PWM Controllers for SMPS
No a PWM controller is not a good fit for this application because it has not regulation capability, it has no feed back. In a true buck regulator, the regulator senses the output and adjust the PWM accordingly to make sure that the output is the correct voltage. This kind of regulation compensates the system across many different loads. If the PWM is fixed then it is only valid for a single load point. 

A prebuilt buck regulator is the easiest solution for you. Add and inductor and some passives and your up and running. If you really want to roll your own there is a topology that uses two opamps and a triangle wave generator.







simulate this circuit &ndash; Schematic created using CircuitLab

These are not the correct values. I will leave it up to you to figure them out if you choose to use this setup.