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Hi,
try mc33063/mc34063 . produced by many (onsemi, fairchild...) it is cheap, under $0,5, but you'll need something around it (few r/c, one coil). search after the datasheet at onsemi (www.onsemi.com) , it has design examples in it and formulas for components values.
you can buy coil from coilcraft for example, or you can made it yourself. just make sure to use good core. as you can see from the design example in onsemi datasheet, for buck converter you will need a coil around tens of microhenry and saturation current must be higher than approx twice nominal output current. (max switching current = 2x nominal out current). in the datasheet is not mentioned, but you will better put a zenner diode at the output (5,6v, 1W) for protection and a fuse at the input. if the switching element in the ic fails and transform itself into a 3 end wire, the input voltage will appear at the output. you will find that powering 5v logic circuits with 12v may be unpleasant . this is a general problem with buck topologies.
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