Datasheet

Functional description ST1S41
8/27 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.
The soft-start is performed by ramping the non-inverting input (V
REF
) of the error amplifier
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 MΩ