L5987A(2008) 查看數據表（PDF） - STMicroelectronics

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L5987A
(Rev.:2008)

3 A step-down switching regulator

STMicroelectronics 

L5987 - L5987A

Functional description

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 9.5 mV 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.

11/42

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