AN458 APPLICATION NOTE

12/19

Figure 15.

Figure 16.

POWER FAIL

as shown in Fig.17. When the supply voltage reaches the typical rising threshold voltage of 22V, set by the in-

ternal voltage divider, the Power Fail comparator output voltage goes low, turning off the output transistor Q1.

This gives an high level on pin.8. As the power Fail output is an open collector transistor (Q1), an external pull-

up resistor is required.

The Power Fail output goes low, giving an alarm signal, when the input voltage decreases reaching the internal

typical Falling threshold voltage level of 18V. It is possible to change the rising and the falling threshold voltages,

connecting a proper external voltage divider on pin.7. In fig. 15 are shown the power fail waveforms.

INHIBIT

The INHIBIT function, available on pin.16, disables the regulator with a TTL logic signal. An high level at this pin

(above 2.2V) switches off the power stage and forces low the output reset. This useful feature, is normally used