USB charging requires only a constant voltage of +5V DC, with constant current. The LM1117MP-5,0 linear voltage regulator is very popular due to its simplicity of circuit wiring. It outputs a +5V voltage, and 1A current. It has low drop down voltage.
A finished circuit soldering
It's constructed with about a dozen of components I have. The LM1117MP-5,0 is an SMD device, but soldering it on PCB is not difficult.
