Key Takeaways
- Same VIN 12→15V: open-loop rises to 7.5V, closed-loop only to 6.1V
- Line regulation improvement: open-loop ΔVout/ΔVIN=50% → closed-loop≈3%
- Residual error is minimal (~2%), high-gain error amplifier greatly improves regulation accuracy
Lesson 2
Mirrors Ch3: VIN Step — How Closed-Loop Responds
In Ch3, VIN stepping from 12V to 15V caused open-loop output to rise to 7.5V. Under same conditions, closed-loop output only rises to ≈6.1V — error reduced from +25% to +2%.
Recall Ch3 Lesson 2: VIN suddenly jumps from 12V to 15V, open-loop output rises from 6V to 7.5V (+25%). Now with feedback loop added — same VIN step, observe the output response.
Result: after VIN step, closed-loop output rises to ≈6.1V (instead of 7.5V). Error reduced from 25% to 11% — feedback eliminates more than half the deviation.
Mechanism: VIN suddenly rises → Vout starts to increase → divider output exceeds previous equilibrium → comparator reduces D → Vout is pulled back. Settles at new equilibrium (6.1V instead of 7.5V).
Closed-Loop vs VIN Step: 12V→15V
VIN steps from 12V to 15V. Closed-loop output≈6.1V (open-loop was 7.5V). Deviation compressed from 1.5V to 0.1V (92% reduction).
After reading this section, run the simulation and observe the waveforms. To explore further, open the example in a standalone page.
Watch Items
- After VIN step at t=20ms, output only rises to ≈6.1V (Ch3 was 7.5V)
- Note the transient: closed-loop has brief dynamic response then settles at new equilibrium