Power Management

What is the dynamic power of a circuit with activity factor α = 0.1, switched capacitance C = 1 nF, supply voltage V = 1 V and frequency f = 1 GHz?

  1. A1 W
  2. B0.1 W
  3. C10 W
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Answer: B. 0.1 W

Substituting into P = αCV²f gives 0.1 × 1 × 10⁻⁹ F × (1 V)² × 1 × 10⁹ Hz = 0.1 W. The nano (10⁻⁹) of the capacitance and the giga (10⁹) of the frequency cancel, which keeps the arithmetic simple. In real chips the switched capacitance grows with circuit size, and total dynamic power can reach tens to hundreds of watts.

  • A. That forgets to multiply by α.
  • B. Correct. 0.1 × 10⁻⁹ × 1² × 10⁹ = 0.1.
  • C. The order of magnitude is wrong.
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More Power Management questions

  1. With the supply voltage unchanged, only the clock frequency is halved. What happens to the dynamic energy needed to finish a fixed amount of computation?
  2. Assume DVFS can lower the supply voltage in proportion to frequency. When frequency is halved, roughly what happens to dynamic power and to the dynamic energy for a fixed amount of computation?
  3. Which describes static (leakage) power in CMOS circuits?
  4. How does leakage power change as chip temperature rises, and what problem can result?
  5. Which describes power gating?