LM9074 Datasheet, PDF (8/12 Pages) National Semiconductor (TI) – System Voltage Regulator with Keep-Alive ON/OFF

Application Information (Continued)
FIGURE 1. Typical Application Circuit
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An output bypass capacitor of at least 0.1µF is required for
stability (47 µF is recommended). An input capacitor of 1 µF
or larger is recommended to improve line transient and noise
performance.
With the Keep-Alive input, a system microprocessor has the
ability to keep the regulator ON (with a logic “0” on Keep-
Alive) after the ON/OFF input has been commanded OFF. A
power-down sequence, when system variables are typically
stored in programmable memory, can be executed and take
as much time as necessary. At the end of the operation the
micro then pulls Keep-Alive high and the regulator and sys-
tem turn OFF and revert to the low quiescent current standby
mode.
For additional system reliability, consideration has been
made for the possibility of a short circuited load at the output
of the regulator. When the regulator is switched ON, conven-
tional current limiting and thermal shutdown protect the regu-
lator. When the regulator is switched OFF however, a
grounded V_CC supply to the micro (due to the shorted regu-
lator output) will force the Keep-Alive input to be low and
thus try to maintain the Keep-Alive mode of operation. With
a shorted load, the drain on the battery could be as high as
1.5A. A separate internal circuit monitors the output voltage
of the regulator. If V_OUT is less than 4V, as would be the case
with a shorted load, the Keep-Alive function is logically dis-
abled to ensure that the regulator turns OFF and reverts to
less than a 60 µA load on the battery.
FIGURE 2. Control Logic Not Used
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Conventional load dump protection is built in to withstand up
to +60V and −50V transients. A 1 kΩ resistor in series with
the ON/OFF and Keep-Alive inputs are recommended to
provide the same level of transient protection for these pins
if required. Protection against reverse polarity battery con-
nections is also built in. With a reversed battery the output of
the LM9074 will not go more negative than one diode drop
below ground. This will prevent damage to any of the 5V load
circuits.
For applications where the control logic is not required the
logic pins should be configured as shown in Figure 2. A
separate device, called the LM9071, can be used. The
LM9071 is available in a 5-lead TO-220 package and does
not provide control logic functions, but still retains all of the
protection features of the LM9074.
RESET FLAG
Excessive loading of the output to the point where the output
voltage drops by 300 mV to 500 mV will signal a reset flag to
the micro. This will warn of a V_CC supply that may produce
unpredictable operation of the system. On power-up and
recovery from a fault condition the delay capacitor is used to
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