Multiphase PWM Controllers Steer Toward More Phases On-Chip

May 1, 2004 12:00 PM
By Ashok Bindra, Editorial Director, Power Electronics Technology

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Since their inception in 1998, the monolithic multiphase pulse-width-modulated (PWM) controllers have made significant strides in cost and performance to become an attractive approach to powering a new breed of multi-gigahertz processors in desktops and workstations/servers. With the ability to distribute total current across phase-shifted PWM channels and associated output MOSFETs and inductors, as well as use of higher switching frequencies, more power supply designers are migrating toward multiphase dc-dc converter solutions. In addition, innovative digital techniques are being implemented to leapfrog existing analog methods by orders of magnitude in transient response.

Although the earlier focus was on meeting the current and voltage demands of desktop and workstation CPUs, the permeation of faster processors in notebooks with CPU load requirements climbing over 50 A, multiphase solutions have also been extended to mobile computing systems. A new breed of multiphase synchronous PWM controllers ensures that the core voltage of multi-gigahertz processors remains within specified guidelines of Intel's VRM/VRD 10.x specs on the desktop front. Furthermore, notebook power supply designers are exploiting these controllers to comply with the latest Intel mobile voltage positioning (IMVP) standards.

With current requirements surging toward 200 A at low voltages, and transient response specifications getting tougher, the need for more phases per chip is growing. While dual-phase solutions have adequately served earlier-generation processors, the new and emerging class of processors is seeking three, four and more phases per chip. In response, some suppliers are readying 6- and 8-phase monolithic controller designs for desktops and workstations.

Depending on power requirements, others have developed techniques to cascade single-chip dual-phase controllers to attain quad and even more phases in a dc-dc converter solution. Others have crafted a flexible architecture for simplicity, scalability and flexibility from one design.

Multiphase controllers also are incorporating clever feedback mechanisms to provide ultrafast recovery times during steep step increases of load current. They can furnish slew rates better than 100 A/ns to cope with the rigorous requirements set by Intel's 10.x specifications for its latest voltage regulator module (VRM) for desktops and workstations. Intel has created VR-Down (VRD) specs for embedding voltage regulators on the same plane as the processor.

While International Rectifier has taken the scalable and flexible route to support 1- to X-phase operation from a single controller chip, Intersil and Primarion have jointly developed a digital programmable controller to provide new ways of delivering multiphase power solutions to present and forthcoming processors. Concurrently, Volterra Semiconductor has implemented its proprietary digital technique to ready regulators for both VRD and VRM specifications. Unlike analog, the digital controller offers the ability to add, eliminate or change any parameter in the system to meet new requirements or to optimize and calibrate the system. Similarly, IR's flexible Xphase lets designers add phases one at a time to increase output current capability as higher-speed processors become available.

Meanwhile, on the analog front, other IC suppliers focusing on packing higher number of phases on-chip include Analog Devices, Fairchild Semiconductor, Linear Technology, National Semiconductor, On Semiconductor, Semtech, STMicroelectronics and Texas Instruments.

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