Application notes ST1S06xx
14/20 Doc ID 12236 Rev 9
7 Application notes
The ST1S06xx 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.
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 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 ST1S06xx 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 + R
1
/ R
2
]
To make the device working, only few component are required: an inductor and two
capacitors and the resistor divider. 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 ST1S06xx 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.
