LM9070M National Semiconductor, LM9070M Datasheet - Page 8

• Manufacturer Part Number: LM9070M
• Description: Low-Dropout System Voltage Regulator with Keep-Alive ON/OFF Control
• Manufacturer: National Semiconductor

www.national.com
Application Information
An output bypass capacitor of at least 10 µF is required for
stability (47 µF is recommended). The ESR of this capacitor
should be less than 3 . An input capacitor of 1 µF or larger
is recommended to improve line transient and noise perfor-
mance.
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 op-
eration the micro then pulls Keep-Alive high and the regula-
tor and system turn OFF and revert to the low quiescent cur-
rent 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
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
with a shorted load, the Keep-Alive function is logically dis-
abled to ensure that the regulator turns OFF and reverts to
only a 50 µA load on the battery.
Conventional load dump protection is built in to withstand up
to +60V and −50V transients. A 1 k
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 LM9070 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 LM9070.
FIGURE 2. Control Logic Not Used
CC
supply to the micro (due to the shorted regu-
OUT
is less than 4V, as would be the case
resistor in series with
(Continued)
DS012831-24
8
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
unpredictable operation of the system. On power-up and re-
covery from a fault condition the delay capacitor is used to
hold the micro in a reset condition for a programmable time
interval to allow the system operating voltages and
clock to stabilize before executing code. The typical delay
time interval can be estimated using the following equation:
INPUT STABILITY
Low dropout voltage regulators which utilize a PNP power
transistor usually exhibit a large increase in current when in
dropout (V
tion characteristics ( reduction) of the PNP transistor. To
significantly minimize this increase in current the LM9070
detects when the PNP enters saturation and reduces the op-
erating current.
This reduction in input current can create a stability problem
in applications with higher load current (
the input voltage is applied through a long length of wire,
which in effect adds a significant amount of inductance in se-
ries with the input. The drop in input current may create a
positive input voltage transient which may take the PNP out
of saturation. If the input voltage is held constant at the
threshold where the PNP is going in and out of saturation, an
oscillation may be created.
This is only observed where significant series inductance is
present in the input supply line and when the rise and fall
time of the input supply is very slow. If the application and re-
moval of the input voltage changes at a rate greater than 500
mV/µs, the input voltage moves through the dropout region
of operation (V
to be established.
MICROPROCESSOR SYSTEM REGULATOR WITH
KEEP-ALIVE INTERVAL AT TURN-OFF
The following circuit illustrates a system application utilizing
both of the logic control inputs of the LM9070. Closing the
ON/OFF switch powers ON the system. Once powered, the
system controller sets the Keep-Alive line low. The NPN
transistor is used only to signal the controller that the ON/
OFF switch has been opened and the system is to be turned
OFF. Upon detecting this high level at the ON/OFF Sense in-
put line, the controller can then perform a power down rou-
tine. The system will remain fully powered until the controller
commands total shut down by taking the Keep-Alive line
high. The system then shuts OFF and reverts to a very low
current drain standby condition until switched back on.
IN
<
IN
5.5V). This increase is caused by the satura-
of 3V to 5.5V) too quickly for an oscillation
CC
supply that may produce
>
100 mA) where