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Ultra-low power system for wearable electronics

25 Feb 2015  | Chris Glaser

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While the brains of the typical wearable gadget might be the embedded microcontroller (MCU), the heart is definitely with power management. Extremely small capacity batteries, diverse array of functions needing power, and the incredibly small solution size force new and innovative power management solutions to make the system work well. But when an ultra-low power optimised MCU and ultra-low power optimised DC/DC converter come together, the result is a well-running, well-oiled machine fit for wearable applications.

Wearable systems
A wearable device brings together multiple facets of engineering, beginning with the MCU and its integrated features and peripherals. Temperature sensors, analogue-to-digital converters (ADCs), display drivers, a Bluetooth Low Energy (BLE) radio, and even encryption are frequently integrated into the MCU. Other sensors such as accelerometers or pressure sensors are usually implemented discretely due to their system-specific nature. The MCU and sensors define the features and capabilities of the wearable device, which gives it its appeal and niche in the market.

For a very small wearable device, the heart of the system is the power management. A wearable device can lose its appeal if it must be recharged multiple times a day or has a heavy battery pack. Achieving multi-day run times and keeping the device small and light requires ultra-low power-optimised power management to efficiently convert the battery's limited energy to useable power by the loads. Figure 1 shows a typical block diagram for an optical heart rate monitor with the MCU, sensors, BLE radio, power management, and battery.

Figure 1: Optical heart rate monitor block diagram with MCU and power supply. The MCU integrates most of the required functions in a typical wearable device. The power supply integrates all the required components for a complete DC/DC converter solution. Included is a load switch for simpler and smaller system integration.

Ultra-low power microcontroller
While the MCU must provide a diverse array of functions for the wearable device, it must not consume too much power. Advanced low-power modes are needed to efficiently use each Coulomb of battery energy. Especially important is ultra-low power consumption in sleep or standby mode, as wearable devices operate in this mode with no user interaction for much of their operating time. Drawing hundreds ofµA or even a single mA in sleep mode is simply too much power consumption for the battery. On the contrary, the active-mode power consumption of the MCU should be in this range. A different type of MCU is needed for such low-power consumption – one that is optimised for ultra-low power applications.

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