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Power Dissipation

Power Dissipation. • Dynamic Power Consumption. Charging and Discharging Capacitors. • Short Circuit Currents. Short Circuit Path between Supply Rails during Switching. • Leakage. Leaking diodes and transistors. Where Does Power Go in CMOS?. Vdd. Vin. Vout. C. L.

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Power Dissipation

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  1. Power Dissipation

  2. • Dynamic Power Consumption Charging and Discharging Capacitors • Short Circuit Currents Short Circuit Path between Supply Rails during Switching • Leakage Leaking diodes and transistors Where Does Power Go in CMOS?

  3. Vdd Vin Vout C L Dynamic Power Dissipation

  4. Vdd Vin Vout C L Dynamic Power Dissipation 2 Energy/transition = C * V L dd 2 Power = Energy/transition * f = C * V * f L dd Not a function of transistor sizes! Need to reduce C , V , and f to reduce power. L dd Dependence with supply voltage is quadratic !!!

  5. Node Transition Activity and Power

  6. Short Circuit Currents I peak is a function of transistor sizes. It is also a strong function of the input and output slopes …

  7. Short Circuit Currents • If the output is too slow, then the P transistor is off and there’s no direct current Vdd Vin Vout C L

  8. Short Circuit Current • If the output is too fast, then the P transistor goes quickly to saturation (Vds = Vcc) and power consumption is maximum Vdd Vin Vout C L

  9. Short Circuit Current • Graph of direct current versus output capacitance

  10. Leakage Sub-threshold current one of most compelling issues in low-energy circuit design!

  11. Reverse-Biased Diode Leakage JS = 10-100 pA/mm2 at 25 deg C for 0.25mm CMOS JS doubles for every 9 deg C!

  12. Subthreshold Leakage Component

  13. Static Power Consumption Wasted energy … Should be avoided in almost all cases, but could help reducing energy in others (e.g. sense amps)

  14. Principles for Power Reduction • Prime choice: Reduce voltage! • Recent years have seen an acceleration in supply voltage reduction • Design at very low voltages still open question (0.6 … 0.9 V by 2010!) • Reduce switching activity • Reduce physical capacitance

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