AN2093 STMICROELECTRONICS | Alldatasheet
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- 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 cu rrent 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. Th e 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 st ep-down DC/DC converter with internal 1.5A power switch. It is a complete 1.5A switching regulator with an internal compensation t hat eliminates additional components. The constant frequency, current m ode, 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. December 2004 1/6 AN2093 APPLICATION NOTE ST1S03 BUCK CONVERTERS FOR HDD POWER SUPPLIES Rev. 2
The minimum efficiency guaranteed in the typical applic ation is higher than 70%. In some conditions the efficiency can reach values equal to 90%. To clamp the error amplifier reference voltage a so ft 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 pr otection are the thermal shut down blocks, which turn off the regulator when the junc tion temperature exceeds 150°C ty pically and the cycle-by-cycle current limit that provides protection against shorted outputs. Being the ST1S03 an adjustable regulator, the outp ut 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 th eir 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.
- INDUCTOR CHOICE The ST1S03 is a PWM voltage regulator with a ty pical working frequency of 1.5MHz. The very high frequency allows the use of small inductor values. The device is able to work with a minimum of 1µH inductor. We suggest using 3.3µH or higher values in order to cover all the parameters specified in the datasheet. Another important parameter to take into account in the inductor choice is the saturation current. Normally the device does not exceed a peak current of 2.3A @ Vin=5V. 4. SCHOTTKY DIODE During the Toff the current in the i nductor can continue to flow thanks to the schottky diode. It is very important the use of a diode with a very low threshold, in order to have high efficiency, and with a good current capability. In our application good results are obtained using the STPS2L30A diode. In some applications, where the switching noise must be reduced, it could be useful to add a snubber circuit in parallel with the schottky diode. If necessary, we suggest to connect a 10Ω resistor in series with a 680pF capacitor. 5. CONCLUSIONS The device can be used in all the applications where a low supply voltage, starting from 0.8V, is needed. The low inductor and capacitors value allows the use of the ST1S03 in all the boards where space saving and power dissipation control is recommended. The high frequency makes the device compatible with the new HDD generation, where the rotation plate is 7200rpm or 10000rpm.
- REVISION HISTORY Table 2: Revision History Date Revision Descri ption of Changes 10-Dec-2004 1 First Release 15-Dec-2004 2 In figure 2 the zener diode has been replaced by a Schottky diode
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