L7981 datasheet(11/44 Pages) STMICROELECTRONICS

tên linh kiện	L7981

Giải thích chi tiết về linh kiện	3 A step-down switching regulator

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L7981

Functional description

Doc ID 15182 Rev 3

11/44

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 7). 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 7.

Soft-start scheme

Soft-start time results:

Equation 2

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

OUT

VREF

--------

⎝⎠

⎛⎞

⋅

TIME

32 64

⋅

Fsw

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

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