[Design Application]
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.
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