LM9073T National Semiconductor, LM9073T Datasheet - Page 12

• Manufacturer Part Number: LM9073T
• Description: Dual High Current Low-Dropout System Regulator
• Manufacturer: National Semiconductor

www.national.com
Application Information
A low to high transition is required during the watchdog timer
interval at the trigger input. If this line is ever fixed high the
timer will time-out and the system will reset.
ON/OFF Input (pin 3)
The ON/OFF Input enables both the Main and External Sup-
ply outputs. In a typical application this input is connected to
the input supply through a series resistor (nominally 22K )
and a switch (Ignition, as an example).When the switch is
closed this input is pulled high and switches ON both regula-
tor outputs. This input is internally clamped to a 7V zener di-
ode through a series 1k
tor together with an optional 0.1µF capacitor to ground
provide filtering and current limiting to withstand transients
that may appear on the input supply to maintain normal op-
eration of the system.
The switching threshold of the ON/OFF comparator has 2
Volts of hysteresis to ensure noise free control of the system.
To turn the regulators ON this input must be taken above 4V.
To turn the system OFF the ON/OFF Input must be open cir-
cuited or pulled below 2 Volts.
System Keep-Alive Operation
Figure 5 illustrates the basic concept of Keep-Alive opera-
tion. The LM9073 provides the regulated supplies to an en-
tire microcontroller based system or module including re-
mote sensors. The system is switched ON or OFF by a
switch connected to the unregulated input supply and the
ON/OFF input, pin 3. 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 in-
put, pin 9, high. The system remains in normal operation un-
til 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.
resistor. The external series resis-
DELAY
(Continued)
.
Figure 4. Remote ON/OFF Control
12
Keep-Alive Input (pin 9)
This CMOS logic level compatible input provides a system
with the ability to control it’s 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-
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 mod-
ules which need information from the module powered by
LM9073. A high CMOS logic level (
power up the system as needed independent from the nor-
mal ON/OFF switch.
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 in-
terval. The system can perform whatever actions required to
obtain the proper OFF state before actually powering down.
These general housekeeping tasks can include putting ex-
ternal devices in the proper OFF condition and storing vari-
ous system variables in EPROM for example. With the con-
troller 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 LM9073
powers down to a low quiescent current mode with less than
100µA drawn from the input supply.
DS101296-19
>
1.25V) on this input will