LM9074 Datasheet, PDF (9/12 Pages) National Semiconductor (TI) – System Voltage Regulator with Keep-Alive ON/OFF

Application Information (Continued)
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:
t_DELAY = ((3.4V x C_DELAY) / I_DELAY))
For a C_DELAY value of 0.1 µF and a typical I_DELAY of 12 µA,
the typical delay time will be:
t_DELAY = ((3.4V x 0.1 µF) / 12 µA)) = 28.3 ms
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 LM9074. 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/
10130526
OFF switch has been opened and the system is to be turned
OFF. Upon detecting this high level at the ON/OFF Sense
input line, the controller can then perform a power down
routine. The system will remain fully powered until the con-
troller 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.
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