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AN487 APPLICATION NOTE

the PWM signal begins to be generated as soon

as the error amplifier output voltage crosses the

ramp; the power stage starts to switch with steadi-

ly increasing duty cycle (fig.9).

Figure 6. Switching frequency vs. Rosc

(L4970A/77A/75A).

Figure 7. Switching frequency vs. Rosc

(L4972A/74A).

The charge of the soft-start capacitor is started ev-

ery time the system begins to work after an anom-

alous

condition

occurred

(undervoltage

and

thermal protection). The Css discharge current is

in the range of about 20mA.

Figure 8. Soft start circuit

Figure 9. Soft start waveforms