Safety Latch-OFF

To address a system FMEA (Failure Mode Effects Analysis)

issue the Keep-Alive high level should be derived from the

Tracking output regulator as shown in

Figure 6. The issue

stems from the “what-if” scenario whereby the system is in

the Keep-Alive state and there is a short to ground on the

Tracking Output regulator. If this output is powering remote

sensors or systems, this becomes a more highly possible

fault condition. Since a short on the Tracking output does not

affect the Main output, which, if used to provide the

Keep-Alive input signal, would remain ON and draw 120 mA,

the short circuit current of the Tracking regulator, from the in-

put supply.

Using the Tracking output supply for Keep-Alive would pre-

vent Keep-Alive operation during a short to ground fault and

the excessive drain on the input supply. The inversion by the

transistor will require a low level from the controller to allow

Keep-Alive operation. The 24 k

Ω pull-up resistor provides

current limiting in the event of a Tracking output short to the

unregulated/battery input supply.

DS012906-21

FIGURE 5. Simple Logic Configuration to Provide ON/OFF Sensing

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