First I started with the major building blocks of a buck regulator:

1. Oscillator
2. PWM
3. Feedback

Poking around on the googles for opamp PWM, I ran across this great video series: Op-Amps, PWM

I quickly fired up LTSpice and simulated the circuits.  It did work, but the waveforms were a little too off for my tastes.  And, more importantly, it required a dual supply to operate.

Basic operation is as follows:

The  first opamp is a oscillator circuit used to make a square wave.  The square wave is then integrated by the second opamp circuit, which produces a triangle wave.

The search continues...