AN937 Datasheet, PDF (9/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

AN937
Power supply, UVLO and voltage reference
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. Maximum soft start capacitance with f_sw = 100 kHz
L
(μH)
400
fsw=100KHz
680nF
D97IN745
470nF
330nF
300
200
220nF
100
100nF
0
15 20 25 30 35 40 45 50 V_CCmax(V)
Figure 13. Maximum soft start capacitance with f_sw = 200 kHz
L
(μH)
fsw=200KHz
D97IN746
56nF
300
47nF
200
33nF
22nF
100
0
15 20 25 30 35 40 45 50 V_CCmax(V)
Doc ID 5655 Rev 15
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