LM5072EVAL/NOPB National Semiconductor, LM5072EVAL/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM5072EVAL/NOPB
• Description: EVAL BOARD FOR LM5072
• Manufacturer: National Semiconductor

Specifications of LM5072EVAL/NOPB

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The term “Auxiliary Dominance” mentioned in Table 3 means
that when the auxiliary power source is connected, it will al-
ways power the PD regardless of the state of PoE power. “Aux
dominance” is achievable only with the RAUX option, as not-
ed 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 un-
der 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 biased, so no
detection signature will be observed. Under this condition,
when the auxiliary supply is removed, power will not be main-
tained 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 al-
ready present but has a higher voltage than PoE, it may
assume power delivery responsibility. Under the second
case, if the supplied voltages are comparable, the load cur-
rent may be shared inversely proportional to the respective
output impedances of each supply. (The output impedance 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 powering 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 with-
out interruption.
If either FAUX or RAUX power is supplied prior to PoE power,
it will prevent the recognition of the PD by the PSE. Conse-
quently, continuity of power delivery cannot be guaranteed
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 auxil-
iary 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
inrush current will revert back to the default 150 mA limit.
Pulling up the ICL_FAUX pin will increase the default DC cur-
rent limit to 540 mA. This increase in DC current limit is
necessary because higher current is required to support the
Hot Swap Protection / Current Limit
Protection
Minimum Auxiliary Voltage
(at the IC pins)
Auxiliary Dominance Over PoE
Use of nPGOOD Pin as “Powered from
PoE” Indicator
Transient Protection
Tradeoff
ICL
TABLE 3. Comparison Between FAUX and RAUX Operation
, if present. In this case, the
Automatically provided by the hot swap
MOSFET.
Limited to 18V by the signature
detection mode, or by the power
requirement (current limit).
Cannot be forced without external
components.
Not applicable as power is delivered
through the hot swap interface in both
PoE and FAUX modes.
Excellent due to active MOSFET
current limit.
FAUX Operation
17
PD output power at the lower input potentials observed with
auxiliary sources. In cases where the auxiliary supply voltage
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
ICL_FAUX pin will not affect the programmed DC current limit.
In other words, programmed DC current limit can be consid-
ered a “hard limit” that will not vary in any configuration.
RAUX Option
The RAUX option is desirable when the auxiliary supply volt-
age 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 volt-
age, 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 domi-
nance will be a desired feature to off load PoE power for other
PDs that do not have auxiliary power available. Aux domi-
nance is achieved by pulling the RAUX pin up to the auxiliary
supply voltage through a lower value (~5 kΩ) resistor that de-
livers 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 actually be
removed by the PSE, although the DC current from the net-
work cable is effectively reduced to zero (< 150 µA). IEEE
802.3af requires the AC input impedance to be greater than
2 MΩ to ensure PoE power removal. This condition is not sat-
isfied when the auxiliary power source is applied. The PSE
Requires a series resistor to limit the inrush
current during hot swap.
Only limited by 9V minimum input requirement.
Can be forced with appropriate RAUX pin
configuration.
Supported.
Fair due to passive resistor current limit.
RAUX Operation
DCCL
, the condition of the
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