ELEC 7770 Advanced VLSI Design Spring 2012 Power and Ground

Spring 2012, Apr 4 . . .Spring 2012, Apr 4 . . . ELEC 7770: Advanced VLSI Design (Agrawal)ELEC 7770: Advanced VLSI Design (Agrawal) 11

ELEC 7770ELEC 7770Advanced VLSI DesignAdvanced VLSI Design

Spring 2012Spring 2012Power and GroundPower and Ground

Vishwani D. AgrawalVishwani D. AgrawalJames J. Danaher ProfessorJames J. Danaher Professor

ECE Department, Auburn UniversityECE Department, Auburn University

Auburn, AL 36849Auburn, AL 36849

[email protected]

http://www.eng.auburn.edu/~vagrawal/COURSE/E7770_Spr12

ReferencesReferences Q. K. Zhu, Q. K. Zhu, Power Distribution Network Design for VLSIPower Distribution Network Design for VLSI, Hoboken, New , Hoboken, New

Jersey: Wiley, 2004.Jersey: Wiley, 2004. M. Popovich, A. Mezhiba and E. G. Friedman, M. Popovich, A. Mezhiba and E. G. Friedman, Power Distribution Power Distribution

Networks with On-Chip Decoupling CapacitorsNetworks with On-Chip Decoupling Capacitors , Springer, 2008., Springer, 2008. C.-K. Koh, J. Jain and S. F. Cauley, “Synthesis of Clock and C.-K. Koh, J. Jain and S. F. Cauley, “Synthesis of Clock and

Power/Ground Network,” Chapter 13, L.-T. Wang, Y.-W. Chang and K.-Power/Ground Network,” Chapter 13, L.-T. Wang, Y.-W. Chang and K.-T. Cheng (Editors), T. Cheng (Editors), Electronic Design AutomationElectronic Design Automation, Morgan-Kaufmann, , Morgan-Kaufmann, 2009. pp. 751-850.2009. pp. 751-850.

J. Fu, Z. Luo, X. Hong, Y. Cai, S. X.-D. Tan, Z. Pan, “J. Fu, Z. Luo, X. Hong, Y. Cai, S. X.-D. Tan, Z. Pan, “VLSI On-Chip VLSI On-Chip Power/Ground Network Optimization Considering Decap Leakage Power/Ground Network Optimization Considering Decap Leakage Currents,” Currents,” Proc. Asia and South Pacific Design Automation Conf.Proc. Asia and South Pacific Design Automation Conf. , , 20052005 , pp. 735-738., pp. 735-738.

Decoupling Capacitors, Decoupling Capacitors, http://www.vlsichipdesign.com/index.php/Chip-Design-Articles/decoupling-capacitors.html


Supply VoltageSupply Voltage


3.0

2.5

2.0

1.5

1.0

0.5

0.00.25 0.18 0.13 0.1

Minimum feature size (μm)

Sup

ply

volta

ge (

V)

Gate Oxide ThicknessGate Oxide Thickness


60

50

40

30

20

10

00.25 0.18 0.13 0.1

Minimum feature size (μm)

Gat

e ox

ide

thic

knes

s (A

)

High gate leakage

Power Supply NoisePower Supply Noise

Transient behavior of supply voltage and ground Transient behavior of supply voltage and ground level.level.

Caused by transient currents:Caused by transient currents: Power droopPower droop Ground bounceGround bounce


Power SupplyPower Supply


+

– Gat

e 1

Gat

e 2

VDD

Rg

R

C

R

C

V(t)

Switching TransientsSwitching Transients Only Gate 1 switches (turns on):Only Gate 1 switches (turns on):

V(t) = VDD – Rg VDD exp[– t/{C(R+Rg)}]/(R+Rg)V(t) = VDD – Rg VDD exp[– t/{C(R+Rg)}]/(R+Rg)


V(t

)

VDD

0 time, t

VDD Rg/(R+Rg)

Multiple Gates SwitchingMultiple Gates Switching


Gat

e ou

tput

vol

tage

VDD

0 time, t

many

Number of gates switching

1 23

Decoupling CapacitorDecoupling Capacitor A capacitor to isolate two electrical circuits.A capacitor to isolate two electrical circuits. Illustration: An approximate model:Illustration: An approximate model:


+

–

VDD = 1Rg

Rd

CdIL

VL(t)

t

i(t)

a

t=0

t=0

Approximate Load Current, ILApproximate Load Current, IL

0,0, t < 0t < 0

at,at, t < tpt < tp

ILIL ==

a(2tp – t),a(2tp – t), t < 2tpt < 2tp

0,0, t > 2tpt > 2tp


Transient Load VoltageTransient Load Voltage

VL(t) = 1 – a Rg [ t – Cd Rg (1 – eVL(t) = 1 – a Rg [ t – Cd Rg (1 – e – t/T – t/T) ], 0 < t < tp) ], 0 < t < tp

