PMBus: Digital Power Control Protocol

Probably the best 2005 development in power is the new PMBus (Power Management Bus) open standard specification. It defines a digital communications protocol for controlling power conversion and management devices. Taking part in this collaboration were power-supply and semiconductor companies such as Artesyn Technologies, Astec/Emerson Network Power, Intersil Corp., Microchip Technology, Summit Microelectronics, Texas Instruments, Volterra Semiconductor, Zilker Labs Inc., and others.

With PMBus, power converters can be configured, monitored, and maintained according to a standard set of commands. Designers can use PMBus commands to set a power supply's operating parameters, monitor its operation, and perform corrective measures in response to faults or operational warnings. The ability to set a power supply's output voltage allows the same hardware to provide different output voltages by merely reprogramming. The ability to monitor and maintain a PMBus system enhances its reliability and availability.

Implementation of the PMBus specification requires the design of power supplies and their associated ICs to adhere to the required interface and commands. For example, the SMBus provides serial communication between the host computer or system manager and the PMBus-compliant devices (see the figure). A variation of the widely used I2C bus, the industry-standard SMBus is a two-wire bus modified several years ago for Smart Battery applications.

When implemented, the PMBus protocol will enable multisourced power-management products. Also, OEMs will be able to control compliant power converters using a standard set of commands. First hardware that complies with PMBus appeared in the second half of 2005. This includes an IC and two power supplies.

WHAT IT COVERS The PMBus specification has two parts. Part I includes the general requirements and defines the transport and electrical interface and timing requirements of hardwired signals. Part II defines the command language for PMBus. The PMBus protocol covers a wide range of power-system architectures and converters. However, not all PMBus devices must support all available features, functions, and commands. Here, "devices" refers to the specific power-management product—that is, IC, power converter, power supply, etc.

To comply with the PMBus specification, devices must:

• Meet all requirements of the specification in Part I.
• Support at least one of the non-manufacturer specific commands given in Part II.
• Execute functions as specified in the PMBus command code.
• Either accept, acknowledge, and execute a PMBus command, or reject it.
• Upon application of power-up, safely start and operate without communication with other PMBus devices.
• Use the System Management Bus (SMBus) for transport, although the specification lists some exceptions.
• Support the Group Command Protocol used to send commands to more than one PMBus device. These commands are received in one transmission. When the devices detect the STOP condition that ends the command, they will all begin executing the received command.
• Respond if there is a change in the state of a hardwired signal, although there are no specific time requirements.
• Each PMBus device will specify in its product literature the accuracy of the output voltage and other parameters that can be set and reported.

The specification does not cover:

• A particular power-conversion device or family of power-conversion devices.
• A specification of any individual or family of integrated circuits.
• Direct device-to-device communication, such as analog current sharing, real-time analog or digital voltage tracking, and switching-frequency clock signals.

Optional functions include:

• Support for the SMBus Packet Error Checking (PEC) protocol.
• Temporarily becoming bus masters and communicating with the host.
• Notifying the host that they want to communicate with it.
• Write Protect (WP) signal inputs.
• Upgrading firmware via the SMBus interface.

ELECTRICAL INTERFACE The specification describes the electrical interface for hardwired signals. The only exception to this is pins used to set the physical address. If the electrical interface to address pins doesn't comply with the specification, the electrical interface shall be described in the PMBus device's product literature.

The CONTROL signal is a power-converter input signal used to turn the device on and off along with commands received via the SMBus. It can be configured as an active high or active low signal. This signal is optional but recommended.

PMBus devices may use pins for programming or configuration. The device's product literature shall describe the function and electrical interface of any such pins. Examples of such pins are a RESET pin or pins that are used to set the output voltage to the high-or low-margin voltages. Pins that provide a binary input (high or low) shall have an electrical interface that adheres to the specification.

DATA COMMAND FORMATS FOR OUTPUT VOLTAGE AND RELATED PARAMETERS Data for commanding or reading the output voltage or related parameters can be in one of three different formats depending on the type of device:

• Linear scale commanded and reported using a two-byte unsigned binary integer with a scaling factor (similar in concept to a mantissa and exponent).
• VID code format of popular microprocessors.
• Direct format that uses an equation and device-supplied coefficients.

Power supplies and power converters generally have no way of knowing whether their outputs are connected to ground. However, power supplies treat most output voltages as " positive." Therefore, all output voltages and output voltage-related parameters of PMBus devices are commanded and reported as positive values.