ST1S03 STMicroelectronics, ST1S03 Datasheet - Page 7

• Manufacturer Part Number: ST1S03
• Description: 1.5 A, 1.5 MHz adjustable, step-down switching regulator
• Manufacturer: STMicroelectronics

Specifications of ST1S03

• Typical Efficiency: > 70% over all operating conditions
• Not Switching Quiescent Current: max 2.5mA over temperature range
• Switch Vds: max 350mV @ ISW=750mA

ST1S03
Application notes
5
Application notes
The ST1S03 is an adjustable current mode PWM step-down DC-DC converter with internal
1.5 A power switch, packaged in 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 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 = 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 ST1S03 an adjustable regulator, the output voltage is determined by an external
resistor divider. The desired value is given by the following equation:
V
= V
[1+R1/R2]
O
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.
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