Electronic Design

Ultra-Portables Bank On Power-Frugal Components

A new breed of components is emerging, significantly reducing power and in turn paving the way for ultra-portable—and ultra-low-power—systems. Needless to say, manufacturers will now look at other products in this vein.

Power consumption is the most important design challenge for ultra-portable devices that operate from a limited energy source, such as a lithium-ion battery, according to Ken Marasco, system applications manager at Analog Devices. This becomes even more important as the use of high-speed microprocessor-based portable systems increases due to the continued demand for computation power, says Marasco.

THE POWER OF THE PORTABLE • Portable devices now have the processing power of a year 2000 desktop computer. This requires very high clock speeds to run the complex application-specific software in a variety of ultra-portable devices, including smart phones, portable media players, digital still cameras, and personal navigation systems. Today, portable medical devices can relay data directly to doctors using a wireless Internet connection. Power systems must be designed from the ground up using a blend of hardware and software power-saving techniques.

Marasco also notes that as ultra-portable devices continue their rapid evolution, large-sized power-management ICs (PMICs) cannot keep up with the system’s power requirements. That’s because additional functions require a long design cycle, which in some cases exceed the end-product design cycle and delay product release. It’s easier to bolt on the additional point-of-load power when required than completely redesign the PMIC, resulting in faster time-to-market for derivative platforms.

The system architect has to design a system with long autonomy for consumer acceptance, meaning the system will operate from the battery before requiring recharging, says Marasco. However, powerful applications require powerhungry processors, which reduce autonomy.

Balancing power consumption and battery life is a key consideration for portable system design. Decreasing processor speed reduces power consumption, extending the battery runtime and lowering the software performance. The system architect must find a way to minimize power consumption while still meeting critical performance requirements.

Power-consuming microprocessors and microcontrollers are prime targets for obtaining more efficient electronic systems. Some of the newer devices operate at lower voltages and dissipate less power, but they have to trade off processing power for operating power. It appears that operating power still needs some work. One interesting approach uses software to reduce processor power when it’s inactive.

To address low-power microcontroller design, Tiempo recently announced its 16-bit TAM16 microcontroller core intellectual property (IP) on a CMOS 130-nm general-purpose process. The chip logic has been entirely designed in the company’s asynchronous and delay-insensitive technology.

The 16-bit microcontroller core offers a power-efficient instruction set, with fast, energy-efficient, and easy-to-program interrupt management and peripheral communications, suiting it for ultra-low-power embedded electronics. The core also integrates various peripherals, such as an interrupt controller, UART, and cascadable timers, and is interfaced to standard ROM (including BIST) and RAM.

The chip core consumes down to 37 µA per MIPS when operating at 0.7 V (47 µA at 1.2 V), including leakage current. Power consumption of instructions that include communication with peripherals and memories—and that typically require two or three clock cycles with a synchronous microcontroller—was measured at 61 µA, i.e., less than 25 µA per (equivalent) MHz. And, the microcontroller instantaneously falls into sleep mode when no activity is detected and wakes up in a few nanoseconds when activity resumes.

Targeted applications are ultra-low-power chips for embedded electronics, e.g., power-management chips, sensor networks, metering devices, RFID, and smartcards. It also suits chips that must operate with low electro-magnetic emissions, such as electronics for the automotive and medical industries.

POWER-SUPPLY EFFICIENCY • One of the more difficult design issues is developing power supplies with higher efficiency, particularly when powering microprocessors and microcontrollers. The multiphase converter has helped in that regard, but its present power MOSFETs still consume too much system power.

Lower on-resistance, thanks to packaging advances made in recent years by Fairchild, International Rectifier, and Vishay Siliconix, has helped reduce power consumption. However, there’s still room for efficiency improvement in power MOSFET technology. One possibility lies in MOSFETs that use materials other than silicon.

Another component that consumes relatively high power is the LED used to backlight LCDs. For example, a highly integrated solution like the ADP5520 LED lighting management system from Analog Devices can improve system efficiency by offloading tasks from the associated microprocessor.

The ADP5520 is a single-chip, white LED backlight driver with a user-configurable I/O expander (see the figure). It fits handsets where the flip or slider section of the phone needs backlighting, I/O signaling and detecting, auxiliary LED lighting, and keypad functions. By incorporating an I2C-compatible serial interface and a single line interrupt, the device significantly reduces the total number of lines required to interface with the baseband processor across the hinge flex.

To extend battery operation, the ADP5520 can detect ambient light levels and adjust the backlight brightness accordingly. Once configured, the chip can control the flip/ slider backlight intensity, on/off timing, dimming, and fading without the intervention of the main processor, saving valuable battery power.

Another low-power aid available today, the ambient light sensor (ALS), further reduces backlight power consumption by lowering the LED backlight intensity when used in an office or dark environments, increasing system autonomy. For instance, Toshiba America Electronic Components’ TPS859 ultra-compact photo-IC ALS incorporates a photodiode, a current amplifier, and a luminous-efficiency correction (LEC) function in a single chip that’s ideal for use in flatpanel displays. It also can be used to turn a keypad or LCD backlight on or off or adjust the brightness according to the ambient light condition.

The TPS859 provides ultra-high sensitivity with light current (IL) of 230 µA compared with 40 µA in the previousgeneration TPS852—an almost 6× improvement. The device also employs a new optical filtering technology that achieves luminous efficiency near to that of the human eye. The filter can reduce the ratio of incandescent to fluorescent light sensitivity from 1.2 (typical) in the TPS852 to 1.0 (typical) for the TPS859.

These approaches can reduce power consumption today. Future prospects, though, will provide major power reductions involving new circuits, topologies, and components.

For more, see “MIT, TI Develop Energy-Efficient Microchip” at www.electronicdesign.com, ED Online 20429.

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