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Introduction to
CMOS VLSI
Design
Lecture 18:
Design for Low Power
Credits: David Harris
Harvey Mudd College
(Material taken/adapted from
Harris’ lecture notes)
Outline




Power and Energy
Dynamic Power
Static Power
Low Power Design
18: Design for Low Power
CMOS VLSI Design
Slide 2
Power and Energy
 Power is drawn from a voltage source attached to
the VDD pin(s) of a chip.
 Instantaneous Power: P(t )  iDD (t )VDD
 Energy:
 Average Power:
18: Design for Low Power
T
T
0
0
E   P(t )dt   iDD (t )VDDdt
T
E 1
Pavg    iDD (t )VDDdt
T T0
CMOS VLSI Design
Slide 3
Dynamic Power
 Dynamic power is required to charge and discharge
load capacitances when transistors switch.




One cycle involves a rising and falling output.
On rising output, charge Q = CVDD is required
On falling output, charge is dumped to GND
VDD
This repeats Tfsw times
iDD(t)
over an interval of T
fsw
18: Design for Low Power
CMOS VLSI Design
C
Slide 4
Dynamic Power Cont.
Pdynamic 
VDD
iDD(t)
fsw
18: Design for Low Power
CMOS VLSI Design
C
Slide 5
Dynamic Power Cont.
T
Pdynamic
1
  iDD (t )VDD dt
T 0
T
VDD

iDD (t )dt

T 0
VDD

TfswCVDD 
T
 CVDD 2 f sw
VDD
iDD(t)
fsw
18: Design for Low Power
CMOS VLSI Design
C
Slide 6
Activity Factor
 Suppose the system clock frequency = f
 Let fsw = af, where a = activity factor
– If the signal is a clock, a = 1
– If the signal switches once per cycle, a = ½
– Dynamic gates:
• Switch either 0 or 2 times per cycle, a = ½
– Static gates:
• Depends on design, but typically a = 0.1
 Dynamic power:
Pdynamic  aCVDD 2 f
18: Design for Low Power
CMOS VLSI Design
Slide 7
Short Circuit Current
 When transistors switch, both nMOS and pMOS
networks may be momentarily ON at once
 Leads to a blip of “short circuit” current.
 < 10% of dynamic power if rise/fall times are
comparable for input and output
18: Design for Low Power
CMOS VLSI Design
Slide 8
Example
 200 Mtransistor chip
– 20M logic transistors
• Average width: 12 l
– 180M memory transistors
• Average width: 4 l
– 1.2 V 100 nm process
– Cg = 2 fF/mm
18: Design for Low Power
CMOS VLSI Design
Slide 9
Dynamic Example
 Static CMOS logic gates: activity factor = 0.1
 Memory arrays: activity factor = 0.05 (many banks!)
 Estimate dynamic power consumption per MHz.
Neglect wire capacitance and short-circuit current.
18: Design for Low Power
CMOS VLSI Design
Slide 10
Dynamic Example
 Static CMOS logic gates: activity factor = 0.1
 Memory arrays: activity factor = 0.05 (many banks!)
 Estimate dynamic power consumption per MHz.
Neglect wire capacitance.
Clogic   20  106  12l  0.05mm / l  2 fF / mm   24nF
Cmem  180  106   4l  0.05mm / l  2 fF / mm   72nF
Pdynamic  0.1Clogic  0.05Cmem  1.2  f  8.6 mW/MHz
2
18: Design for Low Power
CMOS VLSI Design
Slide 11
Static Power
 Static power is consumed even when chip is
quiescent.
– Ratioed circuits burn power in fight between ON
transistors
– Leakage draws power from nominally OFF
devices
Vgs Vt
I ds  I ds 0e
nvT
Vds


