AN938 Datasheet, PDF (12/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

Power supply, UVLO and voltage reference
AN938
Example: For a maximum V_cc of 50 V, at 100 kHz, with an inductor of 140 µH, it is possible
to use a soft-start capacitor lower than 470 µF (see Figure 13). With such a value, the soft-
start time (see Figure 16) is about 10 ms for an output voltage of 5 V.
Figure 15. Soft-start internal circuit
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Soft-start
The soft-start function is needed to generate a correct startup of the system without
overstressing the power stage, avoiding the intervention of current limiting, and having an
output voltage rising smoothly without output overshoots.
The soft-start circuit is shown in Figure 15. The soft-start capacitor is charged at 40 µA, and
quickly discharged at power-off and in case of thermal shutdown intervention.
The output startup time is calculated using the following formula:
Equation 6
Tss = _i--_C----_H-----_â‹---₆-^v---^o_â‹----_D----_m----_a---_x- ⋅ Css
where D_max is 0.95.
The soft-start time versus output voltage and Css is shown in Figure 16. Thanks to the
voltage feed-forward the startup time is not affected by the input voltage. Figure 17 shows
the output voltage startup using different soft-start capacitance values.
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