L5983 datasheet(11/37 Pages) STMICROELECTRONICS

Il numero della parte	L5983

Spiegazioni elettronici	1.5A step-down switching regulator

PDF	37 Pages

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Produttore elettronici	STMICROELECTRONICS [STMicroelectronics]

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Functional description

L5983

11/37

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 and R1, R2 is the resistor divider to

regulate the output voltage (see Figure 7). The soft-start stair case consists of 64 steps of

9.5mV each one, from 0V to 0.6V. 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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