AST1S31HF Datasheet, PDF (11/33 Pages) STMicroelectronics – Internal soft-start and enable

AST1S31HF
Functional description
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.
Table 5. Characteristics of the uncompensated error amplifier
Description
Value
DC gain
gm
Ro
87 dB
236 A/V
98 M
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.
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