CMOS Power Dissipation Explained

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Questions and Answers

What is the total power dissipation in CMOS circuits comprised of?

Pdyn plus Pstat (correct)

Psw plus Psc

Pdyn only

Pstat only

Which component of power dissipation accounts for unnecessary transitions in the output?

Static power dissipation

Glitches (correct)

Short-circuit power dissipation

Leakage power dissipation

What does the equation Psw = CL V²DD fCLK α represent?

Leakage power dissipation

Total power dissipation

Static power dissipation

Switching power dissipation (correct)

Which factor does NOT affect switching power dissipation in CMOS circuits?

Resistance of the channel Signup and view all the answers

What form of energy is dissipated during the output rising transition in a CMOS inverter?

Energy stored in the capacitor Signup and view all the answers

In the context of power dissipation, what does 'Pdyn' refer to?

Dynamic component of power dissipation Signup and view all the answers

What happens to the total power dissipation as supply voltage is reduced in CMOS technology?

It decreases Signup and view all the answers

Which statement accurately describes static power dissipation in CMOS circuits?

It is primarily caused by leakage current Signup and view all the answers

What causes glitches in dynamic gates?

Skew in the input signals Signup and view all the answers

Which of the following represents the total power dissipation in a circuit?

Pdyn + Pstat Signup and view all the answers

How is short-circuit power (Psc) affected by transistor size?

Increases with larger transistor sizes Signup and view all the answers

What condition must be satisfied to prevent short-circuit power consumption in a CMOS gate?

VDD < Vtn + |Vtp| Signup and view all the answers

What effect does load capacitance (CL) have on peak short circuit current?

It reduces the peak short circuit current Signup and view all the answers

Which of the following components of power is NOT included in dynamic power?

Leakage power Signup and view all the answers

Subthreshold leakage current, Isub, dominates in which condition?

VGS < VTH Signup and view all the answers

What is a primary cause for an increase in short-circuit power during a transition?

Faster gate output transitions Signup and view all the answers

Which leakage component is due to tunneling at the gate-drain overlap?

Gate induced drain leakage Signup and view all the answers

What condition is necessary to completely eliminate short-circuit power?

VDD must be scaled down below |Vtp| + Vtn Signup and view all the answers

What happens to short-circuit power when the gate output transition is faster than the input transition?

It increases Signup and view all the answers

What type of leakage current becomes significant with scaling due to thin oxide layers?

Gate tunneling Signup and view all the answers

What approximates the energy drawn during a glitch in a dynamic gate?

The product of voltage swing and number of transitions Signup and view all the answers

Which condition is necessary for short-circuit conduction to occur in a CMOS gate?

Pull-up path through pMOS and pull-down path through nMOS are on Signup and view all the answers

Which of the following is NOT a component of leakage current?

Power supply leakage Signup and view all the answers

What is the effect of a low threshold voltage (VTH) on subthreshold leakage current?

It increases subthreshold leakage current. Signup and view all the answers

Study Notes

Power Dissipation in CMOS

CMOS logic (inverter) has no static leakage path for either 1 or 0 inputs.
Power dissipation consists of dynamic and static components.
Dynamic Component (P_dyn): Switching power (P_sw), logic activity, and glitches.
Short-circuit power (P_sc).
Static Component (P_stat): Leakage power.
Total power (P_total) = P_dyn + P_stat = (P_sw + P_sc) + P_stat

Switching Power (P_sw)

Defined as the power consumed when charging and discharging the load capacitance (C_L) with the supply voltage (V_DD) during a switching cycle (T).
P_sw = C_L V_DD² / T
Switching power depends on capacitance, supply voltage, clock frequency, and activity factor.

Glitches

Temporary changes in the output value (unnecessary transitions).
Caused by skew in input signals to a gate.

Type of Logic Function

Static 2-input NOR gate: Transition probability (0→1) = 3/16
Static 2-input XOR gate: Transition probability (0→1) = 1/4

Dynamic Gate

Dynamic gates consume power during pre-charge and evaluation phases of the clock cycle (CLK waveform).

Dynamic Power Consumption

Dynamic power consumption is data-dependent.
Logic functions with higher transitions have higher activity factors which in turn, leads to higher power consumption in dynamic gates.

Glitch Power Dissipation

Glitch power dissipation depends on the voltage swing of incomplete transitions within a clock cycle.
Formula for approximating glitch power: P_glitch = (1/T) * C_load * V_DD * ΣΔV_n

Short-Circuit Power (P_sc)

Power dissipation during the transition period when both the NMOS and PMOS transistors are partially ON simultaneously.
P_sc = I_mean * V_DD
Short-circuit power increases with transistor size, input transition time, and decreases with load capacitance.

Leakage Power

Leakage power is the continuous power consumption due to various leakage components like:
- Subthreshold leakage (I_sub)
- Reverse bias pn junction leakage (I_D)
- Gate-induced drain leakage (I_GIDL)
- Drain-source punch-through (I_pt)
- Gate tunneling (I_G)

Subthreshold Leakage (I_sub)

Occurs when the gate-source voltage (V_GS) is less than the threshold voltage (V_TH).
Dominated by diffusion current.
Most important OFF-state leakage mechanism in modern CMOS devices.
Formula for subthreshold leakage in long-channel devices: I_sub = μ₀ C_ox (W/L) V_T² exp{(V_GS - V_TH)/ηV_T}

CMOS Gate and Short-Circuit Power

A CMOS gate consumes no short-circuit power when the supply voltage (V_DD) is less than or equal to the sum of the threshold voltages of the nMOS and pMOS transistors (V_DD ≤ V_tn + |V_tp|).
The short-circuit power occurs when both transistors are ON simultaneously during the input transition.

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Description

This quiz explores the fundamentals of power dissipation in CMOS circuits, covering both dynamic and static components. You'll learn about factors affecting switching power and the significance of glitches in digital logic. Test your understanding of how these concepts contribute to overall power consumption in CMOS technology.