DocID025723 Rev 4

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ST1S50

Functional description

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5.1

Programmable soft-start and enable

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

avoids an inrush current surge and makes the output voltage increases monothonically.

from 0 V to 0.8 V. The ramp of the reference voltage can be programmed selecting the soft-

start capacitor that is connected between the EN_SS pin and ground.

The device sources 5

A (typical) in the RC network, ramping the EN_SS pin logarithmically

from 0.34 V to 1.7 V. The reference of the error amplifier ramps, starting when the EN_SS

pin voltage is equal to 0.7 V (typ.), up from 0 V to 0.8 V accordingly.

Once the soft-start time ends, the EN_SS pin is internally tied to 3.3 V to avoid injecting

noise.

5.2

Error amplifier and control loop stability

The error amplifier compares the FB pin voltage with the internal 0.8 V reference and it

provides the error signal to be compared with the output of the current sense circuitry, that is

the high-side power MOSFET current. Comparing the output of the error amplifier and the

peak inductor current implements the peak current mode control loop.

The error amplifier is a trans-conductance amplifier (OTA). The uncompensated

characteristics are listed inTable 5.

Table 5. Error amplifier characteristics

Description

Value

DC gain

95 dB

Gm

250

A/V

240 M

