The input referance voltage is no need to be 1.2V, it can be of 2,205,3V ect. The value we set depends on the feed back gain.
If you look into the internal structure of the PWM controller IC's, they show you an Internal OP-amp with inverting and non-inverting pins. Normally refernce voltage we set at non-inverting pin. The output voltage is fedback to PWM controller inverting pin with some gain( can provide isolated feedback using optocouplers if necessary). Simple logic of PWM control is, when output feedback voltage at inverting pin tries to go more than set refernce voltge, the op-amp output will start reduce and maintain mean value because PWM signal is pulsed.
As this voltage tries to increase more the off-time will increase, i.e dutycycle will reduce to maintain the ouput voltage.
We need to set the referace voltage, feedback gain such that inverting and non-inverting pin voltage should be at same level(magnitude) at the desired voltage level. If you adjust the either of inverting or non-inverting pins the desired ouput voltage level changes.
I hope this is helpfull to you