Datasheet

Functional description ST1S40
8/29 DocID17928 Rev 5
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 causes the output voltage to 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 inTable 5.
Table 5. Error amplifier characteristics
The ST1S40 device embeds the compensation network that assures the stability of the loop
in the whole operating range. All the tools needed to check the loop stability are shown
below.
Parameter Value
DC Gain 95 dB
Gm 251 µA/V
Ro 240 M