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AN1061 APPLICATION NOTE

Figure 12. Maximum Soft Start

Figure 13. Maximum Soft Start

15

20

25

30

35

40

45

0

100

200

300

400

L

(

µH)

D97IN745

680nF

fsw=100KHz

470nF

330nF

220nF

100nF

15

20

25

30

35

40

45

0

100

200

300

L

(

µH)

56nF

fsw=200KHz

D97IN746

47nF

33nF

22nF

During the recharging of the soft start capacitor, the Ton increases gradually and, if the short

cup protection intervenes again. So, the value of the soft start capacitor must not be too high

that during the soft start slope the current exceeds the limit before the protection activation.

The folllowing diagrams of Figure 12 and Figure 13 show the maximum allowed soft-start ca-

pacitor as a function of the input voltage, inductor value and switching frequency. A minimun

value of the soft start capacitance is necessary to guarantee, in short circuit condition, the

functionality of the limiting current circuitry. Infact, with a capacitor too small, the frequency of

the current peaks (see figure 11) is high and the mean current value in short circuit increases.

3.2 Soft Start and Inhibit functions.

The soft start and the inhibit functions are realised using one pin only, pin2. Soft-start is re-

quested to inizialise all internal functions with a correct start-up of the system without over-

stressing the power stage, avoiding the intervention of the current protection, and having an

output voltage rising smoothly without output overshoots.

At Vcc Turn-on or having had an intervention of inhibit function, an initial 5

µA internal current

generator starts to charge the soft-start capacitor, from 0V to about 1.8V. From this hysteretic

threshold, a 40

µA current generator is activated, putting in off state the previous generator.

At this point, the output PWM starts, initiating the rising phase of the output voltage.

The soft-start capacitor is quickly discharged in case of:

The soft-start and inhibit schematic diagram is shown in figure 14.