ST1S14 datasheet(11/46 Pages) STMICROELECTRONICS

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DocID17977 Rev 2

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ST1S14

Function description

4.1

Power supply and voltage reference

The internal regulator circuit consists of a start-up circuit, an internal voltage pre-regulator,

the bandgap voltage reference, and the bias block that provides current to all the blocks.

The starter supplies the start-up current to the entire device when the input voltage goes

high and the device is enabled (inhibit pin connected to ground). The pre-regulator block

supplies the bandgap cell with a pre-regulated voltage that has a very low supply voltage

noise sensitivity.

4.2

Voltage monitor

An internal block continuously senses the Vcc, Vref, and Vbg. If the monitored voltages are

good, the regulator begins operating. There is also a hysteresis on the VCC (UVLO).

Figure 4. Internal circuit

4.3

Soft-start

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

components at the power up. The startup takes place when VIN crosses the selected UVLO

threshold. A internal counter (2816 clks) sets the soft start time (see Figure 5).

The reference of the error amplifier is ramped smootly in 704 steps (one step every 4 clks).

A low pass filter smooths each step to minimize output discontinuity. Considering the typical

850 kHz switching frequency, the phase two duration is 3.3 msec

The device has full load current capability during the soft start time in order to charge the

output capacitor (see Figure 5).

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