Load changes and input fluctuations cause open-loop Buck to deviate from target — automatic D adjustment is essential.
The first two chapters analyzed ideal steady-state conditions: constant VIN, fixed load. In reality, loads change abruptly (CPU idle-to-full) and inputs fluctuate (battery discharge). This chapter demonstrates how open-loop Buck output deviates under these conditions, motivating the need for feedback.
This chapter focuses on comparison: output deviation of the same circuit under different conditions. Each demo keeps D=50% fixed, observe how Vout deviates from target 6V.
Change load from 10Ω to 100Ω (lighter load), the same D=50% Buck output rises from 6V to above 8V — open-loop is helpless.
2VIN steps from 12V to 15V, Vout immediately rises from 6V to 7.5V — open-loop is equally helpless against input changes.
3Open-loop is 'blind' — doesn't know how far output has drifted. Solution: use a feedback loop to detect Vout in real time and adjust D.