Power Management

If only the supply voltage is lowered by 10% (all else equal), by roughly what percentage does dynamic power fall?

  1. AAbout 19%
  2. BAbout 10%
  3. CAbout 50%
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Answer: A. About 19%

Dynamic power is proportional to the square of voltage, so scaling voltage by 0.9 scales power by 0.9² = 0.81, a reduction of about 19%. Even small voltage changes have a large effect on power, making voltage reduction one of the most effective power-saving levers. Lower voltage slows circuits, though, so it must stay within the range that keeps required speed, and doing this according to load is what DVFS does.

  • A. Correct. 0.9² = 0.81, about a 19% reduction.
  • B. Power is proportional to the square of voltage.
  • C. A 10% change does not cut it that much.
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More Power Management questions

  1. In the CMOS dynamic power formula P = αCV²f, what does α represent?
  2. 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?
  3. 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?
  4. 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?
  5. Which describes static (leakage) power in CMOS circuits?