LTC4267CDHC datasheet(17/32 Pages) LINEAR

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LTC4267

4267fc

Diode Bridge

IEEE 802.3af allows power wiring in either of two conﬁgu-

rations: on the TX/RX wires or via the spare wire pairs in

the RJ45 connector. The PD is required to accept power in

either polarity on either the main or spare inputs; therefore

it is common to install diode bridges on both inputs in

order to accommodate the different wiring conﬁgurations.

Figure 9 demonstrates an implementation of these diode

bridges. The IEEE 802.3af speciﬁcation also mandates

that the leakage back through the unused bridge be less

than 28µA when the PD is powered with 57V.

The IEEE standard includes an AC impedance requirement

in order to implement the AC disconnect function. Capaci-

tor C14 in Figure 9 is used to meet this AC impedance

requirement. A 0.1µF capacitor is recommended for this

application.

The LTC4267 has several different modes of operation

based on the voltage present between VPORTN and VPORTP

pins. The forward voltage drop of the input diodes in a PD

design subtracts from the input voltage and will affect the

transition point between modes. When using the LTC4267,

it is necessary to pay close attention to this forward voltage

drop. Selection of oversized diodes will help keep the PD

thresholds from exceeding IEEE speciﬁcations.

The input diode bridge of a PD can consume over 4%

of the available power in some applications. It may be

desirable to use Schottky diodes in order to reduce power

loss. However, if the standard diode bridge is replaced

with a Schottky bridge, the transition points between the

modes will be affected. Figure 10 shows a technique for

using Schottky diodes while maintaining proper threshold

points to meet IEEE 802.3af compliance. D13 is added to

compensate for the change in UVLO turn-on voltage caused

by the Schottky diodes and consumes little power.

Classiﬁcation Resistor Selection (RCLASS)

The IEEE speciﬁcation allows classifying PDs into four

distinct classes with class 4 being reserved for future use

(Table 2). An external resistor connected from RCLASS to

VPORTN (Figure 4) sets the value of the load current. The

designer should determine which power category the PD

falls into and then select the appropriate value of RCLASS

from Table 2. If a unique load current is required, the value

of RCLASS can be calculated as:

RCLASS = 1.237V/(IDESIRED – IIN_CLASS)

where IIN_CLASS is the LTC4267 IC supply current during

classiﬁcation and is given in the electrical speciﬁcations.

The RCLASSresistormustbe1%orbettertoavoiddegrading

the overall accuracy of the classiﬁcation circuit. Resistor

power dissipation will be 50mW maximum and is transient

so heating is typically not a concern. In order to maintain

loop stability, the layout should minimize capacitance at

the RCLASS node. The classiﬁcation circuit can be disabled

by ﬂoating the RCLASS pin. The RCLASS pin should not be

shorted to VPORTN as this would force the LTC4267 clas-

siﬁcation circuit to attempt to source very large currents

and quickly go into thermal shutdown.

Power Good Interface

The ⎯P⎯W⎯R⎯G⎯D signal is controlled by a high voltage, open-

drain transistor. The designer has the option of using this

signal to enable the onboard switching regulator through

the ITH/RUN or the PVCC pins. Examples of active-high

interface circuits for controlling the switching regulator

are shown in Figure 7.

In some applications, it is desirable to ignore intermittent

power bad conditions. This can be accomplished by in-

cluding capacitor C15 in Figure 7 to form a lowpass ﬁlter.

With the components shown, power bad conditions less

than about 200µs will be ignored. Conversely, in other

applications it may be desirable to delay assertion of

⎯P⎯W⎯R⎯G⎯D to the switching regulator using CPVCC or C17

as shown in Figure 7.

It is recommended that the designer use the power

good signal to enable the switching regulator. Using

⎯P⎯W⎯R⎯G⎯D ensures the capacitor C1 has reached within

1.5V of the ﬁnal value and is ready to accept a load. The

LTC4267 is designed with wide power good hysteresis

to handle sudden ﬂuctuations in the load voltage and

current without prematurely shutting off the switching

regulator. Please refer to the Power-Up Sequencing of the

Application Information section.

APPLICATIO S I FOR ATIO

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