Low-Voltage MCU Exploits a Battery of Power Tricks

Apr 1, 2008 12:00 PM
By David Morrison, Editor in Chief

By applying clever power-management techniques on-chip, a family of 8-bit microcontrollers (MCUs) from Silicon Laboratories is able to operate at voltages down to 0.9 V. With this capability, members of the C8051F9xx family become the first MCUs to run directly off of a single battery cell (alkaline or other type), according to the company. Silicon Labs also claims that best-in-class power efficiency for these devices enables the MCUs to provide double the battery life when operated off of two cells in parallel. That improvement is versus existing MCUs that operate off of two cells in series.

The C8051F9xx targets a variety of portable consumer, medical and industrial applications with space constraints and demand for longer run-time. In these applications, the ability to operate off a 0.9-V to 3.6-V supply provides flexibility in the designer's choice of batteries versus existing 8-bit MCUs that may require 1.8 V or higher for the supply.

These applications also may benefit from the C8051F9xx's fast wake-up times, which enable the MCUs to spend more time in sleep mode where typical current consumption is less than 50 nA. These MCUs can wake up from sleep mode operating at 25 MIPS and be ready to make a 10-bit ADC measurement within 2 µs. This is possible because the on-chip bandgap reference settles in less than 1.7 µs, which the company claims is four orders of magnitude faster than competing MCUs. Meanwhile, in active mode, the C8051F9xx's power-efficient architecture consumes as little as 170 µA/MHz.

The key to the MCUs' low-voltage operation is an integrated dc-dc boost converter, which is innovative by virtue of its being small enough to fit on-chip (this MCU is built in 0.18-µm CMOS). This approach saves space versus existing designs that use an external boost converter. Additionally, having the dc-dc converter on-chip means it does not need to operate continuously since its wake-up can be coordinated with other MCU functions.

The boost converter, which requires a 680-nH external inductor, supplies a total of 65 mW for internal as well as external use. The converter is also optimized for MCU-level load currents — an operating region over which many stand-alone boost converters have poor efficiency. With load currents in the 5-mA to 25-mA range, and the battery voltage in the 0.9-V to 1.5-V range, the C8051F9xx's boost converter efficiency ranges from approximately 75% to 87%.

The MCU core requires a 1.7-V supply, which can be supplied by the boost converter. Alternatively, this converter can be programmed to deliver an adjustable 1.8-V to 3.3-V output for powering external devices. In that case, the output of the boost converter can be fed to the MCU's on-chip LDO to generate the core voltage. The same LDO powers the core when battery voltage is above 1.7 V (see the figure).

In addition to its power-management functions, the C8051F9xx features 64 kbytes of Flash and 4 kbytes of RAM (the C8051F93x) within a 4-mm × 4-mm QFN package, providing customers with increased memory for data logging. Alternatively, the MCUs are available with 32 kbytes of Flash (the C8051F92x). The MCUs also feature the aforementioned 10-bit, 300-ksample/s ADC and a smaRTClock oscillator.

This clock allows wake-up times to be adjusted at intervals up to 36 hours. Pricing for the C8051F93x and the C8051F92x starts at $1.99 each in quantities of 10,000. For samples and development tools, see www.silabs.com/point9.

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