L5988D datasheet(18/52 Pages) STMICROELECTRONICS

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Functional description

L5988D

18/52

DocID15324 Rev 4

Figure 8. Input RMS current of two synchronized regulators

5.3

External voltage reference

An external 1.8 V regulated voltage is provided. This reference is useful to set the voltage at

the multifunction pin (see Section 5.1 on page 13) or to source current to ILIM-ADJ and

FSW pins (seeSection 5.2 and Section 5.5.2). The typical current capability is 4 mA.

5.4

Soft-start

When VCC is above the selected UVLO threshold the start-up phase takes place. At start-

up, a voltage ramp is generated charging the external capacitor CSS with an internal current

generator. The device is in inhibit mode as long as SS/INH pin is below the INH threshold.

The L5988D implements the soft-start phase by clamping the output of the error amplifier

and, being based on a voltage mode control, the duty cycle. In fact the comparison between

the output of the error amplifier and the internal saw tooth waveform generates the duty

cycle needed to keep the output voltage in regulation.

Two different current sources charge the external capacitor depending on the pin voltage in

order to reduce the power consumption in INH mode.

Equation 4

The equation for the soft-start time is:

Equation 5

ISS

ISS1 5

A

0 VSS/INH 1



ISS2 2

= 2

A

1 VSS/INH 2.9









TSS



CSS

ISS1

-----------

–





CSS

ISS2

-----------

2.9 1

–





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