LTC4257CS8 datasheet(7/20 Pages) LINER

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LTC4257

4257fb

Table 1. LTC4257 Operational Mode

as a Function of Input Voltage

INPUT VOLTAGE

(VIN with RESPECT to GND)

LTC4257 MODE OF OPERATION

0V to – 1.4V

Inactive

– 1.5V to – 10V

25k Signature Resistor Detection

– 11V to – 12.4V

Classification Load Current Ramps up from

0% to 100%

– 12.5V to UVLO*

Classification Load Current Active

UVLO* to –57V

Power Applied to PD Load

*UVLO includes hysteresis.

Rising input threshold

≅ –39.2V

Falling input threshold

≅ –30.5V

Series Diodes

The IEEE 802.3af defined operating modes for a PD

reference the input voltage at the RJ45 connector on the

PD. However, PD circuitry must include diode bridges

between the RJ45 connector and the LTC4257 (Figure 2).

The LTC4257 takes this into account by compensating for

these diode drops in the threshold points for each range of

operation. Since the voltage ranges specified in the LTC4257

electrical specifications are with respect to the IC pins for

both the signature and classification ranges, the LTC4257

lower end extends two diode drops below the IEEE 802.3af

specification. A similar adjustment is made for the UVLO

voltages.

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

RX –

RX +

TX –

TX +

RJ45

POWERED DEVICE (PD)

INTERFACE

AS DEFINED

BY IEEE 802.3af

4257 F02

SPARE –

SPARE+

TO PHY

BR2

BR1

GND

LTC4257

VIN

Detection

During detection, the PSE will apply a voltage in the range

of –2.8V to –10V on the cable and look for a 25k signature

resistor. This identifies the device at the end of the cable

as a PD. With the terminal voltage in this range, the LTC4257

connects an internal 25k resistor between GND and the VIN

pins. This precision, temperature compensated resistor

presents the proper characteristics to alert the Power

Sourcing Equipment (PSE) at the other end of the cable that

a PD is present and desires power to be applied.

The power applied to a PD is allowed to use either of two

polarities and the PD must be able to accept this power so

it is common to install a diode bridge on the input. The

LTC4257 is designed to compensate for the voltage and

resistance effects of these two series diodes. The signa-

ture range extends below the IEEE range to accommodate

the voltage drop of the diodes. The IEEE specification

requires the PSE to use a

∆V/∆I measurement technique

to keep the DC offset of these diodes from affecting the

signature resistance measurement. However, the diode

resistance appears in series with the signature resistor

and must be included in the overall signature resistance

of the PD. The LTC4257 compensates for the two series

diodes in the signature path by offsetting the resistance

so that a PD built using the LTC4257 will meet the IEEE

requirements.

APPLICATIO S I FOR ATIO

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