L6986ITR datasheet(12/60 Pages) STMICROELECTRONICS

12 / 60 page

Figure 8. Iso-buck primary and secondary current waveforms

4.5

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 SS/INH pin crosses VSS

START. (See figure below).

The internal current generator sources 1 µA typ. current when the voltage of the VCC pin crosses the UVLO

threshold. The current increases to 4 µA 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 VINH threshold.

The start-up 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 SS/INH pin.

L6986I

Soft-start and inhibit

DS13647 - Rev 1

page 12/60

L6986ITR Datasheet(PDF) 12 Page - STMicroelectronics