Application notes ST1S03A
8/14
6 Application notes
The ST1S03A is an adjustable current mode PWM step-down DC-DC converter with internal
1.5 A power switch, packaged in a 6-lead DFN 3x3 mm.
It’s a complete 1.5 A switching regulator with its internal compensation eliminating additional
component.
The constant frequency, current mode, PWM architecture and stable operation with ceramic
capacitors results in low, predictable output ripple. However, in order to maximize the power
conversion efficiency with light load, the regulator reduces automatically the switching
frequency when the output load becomes less than 250 mA typically.
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 = 100 µs ensure the
proper operation when switching on the power supply. other circuits fitted to the device
protection are the thermal shutdown block which turn off the regulator when the junction
temperature exceeds 150 °C typically and the cycle-by-cycle current limiting that provides
protection against shorted outputs.
Being the ST1S03A an adjustable regulator, the output voltage is determined by an external
resistor divider. The desired value is given by the following equation:
V
O
= V
FB
[1 + R1 / R2]
To make the device working, only other four external components are required: an inductor a
schottky and two capacitors. The chosen inductor must be able to not saturate at the peak
current level. Besides, its value can be selected keeping in 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 ST1S03A 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.
Due to the high switching frequency and peak current, it is important to optimize the application
environment reducing the length of the PCB traces and placing all the external component near
the device.
