lm9073 National Semiconductor Corporation, lm9073 Datasheet - Page 11

• Manufacturer Part Number: lm9073
• Description: Dual High Current Low-dropout System Regulator
• Manufacturer: National Semiconductor Corporation

Application Information
The watchdog timer function can be disabled by grounding
pin 2 or replacing Cwatchdog by a resistor with a value less
than 22kΩ. With this only the reset generator can reset the
system.
WATCHDOG TRIGGER (PIN 1)
The Watchdog Trigger input accepts a pulse from the system
controller to refresh the watchdog capacitor and prevent it
from reaching 4V and resetting the system. This positive
pulse must be at least 10µS long and triggers an internal
one-shot pulse. This internal pulse latches ON Qdischarge
Watchdog (figure 2) and discharges Cwatchdog to 0.9V. This
latching action ensures a consistent watchdog timer interval
by not allowing the capacitor to charge positively until it has
been discharged to 0.9V.
As shown in Figure 3, each watchdog trigger input pulse
resets the timer capacitor. If the watchdog trigger signal does
not refresh the timer before Cwatchdog reaches 4V, a sys-
tem reset is generated. Once reset, a full reset delay interval
occurs. At the end of this interval the regulator will automati-
cally try to re-start the system by taking reset high. If the
system does not respond properly by issuing a watchdog
trigger signal in time, the system will once again reset. In this
situation the reset output will continually cycle high (re-
starting the system) for the watchdog time interval and low
(re-setting the system) for the reset delay interval.
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
Supply outputs. In a typical application this input is con-
SYSTEM KEEP-ALIVE OPERATION
Figure 5 illustrates the basic concept of Keep-Alive opera-
tion. The LM9073 provides the regulated supplies to an
entire 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
input, pin 9, high. The system remains in normal operation
DELAY
(Continued)
.
FIGURE 4. Remote ON/OFF Control
11
nected to the input supply through a series resistor (nomi-
nally 22KΩ) and a switch (Ignition, as an example).When the
switch is closed this input is pulled high and switches ON
both regulator outputs. This input is internally clamped to a
7V zener diode through a series 1kΩ resistor. The external
series resistor 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 operation 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
circuited or pulled below 2 Volts.
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
modules which need information from the module powered
by LM9073. A high CMOS logic level (
will power up the system as needed independent from the
normal ON/OFF switch.
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
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1.25V) on this input
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