L5986 データシートの表示（PDF） - STMicroelectronics

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L5986

2.5 A step-down switching regulator

STMicroelectronics 

Functional description

L5986 - L5986A

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

SROUT

=

SRVREF

⋅

⎛

⎝

1

+

RR-----12--⎠⎞

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

SSTIME

=

3----2-----⋅---6---4--

Fsw

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

10/39

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