Lecture 6:
Power
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 2
Outline
 Power and Energy
 Dynamic Power
 Static Power
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 3
Power and Energy
 Power is drawn from a voltage source attached to
the V
DD
pin(s) of a chip.
 Instantaneous Power:
 Energy:
 Average Power:
() () ()Pt ItVt=
0
()
T
E P t dt=
∫
avg
0
1
()
T

C
C
V
C
dV
E I t V t dt C V t dt
dt
C V t dV CV
∞∞
= =
= =
∫∫
∫
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 5
Charging a Capacitor
 When the gate output rises
–
Energy stored in capacitor is
– But energy drawn from the supply is
– Half the energy from V
DD
is dissipated in the pMOS
transistor as heat, other half stored in capacitor
 When the gate output falls
– Energy in capacitor is dumped to GND
– Dissipated as heat in the nMOS transistor
2
1

4th Ed.
7: Power 7
Switching Power
[ ]
switching
0
0
sw
2
sw
1
()
()
T
DD DD
T
DD
DD
DD
DD
DD
P i t V dt
T
V
i t dt
T
V
Tf CV
T
CV f

4th Ed.
7: Power 9
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
 We will generally ignore this component
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 10
Power Dissipation Sources
 P
total
= P
dynamic
+ P
static
 Dynamic power: P
dynamic
= P
switching
+ P
shortcircuit
– Switching load capacitances
– Short-circuit current
 Static power: P
static
= (I

7: Power 12
Solution
( )
( )( )( )
(
)
( )( )( )
( ) ( )
6
logic
6
mem
2
dynamic logic mem
50 10 12 0.025 / 1.8 / 27 nF
950 10 4 0.025 / 1.8 / 171 nF
0.1 0.02 1.0 1.0 GHz 6.1 W
C m fF m
C m fF m
P CC
λ µλ µ
λ µλ µ
=×=
=×=

=+=

CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 13

 Data is often not completely random
– e.g. upper bits of 64-bit words representing bank
account balances are usually 0
 Data propagating through ANDs and ORs has lower
activity factor
– Depends on design, but typically α ≈ 0.1
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 15
Switching Probability
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 16
Example
 A 4-input AND is built out of two levels of gates
 Estimate the activity factor at each node if the inputs
have P = 0.5
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 17
Clock Gating
 The best way to reduce the activity is to turn off the
clock to registers in unused blocks
– Saves clock activity (α = 1)
– Eliminates all switching activity in the block
– Requires determining if block will be used
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 18
Capacitance

quiescent.
– Leakage draws power from nominally OFF
devices
– Ratioed circuits burn power in fight between ON
transistors
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 21
Static Power Example
 Revisit power estimation for 1 billion transistor chip
 Estimate static power consumption
– Subthreshold leakage
• Normal V
t
: 100 nA/µm
• High V
t
: 10 nA/µm
• High Vt used in all memories and in 95% of
logic gates
– Gate leakage 5 nA/µm
– Junction leakage negligible
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 22
Solution
( )
( )( )( )
( )
( )( )



=× ×=


= +× =

( )( )
275 mA
P 584 mA 275 mA 1.0 V 859 mW
static
=+=
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 23
Subthreshold Leakage
 For V
ds
> 50 mV
 I
off
= leakage at V
gs
= 0, V
ds
= V
DD
( )
10
gs ds DD sb

4th Ed.
7: Power 24
Stack Effect
 Series OFF transistors have less leakage
– V
x
> 0, so N2 has negative V
gs
– Leakage through 2-stack reduces ~10x
– Leakage through 3-stack reduces further
( )
( )
( )
21
10 10
x DD x DD x
x DD
V V V V kV
VV
SS
sub off off
NN
II I
γ
η
η
−+ − − −
−
= =
 

++
−


++
−

= ≈
CMOS VLSI DesignCMOS VLSI Design
4th Ed.
7: Power 25
Leakage Control
 Leakage and delay trade off
– Aim for low leakage in sleep and low delay in
active mode
 To reduce leakage:
– Increase V
t
: multiple V
t
• Use low V
t
only in critical circuits
– Increase V
s
: stack effect
• Input vector control in sleep
– Decrease V
b
• Reverse body bias in sleep