTC4265 (Figure 2).

RX–

RX+

TX–

TX+

RJ45

POWERED

DEVICE (PD)

INPUT

4265 F02

SPARE–

SPARE+

TO PHY

BR2

0.1μF

100V

BR1

GND

LTC4265

VIN

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

The input diode bridge introduces a voltage drop that

affects the range for each mode of operation. The LTC4265

compensates for these voltage drops so that a PD built with

the LTC4265 meets the IEEE 802.3af/at-established voltage

ranges. Note that the Electrical Speciﬁcations reference

with respect to the LTC4265 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.

The LTC4265 presents its precision, temperature-compen-

sated 25k resistor between the GND and VIN pins, alerting

the PSE that a PD is present and requests power to be

applied. The LTC4265 signature resistor also compensates

for the additional series resistance introduced by the input

diode bridge. Thus a PD built with the LTC4265 conforms

to the IEEE 802.3af/at detection speciﬁcations.

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