Functional description

ST1S32

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Doc ID 023246 Rev 1

4.1

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 monothonically.

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 monothonically.

4.2

Error amplifier and control loop stability

The error amplifier provides the error signal to be compared with the high-side switch

current through the current sense circuitry. The non-inverting input is connected with the

internal 0.8 V reference, whilst the inverting input is the FB pin. The compensation network

is internal and connected between the E/A output and GND.

The error amplifier of the ST1S32 is a transconductance operational amplifier, with high

bandwidth and high output impedance.

The characteristics of the uncompensated error amplifier are:

The ST1S32 embeds the compensation network that assures the stability of the loop in the

whole operating range. Here below are 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.

Characteristics of the uncompensated error amplifier

Description

Value

DC gain

94 dB

gm

238

μA/V

Ro

96 M

Ω