L5987A 데이터 시트보기 (PDF) - STMicroelectronics

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L5987A

3 A step-down switching regulator

STMicroelectronics 

L5987

5.2

Functional description

Soft-start

The soft-start is essential to assure a correct and safe startup of the step-down converter. It

avoids inrush current surge and makes the output voltage increase monotonically.

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 7). The soft-start staircase 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

SSTIME = 3----F2----s---w-6---4--

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

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