LTC4264IDE datasheet(18/24 Pages) LINER

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LTC4264

4264f

will be drawn from the PSE. On the other hand, if the

adapter voltage is lower, the PSE will continue to supply

power to the PD and the adapter will not be used. Proper

operation will occur in either scenario.

Option 2 applies power directly to the DC/DC converter.

In this conﬁguration the adapter voltage does not need to

exceed the LTC4264 turn-on UVLO requirement and can

be selected based solely on the PD load requirements. It

is necessary to include diode D9 to prevent the adapter

from applying power to the LTC4264. Power priority is-

sues require more intervention. If the adapter voltage is

below the PSE voltage, then the priority will be given to the

PSE power. The PD will draw power from the PSE while

the adapter will remain unused. This conﬁguration is ac-

ceptable in a typical PoE system. However, if the adapter

voltage is higher than the PSE voltage, the PD will draw

power from the adapter. In this situation, it is necessary

to address the issue of power cycling that may occur if

a PSE is present. The PSE will detect the PD and apply

power. If the PD is being powered by the adapter, then

the PD will not meet the minimum load requirement and

the PSE may subsequently remove power. The PSE will

again detect the PD and power cycling will start. With an

adapter voltage above the PSE voltage, it is necessary to

either disable the signature as shown in option 2, or install

a minimum load on the output of the LTC4264 to prevent

power cycling. A 3k, 1W resistor connected between GND

and VOUT will present the required minimum load.

Option 3 applies power directly to the DC/DC converter

bypassing the LTC4264 and omitting diode D9. With the

diode omitted, the adapter voltage is applied to the LTC4264

in addition to the DC/DC converter. For this reason, it is

necessary to ensure that the adapter maintain the voltage

between 42V and 57V to keep the LTC4264 in its normal

operating range. The third option has the advantage of

corrupting the 25k signature resistance when the external

voltage exceeds the PSE voltage and thereby solving the

power priority issue.

Option 4 bypasses the entire PD interface and injects

power at the output of the low voltage power supply. If

the adapter output is below the low voltage output there

are no power priority issues. However, if the adapter is

above the internal supply, then option 4 suffers from the

same power priority issues as option 2 and the signature

should be disabled or a minimum load should be installed.

Shown in option 4 is one method to disable to the signature

while maintaining isolation.

If employing options 1 through 3, it is necessary to ensure

that the end-user cannot access the terminals of the aux-

iliary power jack on the PD since this would compromise

IEEE 802.3af isolation requirements and may violate local

safety codes. Using option 4 along with an isolated power

supply addresses the isolation issue and it is no longer

necessary to protect the end-user from the power jack.

The above power cycling scenarios have assumed the

PSE is using DC disconnect methods. For a PSE using

AC disconnect, a PD with less than minimum load will

continue to be powered.

Wall adapters have been known to generate voltage spikes

outside their expected operating range. Care should be

taken to ensure no damage occurs to the LTC4264 or any

support circuitry from extraneous spikes at the auxiliary

power interface.

Classiﬁcation Resistor Selection (RCLASS)

The IEEE 802.3af speciﬁcation allows classifying PDs

into four distinct classes with class 4 being reserved

for future use (Table 2). The LTC4264 supports all IEEE

classes and implements an additional Class 5 for use in

custom PoE applications. An external resistor connected

from RCLASS to VIN (Figure 6) sets the value of the load

current. The designer should determine which class the

PD is to advertise and then select the appropriate value of

APPLICATIONS INFORMATION

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