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Ultra-low power system for wearable devices

Technology News |
By eeNews Europe


While the brains of the typical wearable device 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 optimized MCU and ultra-low power optimized 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, analog-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-optimized 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 of µA 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 optimized for ultra-low power applications.
 
Figure 2 shows an example of an MCU which integrates the temperature sensor, differential ADC, and encryption, yet draws only 450 nA in standby mode while consuming just 100 µA/MHz in active mode.[1] The integrated FRAM memory (up to 64 KB) enables such an ultra-low standby power consumption, while the package size of down to 2 x 2 mm enables an easy fit into any wearable device.

Figure 2. A block diagram of the MCU (MSP430FR59xx) highlights the differential ADC and encryption, which are needed for a wearable application. The temperature sensor is integrated into the ADC.

Ultra-low power DC/DC converter
 
The DC/DC converter is responsible for converting the battery’s varying voltage and precious energy to the proper voltages required by the various devices in the system. Some sub-systems such as the radio require low noise. Other sub-systems such as sensors may have higher shutdown leakage and need to be disconnected from their supply rail instead of merely being turned off. For all sub-systems in a wearable device, low-current consumption is a must. Therefore, the DC/DC converter must be optimized for ultra-low power operation in order to efficiently power the system. It must remain efficient, even at a very low output power.

Figure 3 shows an example of an ultra-low power DC/DC converter and its system implementation. A common voltage is used for the MCU, BLE radio, and sensors to keep the solution small. The MCU is connected directly to the output of the DC/DC converter and thus remains always powered. The sensors are connected to the output of an integrated load switch. The load switch is closed only when the sensors need to be powered, which is when the user requests data from them. For most of the wearable device’s operating time, sensors are not needed and are turned off.

Opening the load switch eliminates their leakage currents from the output of the DC/DC converter, reducing the battery current drawn. The DCS-Control topology of this DC/DC converter enables a very low-noise output, well-suited for noise-sensitive applications.[2][3] The 2.9 mm x 2.3 mm x 1.1 mm tall size of the DC/DC converter and its fully integrated functionality with no external components support the extremely small size needs of wearable devices.

Figure 3. The DC/DC converter (TPS82740A) integrated into a typical wearable system. Most sub-systems are connected to their supply voltage rail via an integrated load switch, which reduces leakage current. The TPS82740A does not require any passive components for implementation.

Figure 4 shows the efficiency graph of the same DC/DC converter when converting a 3.6-V input to the 2.1 V required by the MCU.[4] Due to its ultra-low-power optimized power-save mode and 360-nA quiescent current (IQ), over 80% efficiency is achieved with just a 10-µA load.[5]

 

Figure 4. DC/DC converter (TPS82740A) efficiency when converting a Lithium-Ion battery (3.6 V nominal) to 2.1 V for a MCU. Over 80% efficiency is achieved above 10 µA of load current.
 
Conclusion
 
An ultra-low power-optimized DC/DC converter paired with an ultra-low power MCU creates an efficient and small solution for a wearable device. The high integration of functionality and components into these two devices result in a very small solution size. The ultra-low 360-nA quiescent current of the DC/DC converter combined with the 450-nA standby current of the MCU allow very long runtimes off of very small batteries.
 
References

  1. MSP430 with FRAM landing page
  2. DCS-Control landing page
  3. High-efficiency, low-ripple DCS-Control™ offers seamless PWM/power-save transitions, Chris Glaser, Analog Applications Journal (SLYT531), Texas instruments, 3Q 2013
  4. Download the TPS82740A datasheet
  5. IQ: What it is, what it isn’t, and how to use it, Chris Glaser, Analog Applications Journal (SLYT412), Texas instruments, 2Q 2013


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