Thin, small, and light are the basic requirements for wearable devices, and explain why the short board of wearable technology is battery life. Traditionally qualified batteries have lithium battery button batteries, which are sufficient for sensors and other low-power wearable devices, but it is difficult to keep up with the growing number of wearable devices, such as health belts and smart watches. . Most smart watch batteries are now only available for one day.
Figure 1: Apple Watch is Apple's first wearable, revolutionary new technology and groundbreaking user interface. (Photo courtesy of Apple)
For these types of devices, increasing battery life is the key to gaining market share, which will be a huge market, with a forecast of 380 million units by 2018. Energy harvesting, wireless charging, battery management, power management and low-consumption solutions are all options for improving battery life in wearable designs. This article will focus on the latest technologies in these areas to help designers of wearable devices create superior power solutions and better power distribution.
Energy harvesting power management
Energy is absorbed in the body, such as body temperature and limb movement, or energy from the environment, such as ambient light, only microwatts to milliwatts of energy—not enough to support the use of smart watches. Texas Instruments has a device, the bq25570, that pushes the collected 300 to 400 milliwatts to 3 to 5 volts that charge the battery. Although this does not currently generate enough power for a smart watch to work, it can at least extend working hours. "Ultra-low-power, high-energy acquisition management ICs," such as the bq25570, also include a highly efficient nano-power buck converter that provides a second power operating system.
Traditional battery charging (usb or adapter)
We can of course charge the battery of the wearable device in the traditional way, via a usb or adapter. TI's new bq25100 single-cell lithium battery charger offers a micro solution that is almost half the size of an existing power solution, while supporting low-cost adapters in a cost-sensitive wearable market. The charger has an input voltage of up to 30V and an input voltage of 6.5V, in addition to other protection parameters.
Maxim's MAX14676/76A provides an alternative to power-charge management for wearable devices. The highly integrated MAX14676/76A, which we can describe as a "wearable power management scheme integrated circuit", includes more than a linear charging circuit, as well as a large amount of low-energy-energy management to save space on the motherboard during extended battery life. These include a 1.8V low quiescent current (lq) 200mA buck regulator, 3.2V low quiescent current 100mA low dropout linear regulator (LDO), 2.0V "always on" 50μALDO, +5V safety output LDO, The 6.6V low Lq120μA charge pump and even the 5V to 17V output boost converters support a wide range of display options.
Both the bq25100 and MAX14676/76A can be paired with a wireless power receiver/transmitter to provide wireless charging capabilities for wearable device designs.
Qi-compliant wireless charging solution
Wireless charging has become very popular with wearable devices due to its convenience. Compared with the traditional linear charging method, consumers prefer to put the smart watch directly on the wireless charging base instead of looking for the charging cable and socket and connecting the three to charge the smart watch. Therefore, many wireless charging solutions are available for a wide variety of products. The following highlights the Texas Instruments program.
TI offers a cutting-edge design for wireless charging solutions, TIDA-00318, for low-power wearable devices. This solution can be combined with the bq25100 single-cell lithium battery linear charger we introduced earlier, and a Qi compliant wireless power receiver to meet the entire Qicompliant wireless charging solution. Qi is an international interoperability standard for wireless charging devices; any Qi-certified wireless power receiving device, such as the Moto 360 smart watch, can be charged on all bases with Qi certification. Therefore, any wearable device that implements TIDA-00318 should be Qi certified to work on the Qi charging dock. The TIDA-00318 is designed for 135mA charging specifications and is very small, only 5x15mm2.
For a smaller wearable device wireless charging receiver solution, TI has a TIDA-00329 reference design. It only has 5.23mm x 5.48mm combined with your bq51003 Qi compliant wireless power receiver or bq51050B/51B Qi compliant wireless charging. This miniature design can deliver up to 2W of power.
TI offers the TIDA-00334 reference design on a wireless power transmitter or on the side of a wearable device wireless charging solution charging dock. The bq500212A IC is used in the design of small wearable device transmitters. The power supply for the mini usb connector is 5V. The low power design supports the receiver's output power up to 2.5W.
