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

Ω)

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

Ω resistor. The external series resis-

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.

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

power up the system as needed independent from the nor-

mal ON/OFF switch.

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.

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.

Figure 4. Remote ON/OFF Control

DS101296-19

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