ELEC 2200-002 Digital Logic Circuits Fall 2008 Power Dissipation

1. ELEC 2200-002Digital Logic CircuitsFall 2008Power Dissipation Vishwani D. Agrawal James J. Danaher Professor Department of Electrical and Computer Engineering Auburn University, Auburn, AL 36849 http://www.eng.auburn.edu/~vagrawal vagrawal@eng.auburn.edu ELEC2200-002 Lecture 10

2. CMOS Logic (Inverter) VDD No current flows from power supply! Where is power consumed? GND F. M. Wanlass and C. T. Sah, “Nanowatt Logic using Field-Effect Metal-Oxide-Semiconductor Triodes,” IEEE International Solid-State Circuits Conference Digest, vol. IV, February 1963, pp. 32-33. ELEC2200-002 Lecture 10

3. Components of Power • Dynamic, when output changes • Signal transitions (major component) • Logic activity • Glitches • Short-circuit (small) • Static, when signal is in steady state • Leakage (used to be small) Ptotal = Pdyn + Pstat = Ptran +Psc+ Pstat ELEC2200-002 Lecture 10

4. Power of a Transition: Ptran V R = Ron i(t) vi (t) v(t) Large resistance C Ground C = Total load capacitance for gate; includes transistor capacitances of driving gate + routing capacitance + transistor capacitances of driven gates; obtained by layout analysis. ELEC2200-002 Lecture 10

5. Charging of a Capacitor R t = 0 v(t) i(t) C V Charge on capacitor, q(t) = C v(t) Current, i(t) = dq(t)/dt = C dv(t)/dt ELEC2200-002 Lecture 10

6. i(t) = C dv(t)/dt = [V – v(t)] /R dv(t) dt ∫───── = ∫ ──── V – v(t) RC - t ln [V – v(t)] = ── + A RC Initial condition, t = 0, v(t) = 0 → A = ln V - t v(t) = V [1 – exp(───)] RC ELEC2200-002 Lecture 10

7. - t v(t) = V [1 – exp(── )] RC dv(t) V - t i(t) = C ─── = ── exp(── ) dt R RC ELEC2200-002 Lecture 10

8. Total Energy Per Charging Transition from Power Supply ∞∞ V 2 - t Etrans = ∫ V i(t) dt = ∫ ── exp(── ) dt 00 R RC = CV 2 ELEC2200-002 Lecture 10

9. Energy Dissipated Per Transition in Resistance ∞ V 2∞ -2t R ∫ i2(t) dt = R ── ∫ exp(── ) dt 0 R20 RC 1 = ─ CV 2 2 ELEC2200-002 Lecture 10

10. Energy Stored in Charged Capacitor ∞ ∞ - t V - t ∫ v(t) i(t) dt = ∫ V [1-exp(── )]─ exp(── ) dt 00 RC R RC 1 = ─ CV 2 2 ELEC2200-002 Lecture 10

11. Transition Power • Gate output rising transition • Energy dissipated in pMOS transistor = CV 2/2 • Energy stored in capacitor = CV 2/2 • Gate output falling transition • Energy dissipated in nMOS transistor = CV 2/2 • Energy dissipated per transition = CV 2/2 • Power dissipation: Ptrans = Etransα fck = α fck CV 2/2 α = activity factor fck = clock frequency ELEC2200-002 Lecture 10

12. Components of Power • Dynamic • Signal transitions • Logic activity • Glitches • Short-circuit • Static • Leakage Ptotal = Pdyn+ Pstat = Ptran + Psc+ Pstat ELEC2200-002 Lecture 10

13. Short Circuit Power of a Transition: Psc VDD isc(t) vi (t) vo(t) CL Ground ELEC2200-002 Lecture 10

14. Short-Circuit Power • Increases with rise and fall times of input. • Decreases for larger output load capacitance; large capacitor takes most of the current. • Small, about 5-10% of dynamic power dissipated in charging and discharging of the output capacitance. ELEC2200-002 Lecture 10

15. Components of Power • Dynamic • Signal transitions • Logic activity • Glitches • Short-circuit • Static • Leakage ELEC2200-002 Lecture 10

16. Static (Leakage) Power • Leakage power as a fraction of the total power increases as clock frequency drops. Turning supply off in unused parts can save power. • For a gate it is a small fraction of the total power; it can be significant for very large circuits. • Static power increases as feature size is scaled down; controlling leakage is an important aspect of transistor design and semiconductor process technology. ELEC2200-002 Lecture 10

17. CMOS Gate Power vi (t) V R = Ron i(t) vi (t) v(t) i(t) Large resistance C isc(t) Ground Leakage current time ELEC2200-002 Lecture 10