The case for digital power management

Digital power ICs are not only the advanced choice with today’s generation of chips, but also the most cost effective ones

BY DEEPAK SAVADATTI
Primarion
Torrance, CA
http://www.primarion.com

Power conversion ICs for computing and communications used to be fairly simple to implement. Analog pulse-width modulation (PWM) ICs had two jobs: power delivery and voltage regulation. Anything more, such as monitoring or diagnostics, was considered superfluous. Or on those rare occasions when needed, they were accomplished through add-on chips.

Today, however, the demand for efficient and reliable power management is ramping up at a rate faster than Moore’s Law-paced development can accommodate. The main processing devices for computing and communications contain billions of transistors, and the power delivery requirement of these devices is ever more precise and complex.

In the datacom space, 36 to 40 voltage rails appear on a board. In the computing world, it is not uncommon for more than 20 voltage rails to operate various ASICs, memory, and processor chipsets on a motherboard. This complexity requires fine-grained diagnostics, control, and monitoring of various parameters, which is beyond the capabilities of analog PWMs. The solution, typically, is to add a separate microcontroller, but this adds significant cost to the system and design complexities, while taking up precious real estate on shrinking boards.

System efficiency

Fine-grained control goes hand in hand with another concern: system efficiency. External factors, such as increasing fuel costs and environmental pressures, put a premium on efficiency.

More than 50% of the electricity supplied for computing applications doesn’t get used for data processing. Instead, it is wasted on things like air conditioning or lost through inefficiencies in the system. The end result is that every wasted watt boosts the total cost of ownership and increases global energy consumption.

Real-time efficiency improvement is nearly impossible without the ability to measure and diagnose the system. The typical analog power solution is a black box, yielding no diagnostic information.

Faced with downward cost pressures, designers are increasingly seeking solutions where diagnostics of the power supply are not left to separate microcontrollers but are instead included with the power supply. They seek cost-effective power solutions able to monitor current, voltage, temperature, and power efficiency on chip. The system designers want to see failure reporting of faults, I2 C communication with the PWM controller and flexible programmability of personalized parameters on a per rail basis for their power implementations.

All this leads to one simple conclusion: market drivers and customer demands mean that Moore’s Law improvements of analog solutions lag behind current customer needs. The only way to meet today’s demands is to shift to digital power solutions (see Fig. 1 ).

Certain markets have already begun the transition from analog to digital. The sophisticated power demands of servers and high-end graphics cards, for instance, necessitate digital power supplies.

Other markets have been slower to make the switch, however. Originally, this was a simple cost-driven decision. Analog PWMs were mature, proven, and cheap. Switching to digital added to the overall system cost.

Fig. 1. The above power management controller block diagram meets today's demands to shift to digital power solutions.

Now, though, the cost equation has shifted. When system intelligence, fault processing, and other programmability features are considered, analog ends up being more expensive than digital. For markets like the datacom, analog solutions and their corresponding extra chips can’t cope with the space constraints either.

Switch to digital?

As with many new technologies, there is resistance to change. That is natural, and resistance will erode over time. Unfortunately, beyond the natural resistance to change, digital power solutions are also plagued by two key misconceptions.

The first misconception, as discussed above, centers on cost. This misconception will take care of itself as developers weigh their options and realize they can now get all of the benefits of digital power management for the same price as analog.

The second misconception is more vexing and presents the most significant obstacle for broader adoption of digital power solutions. This is the mistaken idea that digital solutions are so complex that they require much investment in training and design cycle time to customize them to specific applications. Is the design time investment worth the effort?

It could be argued that it is, but, fortunately, designers needn’t make that choice. Digital power management solutions have emerged that shield the designers from the underlying complexity of the solution.

Just as a PC user needn’t know the underlying code in order to use an application, neither do power designers need to know how to program digital power solutions to reap their rewards. This follows a typical trend in technology adoption.

The early adopters tend to be technical experts, eager to tinker under the hood. For mass adoption to happen, though, those complexities must be abstracted away via better and more intuitive user interfaces.

The latest generation digital power IC sheds light on the black box of digital power. It communicates bidirectionally with the processors, ASICs, and microprocessors coordinating power at a motherboard-level to rack level of power management (see Fig. 2 ).

Fig. 2. The latest-generation of digital power IC communicates bidirectionally with the processors, ASICs, and microprocessors that are coordinating power at a motherboard level to rack level of power management.

Once you digitize the analog information, you can multiply it, divide it, add it, communicate it, compensate it, filter it, and store each piece of information. Nonlinear and asynchronous algorithms can be implemented into these digital power ICs for improved transient behavior since digital algorithms define the overall chip specification and performance.

Since all the data has been digitized, the information is easily communicated on an I2 C bus, and the technology is easily adaptable to high-speed communication buses of the futurewhere one might require a 33- or 66-MHz bus speed. These types of architectural functions create an opportunity for digital power ICs to excel in the coming decade by meeting these complex system requirements.

GUI

Digital power IC solutions also provide flexibility to the system designer. The compensation, set point, OCP level, OVP level and overall key parameters of a power supply design can be completely defined in software.

Through an intuitive GUI, designers can optimize a specific design for the output capacitance and inductors without changing resistors or capacitors on the board. No discrete component changes are required for the design implementation of the power delivery. The end result is not only better system flexibility, but also faster time-to-market for end products.

In addition, once the products are deployed, calibration for component aging and temperature drift can be implemented. The personalization of each voltage rail can be uniquely designed and programmed within the digital power IC.

The digital power IC can easily be implemented into multiple phases and paralleled operation with synchronous timing of phases so that EMI signatures are reduced for easier filtering. The rms input currents are reduced requiring less input capacitance. The flexibility and personalization are again accomplished in software not in hardware.

The key to all of the above features is an intuitive GUI. Now, even a designer unfamiliar with digital power management can easily program the output voltage, current, faults, PID coefficients for loop compensation and other key functions by stepping through screens of various choices.

A designer can program a whole group of voltage rails that would reside on a motherboard so that the timing, tracking, rise times, and programmed delays of each voltage rail are defined uniquely for that system board. The digital power IC thus gives one not only the PWM control, but also provides complete power management on a multiple voltage rail board with real-time communication to the system.

The addition of a GUI may seem obvious, but it is not trivial. Just as better interfaces spur the adoption of Linux, so too do intuitive GUIs take the design time element out of the cost equation for digital power management. Digital power ICs are not only the advanced choice with today’s generation of chips, but also the most cost-effective. ■

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