LM5072 datasheet(17/24 Pages) NSC

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Auxiliary Power Options (Continued)

TABLE 3. Comparison Between FAUX and RAUX Operation

Tradeoff

FAUX Operation

RAUX Operation

Hot Swap Protection / Current Limit

Protection

Automatically provided by the hot

swap MOSFET.

Requires a series resistor to limit the inrush

current during hot swap.

Minimum Auxiliary Voltage

(at the IC pins)

Limited to 18V by the signature

detection mode, or by the power

requirement (current limit).

Only limited by 9V minimum input requirement.

Auxiliary Dominance Over PoE

Cannot be forced without external

components.

Can be forced with appropriate RAUX pin

configuration.

Use of nPGOOD Pin as “Powered

from PoE” Indicator

Not applicable as power is delivered

through the hot swap interface in both

PoE and FAUX modes.

Supported.

Transient Protection

Excellent due to active MOSFET

current limit.

Fair due to passive resistor current limit.

The term “Auxiliary Dominance” mentioned in Table 3 means

that when the auxiliary power source is connected, it will

always power the PD regardless of the state of PoE power.

“Aux dominance” is achievable only with the RAUX option,

as noted in the table.

If the PD is not designed for aux dominance, either the FAUX

or RAUX power sources will deliver power to the PD only

under the following two conditions: (i) If auxiliary power is

applied before PoE power, it will prevent the PD’s detection

by the PSE and will supply power indefinitely. This occurs

because the PoE input bridge rectifiers will be reverse bi-

ased, so no detection signature will be observed. Under this

condition, when the auxiliary supply is removed, power will

not be maintained because it will take some time for the PSE

to perform signature detection and classification before it will

supply power. (ii) If auxiliary power is applied after PoE

power is already present but has a higher voltage than PoE,

it may assume power delivery responsibility. Under the sec-

ond case, if the supplied voltages are comparable, the load

current may be shared inversely proportional to the respec-

tive output impedances of each supply. (The output imped-

ance of the PSE supply is increased by the cable series

resistance).

If PoE power is applied first and has a higher voltage than

the non-dominant aux power source, it will continue power-

ing the PD even when the aux power source becomes

available. In this case, should PoE power be removed, the

auxiliary source will assume power delivery and supply the

DC-DC loads without interruption.

If either FAUX or RAUX power is supplied prior to PoE

power, it will prevent the recognition of the PD by the PSE.

Consequently, continuity of power delivery cannot be guar-

anteed because the PoE supply will not be present when

auxiliary power is removed.

FAUX Option

With the FAUX option, the LM5072 hot swap MOSFET pro-

vides inrush and DC current limit protection for the auxiliary

power source. To select the FAUX configuration for an aux-

iliary voltage lower than nominal PoE voltages, the

ICL_FAUX pin must be forced above its high threshold to

override the VIN UVLO function. Note that when the

ICL_FAUX pin is pulled high to override VIN UVLO, it also

overrides the inrush current limit programmed by R

ICL,if

present. In this case, the inrush current will revert back to the

default 150 mA limit.

Pulling up the ICL_FAUX pin will increase the default DC

current limit to 540 mA. This increase in DC current limit is

necessary because higher current is required to support the

PD output power at the lower input potentials observed with

auxiliary sources. In cases where the auxiliary supply volt-

age is comparable to the PoE voltage, there is no need to

pull-up the ICL_FAUX pin to override VIN UVLO, and the

default DC current limit remains at 440 mA. However, if the

DC current limit is externally programmed with R

DCCL, the

condition of the ICL_FAUX pin will not affect the pro-

grammed DC current limit. In other words, programmed DC

current limit can be considered a “hard limit” that will not vary

in any configuration.

RAUX Option

The RAUX option is desirable when the auxiliary supply

voltage is significantly lower than the PoE voltage or when

aux dominance is desired. The inrush and DC current limits

of the LM5072 do not protect or limit the RAUX power

source, and an additional resistor in the RAUX input path will

be needed to provide transient protection.

To select the RAUX option without aux dominance, simply

pull up the RAUX pin to the auxiliary power supply voltage

through a high value resistor. Depending on the auxiliary

supply voltage, the resistor value should be selected such

that the current flowing into the RAUX pin is approximately

100 µA when the pin is mid-way between the lower and

upper RAUX thresholds (approximately 4V). For example,

with an 18V non-dominant rear auxiliary supply, the pull up

resistor should be:

If the PSE load capacity is limited and insufficient, aux

dominance will be a desired feature to off load PoE power for

other PDs that do not have auxiliary power available. Aux

dominance is achieved by pulling the RAUX pin up to the

auxiliary supply voltage through a lower value (~5k

Ω) resis-

tor that delivers at least 250 µA into the RAUX pin. When this

higher RAUX current level is detected, the LM5072 shuts

down the PD interface. In aux dominant mode, the auxiliary

power source will supply the PD system as soon as it is

applied. PD operation will not be interrupted when the aux

power source is connected. The PoE source may or may not

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