Functional description

ST1S41

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DocID023654 Rev 2

5.1

Internal soft-start

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

avoids inrush current surge and makes the output voltage increase monothonically.

from 0 V to 0.8 V in around 1 ms.

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 transconductance amplifier (OTA). The uncompensated

characteristics are listed in Table 5:

The ST1S41 embeds the compensation network that assures the stability of the loop in the

whole operating range. Here below, all the tools needed to check the loop stability.

In Figure 4 the simple small signal model for the peak current mode control loop is shown.

Table 5. Error amplifier characteristics

DC gain

95 dB

Gm

251 uA/V

Ro

240 M

Ω