Datasheet

+
-
11
10
VCC
nPGOOD
16
PWM
DISABLE
LED to indicate
ARTN
50PA
"Powered from PoE"
LM5072
130:
PGOOD
2.5V
C
PGOOD
R
LED
LM5072
SNVS437D –MARCH 2006–REVISED APRIL 2013 www.ti.com
The maximum recommended DC current limit is 800 mA. While thermal analysis should be a standard part of the
module development process, it may warrant additional attention if the DC current limit is programmed to values
in excess of 400 mA. This analysis should include evaluations of the dissipation capability of LM5072 package,
heat sinking properties of the PC Board, ambient temperature, and other heat dissipation factors of the operating
environment.
Power Good and Regulator Startup
The Power Good status indicates that the circuit is ready for PWM controller startup to occur. It is established
when the input capacitors are fully charged through the hot swap MOSFET. Since the hot swap MOSFET is in
series with the input capacitors of the DC-DC converter, its drain-to-source voltage decreases as the charging
occurs. Power Good is indicated when the following two conditions are met: the MOSFET drain-to-source voltage
drops below 1.5V (with 1V hysteresis), and the gate-to-source voltage is greater than 5V. Circuitry internal to the
LM5072 monitors both the drain and gate voltages (see Figure 14), and issues the Power Good status flag by
pulling down the nPGOOD pin to a logic low level relative to the ARTN pin.
The nPGOOD circuitry consists of a 2.5V comparator, a 130Ω pull down MOSFET, and a 50 µA pull up current
source, as shown in Figure 22. Once the Power Good status is established, the nPGOOD pin voltage will be
pulled down quickly by the MOSFET, and the PWM controller will start as soon as the nPGOOD pin voltage
drops below the 2.5V threshold.
Figure 22. "Powered-from-PoE" Indictor and Power Good Delay Timer
The nPGOOD pin can be configured to perform multiple functions. As shown in Figure 22, it can be used to
implement a “Powered from PoE” indicator using an LED with a series current 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 configuration, 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 below) because both PoE and auxiliary supply current pass
through the hot swap MOSFET. In this configuration, the nPGOOD 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
CC
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
CC
that is higher 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
CC
line will increase the dropout voltage of the V
CC
regulator. This may be
a concern 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 current during such a sudden input voltage change, the nPGOOD 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:
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