Adjusting buck regulator with DAC
You could try a voltage DAC. Problem is, you care about Vref if you’re working in voltage. This poses two problems:


DAC Vout compliance range has to go below Vref
because Vref is close to ground, this puts a bias on the max/min adjustment range.


These two problems get in the way of making that ‘idiot-proof’ range you’re seeking.

A better way is needed. 

The adjustment calculation is most easily done using the sum-of-currents method to figure out the DAC-to-output relationship. This kind of points the way out of the dilemma: use current, not voltage, to set your regulator.

That said, Maxim makes a line of current source/sink DACs that are ideal for this purpose. I’m not shilling for them, but I thus far haven’t found an easier, better way to do this. Being based on current makes the adjustment simple, and they can margin up or down with practically any regulator’s Vref.

Link: https://www.maximintegrated.com/en/products/analog/data-converters/digital-to-analog-converters/DS4424.html

Now, how to make the adjustment ‘idiot-proof’. You set the default voltage to be mid-way between 1.2 and 3.3 (that is, 2.25V), then use the Maxim DAC current setting resistor to define the max/min current. This defines a hard margining limit above and below your set point, regardless of your Vref voltage.



If you only have a voltage DAC (or you have one available and want to use it), here's an alternative.



This approach takes advantage of the fact that the low-side of the voltage divider is at Vref (0.8V), and so the two resistors (10K and 11K) in series will have a known current, and therefore the voltage at the upper 11k is also known. The 11K lifts Vref up to about 3.3V/2,or 1.68V for the adjustment point.

That is:


I (10K) = Vref/10K (- input is at virtual 0.8V due to feedback)
I (11K) = Vref/10K (Kirchhoff's Current Law)
Therefore... V (adj) = Vref/10K * (10K + 11K) = 1.68V


The DAC swing, being centered on 3.3V/2, will therefore change the output symmetrically since it sinks or sources to a load that's also about 3.3V/2.

With an 11K series resistor on the voltage DAC, we get:


DAC 3.3V gives 1.202V
DAC 0V gives about 3.332V
DAC high-Z gives 2.25V


Try it here.

How to find that DAC series resistance? Solve for the current you need to source/sink to/from the adjustment node for the required span. Hint: the sum of the DAC adjustment current and 7.1K feedback current is simply Vref/10K.