This circuit is a voltage regulator based on LM2576 step down regulator. It is able to provide an output of 12V 3A with an input of 16-40V.
LM2576 Switching Regulator 12V 3A - [Link]
A dc-dc regulator/converter or another name known as buck regulator, provides stable regulated output voltage to supply electronic circuits. Schematic, PCB layout and component list are available on this page.
LM2576 circuits perform same function as the commonly known voltage regulator LM7805 from National Semiconductor. The 7805 voltage regulator dissipates a lot heat. The higher input voltage, the more heat is generated. The extra input energy is converted to heat, keeping the output voltage regulated at 5V.
DC-DC converter using LM2576 - [Link]
I build this project when I have a problem to driver stepping motor that draw current 3A at 3V.After search the infomation on the internet I have found LM2576-ADJ which is an adjustable output version of 3A Step-Down Voltage switching Regulator and capable of driving 3A load with excellent line and load regulation.
You can change R1 and R2 for your desired output voltage.In my design I use the circuit from the example circuit in data sheet and it works fine.
3V/3A switching regulator - [Link]