Powerelectronics 1253 S

Benefits of Digital Multiphase PWM Power Control

Oct. 1, 2003
Since the early 1990s, the power requirements for VCC core voltages for leading-edge processors and ASICs have become increasingly demanding, with chip

Since the early 1990s, the power requirements for VCC core voltages for leading-edge processors and ASICs have become increasingly demanding, with chip voltage decreasing and current increasing. Early devices used 5V from the “silver box” to generate VCC core voltage. With the increase in current demand, a shift was made to using 12V. In an attempt to decrease the high ripple associated with regulating the low VCC core voltage from 12V, the use of a multiphase buck converter became the standard. Furthermore, to satisfy the increasing complexity of power specifications, such as soft start, power sequencing, dynamic VID and load-line specification, dedicated analog multiphase controller ICs were introduced.

These ICs are based on analog PWM architectures and process technologies, consequently requiring designers to possess expertise in analog design. Additionally, these ICs tend to be specific to a given application, i.e. applicable to a certain generation of processor. Increasingly, the PWM function in the multiphase controller is becoming a minor part of the controller function. This, coupled with the demand for more functionality from the controller, has compelled many companies to look at digital architectures for advanced PWM control.

Currently, digital solutions range from standard digital signal processors (DSPs) to the latest dedicated fixed-function mixed-signal chipset, such as the ISL6590 and ISL6580. These software-driven digital controllers have many advantages over their analog counterparts, such as ease of design, flexibility, optimization and improved system reliability. In addition, digital offers more elegant solutions to many of today's demanding power requirements.

Fundamentally, digital architectures differ from analog in the fact that digital controllers use analog-to-digital converters to digitize current and voltage information where compensation and regulation is done using DSP techniques (such as programmable filters). Also, digital architectures use some form of program memory, which not only allows for more sophisticated monitoring and control schemes, but also adds a software graphical user interface (GUI) as a design tool. These GUIs are used in the design phase to configure the digital controller for specific design parameters. Finally, digital processes tend to exist in smaller geometries, offering lower-cost solutions where there's a high degree of circuit function integration.

The use of software to change the controller functionality makes a system based on a digital controller flexible.

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. For example, the same voltage regulator module (VRM) can be programmed to meet different design specifications, allowing the supplier to have a single module that meets several design points. It also offers the capability to integrate and cascade multiple systems together because of the ease of integrating communications capability into the digital controller. For example, where multiple VRM boards are used, the need to current share can be implemented through a standard communication bus without additional hardware.

Systems based on digital controllers require fewer components, which decreases the mean time before failure of the system. For example, all the components for the feedback loop are eliminated along with the “select at test” and “select according to design specification” components. The added capability of monitoring protection and prevention also increases system reliability. For instance, an engineer can choose to monitor the system temperature to decrease the current limit level or turn on a fan. This scenario decreases stress on the power components and fans.

Over the years, Moore's Law has continued to push digital gate integration, which, in turn, increasingly adds to the complexity of the power-delivery system. There doesn't appear to be an end to the gate integration curve; therefore, continued innovation is required in power-delivery systems.

Digital controllers are the building blocks of future innovation in power regulation. Digital offers system flexibility and optimization through a communications bus that just isn't achievable with today's analog controllers. Furthermore, digital controllers offer improved time-to-market with ease of design and increased system reliability.

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