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, and simple logic scheme

shown in Figure 6 can be used. With this circuitry the Reset

output form the LM9073 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

LM9073. The inverted Reset signal provides a logic 1 to the

Keep-Alive input of the LM9073.

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 turns 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.

Figure 5. Basic Keep-Alive Operation

DS101296-20

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