I will build a charge-pump on a breadboard and explain along the way, stupid mistakes included!
I simulated an OpAmp based oscillator in everycircuit.com and then build it on a breadboard to test it.
The first op amp circuit will be a square wave generator. A side-effect of this circuit is a triangle wave signal on the negative input.
The second opamp will combine this triangel wave signal with a voltage from a potentiometer into a PWM signal.