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MC34063 converter part value selection confusion

Question asked by morgan.christopher on Apr 2, 2013
Latest reply on Apr 18, 2013 by grimaldi.vincenzo
Hello.

I've gone through the process of selecting part values for a few other dc/dc converters but I'm stumped with the steps to do so for the MC34063. I'm starting with 12V and converting to 5V and 3.3v so I just copied the example circuit values for the 12V->5V step-down but I have no idea where to start on the 3.3V one.

The calculations table seems like it has the key information but I'm not sure how to interpret it. Some of the calculations are asking about forward voltage but don't specify if this is if using an external transistor or if its a part characteristic or what, same for ton/toff etc.

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