ST1S03

4/11

APPLICATION NOTES

The ST1S03 is an adjustable current mode PWM

step-down DC/DC converter with internal 1.5A

power switch, packaged in a 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.

Figure 3: Application Circuit