Ch3_PowerBasics

Ch3_PowerBasics - Low Power Design Essentials 2008 Jan M....

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Unformatted text preview: Low Power Design Essentials 2008 Jan M. Rabaey Low Power Design Essentials 2008 Chapter 3 Power and Energy Basics Low Power Design Essentials 2008 Chapter Outline Metrics Dynamic power Static power Energy-delay trade-offs Low Power Design Essentials 2008 Metrics Delay (sec): Performance metric Energy (Joule) Efficiency metric: effort to perform a task Power (Watt) Energy consumed per unit time Power*Delay (Joule) Mostly a technology parameter measures the efficiency of performing an operation in a given technology Energy*Delay = Power*Delay2 (Joule-sec) Combined performance and energy metric figure of merit of design style Other Metrics: Energy-Delayn (Joule-secn) Increased weight on performance over energy Low Power Design Essentials 2008 Where is Power Dissipated in CMOS? Active (Dynamic) power (Dis)charging capacitors Short-circuit power Both pull-up and pull-down on during transition Static (leakage) power Transistors are imperfect switches Static currents Biasing currents Low Power Design Essentials 2008 Active (or Dynamic) Power Sources: Charging and discharging capacitors Temporary glitches (dynamic hazards) Short-circuit currents Key property of active power: f P dyn with f the switching frequency Low Power Design Essentials 2008 Charging Capacitors 2 1 CV E = 2 2 1 CV E R = R C V 2 2 1 CV E C = Applying a voltage step Value of R does not impact energy! Low Power Design Essentials 2008 Applied to Complementary CMOS Gate One half of the power from the supply is consumed in the pull-up network and one half is stored on CL Charge from CL is dumped during the 10 transition Independent of resistance of charging/discharging network V dd V out i L C L PMOS NETWORK NMOS A 1 A N NETWORK 2 1 DD L V C E = 2 2 1 DD L R V C E = 2 2 1 DD L C V C E = Low Power Design Essentials 2008 Circuits with Reduced Swing C V- V TH V V E 01 = VC dV C dt dt = CV dV C V- V T = CV ( V- V TH ) Energy consumed is proportional to output swing Low Power Design Essentials 2008 Charging Capacitors - Revisited R C E E E + = 1 2 ) ( CV T RC E R = R C I 2 2 1 CV E C = Driving from a constant current source 2 2 ) ( ) ( CV T RC T RI dt RI I E I CV T R = = = = Energy dissipated in resistor can be reduced by increasing charging time T (that is, decreasing I ) Low Power Design Essentials 2008 3. Charging Capacitors Using constant voltage or current driver? Energy dissipated using constant current charging can be made arbitrarily small at the expense of delay: Adiabatic charging Econstant_current < Econstant_voltage if T > 2 RC Note: tp (RC) = 0.69 RC t 090%(RC) = 2.3 RC Low Power Design Essentials 2008 3 Charging Capacitors Driving using a sine wave (e.g. from resonant circuit) Energy dissipated in resistor can be made arbitrarily small if frequency << 1/RC (output signal in phase with input sinusoid) R C v(t) 2 2 1 CV E C = Low Power Design Essentials 2008...
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This note was uploaded on 03/13/2010 for the course CSE cse241a taught by Professor Cheng during the Spring '10 term at UCSD.

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Ch3_PowerBasics - Low Power Design Essentials 2008 Jan M....

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