AN2093 - APPLICATION NOTE

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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=25K

Ω, 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.