LTC4267CDHC datasheet(11/32 Pages) LINEAR

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LTC4267

4267fc

Series Diodes

The IEEE 802.3af-deﬁned operating modes for a PD refer-

ence the input voltage at the RJ45 connector on the PD.

The PD must be able to accept power of either polarity

at each of its inputs, so it is common to install diode

bridges (Figure 2). The LTC4267 takes this into account

by compensating for these diode drops in the threshold

points for each range of operation. A similar adjustment

is made for the UVLO voltages.

Detection

During detection, the PSE will apply a voltage in the

range of – 2.8V to –10V on the cable and look for a 25kΩ

signature resistor. This identiﬁes the device at the end of

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

the LTC4267 connects an internal 25kΩ resistor between

the VPORTP and VPORTN pins. This precision, temperature

compensated resistor presents the proper signature to

alert the PSE that a PD is present and desires power to be

applied. The internal low-leakage UVLO switch prevents

the switching regulator circuitry from affecting the detec-

tion signature.

The LTC4267 is designed to compensate for the voltage

and resistance effects of the IEEE required diode bridge.

The signature range extends below the IEEE range to ac-

commodate the voltage drop of the two diodes. The IEEE

speciﬁcation requires the PSE to use a ΔV/ΔI measurement

technique to keep the DC offset of these diodes from af-

fecting 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 LTC4267 compensates for the

two series diodes in the signature path by offsetting the

resistance so that a PD built using the LTC4267 will meet

the IEEE speciﬁcation.

In some applications it is necessary to control whether or

not the PD is detected. In this case, the 25kΩ signature

resistor can be enabled and disabled with the use of the

SIGDISA pin (Figure 3). Disabling the signature via the

SIGDISA pin will change the signature resistor to 9kΩ

(typical) which is an invalid signature per the IEEE 802.3af

speciﬁcation. This invalid signature is present for PD input

voltages from –2.8V to –10V. If the input rises above –10V,

the signature resistor reverts to 25kΩ to minimize power

dissipation in the LTC4267. To disable the signature, tie

SIGDISA to VPORTP. Alternately, the SIGDISA pin can be

driven high with respect to VPORTN. When SIGDISA is high,

all functions of the PD interface are disabled.

APPLICATIO S I FOR ATIO

RX–

RX+

TX–

TX+

RJ45

POWERED DEVICE (PD)

INTERFACE

AS DEFINED

BY IEEE 802.3af

4267 F02

SPARE–

SPARE+

TO PHY

BR2

BR1

VPORTP

LTC4267

VPORTN

Figure 2. LTC4267 PD Front End Using

Diode Bridges on Main and Spare Inputs

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