Load-line regulation with estimated load-current feedforward: Application to microprocessor voltage regulators

A consistent framework for load-line regulation design is presented, applicable to microprocessor voltage regulators (VRs) using either electrolytic or ceramic output capacitors. With conventional feedback control, the loop bandwidth is limited by stability constraints linked to the switching frequency. The output capacitor has to be chosen sufficiently large to meet the stability requirement. Load-current feedforward can extend the useful bandwidth beyond that imposed by feedback stability constraints. With load-current feedforward, the size of the output capacitor can be reduced, since it is determined solely by large-signal and switching-ripple considerations which are shown to be less constraining than the feedback stability requirement. This work points to the feasibility of microprocessor VR implementations using only a small number of ceramic output capacitors, while running at sub-megahertz switching frequencies.