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-

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

the LM9072,

this input will power up the system as needed independent

from the normal ON/OFF switch.

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 con-

nected to the unregulated input supply and the ON/OFF in-

put, pin 1. When closed the regulators turn ON and the sys-

tem 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 8, 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 LM9072

powers down to a low quiescent current mode with less than

100 µA drawn from the input supply.

To initiate the Keep-Alive routine before actual power down,

it is important for the system controller to know when the sys-

tem has been switched OFF. To eliminate any interface be-

tween the controller and the ON/OFF switch and potentially

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 indi-

cation 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 sys-

tem 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

FIGURE 3. Remote ON/OFF Control

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