As integrated circuit clock frequencies and circuit densities continue to scale dramatically upward, the importance of a stable power supply voltage becomes critical for reliable operation. Power supply current demand and therefore voltage fluctuation is affected by changes in power consumption on the chip. Sustained changes of on-chip switching activity for more than one clock cycle change the average current demand (ΔI) of CMOS chips and create power voltage noise in the mid-and low frequency range. Thus, switched bypass resistors are often connected directly to the power supply.
The power supply noise is created because the power supply and voltage regulation functions are physically displaced from the chip, resulting in additional power loading on the power supply. Chip modules, circuit card assemblies (CCA) and printed circuit boards (PCB) often combine to present a complex distribution network for on-chip power supply regulation. The un-avoidable inductances in the power delivery network routed from the power supply to the on-chip circuits are a primary source of power supply noise. Increased switching activity causes a drop of the on-chip supply voltage and decreasing switching activity can result in voltage supply overshoot. The on-chip voltage fluctuations are attributable to high switching activity of a large percentage of active devices and their corresponding capacitive loads. This type of fluctuation in power supply voltage tends to occur over a period of about 5 nanoseconds.
Power supply noise impacts chip performance and can cause false switching in logic circuits. Power supply noise becomes more and more critical with increasing ΔI and decreasing supply voltage because noise margins for low voltage circuits are commensurately reduced.
Large leakage currents in the range of about 70 A can operate to reduce power supply noise in CMOS integrated circuits due to a damping effect in supply perturbations and also because of the relatively small dependency of the supply voltage to leakage resistance. However, in current process technologies, leakage currents present a major source of power dissipation as well as unique challenges to chip cooling, thereby mitigating any reliance on leakage currents as a means for reducing power supply noise.
Another common practice to reduce power supply noise is to place decoupling capacitors on-chip as well as on the module, CCA or PCB. On-chip decoupling capacitors are most efficient for mid-frequency power supply noise reduction, but the total amount of decoupling capacitance is limited by the chip size. Moreover, the path inductances of the power delivery network and of the decoupling capacitors themselves are reduced by appropriate design and technology. However, low inductance capacitors (LICA) placed on the module are significantly more expensive than general purpose capacitors.