Stability analysis of voltage regulators versus different digital control strategies by analog-mixed-signal circuit simulation

Voltage regulator modules (VRMs) used in modern computers can work in “extreme” conditions since they supply high currents at a very low voltage and keep voltage swing inside a small tolerance interval against large variations of load currents. This is done by designing a digital controller that allows the VRM to behave as much as possible as an ideal voltage source with a very small series resistor. This can be achieved through different possible choices of controlling schema. We show that, considering also effects due to parasitics, controlling schema reflect on stability and on the admissible minimum size of the capacitor bank filtering the VRM output voltage. Moreover we show how the output impedance of the VRM degrades versus lowering of the bank size. This is done by resorting to a general and efficient technique, already presented in the literature, allowing to (i) determine the periodic steady state behaviour of these circuits together with their stability and (ii) derive time varying transfer functions such as, for example, the output impedance of VRMs.