ST1S40

Functional description

Doc ID 17928 Rev 4

7/30

5

Functional description

The ST1S40 is based on a “peak current mode”, constant frequency control. The output

providing an error signal that, compared to the output of the current sense amplifier, controls

the ON and OFF time of the power switch.

The main internal blocks are shown in the block diagram in Figure 3. They are:

●

A fully integrated oscillator that provides the internal clock and the ramp for the slope

compensation avoiding sub-harmonic instability

●

The soft-start circuitry to limit inrush current during the startup phase

●

The transconductance error amplifier with integrated compensation network

●

The pulse width modulator and the relative logic circuitry necessary to drive the internal

power switches

●

The drivers for embedded P-channel and N-channel Power MOSFET switches

●

The high side current sensing block

●

The low side current sense to implement diode emulation

●

A voltage monitor circuitry (UVLO) that checks the input and internal voltages

●

A thermal shutdown block, to prevent thermal run-away.

Figure 3.

Block diagram

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.

OSC

E/A

DRIVER

DRIVER

DMD

OTP

MOSFET

CONTROL

LOGIC

REGULATOR

SHUT-DOWN

I_SENSE

COMP

COMP

OCP

REF

0.8V

SOFTSTART

Vsum

Vc

OCP

UVLO

Vdrv_p

Vdrv_n

I2V

VINA

VINSW

SW

GNDP

GNDA

EN

FB