But is digital control really needed? Digital proponents say the trend is inevitable, especially given the shortage of analog-savvy engineers, while the analog camp points out that analog designs are less costly.

"Digital power management comes in two flavors: digitally controlled power conversion and system-level digital power management," said Ravi Ambatipudi, director of advanced technology for portable power systems at National Semiconductor Corp. Power converters are traditionally designed with analog control loops and analog circuitry, he said. By contrast, "In the most basic form of digital power management, the power converters are designed with complete digital

control loops and digital circuitry. No analog components other than external output filters are used," he said. "System- level digital power management involves intelligence in the system-on-chip ICs that can intelligently control voltage regulators using a dedicated interface for advanced power management."

Analog Devices Inc. (ADI) maintains that there are two ways to define digital power control: when the power controller's converter has a serial bus port for communication, or in cases defined by the control loop itself. Some companies, including ADI, offer controllers that provide full loop control and have an SMBus interface, digital communication reporting and status information, but have a loop that is controlled in an analog fashion.

So, if there is no consensus and analog has served well for so long, then what is driving the digital control market?

"First, everything you buy is getting smaller and more feature-dense," said Don Alfano, director of marketing for Silicon Laboratories Inc. "That means there is more circuitry, or there is the same amount of circuitry but it is running faster, which leads to more current. Then you have a problem with space and efficiency, because if you are pushing that much current the devices tend to overheat. The shrinking process technology leads to a need for very precise power regulation."

There is no clear-cut decision on which approach is better, said Ahmad Ashrafzadeh, business manager for Maxim Integrated Products. The needs of complex systems have paved the way for considering and studying digital approaches to power management, which seemed farfetched not too long ago, he observed. "Unfortunately, like any new technology, there is a lot of myth and hype, which will clear up once the technology settles over its natural course," Ashrafzadeh added.

"In 2004 the industry reached an inflection point in embedded designs where it was dedicating more time, money and board space to power management than it was to power conversion," said Jim Templeton, founder and vice president of marketing for Zilker Labs Inc. "That's when it became a very interesting problem for semiconductor companies to go to work on. Now it's a big enough problem to warrant integration."

The first step in doing that is to get the power converter into the digital domain. That would support the logic density required to combine the power-management and conversion functions.

The only time it makes sense to mention digital in the context of power, Templeton said, is when discussing the conversion loop itself--the feedback loop

that controls power regulation. "Digital power must include that aspect of power conversion. In digital power you must convert the signal to the digital domain and process the error signal in the digital domain and process the compensation loop in the digital domain, and then output it to the PWM [pulse-width modulator] that closes the loop," he said.

Steve Pullen, vice president of engineering for Primarion Corp., said the most daunting problem for most power customers is simply finding the time and resources to develop solutions to their complex power-management problems. Additionally, he said, "the top five drivers in selecting a power solution are cost, cost, cost, cost--and size."

National Semiconductor's Ambatipudi sees four challenges for engineers looking to implement a digital power design:

• Converting the analog output voltage to the digital domain. The output voltages are analog in nature. Converting one to an accurate digital representation requires a high-speed, high-resolution A/D converter.

• A digital PWM is needed to convert the digital data from the digital loop filter to a pulse-width modulated signal. Significant power can be consumed to achieve the required time resolution. The resolution of the digital PWM must be greater than the resolution of the output voltage A/D to avoid limit-cycle oscillations.

• Certain control functions, such as input supply voltage feed-forward, and system functions such as current limit, oscillator, voltage and current references are difficult to implement using only digital techniques.

• A dual-voltage silicon process technology is needed to support low voltage for the dense digital circuitry and higher voltage for the output power FETs or drivers.

"One of the biggest challenges customers have in portable applications is to conserve power, because they use batteries," said Tom DeLurio, director of applications engineering for Summit Micro- electronics Inc. "They need to disable certain functions and change voltages dynamically to stop the drain on the battery. Additionally, customers also need to be able to turn off different voltage levels for different functions that aren't presently needed, and they need to minimize component count because designs need to go into smaller equipment."

So what's the best design--digital or analog? For now, the answer depends quite a bit on the application. It also depends on the designer's level of expertise in the digital and analog domains. It is necessary to consider what to integrate, and that requires access to process technology.

A number of digital solutions are available in the market. "The most complex are DSP-based solutions," said Primarion's Pullen. "These products offer the ultimate in flexibility, but are difficult to use. DSP-based solutions require power engineers, already strapped for time, to learn how to program a DSP core."

Many analog players are adding "digital wrappers" to their analog controllers, which results in a cost increase. Other interim two-chip solutions combine a separate microcontroller with a traditional analog controller to provide bus telemetry.

Some will wonder how, if the design phase for digital costs more than analog, digital can be cheaper. "When you talk about cost, it isn't about just the bill of materials and the overhead to build and test the power supply," Silicon Labs' Alfano said. "The cost is determined from conception to delivery, including the installation."

What's more, he said, "digital supplies are capable of accepting configuration and reconfiguration over the network wires, so you have to look at the cost required to dispatch personnel from your factory or local field office to visit the installations."

Additionally, there are attractive features that are intrinsic to digital. For example, digital technology allows designers to use new control topologies or algorithms--they are theoretically possible with analog, but very expensive to implement. Digital control can use those new ideas and make them possible for a reasonable cost.

"Digital control will get cheaper and it will open a new application area," said Anton Bakker, senior staff design engineer for power and thermal for computing at ADI. "You can compare it to digital photography--it also took a long time to be accepted. In the beginning of digital cameras, you could do the same thing with any film-based or analog camera."

The same is happening in power supplies, Bakker said. "Customers need to look at the applications where they can benefit from the additional features of digital controllers. Currently it's high-end applications that need monitoring functions, customization and programmability, which are slightly more expensive than analog controllers. The answer will be completely different in three to five years."

One trade-off as seen by designers of point-of-load products results from a need to operate at 2 MHz and concurrently convert from 8 volts to 1 V, said Dave Freeman, applications manager for power- management products at Texas Instruments Inc. To do this you need a subnanosecond duty cycle resolution and a digital product that uniquely addresses the issue, not the typical digital PWM.

In 2005 there were only a few introductions, Freeman said. This year, companies will start getting used to the digital designs and play with them. But in 2007, he predicted, there should be enough choices and competition and leveraging between companies to start a real upswing in digital power designs.