Radio Architecture Takes Less than 1 Microamp
Low-power device aimed at implant devices
Many of today's implant devices with integrated sensors use inductive techniques to communicate data with external circuitry. This requires holding an activating device in very close proximity to the body, typically pressed against the skin. The communication data rate is low, just a few kbits/sec, so the diagnostic tool has to be held against the patient for a long time to completely receive the data. Power consumption is critical in these applications, since replacing the battery requires a surgical procedure. Typical battery life ranges from five to 10 years.
To improve the performance and put an active radio in the body, power consumption is one of the primary considerations. Using technology derived from paging and ultra-low power automation devices, Cambridge Consultants designed its SubQore architecture to have an average current of less than 1 µA, and less than 1.7 mA peak, for 0.05 percent duty-cycle, 400 kbits/sec bidirectional communications. In contrast, even the newest "standard" wireless protocol, ZigBee, typically draws 20 mA. Engineers estimate the units will deliver more than 10 years of useful battery life from a lithium-ion cell.
Matching the Application
The SubQore architecture takes full advantage of the unique requirements of medical implants. In a typical application, another wireless communication device, an interrogator, is located within 6 ft of the patient. To reduce power consumption, the link budget can be asymmetric, so all the complexity and associated higher power can be handled in the external wireless unit. The wake-up scheme minimizes current draw for even further battery life.
Operating in the Medical Implant Communications Service (MICS) frequency range of 402-405 MHz, the architecture can handle several different data rates. At the high end, 400 kbits/sec, the unit consumes more power but the typical application would be a swallowed video probe. A more common application, such as a pacemaker, would be 20 to 100 kbits/sec. The architecture's design facilitates manufacturing as a single chip in a chip-scale package or as a directly bonded chip to a substrate with only a few millimeters footprint.
A Mere Microamp: Using BiCMOS IC technology, SubQore exploits an ultra-lean radio architecture and advanced power management and wakeup schemes to minimize power consumption. A low power variation of Cambridge Consultants' 16-bit RISC processor core, called XAP, provides the computing portion of the architecture.
CONTACT: Richard Traherne, Cambridge Consultants Tel: +44 (0)1223 420024; e-mail: Richard.Traherne@CambridgeConsultants.comhttp://rbi.ims/ca/4391-507
SMALL MICROCONTROLLERS OPEN NEW APPLICATIONS
The digital control can displace analog circuitry
Breaking with tradition can be a difficult decision for an analog designer, but one that should get easier as microcontrollers (MCUs) get smaller and cheaper. To add control circuitry to a traditionally analog system, the analog engineer would normally look for the best-fit analog integrated circuit or design the circuit with discrete components. A microcontroller would not be an option. However, with its flexibility and reprogrammability, the MCU can provide a number of advantages. For example, a microcontroller could be an attractive alternative for power-up sequencing for a switching power supply.
The programming of the MCU is an additional step and a trade-off to calculating the resistor values in the analog design. However, the MCU is digital instead of analog, and its logic circuitry provides three distinct advantages. The most obvious advantage is programming with software instead of resistors. The software does not take additional board space and can easily be changed. The second is the MCU's well-known timing capability for sequencing events. Timing control with the MCU's clock and counter is easier than with a timing ramp in an analog system. Finally, an MCU is deterministic, a state machine, so there is much greater control for sequencing steps with if-then decision making. In contrast, in the analog design, component tolerances, voltage swings, and temperature variations can provide uncertainty.
The sequencing control for the switching power supply would require less than 256 instructions to replace the resistors in the analog design. Today, a SOT-23, 6-pin MCU has from 256 to 512 words of Flash memory, 16 to 24 bytes of SDRAM, an 8-bit timer, comparator, and four I/O channels to perform the sequencing function. The factory-calibrated internal 4 MHz oscillator's precision of ±1 percent is sufficient for many applications.
Once the digital approach is understood, revising the program for an upgrade or a new application is rather straightforward. This avoids changing, buying, stocking, and placing new resistors. In general, the MCU requires fewer resistors and external components, saving board space and the placement cost for those components. Equally important, other functions can easily be added with software. For example, another microcontroller-based function for a switching power supply is a Soft-Start Controller circuit. The code for this digital design has already been developed.
Digital Replacement: By replacing the analog block in the sequencing control for a switching power supply with a digital block, a small microcontroller, the same function and more can be performed in a footprint as small as a 6-pin SOT-23 package (2.8 x 2.95 mm). With the MCU, changing the performance or adding other functionality is as simple as writing new code.
CONTACT: Fanie Duvenhage, Microchip Technology Inc. Tel: 480-792-7987; e-mail: firstname.lastname@example.org For info on the Soft-Start Controller circuit: http://rbi.ims.ca/4391-508