ST AN2093 APPLICATION NOTE

AN2093

APPLICATION NOTE

ST1S03

BUCK CONVERTERS FOR HDD POWER SUPPLIES

1. ABSTRACT

The ST1S03 is a step down DC-DC converter optimized for powering the low-voltage digital core in HDD
applications and, generally, replacing the high current linear solution when the power dissipation may
cause excessive heating of the application environment. It provides up to 1.5A over an input voltage range
of 3V to 16V. A high switching frequency (1.5MHz) allows the use of tiny surface-mount components, as
well as the use of a resistor divider to set the output voltage value. Only an inductor, a Schottky diode and
two capacitors are required. Besides, a low output ripple is guaranteed by the current mode PWM
topology and by the use of low E.S.R. SMD ceramic capacitors. The device is thermal protected and
current limited to prevent damages due to accidental short circuit. The ST1S03 is available in DFN6 (3x3)
package.

Figure 1: Assembled Board

2. INTRODUCTION

Purpose of this application note is to provide a brief summary of the specifications and the functionality of
the application board.
The ST1S03 is an adjustable current mode PWM step-down DC/DC converter with internal 1.5A power
switch.
It is a complete 1.5A switching regulator with an internal compensation that eliminates additional
components.

The constant frequency, current mode, PWM architecture and stable operation with ceramic capacitors
results in a low, predictable output ripple. In order to maintain the regulation with light load, the device
starts to work in pulse skipping. In a typical application where the input voltage is 5V and Vout is 1.2V, the
device goes in pulse skipping for Iout lower than 15mA.

Rev. 2

December 2004

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AN2093 - APPLICATION NOTE

The minimum efficiency guaranteed in the typical application is higher than 70%. In some conditions the
efficiency can reach values equal to 90%.

To clamp the error amplifier reference voltage a soft start control block generating a voltage ramp has
been implemented. Besides, an on-chip power on reset of 50=100µs ensures a proper operation when
switching on the power supply. Other circuits fitted to the device protection are the thermal shut down
blocks, which turn off the regulator when the junction temperature exceeds 150°C typically and the
cycle-by-cycle current limit that provides protection against shorted outputs.
Being the ST1S03 an adjustable regulator, the output voltage is determined by an external resistor
divider. The desired value is given by the following equation:

Vout=VFB[1+R1/R2]

According to the previous formula, being VFB=0.8V, if 1.2V is needed, the external divider must be
R1=25KΩ, R2=50KΩ

Figure 2: Application Schematic

To make the device work, only other four external components are required: a Schottky diode, an inductor
and two capacitors. The chosen inductor must be able not to saturate at the peak current level, that for
this device typical value is 2.3A.
Besides, its value can be selected taking into account that a large inductor value increases the efficiency
at low output current and reduces output voltage ripple, while a smaller inductor can be chosen when it is
important to reduce the package size and the total cost of the application.
Finally, the ST1S03 has been designed to work properly with X5R or X7R SMD ceramic capacitors both
at the input and at the output. This kind of capacitors, thanks to their very low series resistance(ESR),
minimize the output voltage ripple. Other low ESR capacitors can be used according to the need of the
application without invalidate the right functioning of the device.

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