L5986ATR datasheet(10/39 Pages) STMICROELECTRONICS

10 / 39 page

Functional description

L5986 - L5986A

10/39

5.2

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.

The soft-start is performed by a staircase ramp on the non-inverting input (VREF) of the Error

Amplifier. So the output voltage slew rate is:

Equation 1

where SRVREF is the slew rate of the non-inverting input, while R1and R2 is the resistor

divider to regulate the output voltage (see Figure 6). The soft-start stair case consists of 64

steps of 9.5mV each one, from 0 V to 0.6 V. 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 6.

Soft-start scheme

Soft-start time results:

Equation 2

For example with a switching frequency of 250 kHz the SSTIME is 8 ms.

OUT

VREF

--------

⎝⎠

⎛⎞

⋅

TIME

32 64

⋅

Fsw

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

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