Design of ceramic-capacitor VRM's with estimated load current feedforward
For voltage regulator module (VRM) designs with ceramic output capacitors, the capacitor size has to be chosen sufficiently large to allow for the use of relatively large inductor values. This enables operation at conventional switching frequencies, while meeting load transient response specifications. Due to the small effective series resistance (ESR) time constant of ceramic capacitors, this may result in designs with output capacitor ESR substantially lower than the desired output impedance. This is in contrast to conventional VRM implementations with electrolytic capacitors, where the desired output impedance is closely related to the output capacitor ESR. In ceramic capacitor designs with conventional feedback control, the required loop bandwidth is inversely proportional to the output capacitor size. The feedback bandwidth is limited by stability constraints linked to the switching frequency. The use of load current feedforward can extend the useful bandwidth beyond the limits imposed by feedback stability constraints. Load current feedforward is used to handle the bulk of the regulation action, while feedback is used only to compensate for imperfections of the feedforward and to ensure tight DC regulation. An experimental converter demonstrates tighter output regulation with estimated load current feedforward, than with pure feedback control.
