Electronic Design

PMBus: The New Power-Management Paradigm

PMBus Rocks The System-Power-Management World. Such a headline may seem trite. But not since embedded computer systems moved to the distributed power architecture (DPA) and intermediate bus architecture (IBA) has this arena seen such a change. The change was needed, though. Once DPA and IBA matured, it became apparent that one link was missing—the ability to control, monitor, and maintain the power-management subsystem.

The PMBus (power management bus) standard specification, developed by a consortium of power-supply and semiconductor companies, delivered that missing link in 2005. An open-standard specification, PMBus defines a digital communications protocol that enables power converters to be configured, monitored, and maintained according to a standard set of commands.

PMBus-compliant product introductions began with a trickle in the second half of 2005. This year, though, the floodgates will open as more semiconductor and power-supply companies participate in the product parade.

Using PMBus commands, designers 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 enables the same hardware to provide different output voltages by merely reprogramming. Plus, the ability to monitor and maintain a power-management subsystem enhances its reliability and availability.

The first PMBus-compliant IC to emerge was Zilker Labs' ZL2005, an integrated power-management and conversion controller (see "PMBus Controller Takes A State-Machine Approach," Dec. 15, 2005, p. 35, ED Online 11614). It combines a synchronous dc-dc controller, adaptive drivers, and PMBus power-management functions (Fig. 1). It's also compatible with designs ranging from a single-phase power supply operating from a 3.3-V input to a multiphase supply operating from a 12-V input.

The DPL20C was the first PMBus-compliant dc-dc converter, says Todd Hendrix, VP of Artesyn Technologies. He also says the company is a longtime advocate of open architecture systems, which help customers reduce time-to-market and streamline life-cycle product costs.

The DPL20C is a 20-A output, non-isolated point-of-load (POL) dc-dc converter. Its extensive digital configuration, monitoring, and diagnostic facilities are accessible via the PMBus interface. The converter stores all configuration and setup data in nonvolatile memory. It also powers up with these pre-programmed default settings, eliminating the need for external power controllers. Intended for computing, storage, and networking applications, the DPL20C is rated for forced-air environments-with ambient operating temperatures of 0°C to 70°C.

The company offers an evaluation board to help engineers familiarize themselves with these PMBus-compliant, digitally programmable POL converters. The board consists of a DPL20C POL converter mounted on a pc board with a PMBus bus connector. It comes with a USB cable to connect the board to a PC, a CD-ROM containing the configuration software and installation instructions, and a printed copy of the user guide.

It also features a 4.5- to 13.8-V input, 0.6- to 5.5-V programmable output, 20-A output with 91% typical efficiency (12 V input), and differential remote sense. Protection features include overcurrent and overvoltage.

Astec Power has announced its DTX digital dc-dc converter for use with the open-source PMBus communication protocol as well as the industry-standard I2C bus. This control platform is intended for an integrated digital system environment that will ultimately include a spectrum of devices, from front ends to eighth-bricks, bus converters, and POL regulators. The DTX42K48 dc-dc converter is a low-profile, open-frame package with surface-mount termination.

These supplies will be able to connect to virtually any I²C-based power control and monitoring system, but the communication bus isn't required for operation. These devices will operate in standalone mode, too. Customers will be able to adapt these products to their individual needs on their own, using the company's support tools. Designers will choose from an array of preprogrammed modules that emulate existing industry-standard "bricks," with or without a menu of additional features.

Featuring integrated digital control with bidirectional PMBus communication, the unit also includes self-diagnostics, efficiency optimization, and output impedance control, as well as dynamic feedback-loop compensation and configuration. Designers can program limits and modes relating to undervoltage, overvoltage, overcurrent, and overtemperature. The unit includes more than 4 kbytes of memory to record historical data, including tracking and serial-number information.

The PMBus communication is an open-source protocol, so designers will have access to a variety of software tools directly from Astec and other sources. The open-source flexibility also will let designers use these tools with other PMBus devices as they're released and easily customize them to meet specific needs. Astec offers users a free executable file with a GUI that provides extensive management capabilities, including custom controls.

The DTX42K48 features a 36- to 75-V dc input range, programmable 0.96- to 1.44-V output, and 42-A output capability at 86% typical efficiency. It also provides programmable undervoltage lockout, overvoltage protection, overcurrent protection, and overtemperature protection.

Though it wasn't designed to comply with the PMBus specification, Linear Technology's LTC2970 is similar (Fig. 2). The IC, set for the first quarter of 2006, controls and monitors two power supplies.

Similar to the PMBus, digital communication with the LTC2970 is over an industry-standard I2C, SMBus-compatible serial bus. Two general-purpose I/O pins provide added fault information or designer-defined system control.

Tailored for high-availability systems, its 14-bit, delta-sigma analog-to-digital converter (ADC) and a highly accurate internal reference enable the IC to control power-supply output to better than 1% accuracy. And thanks to the ADC's seven-channel multiplexer, it can monitor current, temperature, and other analog parameters. The high-resolution ADC also lets it perform precision margining.

Its intelligent ADC architecture averages power-supply noise. A servo algorithm limits its IDAC (current output digital-to-analog converter) step size to 1 LSB per iteration, which minimizes power-supply transients. By selecting two resistor values, designers can choose the appropriate resolution while providing a hardware range limit that the supply will not be driven beyond.

The LTC2970 also can command each IDAC to connect to the power supply's feedback node. It would use the IDAC code that most closely approximates the feedback node's regulation voltage (Soft Connect) or a user-selected IDAC code (Hard Connect). It can change the code of a previously connected IDAC under processor control or automatically servo to a programmed voltage as measured by the ADC.

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