PMBus Defines Standard For Digital Control Of Power Management Subsystems

PMBus, or Power Management Bus, is a new open standard specification that defines a digital communications protocol for controlling power conversion and management devices. It is a collaborative effort involving both power supply and semiconductor companies. Included in this effort are Artesyn Technologies, Astec/Emerson Network Power, Intersil Corporation, Microchip Technology, Texas Instruments, Volterra Semiconductor, Summit Microelectronics and Zilker Labs, Inc..

The PMBus allows power converters to be configured, monitored and maintained according to a standard set of commands. Using PMBus commands, a designer can 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 re-programming. 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. As shown in Fig. 1, the SMBus provides serial communication between the host computer or system manager and the PMBus compliant devices. A variation of the widely-used I²C bus, the industry-standard SMBus is a two-wire bus modified several yeas ago for Smart Battery applications.

When implemented, the PMBus protocol will allow multi-sourced power management products. In addition, OEMs will be able to control compliant power converters using a standard set of commands. First hardware that complies with PMBus is expected by mid 2005. This includes ICs and POL (point-of-load) converters.

What the specification covers There are two parts to the PMBus™ specification. 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 of the available features, functions and commands. Here, the term "devices" refers to the specific power management product; that is, IC, power converter, power supply, etc.

To be compliant with the PMBus specification devices must:

Meet all of the 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 is not in compliance 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. Output voltage and related commands include:

Set the device to use either the Linear, VID or Direct modes for output voltage related commands. Also includes information about the selected mode, such as which manufacturer's VID codes are being used.
Set the device:

Output voltage.
Output voltage above which an output overvoltage fault is declared.
Upper limit on the output voltage regardless of any other commands or combinations.
Rate in mV/μs at which the output should change voltage.
Rate, in mV/A (mΩ) at which the output voltage decreases with increasing current.
Output power (watts) that triggers regulation in constant power mode instead of constant voltage.
Maximum allowed duty cycle (%) of the power conversion stage.
Switching frequency (kHz).
Input voltage where power conversion starts and stops.
Ratio of the voltage at the current sense pins to the sensed current.
Fan voltage.
Fixed output offset voltage.

Load the device with the voltage to which the output is to be changed when the OPERATION command is set to "Margin High" or "Margin Low."
Trim the output voltage.
Map PMBus devices between the commanded voltage and the voltage at the control circuit input, divided down to match a reference voltage.
Arrange multiple devices to distribute their switching periods in time.
Null out any offsets in the output current (amperes) sensing circuit.

Output Voltage Sequencing Commands

Set the device time (ms):

From receipt of a start condition until the output voltage starts to rise.
From when the output starts rising until the voltage starts regulating.
From receiving a stop condition until the device stops transferring energy to the output.
From the end of the turn-off delay time until the output voltage is zero.

Set the upper limit of time (ms):

That it can attempt to power up the output without reaching its output under-voltage fault limit
That it can attempt to power up the output.
That it can attempt to power down the output without reaching 12.5% of the output voltage programmed at the time the converter is turned off.

Instruct the device on action to take in response to a maximum on-time fault or a maximum off-time limit.

Fault Management and Reporting The protocol includes the ability to program fault or warning levels for a power conversion device. Fault conditions are more serious than warning conditions and may require the PMBus device to disable the output and stop the transfer of energy to the output. For fault conditions, you can program the PMBus device to respond by shutting down immediately and latching off, shutting down and retrying or continuing to operate for a specified delay time before shutting down. Commands for fault management and reporting include:

Clear any fault bits that have been set.
Set the device output voltage threshold measured at the sense or output pins for:

Over-voltage or under-voltage faults.
Low or high voltage warnings.

Set the device:

Output voltage for asserting and negating a power good signal.
Output current (amperes) threshold to indicate an over-current warning or fault.
Maximum allowable sink current (amperes) before taking action.

Set temperature (°C) for device:

Over-temperature warning or fault.
Under-temperature warning or fault.

Set device input voltage threshold to cause:

Low or high input voltage warning.
Input under-voltage or over-voltage fault.

Set device input current threshold to cause:

High input current warning.
Input over-current fault.

Instruct the device on action to take in response to:

Output under-voltage or over-voltage fault.
Output under-current or over-current fault.
Under-temperature or over-temperature fault.
Input under-voltage or over-voltage fault.
Input over-current fault.

Specify the voltage threshold for the case where the response to an over-current condition is to operate in a constant current mode unless the output voltage is pulled below the specified value

Device Status Commands Read only device status commands retrieve binary status information from PMBus devices. Device status command data includes:

Device busy
Device not providing power to the output
Output over-voltage or over-current fault
Input under-voltage fault
Temperature fault or warning
Communications, memory or logic fault
Output voltage fault or warning
Output current fault or warning
Input voltage fault or warning
Input current fault or warning
Negate Power Good signal, if present
Output over-voltage fault or warning
Output under-voltage fault or warning
Attempt made to set the output voltage higher than allowed.
Tracking error on power-up or power-down.
Output over-current fault.
Output over-current and low voltage shutdown fault.
Output over-current warning
Output undercurrent fault
Output current share fault
Device operating with the output in constant power mode at the power set by the maximum output power commands
Input over-voltage fault or warning
Input under-voltage fault or warning
Device is off due to insufficient input voltage
Input over-current fault or warning
Over-temperature fault or warning
Under-temperature fault or warning
Invalid or unsupported command or data
Packet error check failed
Memory or processor fault
Communication fault other than those listed
Fault of undetermined type
Fan fault
Input fuse or circuit breaker fault
Input OR-ing device fault
Output OR-ing device fault
Unspecified or unknown fault
Manufacturer specific fault or warning

Reading Parametric Information The host or power system manager can read parametric values, including:

Input voltage (volts).
Input current (amperes).
Voltage on the energy storage (hold-up) capacitor (volts).
Actual, measured output voltage in the same format as set by the output voltage mode command.
Measured output current (amperes).
Device temperature readings (°C).
Device fan speed readings.
Device fan voltages (volts).
Duty cycle of the PMBus device's power converter (%).
Actual switching frequency of the PMBus device's power converter (kHz).

Manufacturer Information There are commands to store and retrieve the device manufacturer's inventory information. Typically, this is for the manufacturer of an assembled power supply or dc-dc converter rather than an IC, including:

Manufacturer's ID
Manufacturer's model number.
Manufacturer's revision number.
Facility that manufactured the device.
Device manufacture date.
Device serial number.

Manufacturer Ratings Several commands allow manufacturers to return device rating information, which serves as an electronic nameplate for the user's convenience. The PMBus device does not enforce or report as a warning or a fault any violation of these ratings. These commands include:

Minimum or maximum rated input voltage (volts).
Maximum rated input current (amperes).
Maximum rated input power (watts).
Minimum or maximum rated output voltage setting (volts).
Maximum rated output current (amperes) or power (watts).
Minimum or maximum rated ambient operating temperature (°C).

References

PMBus™ Power System Management Protocol Specification, Part I – General Requirements, Transport and Electrical Interface. http://www.pmbus.com/PMBus-Specification1.pdf.
PMBus™ Power System Management Protocol Specification, Part II – Command Language. http://www.pmbus.com/PMBus-Specification2.pdf.