Question 22.14: The LM7805 can regulate to specifications with an input volt...

The LM7805 produces an output of 5 V. With Eq. (22-13), the maximum efficiency is:

Efficiency ≅\frac{V_{out}}{V_{in}} \times 100\%=\frac{5 \ V}{7.5 \ V} \times 100\% =67\%

This high efficiency is possible only because the headroom voltage is approaching the dropout voltage. On the other hand, the minimum efficiency occurs when the input voltage is maximum. For this condition, the headroom voltage is maximum and the power dissipation in the pass transistor is maximum. The minimum efficiency is:

Efficiency  ≅ =\frac{5 \ V}{20 \ V} \times 100\% =25\%

Since the unregulated input voltage is usually somewhere between the extremes in input voltage, the efficiency we can expect with an LM7805 is in the range of 40 to 50 percent