ST1S03
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.
resistor divider. The desired value is given by the
following equation:
V
= V [1+R1/R2]
OUT
FB
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.
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
Figure 3: Application Circuit
4/11
STMICROELECTRONICS [ ST ]
