Question E7D12:
What is the dropout voltage of a linear voltage regulator?  

Choices:  
A: Minimum input voltage for rated power dissipation  
B: Maximum output voltage drop when the input voltage is varied over its specified range  
C: Minimum input-to-output voltage required to maintain regulation  
D: Maximum that the output voltage may decrease at rated load    

Correct Answer is  C: Minimum input-to-output voltage required to maintain regulation  

Correct answer: C — Minimum input-to-output voltage required to maintain regulation

1) Why C is correct
- Dropout voltage is the smallest headroom the regulator needs between input and output to keep the output in regulation.
- If $V_{IN}$ falls below this headroom, the pass element saturates and $V_{OUT}$ starts to fall.
- Design rule:
  $$V_{IN,\;min} = V_{OUT} + V_{dropout}$$
  Example: If $V_{OUT} = 5\ \text{V}$ and $V_{dropout} = 2\ \text{V}$ (typical old 7805), the input must be at least $7\ \text{V}$ to regulate.

2) Why the other choices are incorrect
- A: “Minimum input voltage for rated power dissipation”  
  Power dissipation depends on load current and the voltage drop:  
  $$P_{diss} = (V_{IN} - V_{OUT})\, I_{OUT}$$  
  This has nothing to do with dropout; there’s no single “minimum input” tied to “rated power dissipation.”

- B: “Maximum output voltage drop when input is varied over its range”  
  That describes line regulation (change in $V_{OUT}$ vs. $V_{IN}$ variations while still in regulation), not the threshold at which regulation is lost.

- D: “Maximum that the output voltage may decrease at rated load”  
  That describes load regulation (change in $V_{OUT}$ from no-load to full-load), not the input-to-output headroom requirement.

3) Useful formulas and rules of thumb
- Basic requirement:
  $$V_{IN} \ge V_{OUT} + V_{dropout}$$
- Non-LDO (e.g., 78xx series) typical $V_{dropout} \approx 2$–$3\ \text{V}$.
- LDO regulators: $V_{dropout}$ can be tens to hundreds of mV and often varies with load current:
  - For PMOS pass devices: approximately  
    $$V_{dropout} \approx I_{OUT} \times R_{DS(on)}$$
  - For PNP/NPN pass devices: approximately the saturation voltage across the pass element (e.g., $V_{CE(sat)}$ or $V_{E(sat)}$).
- When using rectified supplies with ripple, include ripple headroom:  
  $$V_{IN,\;min} = V_{OUT} + V_{dropout} + V_{ripple,pk}$$

Key idea: Dropout is the minimum input-to-output voltage headroom needed for regulation, not a measure of line/load regulation or power dissipation.