TT == Cd (Rg + Rd)Cd (Rg + Rd)


Realizing Decoupling CapacitorRealizing Decoupling Capacitor


GND

S B D

VDD

GND

S B D

VDD

OR

CapacitanceCapacitanceCdCd == γ×γ×WLWL×ε×ε×ε×ε00/Tox/Tox

≈≈ 0.26fF, for 70nm BSIM0.26fF, for 70nm BSIM

LL == 38nm,38nm, WW == 200nm200nm

γγ == 1.54621.5462

εε == 44


Leakage ResistanceLeakage Resistance

IgateIgate == αα ×× e e – – ββToxTox ××WW

where where αα and and ββ are technology parameters. are technology parameters.

RdRd == VL(t)/IgateVL(t)/Igate

Because V(t) is a function of time, Rd is Because V(t) is a function of time, Rd is difficult to estimate. The decoupling difficult to estimate. The decoupling capacitance is simulated in spice.capacitance is simulated in spice.


Power-Ground LayoutPower-Ground Layout


Vss VssVdd

VssVddVdd

Solder bump pads

M5

M4

Via

Vdd/Vss supply

Vdd/Vssequalization

Power GridPower Grid


+

–

Nodal AnalysisNodal Analysis


V1

V2

V3

V4

Ci

Vi

BiApply KCL to node i:

4 ∑ (Vk – Vi) gk – Ci ∂Vi/∂t = Bik=1

g1g2

g3

g4

Nodal AnalysisNodal Analysis


G V – C V’ = B

Where G is conductance matrixV is nodal voltage vectorC is admittance matrixB is vector of currents

V(t) is a function of time, V(0) = VDD

B(t) is a function of time, B(0) ≈ 0 or leakage current

Wire Width ConsiderationsWire Width Considerations

Increase wire width to reduce resistance:Increase wire width to reduce resistance: Control voltage drop for given currentControl voltage drop for given current Reduce resistive lossReduce resistive loss

Reduce wire width to reduce wiring area.Reduce wire width to reduce wiring area. Minimum width restricted to avoid metal Minimum width restricted to avoid metal

migration (reliability consideration).migration (reliability consideration).


A Minimization ProblemA Minimization Problem


Minimize total metal area: n n

A = ∑ wi si = ∑ | ρ Ci si2 | / xi

i=1 i=1

Where n = number of branches in power networkwi = metal width of ith branchsi = length of ith branchρ = metal resistivityCi = maximum current in ith branchxi = voltage drop in ith branch

Subject to several conditions.

Condition 1: Voltage DropCondition 1: Voltage Drop


Voltage drop on path Pk:

∑ xi ≤ Δvk

i ε Pk

Where Δvk = maximum allowable voltage drop on kth path

Condition 2: Minimum WidthCondition 2: Minimum Width


Minimum width allowed by fabrication process:

wi = ρ Ci si / xi ≥ W

Where wi = metal width of ith branchsi = length of ith branchρ = metal resistivityCi = maximum current in ith branchxi = voltage drop in ith branchW = minimum line width

Condition 3: Metal MigrationCondition 3: Metal Migration


Do not exceed maximum current to wire-width ratio:

Ci / wi = xi /(ρ si) ≤ σi

Where wi = metal width of ith branchsi = length of ith branchρ = metal resistivityCi = maximum current in ith branchxi = voltage drop in ith branchσi = maximum allowable current density

across ith branch

Decoupling CapacitanceDecoupling Capacitance


+

–

VDDRg

Cd I(t)


Initial charge on Cd, QInitial charge on Cd, Q00 = Cd VDD = Cd VDD

I(t): current waveform at a nodeI(t): current waveform at a node T: duration of currentT: duration of current Total charge supplied to load:Total charge supplied to load:

TT

Q = ∫ I(t) dtQ = ∫ I(t) dt

00



Assume that charge is completely supplied by Cd.Assume that charge is completely supplied by Cd. Remaining charge on Cd = Cd VDD – QRemaining charge on Cd = Cd VDD – Q Voltage of supply node = VDD – Q/CdVoltage of supply node = VDD – Q/Cd For a maximum supply noise For a maximum supply noise ΔΔVDDmax,VDDmax,

VDD – (VDD – Q/Cd) ≤ VDD – (VDD – Q/Cd) ≤ ΔΔVDDmaxVDDmax

OrOr CdCd ≥≥ Q /Q / Δ ΔVDDmaxVDDmax


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ELEC 7770 Advanced VLSI Design Spring 2012 Power and Ground