Electronic Design

Crossing The 3-V DSP Barrier Means Bending The Rules

Technology Won't Let You Build A True 1-V-DC Battery-Operated Product, But Lower-Voltage Design Offers Interesting Possibilities.

Dick Tracy's wrist radio was always the goal for consumer products to meet. In fact, the semiconductor industry has already delivered Tracy's radio—and added a clock, alarm, calendar, and pager to it, too. The challenges faced in making this and a host of other tiny consumer products are strewn with tradeoffs in processing power, information storage, size, weight, and features. All of this effort goes to achieving the final goal—optimum battery life.

Right now, typical battery-operated consumer devices operate from two AA or AAA batteries. A major leap forward will occur when these devices no longer require two batteries, but can employ a single cell to deliver acceptable operating times. To achieve this, DSPs, nonvolatile memory, digital-to-analog and analog-to-digital converters, power-management devices, LCDs, and mixed-signal devices must ultimately operate at lower voltages.

In order to select the appropriate battery system, it's important to understand the environment, requirements, and operating conditions under which the system will be used: disposable (primary) or rechargeable. Consider how often the device will be used, whether time will be available to recharge it, and if the device will be left idle for long periods. All of these issues impact the battery-technology selection. Rechargeable batteries have the advantage of a long service life, but they're more expensive. There's also the inconvenience of recharging and the fact that they have a lower capacity than same-sized alkaline batteries. That is, in a given application a fully recharged battery will not last as long as the same size alkaline battery.

As a result, most portable consumer products utilize AA or AAA alkaline batteries. They offer the best tradeoff in terms of high availability, low cost, and large energy storage capacity. And unlike NiCd batteries, which discharge fairly rapidly, an alkaline battery retains its energy when not used for long periods. The wide availability of alkaline batteries is a tremendous convenience for users, but their cylindrical shape limits the size of the portable product in which they can be used. Table 1 outlines most of the common battery technologies available today.

A Series Of Tradeoffs
Battery-operated products in which all components operate at 1 V dc or below aren't yet achievable. Several 1-V-dc, state-of-the-art mixed signal, DSP, and SRAM devices have been presented in various technical conferences and publications such as ISSCC, JSSC, and the VLSI Symposium. But none are yet in production. It will be 2000 before the first 1-V-dc DSPs are offered, and later for low-voltage, precision ADCs. Devices currently are available, however, for designing a low-power, 1.8-V-dc system (Table 2).

Designers creating products today are faced with prudently combining components with differing voltage requirements to achieve the best combination of form, function, and power consumption. The key to navigating this maze is to gain as much information as possible about the technologies to be employed.

Most current battery-powered consumer products operate at 3 V. This "3-V barrier" has been broken by some components (most notably DSPs and SRAMs). But it remains the standard because the majority of MCUs, solid-state storage media, and analog circuits operate at this voltage. A few of the hottest products in the consumer marketplace, like solid-state audio players and pagers, operate from a single AA cell. But they're not true 1-V systems. They use step-up regulators to increase the voltage to accommodate some of their components.

In a solid-state audio player, for example, the biggest obstacles to breaking the 3-V barrier are the storage medium (CompactFlash, MultiMedia, SmartMedia, or MediaStick) and the LCD display. Both operate at 3 V. Other applications, such as medical sensors and pagers, may reduce the overall voltage to 1.8 V depending on their ADC, storage, and display requirements. Stepping up voltage increases circuit complexity through the addition of regulators. Due to their conversion inefficiency, these regulators increase power dissipation.

Two-Cell Battery Systems
A two-cell battery system provides a voltage range of 1.8 to 3.0 V. Each battery delivers from 1.5 V when new to 0.9 V at the end of its operating life. If the system is designed to operate at 1.8 V, the designer can choose to have no regulation. That requires a tradeoff in power dissipation, because the system operates at higher voltages. The other choice is regulating the system to operate at 1.8 V all the time. This approach is the most common, considering that analog components require a clean, unvarying supply voltage to achieve their rated performance.

Digital devices are far less challenging in this regard than their analog counterparts. The digital parts can tolerate a varying voltage supply at the expense of higher current consumption at higher voltages. In contrast, analog components are usually internally regulated to maintain their supply-current consumption across supply voltage variations. Or, in the case of regulators, their current consumption increases as the operating voltage decreases.

A low-dropout (LDO) voltage regulator is usually the choice for powering analog circuits, with their need for clean, stable power. The DSP and other digital devices aren't as sensitive to voltage stability and quality, and require no LDO regulator. But a step-down regulator may be used to maintain a constant 1.8 V from the varying battery supply. Because they're more power-efficient than the LDO regulator (90% versus 75%), these regulators are preferred for digital circuitry.

In most cases, the operating voltage is determined by the accuracy required of the ADCs. Sensor systems with 8- or 10-bit resolution and lower sample rates, such as those in medical-monitoring devices, can use 1.8-V converters. But if audio-quality converters are required, today's component availability raises the minimum voltage to 2.5 V.

In a single-cell system, a step-up regulator is needed to achieve the 1.8 or 3 V required by many components. This doubling and often tripling of the voltage increases the parts count and power dissipation. And it usually requires further regulation for the analog components.

Incremental reductions in power dissipation and parts count can be obtained, however, if the 0.9-V digital components receive their operating voltage directly from the battery at 0.9 to 1.5 V. There is a problem of higher power consumption in the digital components, though, because of the battery's range from the start until the end of life. Compare this extra power consumption to the power savings provided by eliminating an additional regulator.

Since alkaline-battery discharge curves are nonlinear, a single-cell system will discharge twice as fast as a two-cell system. For a typical application, a system using a single AAA cell will deliver only about 35% of the operating time of a two-AAA-cell system. So battery life with a single cell can become a significant limiting factor, depending on the application.

