lm9072 National Semiconductor Corporation, lm9072 Datasheet - Page 8

• Manufacturer Part Number: lm9072
• Description: Dual Tracking Low-dropout System Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
KEEP-ALIVE INPUT (Pin 8)
This CMOS logic level compatible input provides a system
with the ability to control its own ON/OFF sequencing. The
Keep-Alive Input is OR’ed with the ON/OFF Input so either
one can independently control the regulators.
As shown in the Operational Characteristics a system con-
troller can take the Keep-Alive input high at any time. If the
ON/OFF switch is opened, this high level on Keep-Alive will
keep the regulators ON and the entire system operational.
This control is useful for providing as much time as neces-
System Keep-Alive Operation
Figure 4 illustrates the basic concept of Keep-Alive opera-
tion. The LM9072 provides regulated supplies to an entire
microcontroller based system or module including remote
sensors. The system is switched ON or OFF by a switch
connected to the unregulated input supply and the ON/OFF
input, pin 1. When closed the regulators turn ON and the
system is held in a reset state for the duration of the delayed
reset interval controlled by C
Once normal operation of the system begins, the controller
needs to set an output line connected to the Keep-Alive
input, pin 8, high. The system remains in normal operation
until switched OFF by opening the ON/OFF switch. With
Keep-Alive high the entire system remains normally biased
and will remain operational until the Keep-Alive input is taken
low.
Transistor Q1 is shown as a means to inform the controller
that the ON/OFF switch has been opened. This high level on
an input line tells the controller that the system has been
switched OFF. This indicates the start of the Keep-Alive
interval. The system can perform whatever actions required
to obtain the proper OFF state before actually powering
down. These general housekeeping tasks can include put-
ting external devices in the proper OFF condition and storing
various system variables in EPROM for example. With the
controller in command of the Keep-Alive interval these tasks
can take whatever time necessary to complete.
When completed the controller takes the Keep-Alive input to
a low level and the entire system shuts down. The LM9072
powers down to a low quiescent current mode with less than
100 µA drawn from the input supply.
DELAY
.
(Continued)
FIGURE 3. Remote ON/OFF Control
8
sary for a system to perform “housekeeping” chores such as
programming EEPROM with system information prior to
turning itself OFF (by taking the Keep-Alive Input low) and
reverting to the low quiescent current state.
A second use of the Keep-Alive Input can be from other
modules which need information from the module powered
by the LM9072, Figure 3 . A CMOS logic high level (
this input will power up the system as needed independent
from the normal ON/OFF switch.
To initiate the Keep-Alive routine before actual power down,
it is important for the system controller to know when the
system has been switched OFF. To eliminate any interface
between the controller and the ON/OFF switch and poten-
tially noisy unregulated input supply, a simple logic scheme
shown in Figure 5 can be used. With this circuitry the Reset
output from the LM9072 provides the ON/OFF sensing input
to the controller.
When switched OFF, the main regulator output will fall out of
regulation and generate a low logic level on the Reset out-
put. This input to the controller provides the switch OFF
indication and initiates the Keep-Alive interval.
Control of the Keep-Alive duration is set by a logic 1 on the
Keep-Alive output line from the system controller. This high
level prevents the Reset output from resetting the entire
system and also gates the Keep-Alive input signal to the
LM9072. The inverted Reset signal provides a logic 1 to the
Keep-Alive input of the LM9072.
The Main output will only drop out of regulation for a very
short time before the Keep-Alive input turns it back on.
The Reset output remains low for the delay time interval.
When it returns high the Main output switches OFF and back
ON again very quickly. This continues until the system con-
troller takes the Keep-Alive output line to a logic low level.
DS012906-19
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