System configurations for power systems based on PMBus 1.3

Michael Jones, Linear Technology
EDN (February 27, 2014)

This article shows several ways to implement a power management controller using the new PMBus Revision 1.3 specification, and addresses several new considerations with architecting a system using this new specification.

Do you find yourself working with a PMBus-based power system for a product which uses FPGAs, ASSPs, or ASICs? Or maybe you need to design from scratch a power management controller for your product, and you are evaluating various ways of implementing it. The examples that follow may give you a starting point for the system design and compare some consequences of the different choices.

Terminology and Background on AVS
The PMBus Rev. 1.3 specification, which is being drafted now and should be released by the end of March 2014, incorporates significant changes intended to enhance the effective bandwidth of the bus for usage in controlling multiple power supplies in a system (see references at the end). This allows very capable and extensible implementations of system power controllers based on the physical layer definition and command set of the new specification.

For the purposes of this article, we will refer to circuits or subsystems used for the purpose of controlling, coordinating, and monitoring multiple power supplies as a "power management controller (PMC)" to distinguish these functions from the more traditional "PWM power controller ICs" (which may regulate one or more voltage rails), and from "system management controllers (SMCs)" which may also deal with other functions such as fans, interlocks, displays, boot management, etc. A PMC defined in this way may be implemented with one or more ICs, and may be on the same or a different printed circuit assembly (PCA) as the power supplies it controls.

With the new PMBus 1.3 specification, a significant new function is available to enhance the control of power for high-density logic devices such as System on a Chip (SoC) Processors, Field Programmable Gate Arrays (FPGAs), Application Specific Standard Products (ASSPs), and Application Specific Integrated Circuits (ASICs). This new function is a dedicated high-speed bus (5-50MHz) which allows immediate control of one or more regulated voltages to the logic device for a technique called Adaptive Voltage Scaling (AVS). This three-wire bus is similar to the SPI bus, and will be referred to as the AVSBus. PMBus 1.3, with its speed improvements and the new AVSBus, are known as PMBus+™.

As the logic devices progress to smaller semiconductor process nodes along with recent leading-edge microprocessors, it follows that the power control requirements will become similar to those that have been used for microprocessors for several years, including AVS. An effective implementation of AVS for these devices can result in a very significant reduction in power consumption for many speed grades of the logic devices and can enable unprecedented integration of power control and monitoring with these devices.

With the introduction of the new AVSBus, there are new architectural questions and considerations which should be dealt with in the design of the power controller system. The rest of this article addresses some of these considerations. We will refer to the high-density logic devices as FPGAs, but similar arguments can be made for the other types of logic devices.

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