Mobile Computing Power Management

Getting from 5 volts to 3 volts and less

by Thomas W. Starnes

Starnes is manager of Motorola’s 68300 marketing and applications team. He can be contacted through Candace Fitzgerald of Cunningham Communications at 617-494-8202.

The two issues that consistently come to the forefront in portable computing power management discussions are battery life and weight. While both of these challenges must be met, there remain a number of more subtle issues which system designers must address before mobile computing can truly achieve its potential. Among the factors which come into play are: heat, operating voltage, general power consumption, functionality, weight, size, and clock speed.

Thermal Factors

Each watt of power consumed generates a watt of heat which must be dissipated. In systems where high heat is generated, thermal management must be factored in. Heat must be drawn from the chip quickly enough to maintain a reasonable operating temperature on the silicon. This requires adequate package design (more expensive and heavy Pin Grid Array, “PGA”), and sometimes add-on heat sinks.

Heat sinks occupy space which may limit the distance between boards in a backplane. Dissipated heat must also be drawn away from other components to keep them in reasonable operating temperature range. This is usually accomplished with fans. As mechanical devices, fans are bulky, electronically noisy, and susceptible to physical shock and reliability problems. While desktop PC users are willing to put up with the additional motherboard real estate heat sinks demand and the noise and bulk of fans, mobile computer users will not afford system designers these luxuries.

Low Voltage

Lowering the operating voltage of an electronic system has a very direct affect on power consumption/heat dissipation. If the internal resistance in a chip is assumed to be relatively stable within the voltage range, it can be shown that a reduction of voltage from 5.0v to 3.3v gives a 55 percent reduction in the power consumed.

3.3 volts has some additional advantages in battery-powered devices: battery operation is typically used in portable products. Portability is defined in part by the size and weight of the device. Batteries contribute both volume and weight. Each battery eliminated makes the product more portable. A good example is Sony’s Walkman, which showed that a huge market can be created by radically shrinking a product — even in a fairly saturated existing market. The wireless residential telephone provides another example: people can talk on the phone anywhere in the house, not just within 10 feet of the telephone jack. And portable computers enable people to write letters and work on spreadsheets on airplanes, on the couch, or in the backyard.

Primary batteries (dry cells available in supermarkets) pack 1.5v of power. Secondary batteries (such as rechargeable Ni-Cads) are generally thought to be replacements for dry cells, although they actually deliver only 1.2v (and 7.2v for the 9v variety). Both types of batteries lose power quickly once they drain down to about 0.9v. This, then, becomes the worst case for which a circuit must be designed.

For instance, to supply a 5-volt/5 percent (4.75v) electronic circuit from batteries, five batteries are actually required (5×0.9=4.5v, which is slightly less than the needed 4.75v, but still allows the components to pull 92 percent of the usable energy from the batteries).

A more attractive approach would be to use just three batteries which would supply from 2.7 to 3.6 volts (or 4.5v for dry cells), depending upon the batteries’ state of discharge. This would imply that a circuit requiring 3.3v— 0.3v is just about right, though a 3.0v — 10 percent is significantly better.

The ideal component for a 3-battery system would have an operating range of 4.5v down to 2.7v, allowing the computer to operate consistently for the maximum possible time without the need for voltage limiting circuitry.

Overall power consumption determines battery life. Since user behavior and the demands on the electronics will vary, one excellent way to extend battery life is to minimize use of circuitry during idle periods and to slow down the operating frequency when demands are low. Each portion of circuitry which is not drawing current, leaves that power for later use. For example, Motorola’s M68300 processors allow software to adjust operating frequency and to turn off selected peripherals to fine tune power consumption against performance needs, enhancing battery life.

Other Considerations

Each system design has its own power considerations. Some of these could be changed dynamically during operation with the addition of control circuits external to the processor chip. Higher capacitive loading of the address, data, and control buses of the processor causes higher current consumption to drive the buses. Fanout, signal length, termination, ground plane, and board design all contribute to capacitive loading. The transistor junction temperature of the silicon chip affects power consumption. In fact, as power consumption rises, so does the heat generated by the chip, which can raise the chip’s temperature.

Specific Low-voltage Applications

There are many different types of applications for low voltage circuits with high functional integration. As more designers become aware of the benefits of the lower power consumption, more creative applications will emerge.

