Is it possible to construct an adjustable output switching power supply based on the MC34063? I read over the On Semi datasheet and app notes but didn't see any examples of an adjustable output application.
Is it just a matter of replacing the "R1" and "R2" resistors with a potentiometer and then calculating the other components values based on a Vout in the middle of the the target Vout range (i.e. so for a Vout of 5-12V, you'd base calculations on 8.5V)?
It is fairly trivial to make a power supply adjustable - as you said, the voltage feedback divider can be modified to give you a range of output.
You have to decide on what adjustment range you want to have, and do calculations at both the maximum and minimum voltages to make sure the power train is capable of working.
If you're designing a buck, for example, the highest voltage you want will dictate the voltage rating of the output capacitors, rectification and series transistor / MOSFET; the lowest voltage will dictate the minimum duty cycle and peak current capabilities of the inductor / MOSFET as well as the capacitor ripple voltage.
The exact point at which you're operating dictates how much conduction loss and switching loss you'll have - moving the output up and down will change the losses.
So, in a nutshell, designing for the "middle" of the range is not a valid approach. You need to understand how your intended converter works and design such that at the worst corners of input and load, your devices are in safe operating areas.
For large output voltage ranges, you could replace R1/R2 with a pot (digital or otherwise). You can also use a DAC (hooked up to a microcontroller) and an op-amp as a sort of voltage comparator.
The main problem that you'll find is that the switching DC/DC converter is optimized for a certain input to output voltage ratio at a given current. As you stray away from that optimized operation point the converter will degrade in performance. And by degrade, I mean that at best it will loose efficiency and at worst it will cease to function. In between those two extremes, you'll have increased ripple, increased noise, and possibly unstable operation. All switching DC/DC converter chips will have these issues, but some will have them worse than others.
