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1. Design and Implementation of VLSI Systems
(EN1600)
Lecture 14: Power Dissipation
S. Reda EN160 SP’08

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
T T
• Energy: E = ∫ P(t )dt = ∫ iDD (t )VDD dt
0 0
T
E 1
• Average Power: Pavg = = ∫ iDD (t )VDD dt
T T 0
S. Reda EN160 SP’08

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
C
fsw
S. Reda EN160 SP’08

4. Dynamic power dissipation
Vdd
Vin Vout
load capacitance
CL (gate + diffusion +
interconnects)
Energy delivered by the supply during input 1 → 0 transition:
Energy stored at the capacitor at the end of 1 → 0 transition:
dissipated in NMOS
during discharge
S. Reda EN160 SP’08 (input: 0 → 1)

5. Capacitive dynamic power
 If the gate is switched on and off f0→1 (switching factor) times
per second, the power consumption is given by
 For entire circuit
where αi is activity factor [0..0.5] in comparison to the clock
frequency (which has switching factor of 1)
Pdynamic = α CVDD 2 f
S. Reda EN160 SP’08

6. 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
S. Reda EN160 SP’08

7. Dynamic power breakup
Gate
34%
Interconnect
51%
Diffusion
15%
Total dynamic Power
[source: Intel’03]
S. Reda EN160 SP’08

8. Static (leakage) power
• Static power is consumed even when chip is
quiescent.
– Leakage draws power from nominally OFF
devices
Vgs −Vt
 −Vds

I ds = I ds 0e nvT
1 − e 
vT

 

S. Reda EN160 SP’08

9. Techniques for low-power design
• Reduce dynamic power Pdynamic = α CVDD 2 f
– α: clock gating, sleep mode
– C: small transistors (esp. on clock), short wires
– VDD: lowest suitable voltage
– f: lowest suitable frequency
I1
I2 O1
Clock I3
I4
I5 O2
Enable
I critical
6 path
Clock Gating
only reduce supply voltage of
non critical gates
S. Reda EN160 SP’08

10. Dynamic power reduction via dynamic VDD
scaling
• Scaling down supply voltage Pdynamic = α CVDD f
2
– reduces dynamic power
– reduces saturation current
→ increases delay → reduce the frequency
Dynamic voltage scaling (DVS): Supply and voltage of the
circuit should dynamic adjust according to the workload of
criticality of the tasks running on the circuits
S. Reda EN160 SP’08

11. Leakage reduction via adjusting of Vth
• Leakage depends exponentially on Vth. How to control Vth?
– Remember: Vth also controls your saturation current → delay
1. Oxide thickness 2. Body Bias
Sol1: statically choose high Sol2: dynamically adjust the bias of
Vt cells for non critical gates the body
I1
• idle: increase Vt (e.g. by applying
I2 O1
–ve body bias on NMOS)
I3
I4 • Active: reduce Vt (e.g.: by
I5 O2 applying +ve body bias on NMOS)
I critical
6 path
S. Reda EN160 SP’08

12. Leakage reduction via Cooling
 Impact of temperature on leakage current
S. Reda EN160 SP’08

13. Summary
We are still in chapter 4:
 Delay estimation
 Power estimation
 Interconnects and wire engineering
 Scaling theory
S. Reda EN160 SP’08