Functional description

L5980

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4.2

Soft-start

The soft-start is essential to assure correct and safe start up of the step-down converter. It

avoids inrush current surge and makes the output voltage increases monothonically.

Error Amplifier. So the output voltage slew rate is:

Equation 1

regulate the output voltage (see Figure 7). The soft-start stair case consists of 64 steps of

9.5mV each one, from 0V to 0.6V. The time base of one step is of 32 clock cycles. So the

soft start time and then the output voltage slew rate depend on the switching frequency.

Figure 7.

Soft start scheme.

Soft start time results:

Equation 2

SR

OUT

SR

VREF

1

R1

R2

--------

+

⎝⎠

⎛⎞

⋅

=

SS

TIME

32 64

⋅

Fsw

-----------------

=