Efficient Placement of Distributed On-Chip Decoupling Capacitors

Decoupling capacitors are widely used to manage power supply noise [242] and are an effective way to reduce the impedance of power delivery systems operating at high frequencies [26], [27]. A decoupling capacitor acts as a local reservoir of charge, which is released when the power supply voltage at a particular current load drops below some tolerable level. Since the inductance scales slowly [207], the location of the decoupling capacitors significantly affects the design of the power/ground networks in high performance integrated circuits such as microprocessors. At higher frequencies, a distributed system of decoupling capacitors are placed on-chip to effectively manage the power supply noise [334].

The efficacy of decoupling capacitors depends upon the impedance of the conductors connecting the capacitors to the current loads and power sources. As described in [200], a maximum parasitic impedance between the decoupling capacitor and the current load (or power source) exists at which the decoupling capacitor is effective. Alternatively, to be effective, an on-chip decoupling capacitor should be placed such that both the power supply and the current load are located inside the appropriate effective radius [200]. The efficient placement of on-chip decoupling capacitors in nanoscale ICs is the subject of this chapter. Unlike the methodology for placing a single lumped on-chip decoupling capacitor presented in Chapter 18, a system of distributed on-chip decoupling capacitors is described in this chapter. A design methodology to estimate the parameters of the distributed system of on-chip decoupling capacitors is also presented, permitting the required on-chip decoupling capacitance to be allocated under existing technology constraints.