Power supply, UVLO and voltage reference

AN938

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Doc ID 5410 Rev 10

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

2.3

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

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.

Tss

v

o

i

CH

6D

max

⋅⋅

----------------------------------- Css

⋅

=