EE5311- Digital IC Design Module 3 - The Inverter Janakiraman V Assistant Professor Department of Electrical Engineering Indian Institute of Technology Madras Chennai September 6, 2017 Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 1/30
Learning Objectives Explain the functioning of a CMOS inverter Explain the Voltage Transfer Characteristics of an inverter Derive an expression for the trip point of an inverter Derive an expression for the delay of an inverter driving a load Derive expressions for Static, Dynamic and Short Circuit power of an inverter. Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 2/30
Outline Switch Model Transfer Characteristics Switching Threshold Noise Margin Supply Voltage Scaling Propagation Delay Power Dynamic Short circuit Leakage Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 3/30
Switch Model V in = 0 V in = V DD Vin V out R p R n C L Figure: The CMOS Inverter The high and low logic levels are V DD and GND(0) Logic levels are independent of sizes - Ratioless Logic Low output impedance (kω) - Immune to noise Large input impedance - Infinite fanout No conduction path from supply to ground in steady state Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 4/30
Load Line V in G S D Vout D S Figure: The CMOS Inverter I DSp = I DSn V GSn = V in V GSp = V in V DD V DSn = V out V DSp = V out V DD Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 5/30
Load Line I Dn V in = 0 V in = 0.4V DD V in = V DD V in = 0.7V DD V in = 0.5V DD V in = 0.7V DD V in = 0.4V DD V in = V DD V in = 0 V out Figure: Solid lines- NMOS, Dashed lines - PMOS Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 6/30
Voltage Transfer Characterisitcs V out V in V out V in = V Out = V M C L 1 2 3 4 5 V in Figure: VTC of a CMOS Inverter Region NMOS PMOS 1 Off Lin 2 Sat Lin 3 Sat Sat 4 Lin Sat 5 Lin Off Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 7/30
Switching Threshold V out V in V out V in = V Out = C L 1 2 3 4 5 V in Figure: Switching Threshold Both NMOS and PMOS are in saturation Assume velocity saturation Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 8/30
Switching Threshold I DSp = I DSn V GSn = V in V GSp = V in V DD V DSn = V out V DSp = V out V DD V in = V out Both NMOS and PMOS are in saturation Assume velocity saturation Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 9/30
Switching Threshold Ignoring channel length modulation I DSn = k n W n L (V M V Tn V DSATn 2 I DSp = k p W p L (V M V DD V Tp V DSATp 2 )V DSATn )V DSATp V M = V Tn + V DSATn 2 +r(v DD +V Tp + V DSATp 2 ) 1+r r = k p(w p /L)V DSATp k n(w n /L)V DSATn = W pv satp W n v satn V M r r +1 V DD Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 10/30
Switing Threshold V out V in = V Out = V M Increasing r Figure: VTC Trip Point V in Ratio of Wp W n determines V M Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 11/30
Switing Threshold Without Velocity Saturation Left as an exercise V M = V Tn +r(v DD +V Tp ) 1+r kp r = k n Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 12/30
Noise Margin V out V OH V OL V IL V IH Vin V OH V OL NM H NM L V IH V IL Figure: Noise Margin of a CMOS Inverter Logic levels from the driver should be recognized by the load Points of slope -1 provide the noise margin levels NM H = V OH V IH NM L = V IL V OL Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 13/30
Noise Margin Approximation V out V M V IL V IH V in Figure: Noise Margin approximation of a CMOS Inverter Extend the tangent at V M Slope is the gain (g) of the VTC NM H = V DD V IH = V DD V M V M g NM L = V IL = V M + V DD V M g Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 14/30
Noise Margin Calculations Need to consider channel length modulation Slope is the gain (g = dvout dv in ) of the VTC I DSn = I no clm DSn (1+λ n V out ) I DSp = I no clm DSp (1+λ p (V out V DD )) g = 1 k n V DSATn +k p V DSATp I D (V M ) λ n λ p 1+r g (V M V Tn V DSATn /2)(λ n λ p ) r = k pv DSATp k n V DSATn Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 15/30
Inverters - Robust Configuration V DD 0 V DD V Tp V DD V DD 0 V DD V Tn 0 V DD Pull down to GND with NMOS Pull up to V DD with PMOS Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 16/30
Pass Transistors V D v C (0 ) = 0 V D V G v C (0 ) = V DD V G v C (t) = min(v G V Tn,V D ) v C (t) = max(v G V Tp,V D ) Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 17/30
Switch Model Dynamic Behaviour V in 0 V in V DD R p C L R n C L Low High High Low Figure: Dynamic Behaviour of CMOS Inverter τ rise = R p C L τ fall = R n C L Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 18/30
Delay Propagation Delay - 50% input to 50% output Rise delay. Output goes from L H - tplh Fall delay. Output goes from H L - tphl Propagation delay is defined as tp = t phl+t plh 2 Slew Rise time - Time taken for output to go from 10% to 90% Fall time - TIme taken for output to go from 90% to 10% Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 19/30
Delay V in V out V in V out C L t phl t plh t Figure: Delay t phl = R eqn C L t plh = R eqp C L t p = C L R eqn +R eqp 2 Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 20/30
Transistor Sizing - Symmetric delay Rising propagation delay should be identical to falling propagation delay. This also ensures a symmeteric VTC t phl = t plh R eqn = R eqp C L V DD 2I DSATn = C LV DD 2I DSATp W n µ n = W p µ p W p 2W n Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 21/30
Transistor Sizing - Minimum delay C L = C dp1 +C dn1 +C gn2 +C gp2 +C wire t p = C L R eqn +R eqp 2 t p = (0.5)[(1+β)(C gn2 +C dn1 )+C wire ]R eqn (1+ α β ) β opt = α = k nv DSATn /k pv DSATp t p β = 0 ( ) C wire α 1+ C dn1 +C gn2 Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 22/30
Power Dissipation Energy lost as heat disspation in the devices Dynamic - Charge/ Discharging of cpacitance Short Circuit - Conductive path from V DD GND Static - Leakage even when no activity happens Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 23/30
Dynamic Power V in 0 V in V DD R p C L R n C L Low High High Low Figure: Capacitor charge and discharge E VDD = E C = 0 0 L H i VDD (t)v DD dt i VDD (t)v out dt Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 24/30
Dynamic Power E VDD = E VDD = E VDD = 0 0 VDD 0 L H i VDD (t)v DD dt dv out C L V DD dt dt C L V DD dv out E VDD = C L V 2 DD E C = C LV 2 DD 2 Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 25/30
Dynamic Power L H - Load capacitor charges and disspates C LV 2 DD 2 in PMOS H L - Load capacitor discharges and disspates C LV 2 DD 2 in NMOS Note that the energy dissipated is independent of size Depends on Probability of switching (P0 1 ) - Activity factor Frequency of operation (f) P dyn = C L V 2 DDf 0 1 P dyn = C L V 2 DDP 0 1 f Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 26/30
Short Circuit Power V DD V T V in I sc V out V T I peak C L Figure: Both NMOS and PMOS conduct I peak t sc I Peak t sc E sc = V DD +V DD 2 2 P sc = t sc V DD I peak f t sc = V DD +V Tp V Tn t s V DD Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 27/30
Static Power 0 ON V DD V DD 0 ON I subn I subp Figure: NMOS or PMOS leaks current P stat = I stat V DD P tot = P dyn +P sc +P stat P tot = (C L V 2 DD +V DD I peak t s )f 0 1 +V DD I leak Janakiraman, IITM EE5311- Digital IC Design, Module 3 - The Inverter 28/30