DocID026968 Rev 2

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AST1S31HF

Functional description

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6.2

Soft-start

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

It avoids the inrush current surge and makes the output voltage rise monothonically.

The soft-start is managed 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.

6.3

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 AST1S31HF is a transconductance operational amplifier, with high

bandwidth and high output impedance.

The AST1S31HF device embeds the compensation network that assures the stability of the

loop in the whole operating range. In Section 6.7 on page 18 all the tools needed to check

the loop stability are shown.

In Figure 5 is shown the simple small signal model for the peak current mode control loop.

Table 5. Characteristics of the uncompensated error amplifier

Description

Value

DC gain

87 dB

gm

236

A/V

Ro

98 M

