DocID022998 Rev 4

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ST1S31

Functional description

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4.1

Output voltage adjustment

The error amplifier reference voltage is 0.8 V typical. The output voltage is adjusted

according to the following formula (see Figure 1 on page 1):

Equation 1

The internal architecture of the device requires a minimum off time, cycle-by-cycle, for the

output voltage regulation. The minimum off time is typically equal to 94 ns.

The control loop compensates for conversion losses with duty cycle control. Since the

power losses are proportional to the delivered output power, the duty cycle increases with

the load current request.

Figure 4 shows the maximum regulated output voltage over the input voltage range at

different loading conditions.

Figure 4. Maximum output voltage over loading conditions

4.2

Soft-start

The soft-start is essential to assure the correct and safe startup of the step-down converter.

It avoids inrush current surge and makes the output voltage rise monotonically.

The soft-start is managed by ramping the reference of the error amplifier from 0 V to 0.8 V.

The internal soft-start capacitor is charged with a resistor to 0.8 V, then the FB pin follows

the reference so that the output voltage is regulated to rise to the set value monotonically.

0.8

1

-------

+







=