AN937

Power supply, UVLO and voltage reference

Doc ID 5655 Rev 15

9/29

The following diagrams of Figure 13 and Figure 14 show the maximum allowed soft-start

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

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

condition, the functionality of the limiting current circuitry.

In fact, 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.

Example: for a maximum input voltage of 55 V at 100 kHz, with an inductor of 260 mH, it is

possible to use a soft start capacitor lower than 470 µF. With such a value, the soft start time

(see Figure 19) of about 10 ms for an output voltage of 5 V.

Figure 12.

Figure 13.

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