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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 Energydelay tradeoffs 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 (Joulesec) Combined performance and energy metric figure of merit of design style Other Metrics: EnergyDelayn (Joulesecn) Increased weight on performance over energy Low Power Design Essentials 2008 Where is Power Dissipated in CMOS? Active (Dynamic) power (Dis)charging capacitors Shortcircuit power Both pullup and pulldown 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) Shortcircuit 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 pullup 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.
 Spring '10
 Cheng

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