Transcript ppt

ESE370:
Circuit-Level
Modeling, Design, and Optimization
for Digital Systems
Day 18: October 21, 2011
Energy and Power Optimization
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Penn ESE370 Fall2011 -- DeHon
Previously
• Three components of power
– Static
– Short circuit
– Capacitive switching
• Ptot = Pstatic + Psc + Pdyn
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Penn ESE370 Fall2011 -- DeHon
Today
• Power Sources
– Static
– Short Circuit
– Capacitive Switching
• Reducing Switching Energy
• Energy-Delay tradeoffs
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Penn ESE370 Fall2011 -- DeHon
Short Circuit Power
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Penn ESE370 Fall2011 -- DeHon
Preclass 1
• Vin vs. Ipwr,gnd?
– 140mV
– 400mV
– 500mV
– 600mV
– 840mV
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Penn ESE370 Fall2011 -- DeHon
Short Circuit Power
• Between VTN and Vdd-VTP
– Both N and P devices conducting
• Roughly:
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Penn ESE370 Fall2011 -- DeHon
Peak Current
• Ipeak around Vdd/2
– If |VTN|=|VTP| and sized equal rise/fall
IDS

VDSAT 
  satCOX W VGS  VT 


2 

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Penn ESE370 Fall2011 -- DeHon
Short-Circuit Energy
E  Vdd
 I(t)dt 
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 I(t)dt  I peak  tsc  2 
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Penn ESE370 Fall2011 -- DeHon
Short-Circuit Energy
E  Vdd
 I(t)dt 
1
 I(t)dt  I peak  tsc  2 
1 
E  Vdd  I peak  t sc   
2 
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Penn ESE370 Fall2011 -- DeHon
Short Circuit Energy
• Looks like a capacitance
– Q=I×t
– Q=CV

1 
E  Vdd  I peak  t sc   
2 

E  Vdd  Qsc

E  CscV dd
2
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Penn ESE370 Fall2011 -- DeHon
Short Circuit Energy and
Power
• Every time switch
– Also dissipate short-circuit energy: E = CV2
– Different C = Csc
– Ccs “fake” capacitance (for accounting)
• Largely same dependence as charging
Psc = aCscV2 f
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Penn ESE370 Fall2011 -- DeHon
Reduce Short-Circuit Power?
• Psc = aCscV2 f

1 
E  Vdd  I peak  t sc   
2 


Penn ESE370 Fall2011 -- DeHon
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Preclass 2
• Vin vs. Ipwr,gnd vs. Vin @ Vdd=500mV?
– 140mV
– 400mV
– 500mV
– 600mV
– 840mV
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Penn ESE370 Fall2011 -- DeHon
Total Power
• Ptot = Pstatic + Psc + Pdyn
• Pdyn + Psc = a(½Cload+Csc)V2f
• Ptot ≈ a(½Cload+Csc)V2f+VI’s(W/L)e-Vt/(nkT/q)
IDS
W 
 IS e
 L 
Penn ESE370 Fall2010 -- DeHon
VGS VT 


 nkT / q 
 VDS 

kT / q 
1  e  1 VDS 


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Dynamic Power
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Penn ESE370 Fall2011 -- DeHon
Reduce Dynamic Power?
• Pdyn = a × ½CV2 f
• How do we reduce dynamic power?
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Penn ESE370 Fall2011 -- DeHon
Slow Down
• What happens to power contributions as
reduce clock frequency?
• What suggest about Vth?
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Penn ESE370 Fall2011 -- DeHon
Reduce V
• What happens as reduce V?
– Delay?
– Energy?
• Static
• Switching
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Penn ESE370 Fall2011 -- DeHon
Old Reduce V (no vsat)
 tgd=Q/I=(CV)/I
 Id=(mCOX/2)(W/L)(Vgs-VTH)2
 tgd impact?
 tgd α 1/V
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Penn ESE370 Fall 2010 -- DeHon
Saturation Observe
• Ignoring leakage
Et  Const
2
E V
2
t V
1
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Penn ESE370 Fall2011 -- DeHon
Reduce V (velocity saturation)
 tgd=Q/I=(CV)/I
 Ids=(satCOX)(W)(Vgs-VTH-VDSAT/2)
 Preclass 3
Vdd
Tau/Tau(Vdd=1V) Esw/Esw(1V)
1V
1
1
700mV
1.75
0.5
500mV
3.5
0.25
350mV
14
0.13
250mV
1200
0.06
Penn ESE370 Fall2011 -- DeHon
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Energy vs. Power?
• Which do we care about?
– Battery operated devices?
– Desktops?
– Pay for energy by kW-Hr?
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Penn ESE370 Fall2011 -- DeHon
Increase Vth?
• Recall increasing threshold voltage
decreased leakage
– (9W vs. 4.5mW for 4BT chip)
• What is impact of increasing threshold
on delay?
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Penn ESE370 Fall2011 -- DeHon
Increase Vth
 tgd=Q/I=(CV)/I
 Ids=(satCOX)(W)(Vgs-VTH-VDSAT/2)
 Preclass 4
Vth=-Vthp
Tau/tau(|Vth|=300mV)
300mV
1
500mV
1.4
700mV
2.3
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Penn ESE370 Fall2011 -- DeHon
Admin
• Project 1 out
– Two weeks
– That means you need to be starting on it
now…and working on it all next week
• Read assignment today
• Baseline design no later than Wed (10/26)
• Set of opt. ideas no later than Fri (10/28)
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Penn ESE370 Fall2011 -- DeHon
Idea
• Short circuit energy looks like more
capacitance for switching energy
• Tradeoff
– Speed
– Switching energy
– Leakage energy
• Energy-Delay tradeoff: Et2 ? Et
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Penn ESE370 Fall2011 -- DeHon