May 2010

Doc ID 13883 Rev 3

1/20

AN2620

Application note

3 A high-frequency synchronous 900 kHz

step-down converter based on the ST1S10

Introduction

The ST1S10 is a step-down DC-DC converter with an optimized inhibit function for powering

high-voltage LCD applications and low-voltage digital core HDD applications. Generally, it

replaces the high current linear solution when high power dissipation is a problem. It

provides up to 3 A over an input voltage range of 2.5 V to 18 V and synchronous rectification

saves the external Schottky diode. A high internal switching frequency (0.9 MHz) allows it to

use tiny surface-mount components, as well as the resistor divider, to set the output voltage

value. Only an inductor and 3 capacitors are required. The current PWM mode architecture

and stable operation with low E.S.R SMD ceramic capacitors results in low, predictable

output ripple. To maximize the power conversion efficiency in light load, the regulator can

work in burst mode automatically. The device can operate in PWM mode at a fixed

frequency or synchronized to an external frequency. It switches at a frequency of 900 kHz

when SYNC is connected to ground or a fixed voltage (less than 5.5 V) and synchronizes

the switching frequency between 400 kHz to 1.2 MHz from the external clock that is applied

to SYNC. A thermal shutdown circuit is integrated and activates at 150 °C. Cycle-by-cycle

current limitation provides protection against shorted outputs. The on-chip 260 µs power-on

reset ensures the proper operation when switching on the power supply. The quiescent

current is less than 6 µA in the inhibit state. The device is available in MLP4x4 and SO-8

ePad packages.

Figure 1.

Simplified schematic

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