Flexible controllers adopt power-efficient strategies

Innovative digital control technologies make for high-performance gear.

Given rapidly rising energy costs, it's not surprising that power efficiency has become a leading differentiator for designers of high-performance servers and desktop systems. However, managing power to the current generation of low-voltage, high-frequency processors poses many new challenges.

Today's microprocessors, graphics processors and memory modules shift from low to high frequency at increasing rates, forcing power supply designers to meet ever-tighter power regulation requirements. Accurate specifications are also increasing. While earlier 3.3V processors may have operated off a 15A load, today's low-voltage CPUs more often than not require as much as 150A at under 1V.

Despite these rapid changes, systems designers still rely largely on the same analog approaches to power management they have always used. Generally, these schemes use monitoring techniques and feedback loops that need many external components in the control loop to maximize power efficiency. Moreover, as performance requirements rise and new functions are added, these power management designs require more external components, further driving up design complexity, board real estate and cost.

Digital power systems
Managing power using digital controls promises to deliver dramatic advantages based on power monitoring and control. If an operating frequency changes, the designer can simply select a value from a register rather than add another component to the design. Precise digital approaches provide significantly better accuracy than widely used analog schemes. In highly competitive, time-constrained markets such as graphics boards, flexibility might allow system designers to keep optimizing their design while the OEM is building the system. Because you can optimize the system without changing components, "all the designer has to do is e-mail a config file loaded into the chip via the I²C interface," noted Deepak Savadatti, VP of marketing at Primarion Corp., a subsidiary of Infineon Technologies.

However, early attempts at digital implementations faced significant obstacles. With the escalating transition rates of today's processors, transient response has posed a major problem. While analog schemes are limited only by the capabilities of the resistors and capacitors in the feedback loop, digital schemes must overcome significant latencies as they feed the output voltage through an ADC, process the digital output and convert the signal back to analog form. To make matters worse, designers who considered implementing digital control schemes faced a major cost premium compared with analog alternatives.

Variations
Designers at many smaller power management IC makers have begun to develop new controllers that address some of these challenges. By combining innovative digital circuitry with mixed-signal multiphase synchronous buck controllers, these new tools give system designers opportunities to more efficiently manage core voltage and load requirements in system CPUs, GPUs and memory modules. Moreover, with their new digital control techniques, these multiphase buck controllers give designers options in their ongoing efforts to improve performance, manage thermal issues and minimize component count.

"Analog approaches have been the preferred solution for a long time," said Savadatti. "But digital controllers are becoming a reality in some market segments, such as high-performance servers, where manufacturers have long looked outside the box for new ways to add intelligence to their systems and make them more reliable."

Last July Primarion introduced a new digital multiphase controller for VR11.1-based systems that delivers more than 90 percent efficiency across all load levels. The new PX3560 controller has up to six phases, each capable of 1MHz operation and usable to build DC/DC converters capable of delivering up to 200A.

To maximize efficiency across the entire load range, these new controllers drop, or "shed," phases at lighter loads based on average current defined by the user. "When traffic slows down and the processor goes from sucking 130A to 25A, data center managers want the voltage regulator to figure out how to maximize efficiency," Savadatti said. "We get about 4 percent to 6 percent efficiency improvement using phase shedding."

Other vendors, such as Volterra Semiconductor Corp., split more of the regulator's functionality onto the mixed-signal side. The company offers both scaleable chipsets and discrete regulators with varying degrees of digital control.

Power gains
As IT managers fight to rein in data center energy costs, these new multiphase controllers give enticing power savings. "In our initial tests we estimate by improving power efficiency we can save up to 150W in a typical eight-blade server configuration while meeting fast transient response requirements," said Larry Spaziani, VP of marketing at CHiL Semiconductor, a power management IC developer.

CHiL has pushed the bar a little higher with its announcement of both a six-phase and the industry's first eight-phase VR11.1-compliant buck controller. Like its competitors, the company claims more than 90 percent efficiency across all load ranges. To address the fast transients in VR11.1-compliant CPUs, the new controllers use an optimized ADC architecture capable of a transient response of more than 2,000A/?s.

To support higher-performance gaming/enthusiast desktop PC applications, CHiL's eight-phase CS8618 controller has capabilities such as custom digital overclocking features that provide users to extend voltage range up to 2.4V.

- John Mayer
EE Times