Certainly, smaller, lighter laptop computers benefit greatly from lower voltage components. Completely new innovations in these devices are being designed with the advent of more powerful, lower voltage processors and components. Pen-based computers, voice response computers, and powerful personal data communicators will make the mobile office more versatile (and attractive) than ever.

There are myriad applications in diverse markets that are using lower voltage devices effectively. For example, global positioning systems (GPS) are becoming more popular and more useful. The Desert Storm military operation made excellent use of hand-held GPS receivers to help soldiers pinpoint their locations in the featureless desert of the Persian Gulf. Here, small size and battery operation become paramount features to keep from overburdening the troops with heavy equipment.

In addition, foresters are precisely locating stands of timber, and staking out disease and insect infestations with the help of GPS receivers. At the consumer level, hikers and hunters can use GPS to re-orient themselves when they lose their bearings in the wild. None of these outdoorsmen want to lug bulky, weighty equipment (and spare batteries) on their backs, and new smaller models of GPS receivers running for a longer time on fewer batteries eliminates the need.

Applications such as bar code readers and inventory control instruments benefit greatly from lighter weight. Already, smaller, more accurate instruments are being carried by large numbers of stock clerks in grocery stores to monitor shelf supplies.

The Downside

The capacitive loading of internal circuits and external busses is essentially the same on a 5v- and 3v-system. Charging up these capacitors with low voltage outputs takes more time than with 5 volts, leading to more delays in signal transitions. This leads to components that may run 25 MHz at 5 volts not being able to run more than 16 MHz or even 12.5 MHz at 3v levels. Often this is an acceptable trade-off for portability and battery weight and space, but designers can always use a little more headroom.

Another frustration when using low voltage parts can be the scarcity of components available. Certain interface parts, some memories, and a few microprocessors are available to run at low voltage, but it is likely that a designer’s first choice for a component is unavailable. Time will correct this, but for now we live in a 5v world, and there are thousands of devices to be re-characterized or redesigned.

For example, one way Motorola uses to minimize the impact of few available 3v components is to use components which are most highly integrated. The fewer total components that are needed, the less likely it is that the desired one is unavailable at low voltage. Finding a few memory chips, an integrated processor, and a couple of interface devices that run at 3 volts is easier than hunting up dozens of peripheral, support, and logic chips that all run 3 volts.

Each part in the 68300 family contains a 68000-compatible microprocessor, a number of peripherals, and most of the typical glue logic used in a set of applications. Therefore, with a 3.3v 68340V at the heart of the design containing the bulk of the functional parts needed, only the memory and their support chips, and a very few SSI (small scale integration) chips would have to be found that operate at 3.3 volts.

Interfacing with 3.3v circuits raises many issues. Achieving TTL compatibility is most desirable. TTL standards dictate that a high (or logical one) output is indicated by a signal driven to 2.4 volts. When the supply voltage is near 5v, this is much easier than when the supply voltage is 3.3v, which is only 0.9v above the desired level. The result is that signal transitions take longer. This becomes even more severe for 3.0 –10 percent supply voltages. The worst case here is only a 0.3v difference. This is such a low noise margin that very careful system design is required.

On the other hand, most electronic circuits actually pull their high outputs up close to the supply voltage level, rather than stopping at just 2.4 volts. Thus the high TTL signal is typically over 4.8 volts, double what it need be. In a 3.3v system, the high output ends up over 3 volts. This lower high radiates less EMF (electro-magnetic force) noise in the system. This can be a major advantage in allowing more densely-packed PCB (printed circuit boards) traces with lower crosstalk — a definite advantage in small systems. It also means less noise is being generated to interfere with the nearby TV set, which may eliminate space and weight for shielding, while easing FCC certification of the final product — which is often the final hurdle that delays a new product’s introduction.

Conclusions

Trends in electronic design have been moving to smaller form factors since the invention of the first transistor. In the last five years, the world has seen truly portable designs of applications never before considered possible or practical.

Lighter, smaller, and longer lasting products will continue to create and open major new markets in electronics. These applications will be made possible by electronic components and by careful system design operating at ever lower voltage supplies. As soon as engineers resolve the challenges at the 3v to 3.3v level, they will have to struggle with similar problems at the 2.7v and later 1.2v levels.