If I want to control a non-I2C buck output dynamically, can I use its feedback pin to do this?
Connect a resistor between the FB pin and the DAC output of the MCU/SoC or use the PWM output of the MCU/SoC and filter it with RC then connect a resistor between FB pin and RC filter. The analog voltage (from DAC or filtered PWM) will sink or source current into/from the FB pin through the series resistor thereby controlling the buck output voltage.
If there are only 2 predetermined output voltages that need to be dynamically switched, another option is to use a digital signal to control a transistor (or FET) to switch in/out an extra resistor in parallel to change to total FB resistance. This can be extended beyond just two voltages with more switches and other combinations of series/parallel R’s.
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The use of a third resistor from the FB node to a variable voltage to control the Vout is superior, because the loop response (Bode plot) does not change with output voltage.
If you change the divider ratio with switching resistors instead, the loop gain changes and you need to careful check your stability in all situations.
BR
Christian