ST1S03

Application notes

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Application notes

The ST1S03 is an adjustable current mode PWM step-down DC/DC converter with internal

1.5A power switch, packaged in 6-lead DFN 3x3.

It’s a complete 1.5A 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 keep the output

regulated, the devices goes in pulse skipping mode when a very light load is required.

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 ensure

the proper operation when switching on the power supply. Other circuits fitted to the device

protection are the Thermal Shut down 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 ST1S03 an adjustable regulator, the output voltage is determined by an external

resistor divider. The desired value is given by the following equation:

To make the device working, only other four external components are required: a Schottky

diode, an inductor 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 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 invalidating 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.