This circuit is based on
mobile phone chargers. These chargers are based on the Motorola MC34063
switchmode IC. By changing the values of the feedback resistors (R1
& R2), the output voltage can be varied over a wide range. Just
modify R1 and R2 according to the formula: Vout = 1.25 (1+R2/R1). The
values shown give an output of 3V.
Author: Timo Mahoney – Copyright: Silicon Chip Electronics Magazine