L5981TR datasheet(10/37 Pages) STMICROELECTRONICS

Part #	L5981TR

Description	1 A step-down switching regulator

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Functional description

L5981

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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.

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 R1 and R2 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 250 kHz the SSTIME is 8 ms.

SR

OUT

SR

VREF

1

R1

R2

--------

+

⎝⎠

⎛⎞

⋅

=

SS

TIME

32 64

⋅

Fsw

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

=

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