LTC4265 datasheet(16/20 Pages) LINER

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LTC4265

4265f

APPLICATIONS INFORMATION

Shutdown Interface

To corrupt the signature resistance, the SHDN pin can be

driven high with respect to VIN or connected to GND. If

unused, connect SHDN directly to VIN.

Exposed Pad

The LTC4265 uses a thermally enhanced DFN12 package

that includes an Exposed Pad. The exposed may be elec-

trically connected to VIN and must connect to a printed

circuit board heat sink.

Auxiliary Power Source

In some applications, it is desirable to power the PD from

an auxiliary power source such as a wall adapter.

Auxiliary power can be injected into an LTC4265-based PD

at the input of the LTC4265, the output of the LTC4265, or

even the output of the DC/DC converter. In addition, some

PD application may desire auxiliary supply dominance

or may be conﬁgured for PoE dominance. Furthermore,

PD applications may also opt for a seamless transition

— that is, without power disruption — between PoE and

auxiliary power.

The most common auxiliary power option injects power

between the LTC4265 and the DC/DC converter. Figure 10

presents an example of this application.

In this example, the auxiliary port injects 48V onto the line

via diode D1. The components surrounding the SHDN pin

are selected so that the LTC4265 disconnects power to the

output when the auxiliary supply reaches 36V.

This conﬁguration is an auxiliary-dominant conﬁguration.

That is, the auxiliary power source supplies the power even

if PoE power is already present. This conﬁguration also

provides a seamless transition from PoE to auxiliary power

when auxiliary power is applied, however, the removal of

auxiliary power to PoE power is not seamless.

Contact Linear Technology applications support for detail

information on implementing a custom auxiliary power

supply.

Figure 10. Auxiliary Power Dominant PD Interface

4265 F10

TVS

TO PHY

36V

100k

10k

BR1

–

BR2

–

0.1μF

100V

GND

LTC4265

VIN

SHDN

VOUT

–

ISOLATED

WALL

TRANSFORMER

LOAD

RX–

RX+

TX–

TX+

RJ45

SPARE–

SPARE+

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