L7987 datasheet(15/37 Pages) STMICROELECTRONICS

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L7987

Functional description

4.2

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 charging an external capacitor, connected between the SS pin

and ground, with a constant current (5 µA typ.). The SS voltage is used as reference of the

switching regulator and the output voltage of the converter tracks the ramp of the SS

voltage. When the SS pin voltage reaches 0.8 V level, the error amplifier switches to the

internal 0.8 V ± 1% reference to regulate the output voltage.

Figure 7. Soft-start

During the soft-start period the current limit is set to the nominal value.

The dVSS/dt slope is programmed in agreement with Equation 2:

Equation 2

Before starting the CSS capacitor charge, the soft-start circuitry turns-on the discharge

switch shown in Figure 7 for TSSDISCH minimum time, in order to completely discharge the

CSS capacitor.

As a consequence, the maximum value for the soft-start capacitor, which assures an almost

complete discharge in case of EN signal toggle, is provided by:

Equation 3

given TSSDISCH = 530 s and RSSDISCH = 380  typical values.

The enable feature allows to put the device into standby mode. With the EN pin lower than

0.32 V the device is disabled and the power consumption is reduced to less than 11

A

CSS

ISS TSS



VREF

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

AT



0.8V

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

CSS MAX

–

TSSDISCH

5RSSDISCH



------------------------------------ 270nF





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