LM5072EVAL/NOPB National Semiconductor, LM5072EVAL/NOPB Datasheet - Page 15

• 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 nPGOOD pin can be configured to perform multiple func-
tions. As shown in Figure 9, it can be used to implement a
“Powered from PoE” indicator using an LED with a series cur-
rent limiting resistor connected to the VCC pin. This may be
useful when the auxiliary power source is directly connected
to the DC-DC converter stage, a situation known as
“RAUX” (see Auxiliary Power Options below). In such a con-
figuration, the nPGOOD pin will be active when the PD is
operating from PoE power but not when it is powered from the
auxiliary source. However, the “Powered from PoE” indicator
is not applicable in systems implementing the front auxiliary
power configuration “FAUX” (see Auxiliary Power Options be-
low) because both PoE and auxiliary supply current pass
through the hot swap MOSFET. In this configuration, the nP-
GOOD pin is active when either PoE power or auxiliary power
is applied. The designer should ensure that the current drawn
by the LED is not more than a few milliamps, as the V
regulator’s output current is limited to 15 mA and must also
supply the LM5072’s bias current and external MOSFET’s
gate charging current. Supplying an external V
er than the regulated level with a bench supply is an easy way
to measure VCC load during normal operation. It should also
be noted that an external load on the V
the dropout voltage of the V
cern when operating from a low voltage rear auxiliary supply.
The nPGOOD pin can also be used to implement a delay timer
by adding a capacitor from the nPGOOD pin to the ARTN pin.
This delay timer will prevent the interruption of the PWM
controller’s operation in the event of an intermittent loss of
Power Good status. This can be caused by PoE line voltage
transients that may occur when switching between normal
PoE power and a backup supply system (e.g. a battery or
UPS). This condition will create a new “hot swap” event if
there is a voltage difference between the backup supply and
PoE supply. Since the hot swap MOSFET will likely limit cur-
rent during such a sudden input voltage change, the nP-
GOOD pin will momentarily switch to the ”pull up” state. A
capacitor on this pin will delay the transition of the nPGOOD
pin state in order to provide continuous operation of the PWM
controller during such transients. The Power Good filter delay
time and capacitor value can be selected with the following
equation:
FIGURE 9. "Powered-from-PoE" Indictor and Power Good Delay Timer
CC
regulator. This may be a con-
CC
line will increase
CC
that is high-
CC
15
For example, selecting 1000 nF for C
will be 50 ms if no LED is used and about 0.83 ms when an
LED, drawing 3 mA, is used. The delay required for continued
operation will depend on the amplitude of the transient, the
DC current limit, the load, and the total amount of input ca-
pacitance. Note that this delay does not guarantee continued
operation. If the hot swap MOSFET is in current limit for an
extended period, it may cause a thermal limit condition. This
will result in a complete shutdown of the switching regulator,
though no elements in the system will be permanently dam-
aged and normal operation will resume momentarily.
The Power Good status will also affect the default DC current
limit. Should the sensed drain to source voltage of the hot
swap MOSFET (from ARTN to V
will increase the DC current limit from the default 440 mA to
540 mA, thus allowing the PD to continue operation through
the transient event. This higher current limit will remain in ef-
fect until one of the following events occur: (i) the duration of
loss of Power Good status exceeds t
the PWM controller will be disabled, (ii) the increased power
dissipation in the hot swap MOSFET causes a thermal limit
condition as previously discussed, or (iii) the MOSFET drain
to source voltage falls below 1.5V to re-establish Power Good
status. Under this condition, the LM5072 will revert back to
the default 440 mA DC current limit once Power Good status
is restored. Note that if the DC current limit has been pro-
grammed externally with R
section), the DC current limit will remain at the programmed
level even when the Power Good status is lost.
Auxiliary Power Options
The LM5072 based PD can receive power from auxiliary
sources like AC adapters and solar cells in addition to the PoE
enabled network. This is a desirable feature when the total
system power requirements exceed the PSE’s load capacity.
Furthermore, with the auxiliary power option the PD can be
used in a standard Ethernet (non-PoE) system.
For maximum versatility, the LM5072 accepts two different
auxiliary power configurations. The first one, shown in Figure
DCCL
20184625
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) exceed 2.5V, the LM5072
(see the DC current limit
PG_Delay
PGOOD
, the delay time
, at which time
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