L6985F datasheet(14/65 Pages) STMICROELECTRONICS

14 / 65 page

Functional description

L6985F

14/65

DocID027836 Rev 2

Figure 4. Internal circuit

4.3

Soft-start and inhibit

The soft-start and inhibit features are multiplexed on the same pin. An internal current

source charges the external soft-start capacitor to implement a voltage ramp on the SS/INH

pin. The device is inhibited as long as the SS/INH pin voltage is lower than the VINH

threshold and the soft-start takes place when the SS/INH pin crosses VSS START. (See

Figure 5).

The internal current generator sources a 1 A typ. current when the voltage of the VCC pin

crosses the UVLO threshold. The current increases to 4

A typ. as soon as the SS/INH

voltage is higher than the VINH threshold. This feature helps to decrease the current

consumption in inhibit mode. An external open collector can be used to set the inhibit

operation clamping the SS/INH voltage below the VINH threshold.

The startup feature minimizes the inrush current and decreases the stress of the power

components during the power-up phase. The ramp implemented on the reference of the

error amplifier has a gain three times higher (SSGAIN) than the external ramp present at the

SS/INH pin.

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