Multiphase Constant On-Time Control With Phase Overlapping—Part I: Small-Signal Model

Multiphase voltage regulators (VRs) that power multicore processors must have a fast load transient response and good light-load efficiency. Multiphase current-mode constant on-time (COT) control based on pulse distribution method eliminates the phase-locked loops and interleaves the phases through a simple phase manager. Hence, power management industries widely use this control to meet VR requirements with reduced complexity. To handle large load currents, the VR's phase count continues to increase. With increased phase count, these converters operate with overlapping phases within the practical duty cycle range. This article finds that the total current exhibits variable on-time and off-time modulation under a small sinusoidal perturbation in the phase-overlapping region. Hence, the small-signal model of the single-phase COT control does not apply in this region. Therefore, Part I of this article presents a general small-signal model of this control applicable to the whole duty cycle range using the describing function method. Also, the proposed model is verified using SIMPLIS simulations and experimental results from a six-phase COT-controlled buck platform. Part II of this article identifies that the total current loop of this control will become unstable with overlapping phases and derives the critical conditions for stability using the model from Part I.

I. Introduction

Modern computers require their CPUs and GPUs to have faster data processing with minimum thermal design power (TDP). As a result, modern processors integrate many transistors and use multiple cores to maximize their speed and minimize the TDP [1], [2]. However, these two features introduce new challenges to the design of the multiphase interleaved buck regulators that power these processors. First, with the increase in transistor count the processor's power and current requirement increases. Second, the multicore technology introduces large step load transients with very high slew rates (up to 1000 A/s). Hence, to meet these requirements, the phase count of multiphase buck regulators has continued to increase to: 1) limit the per phase current, reduce converter losses, and maximize the efficiency; 2) reduce the equivalent inductance and achieve fast transient response with a reduced output capacitor requirement.