April 2008

Rev 1

1/20

20

AN2728

Application note

ST1S12

small synchronous buck converter

Introduction

The ST1S12 family of synchronous step-down DC-DC converters optimized for powering

low-voltage digital cores in HDD applications is generally used to replace the high-current

linear solution when the power dissipation may cause high heating of the application

environment. It provides up to 0.7 A over an input voltage range of 2.5 V to 5.5 V.

A high switching frequency (1.7 MHz) allows the use of tiny surface-mount components. A

resistor divider to set the output voltage value, an inductor, and two capacitors are required

for the adjustable version. Only an inductor and 2 capacitors are needed for the 1.2 V and

1.8 V fixed version. A low output ripple is guaranteed by the current mode PWM topology

and by the use of low ESR surface-mount ceramic capacitors.

The device is thermal protected and current limited to prevent damages due to accidental

short-circuit. This family of products is available in the TSOT23-5L package.

Figure 1.

ST1S12 - simplified schematic

Driver

OSC

Σ

SW

MOSFET

Comp

Driver

CONTROL

LOGIC

SOFT START

REF1

Gnd

DMD

I_SEN

DMD

UVLO

SHUT DOWN

EN

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