Hi, Dear Mohammad

The output power of the DC-DC converter is equal to the input power multiplied by converter efficiency. I assumed your DC-DC converter has an efficiency off 80%.

Pout=Pin⋅80%

3Iout=5Iin⋅0.8

Iout=1.33Iin

But not always. You get the maximum current that is supported by the converter. If that limit is reached, then this equation is not true anymore.

Hi, Dear Mohammad

The output power of the DC-DC converter is equal to the input power multiplied by converter efficiency. I assumed your DC-DC converter has an efficiency off 80%.

Pout=Pin⋅80%

3Iout=5Iin⋅0.8

Iout=1.33Iin

But not always. You get the maximum current that is supported by the converter. If that limit is reached, then this equation is not true anymore.

Hi, 

Well, Mr Mohammad Malek  answer your second question. I´ll try to figure out the first one. 

Do you want to make from scratch your own DC-DC converter?

If yes, there are several publications about it, You will find diverse topologies as:

• Buck converter
• Bridge converter 
• Watkins- Johnson Converter
• and others...

MOre information in this books: 

• Power Electronics by Ned Mohan
• Design and integration of Fully_Integrated Inductive DC converters by Mike Wens
• POwer Electronics by M. H. Rashid

And you'll find diverse switching control techniques over one single  topology.

If not, I'm sure this is the best for you, you will find a wide variety of DC-DC IC´s step down in market. As an example:

http://www.onsemi.com/pub_link/Collateral/AND8117-D.PDF

here you´ll find a good example on design:

http://www.atmel.com/Images/Atmel-42183-Design-a-Buck-Converter-with-XMEGA-E_AP-Note_AT04204.pdf

Acopian’s low profile wide adjust DC-DC converter can be used in voltage regulation or current regulation mode. Typical converters are voltage regulated, which means that the output voltage will remain constant throughout the output current range. Acopian’s low profile wide adjust converters can be voltage regulated or current regulated, depending on the load and power supply settings. For example, imagine a 10VDC, 5A DC-DC converter with a connected load that draws 3A at 10V. If the Vadj and Iadj pots are fully clockwise, the converter will regulate the voltage at 10VDC and the output current will be 3A. If the Vadj pot is slowly turned counterclockwise, the output voltage will track the pot setting. If it is left at 5VDC, the output will remain at 5VDC and the current will be reduced to 1.5A. Current regulation mode is different. Leave the Vadj pot fully clockwise and turn the Iadj pot counterclockwise. When the Iadj reaches 3A, the power supply will start to current regulate. As the Iadj pot is turned lower, the output voltage will start to decrease to maintain the current that the Iadj pot is set at. If it is left at 2A, the output current will remain at 2A, the output voltage adjusting to the voltage required to keep it at 2A (6.7V in this case).