vT
1  e 


Vt  Vt 0  Vds  
18: Design for Low Power

s  Vsb  s
CMOS VLSI Design

Slide 12
Ratio Example
 The chip contains a 32 word x 48 bit ROM
– Uses pseudo-nMOS decoder and bitline pullups
– On average, one wordline and 24 bitlines are high
 Find static power drawn by the ROM
– b = 75 mA/V2
– Vtp = -0.4V
18: Design for Low Power
CMOS VLSI Design
Slide 13
Ratio Example
 The chip contains a 32 word x 48 bit ROM
– Uses pseudo-nMOS decoder and bitline pullups
– On average, one wordline and 24 bitlines are high
 Find static power drawn by the ROM
– b = 75 mA/V2
– Vtp = -0.4V
2
 Solution:
VDD  Vtp 
I pull-up  b
2
 24μA
Ppull-up  VDD I pull-up  29μW
Pstatic  (31  24) Ppull-up  1.6 mW
18: Design for Low Power
CMOS VLSI Design
Slide 14
Leakage Example
 The process has two threshold voltages and two
oxide thicknesses.
 Subthreshold leakage:
– 20 nA/mm for low Vt
– 0.02 nA/mm for high Vt
 Gate leakage:
– 3 nA/mm for thin oxide
– 0.002 nA/mm for thick oxide
 Memories use low-leakage transistors everywhere
 Gates use low-leakage transistors on 80% of logic
18: Design for Low Power
CMOS VLSI Design
Slide 15
Leakage Example Cont.
 Estimate static power:
18: Design for Low Power
CMOS VLSI Design
Slide 16
Leakage Example Cont.
 Estimate static power:
6
6
– High leakage:  20  10   0.2 12l  0.05mm / l   2.4  10 mm
– Low leakage:  20  106   0.812l  0.05mm / l  
180  106   4l  0.05mm / l   45.6  106 mm
I static   2.4  106 m m   20nA / m m  / 2   3nA / mm   
 45.6  10 mm   0.02nA / mm  / 2   0.002nA / mm 
6
Pstatic
 32mA
 I staticVDD  38mW
18: Design for Low Power
CMOS VLSI Design
Slide 17
Leakage Example Cont.
 Estimate static power:
6
6
– High leakage:  20  10   0.2 12l  0.05mm / l   2.4  10 mm
– Low leakage:  20  106   0.812l  0.05mm / l  
180  106   4l  0.05mm / l   45.6  106 mm
I static   2.4  106 m m   20nA / m m  / 2   3nA / mm   
 45.6  10 mm   0.02nA / mm  / 2   0.002nA / mm 
6
Pstatic
 32mA
 I staticVDD  38mW
 If no low leakage devices, Pstatic = 749 mW (!)
18: Design for Low Power
CMOS VLSI Design
Slide 18
Low Power Design
 Reduce dynamic power
– a:
– C:
– VDD:
– f:
 Reduce static power
18: Design for Low Power
CMOS VLSI Design
Slide 19
Low Power Design
 Reduce dynamic power
– a: clock gating, sleep mode
– C:
– VDD:
– f:
 Reduce static power
18: Design for Low Power
CMOS VLSI Design
Slide 20
Low Power Design
 Reduce dynamic power
– a: clock gating, sleep mode
– C: small transistors (esp. on clock), short wires
– VDD:
– f:
 Reduce static power
18: Design for Low Power
CMOS VLSI Design
Slide 21
Low Power Design
 Reduce dynamic power
– a: clock gating, sleep mode
– C: small transistors (esp. on clock), short wires
– VDD: lowest suitable voltage
– f:
 Reduce static power
18: Design for Low Power
CMOS VLSI Design
Slide 22
Low Power Design
 Reduce dynamic power
– a: clock gating, sleep mode
– C: small transistors (esp. on clock), short wires
– VDD: lowest suitable voltage
– f: lowest suitable frequency
 Reduce static power
18: Design for Low Power
CMOS VLSI Design
Slide 23
Low Power Design
 Reduce dynamic power
– a: clock gating, sleep mode
– C: small transistors (esp. on clock), short wires
– VDD: lowest suitable voltage
– f: lowest suitable frequency
 Reduce static power
– Selectively use ratioed circuits
– Selectively use low Vt devices
– Leakage reduction:
stacked devices, body bias, low temperature
18: Design for Low Power
CMOS VLSI Design
Slide 24