Noise coupling in multi-voltage power distribution systems with decoupling capacitors

Multiple supply voltages are often used in modern high performance ICs, such as microprocessors, to decrease power consumption without affecting circuit speed. A decoupling capacitor system for use in multiple supply power distribution systems is described. In order to minimize the total impedance of a multi-voltage power delivery system, as seen from a particular power supply, a decoupling capacitor is placed between the power supplies. The noise at one power supply can propagate to the other power supply, causing power and signal integrity problems in the overall system. With the introduction of a second power supply, therefore, the interaction between the two power distribution networks must be considered. The dependence of the magnitude of the voltage transfer function on the parameters of the power distribution system is investigated. It is shown that the magnitude of the voltage transfer function is strongly dependent on the parasitic inductance of the decoupling capacitors, decreasing with smaller inductance. It is also shown that it is highly desirable to maintain the effective series inductance of the decoupling capacitors as low as possible to decrease the overshoot of the response of the dual voltage power distribution system over a wide range of operating frequencies. Finally, a criterion for an overshoot-free voltage response is presented.