L6986H datasheet(12/68 Pages) STMICROELECTRONICS

12 / 68 page

4.1

Power supply and voltage reference

The internal regulator block consists of a start-up circuit, the voltage pre-regulator that provides current to all the

blocks and the bandgap voltage reference. The starter supplies the start-up current when the input voltage goes

high and the device is enabled (SS/INH pin over the inhibits threshold).

The pre-regulator block supplies the bandgap cell and the rest of the circuitry with a regulated voltage that has a

very low supply voltage noise sensitivity.

Switchover feature

The switchover scheme of the pre-regulator block features to derive the main contribution of the supply current for

the internal circuitry from an external voltage (3 V < VBIAS<5.5 V is typically connected to the regulated output

voltage). This helps to decrease the equivalent quiescent current seen at VIN. (Please refer to Switchover

feature).

4.2

Voltage monitor

An internal block continuously senses the VCC,VBIAS and VBG. If the monitored voltages are good, the regulator

starts operating. There is also a hysteresis on the VCC (UVLO).

Figure 4. Internal circuit

STARTER

IC BIAS

PREREGULATOR

BANDGAP

VREG

VREF

D00IN1126

VCC

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 SS/INH pin crosses VSS

START. (See Figure 5. Soft-start phase).

The internal current generator sources 1 mA 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.

L6986H

Power supply and voltage reference

DS12905 - Rev 2

page 12/68

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