The most common method of reducing power consumption in portable products is through system partitioning, which lets individual components and functional blocks be powered down when not in use. Entire ICs or individual functions can be powered up or down under software control.

A DSP waiting to load data can be powered down in sections. For example, the CPU can be powered down while the serial ports continue collecting the data. The headphone amplifiers, keypad, and other analog circuits also can be placed in standby mode when not in use. In a medical-monitoring system, many functions can be powered down or placed in standby during the various stages of sample collection and processing and data storage or transmission.

As much as any semiconductor device, battery technology greatly determines the performance achieved by consumer products. As mentioned earlier, the vast majority of battery-operated products rely on alkaline AA or AAA cells. Recent advancements by Duracell and Energizer have extended the operating life of alkaline batteries by as much as 30% when used in some products.

In the rechargeable area, NiCd batteries are slowly being replaced by nickel-metal-hydride (NiMH) and lithium-ion technologies. These newer entrants have greater power densities and are lighter. Plus, they eliminate the two most undesirable NiCd characteristics: the "memory" effect and the use of heavy metals.

The latest battery technology to reach the market is lithium polymer. It's championed by Matsushita (among others), which introduced its first commercial products in January 1999. These batteries are very thin (Matsushita's is 3.6 mm thick) and weigh about 15 g. They have a capacity of 500 mAh and a minimum lifetime of 500 charge cycles.

Even more significant, the lithium-polymer battery operates at 3.7 V, which has the potential to somewhat change the portable-product design scenario. Instead of operating from a lower voltage source, products can be manufactured to operate from 3 V. Voltages could be lowered by using highly efficient regulators to power digital circuitry. But the AA alkaline battery still enjoys a considerable advantage in cost over rechargeables in most applications, and the future of lithium-polymer technology depends on its acceptance by major manufacturers.

To illustrate the issues involved, a 1.8-V development platform was created based on TI's TMS320C54x fixed-point DSP family. This platform can be used as a broad-based tool for multiple applications. The problems and solutions encountered in its design represent those likely to be found in other low-power products, as well.

From the beginning, the goal was to provide a flexible platform for low-power operation. But due to analog-component availability, some targeted applications prevented the use of a pure 1.8 V dc. To accommodate the different voltage supplies required for those applications, the system was designed to allow the user to select up to three voltage supplies: 1.8, 2.5, and 3 V.

This system is flexible enough to provide either single-supply-voltage operation or a combination of two supply voltages. For example, it could run at a single supply of 1.8 V or with dual 1.8- and 3.3-V supplies. The parts list with voltage and current characteristics is shown in Table 3 and the block diagram in Figure 1. To support these multiple-voltage modes, the selected dc-dc converter can provide either a 1.8- or 3-V stepped-up voltage by a simple change in the external resistor network. The LDO regulator's network (included in the same part as the dc-dc converter) is similarly designed to provide either a 1.8- or 2.5-V regulated output via a resistor-network modification.

To allow flexibility and easy interface to buttons, daughtercards, and Compact Flash, a "zero-power" CPLD was chosen. This CPLD dissipates very little power while operating and allows the system to be reconfigured for many applications without board modifications. Since the goal was to produce a broad-based development platform, the extra expense and small additional power consumption of the CPLD was a great tradeoff—especially compared to extensive board modifications or the extra power consumed by an FPGA (Fig. 2). For a dedicated application, the system cost and power could be optimized by utilizing low-power discrete glue logic (NAND gates, flip-flops, etc.) that currently operates from 0.9 to 3 V.

Think about how the development platform would be configured for both solid-state audio and medical-monitoring systems. The major requirements for solid-state audio are the need for an audio-quality codec, a standard flash-based storage media, and an LCD readout. The solid-state media and LCD require 3 V, and the audio codec requires 2.5 to 3 V. The other components required for this application are available at lower voltages. But to simplify the interface logic, a 3-V supply is utilized for all I/O.

For example, although the boot flash is available at 1.8 V, a 3-V device is selected. Because it shares the parallel bus with the solid-state media, it simplifies the interface logic. The headphone amplifier operates from 1.8 to 3 V. But again, its supply voltage is set to 3 V because it interfaces directly to the data generated by the 3-V codec. The DSP core operates at 1.8 V and its I/O is set at 3 V.

Surprisingly, the choice of regulators was quite limited due to low availability of components offering dc-to-dc conversion and regulation in the 200- to 300-mA range. Still, many devices were available for operation below 100 mA and from 500 mA to 1 A.

In contrast, a medical-monitoring device requires much lower sample rates and precision solid-state audio. To support this application, a daughtercard containing low-noise preamplifiers, a low-voltage LCD readout, and 1.8-V, 8-bit codecs to sample data from sensors is added to the system. As it provides sufficient capacity, a 1.8-V version of the boot flash is used as the storage media. The motherboard codec, headphone amplifier, and CPLD aren't populated. This allows the whole system to operate from a single 1.8-V supply.

Coming Soon...
By early 2000, advances in semiconductor technology will enable DSP cores to operate at 1 V and companion analog components to operate at 1.8 V. Eventually, 1-V analog components will follow, probably using internal step-up regulators. This will enable DSPs to lower their I/O voltages to 1 V. Since power dissipation is related to the load capacitance, voltage swing, and frequency of operation, this will further reduce the power consumption of the complete system.

Of course, regulators will still need to increase voltages for many other devices, such as LCDs, storage media, and interface circuitry to PCs. As a result, until a true 1-V product is achievable, designers must make the best use of current technology to accomplish their goals.

TAGS: Digital ICs
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