Digital VLSI Design

• Full Automation

• Maximum benefit of scaling

• High speed ,

• low power

• Robustness

Power dissipation

Why worry about power?

-- Heat Dissipation

DEC 21164

microprocessor power dissipation

Why worry about power — Portability

Multimedia Terminals

Laptop Computers

Digital Cellular Telephony

BATTERY(40+ lbs)

Year

No

min

al C

ap

acity (

Wa

tt-h

ours

/ lb)

Nickel-Cadium

Ni-Metal Hydride

65 70 75 80 85 90 95

0

10

20

30

40

50

Rechargable Lithium

Expected Battery Lifetime increase

over next 5 years: 30-40%

Where Does Power Go in CMOS?

• STATIC POWER---NIL

• Dynamic Power Consumption

• Short Circuit Currents

• Leakage

Charging and Discharging Capacitors

Short Circuit Path between Supply Rails during Switching

Leaking diodes and transistors

Power consumption

• 4 components

Static power consumption

Short circuit power consumption

Leakage power consumption

Dynamic power consumption

• The total power in a CMOS circuit is given by Ptotal = Pd + Psc + Ps where

Pd is the dynamic average power (previous chart),

Psc is the short circuit power,

and Ps is the static power due to ratio circuit current, junction leakage, and sub-threshold Ioff leakage current

• Short circuit current flows during the brief transient when the pull down and pull up devices both conduct at the same time where one (or both) of the devices are in saturation

Static power consumption

Short circuit power

CMOS Short-Circuit Power Dissipation

Derivation

Short Circuit Path

Modelling

t1- t2, Mos operates in saturation

At t2, current reaches its maximum value

At this point vin=vdd/2, because inverter is

symmetrical

I mean= 2x [2/T] x ∫Isat dt : Limits(t1, t2)

Conditions—Vin(t)=(Vdd/τ) t; --assume vin increases linearly with time

tr = tf = trf

Psc = (/12) (Vdd – 2Vt)3 (trf/tpin)

• For a balanced CMOS inverter with

n=p= , and Vtn = |Vtp|, the short

circuit power can be expressed by

Psc = (/12)(Vdd – 2Vt)3 (tr/f/tpin)

where tpin is the period of the input

waveform and trf is the input rise time

(or fall time) tr = tf = trf

Effect of load cap on short circuit

power

• P short circuit reduces

• Reason---- output start switching after

input has completely stabilized

Effect of Cload

Dynamic energy consumption

Energy stored across capacitor

Dynamic power consumption-derivation

Average Dynamic Power in CMOS

Inverter • Average dynamic power derivation:

– On negative going input, pull-up

device charges the load

capacitance. On positive going

input, pull-down device discharges

the load into ground.

– Average power given by

Pave = (1/T)CL (dvout/dt) (Vdd – vout)dt

+ (1/T)(-1) CL (dvout/dt) vout dt

where the first integral is taken from

0 to T/2 and the second integral is

from T/2 to T

• completion of the integral yields

Pave = CL Vdd2 f where f = 1/T

• Note that the dynamic power is

independent of the typical device

parameters, but is simply a

function of power supply, load

capacitance and frequency of

the switching!

Vin Vout

CL

Energy/transition = CL * Vdd2

Power = Energy/transition * f = CL * Vdd2 * f

Need to reduce CL, Vdd, and f to reduce power.

Vdd

Not a function of transistor sizes!

Reduce power consumption

• Reduce Vdd

• Reduce swing at the output

• Reduce CL

• Reduce Switching activity

To keep same speed, can we reduce Vdd, increase (w/L)? No

Inc in W inc in CL

Dynamic Power Consumption - Revisited

Power = Energy/transition * transition rate

= CL * Vdd2 * f01

= CL * Vdd2 * P01* f

= CEFF * Vdd2 * f

Power Dissipation is Data Dependent

Function of Switching Activity

CEFF = Effective Capacitance = CL * P01

Power Consumption is Data Dependent uniform distribution of inputs

Example: Static 2 Input NOR Gate

Assume:

P(A=1) = 1/2

P(B=1) = 1/2

P(Out=1) = 1/4

P(01)

= 3/4 1/4 = 3/16

Then:

= P(Out=0).P(Out=1)

CEFF = 3/16 * CL

Transition Probabilities for Basic Gates

Non-uniform distribution of inputs

No feedback

Power consumption—Correlated signals

½ 1

Sizing for min. power

consumption

For a given delay constraint

Sizing for power consumption

Vdd=Vddref

Vdd=Vddref

Vdd≠ Vddref

Graphical solution

Why energy reduces for F increasing?

• Assume delay reqd is tpref=5ns.

• As F inc CL inc. delay (tp) inc. and dyn. energy inc.

linearly

• But as f inc delay reduces exponentially, energy inc.

• for F= 1 delay is already small and close to tpref). Inc in f

does not cause much reduction rather energy increment is

more

• For F large, delay and energy are large values

• Hence as f inc., delay reduces drastically (become less than

tpref ). Hence to have given delay= tpref, energy is dec.

which inc. delay to tpref.

• As f is increased further, delay reduction reduces, only

energy increases

Design example—0.25um technology, find f, Vdd for

tpref=0.2ns. Cext=10Cg1, γ=1, Vref=2.5v

Design a chain of inv for min delay, min energy

Power delay product

Indicates that energy required 0 for Vdd 0 erroneous

Energy delay product

Shd. Be minimum

Energy delay product optimum Vdd