Wearable Electronics
Prolonging the battery life of wearable devices is crucial if they’re to be more widely accepted
Wearable consumer electronic devices have, by their very nature, a number of physical design limitations. Chief among these are that the device must be small and lightweight. These constraints have a major impact on the size of the battery, its capacity and, by extension, the device’s battery life. Mark Patrick, Mouser Electronics, talks about the challenges of making batteries in wearable devices last longer
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n the past, lithium-ion (Li-Ion) coin-type cells have offered sufficient capacity to power simple sensor-based fitness bands, but the introduction of more sophisticated and power-hungry smartwatches – many with colour OLED displays – means that the battery life of these devices from a full charge is now measured in days. Consumers are becoming aware that many wearables need to be recharged regularly. And because this is seen as an inconvenience, people are becoming far more savvy when choosing a new device. For manufacturers, extending the battery life of their devices is critical when it comes to gaining market acceptance. As a consequence, developers are investigating techniques such as power management, wireless charging, energy harvesting, battery management and many other low-power and energy-conservation techniques, as ways of extending the time between charges. Let’s look at how these techniques work.
Energy-harvesting power management
Energy-harvesting techniques have been around for some time, and although it is possible to generate a few milliwatts from
an ambient light source, this isn’t enough to power a smartwatch. However, there are devices that can help. The bq25570 ultra low power harvester power management IC from Texas Instruments uses a combined boost-and-buck controller approach to convert harvested energy as low as 120 mV up to 3-5 V, which is enough to charge a battery. While it would never be enough to power a smartwatch on its own, it can extend the battery life. The bq25570 device also includes a nano- power buck converter that can provide a secondary power source to the application.
Battery-charging from USB source or mains adapter
Using a conventional method of charging, such as a USB port or mains adapter, does not mean the product design can’t be improved. For example, TI’s relatively new bq25100 single-cell Li-Ion battery charger IC occupies just 1.6 x 0.9 mm, half the footprint of previous devices. It also enables the use of low-cost unregulated wall adapters. This can help cut the overall BOM in the highly competitive and cost- sensitive wearables market. Able to accommodate an input voltage of up to 28 V DC, with overvoltage protection
features operational from 6.5 V DC, the bq25100 is a highly integrated linear charger, suitable for Li-Ion and Lithium- Polymer (Li-Pol) batteries. Another example of a battery charge-management device is the XC6803A4 IC from Torex. Designed for a broad range of wearables, such as fitness trackers, GPS watches and smartwatches, this device can be configured for constant-voltage or constant-current charging.
Qi-compliant wireless charging solutions
In the past few years, wireless charging has become popular for wearable devices. Consequently, wireless charging capabilities are being incorporated into desk lights and other home and office equipment. Consumers generally find this approach to charging far more convenient than having to carry a cabled charger. Many of the major semiconductor vendors are now starting to provide wireless charge controllers, and a lot of these extend to complete wireless charging reference designs. Aiding the design process and
converter module that offers 95 per cent conversion efficiency and consumes only 360 nA Iq during active operation and 70 nA during standby. The tiny module is packaged in a fully integrated, 9-bump MicroSiP format, which incorporates a switching regulator, inductor and input/output capacitors to achieve a solution size of only 6.7 mm2.
Bluetooth, microcontrollers, and other low-power solutions When it comes to extending the battery life of a wearable design, a holistic approach is best, to ensure every part of the design helps reduce overall consumption. A popular technique is to offload power-hungry functions, such as intensive compute, data analysis and display functions, to a device the wearable attaches to, such as a smartphone or PC. For the embedded design engineer, venturing into the analogue and often unpredictable world of RF can be daunting. However, incorporating Bluetooth connectivity into a wearable device is made simpler by a number of
Figure 2: Maxim MAX8627 typical operating circuit
incorporating a bq25100 single-cell linear charger and the bq51003 Qi-compliant wireless power receiver, TI has published the complete test results and design files of a reference design termed TIDA-00318. Any wearable implementing the TIDA- 00318 design should be able to gain Qi certification and work with any Qi charging base. The TIDA-00318 is for 135 mA charge current applications, and the supplied Gerber files provide an ultra-small footprint of 5 x 15 mm.
Power management
Ultra-low power conversion is critical to achieving optimal battery life in wearable devices. There are different ways of achieving this. One approach is to use a low-drop-out regulator (LDO) such as the TI TPS727xx series.
Figure 1: Texas Instruments bq25100 typical application diagram
www.cieonline.co.uk
This has an ultra-low quiescent current, Iq, of just 7.9 µA, very low drop-out (65 mV typical at 100 mA, 130 mV typical at 200 mA, and 163 mV typical at 250 mA), and excellent line and load transient response. The LDOs also feature high power supply rejection ration (PSRR) of 70 dB at 1 kHz for quiet performance in RF applications, and are stable, with small, low-cost 1.0 µF ceramic capacitors. Another approach is using a step-down converter method, such as the TPS82740A from TI. This is a 200 mA step-down
radio regulatory pre-certified wireless modules and wireless microcontrollers. For short-range, very-low-power applications, the most common method is via Bluetooth Low Energy (BLE). Examples include the BGM113 series from Silicon Labs. The choice of microcontroller unit (MCU) is another important factor when it comes to power management of wearables. Efficient MCUs can process data quickly, then go to sleep to save power. Designers of wearable devices today have a broader range of low-power MCUs available to them than ever before, as 32-bit MCUs have become cost- competitive with 16-bit MCUs. ARM’s Cortex-M series of 32-bit processor cores, optimised for cost- and power-sensitive MCUs, are seeing success in the wearables market. Ranging from the ultra-low-power Cortex-M0 and M0+ to the high- performance Cortex-M7, the ARM Cortex- M series is a broad offering to meet the differing needs of various wearables. Achieving a low-power wearable design
requires the engineering team to fully understand the power profiles of every component and part of the development. Taking a holistic approach will help them prolong their device’s battery life by saving every available µA of energy.
www.mouser.co.uk Components in Electronics July/August 2017 37
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