Small-Signal Modeling of Multiphase V2 Constant On-Time Control with Phase Overlapping

Multiphase V² constant-on time control (COT) based on pulse distribution method is increasingly used in voltage regulators for CPUs and GPUs because of its good light-load efficiency, fast transient response, and simple structure. When the duty cycle of an N phase V² COT controlled buck exceeds 1/N, the output voltage exhibits variable on-time and off-time modulation from a small-signal perspective. Hence, single-phase V² COT control models cannot be extended to the multiphase V² control in the phase overlapping regions (or when D>1/N). To address this issue, this work presents a general small-signal model of multiphase V² COT control architectures for the whole duty cycle range based on describing function method. The proposed models for multiphase V² COT control architectures are verified using SIMPLIS simulation and experimental results from a twelve phase V² COT controlled buck platform.

I. Introduction

The multicore processors in CPUs and GPUs of laptops, servers, and other computing devices operate primarily in sleep mode, introduce fast-changing load current profiles in turbo-boost mode, and have tight voltage tolerances. According to Intel’s VR 12.5 and 13 specifications, the load current step and slew rate in turbo-boost mode can reach 220A and 1000A/us, respectively [1]. Moreover, the voltage tolerance band is less than 3% of their nominal output voltage. Hence, these processors require voltage regulators (VRs) with good light-load (sleep-mode) efficiency, high voltage loop bandwidth, and a fast load transient response.