The TIDA-00334 Wireless Transmitter reference design is displayed in a 30mm area that matches the diameter of the Wurth coil 760308101103 and is only a little larger than a 15 cent or 2 Euro coin.
Figure 2: Wurth Elektronik's WE-WPCC Wireless Charging Receiver Coils meet the WPC's Qi standard.
Power management technology in wearable devices
Ultra-low power conversion is the key to achieving the best power life for wearables. Here are some of the latest low-power products or high-efficiency DC-DC converters.
TI's TPS727xx series, 250mA LDOs features a very small quiescent current of only 7.9μA, low leakage current (100mV typical for 100mA, 130mV for 200mA, 163mV for 250mA), wide output voltage and load transients response. Another feature of LDO is its high supply voltage rejection ratio (PSRR), a 1kHz 70dB smooth performance in RF applications, and a small, low cost 10μA ceramic capacitor.
Now TI also introduced the TPS82740B 200mA buck-boost conversion module, which provides 95% conversion rate, which consumes only 360nA lq while working, and only 70nA when quiet. The small module can be used for full integration, incorporating the switching regulator, inductor and input/output capacitor 9-bump MicroSiPTM components, achieving a size of only 6.7mm2.
Figure 3: Low Power TPS82740x 360nA Micro SIP Buck-Boost Converter Module
Boost conversion is usually not as efficient as buck conversion. However, it is common to boost the battery in various system circuits, especially the display circuit. Maxim has a new 1A boost converter. The MAX8627 can increase the output voltage of a single-cell lithium battery from 3V to 5V. 95% conversion rate and only consume 20μA lq. Silicon Labs now has a TS33x boost converter with industry-leading lq down to 150nA. The TS33x increases the input voltage from 0.9V to 3.6V and has eight selectable output voltages ranging from 1.8V to 5V.
Bluetooth, microcontrollers and other low power solutions
In fact, when trying to extend the life of a wearable device battery, everything needs to be considered.
A common way to save power is to turn off some power-hungry features, such as some processing and display features, such as smart watches, tablets, or computers. Bluetooth® Smart, or low-power Bluetooth, is standard on most new smartwatches and is therefore the standard for wireless communication for wearable devices. Bluetooth can also be used to transfer information from smartphones to smart watches, and TI offers a “Bluetooth Wearable Watch Development System†called TI Meta WatchTM to ensure the rapid development of related watch devices. The Meta Watch SDK/API makes it easy to receive information from a mobile app or web service on a watch. The development system includes a smart watch with a display, and a 3 ATM waterproof stainless steel case, belt watch, crystal mirror, vibration motor, three-axis accelerometer and ambient light sensor.
Figure 4: The Texas Instruments Meta WatchTM Bluetooth® Wearable Watch Development System ensures the rapid growth of “Connectable Watches†applications.
The Meta Watch Platform has been optimized for low power, based on TI's 16-bit MSP430TM ultra-low power microcontroller (MCU) and CC2564 Bluetooth master interface solution.
Choosing an MCU is important for power management of wearable devices. An efficient MCU can speed up the import of data and quickly go to sleep and conserve power. Low-power sleep mode can effectively reduce power consumption. Designers of wearable devices have more choices of MCUs than before, and 32-bit is more cost-competitive than 16-bit. Optimized for cost and power-sensitive MCUs, ARM's Cortex-M family of 32-bit processor cores has foreseen success in the wearable market. From the ultra-low-power Cortex-M0 and M0+ to the high-performance Cortex-M7, the ARM Cortex-M series offers a wide range of wearable devices to meet different needs. MCUs based on the ARM-Cortex-M family are now available from many vendors, including Texas Instruments, and STMMicroelectronics, with a huge line of STM32 MCUs, including STM32L1 and L0 ultra-low-power MCUs.
Finally, it is important to consider the power management of countless sensors in wearable devices. Sensor technology is a catalyst for accelerating the development of the wearable market. But we can't forget the peripheral circuit of the sensor. STMicro can be used in wearable sensors with low-power sensor signal conditioning. Its QA4NP low-power quad op amp consumes only 580nA per channel (at 1.8V supply).
The above is just a glimpse of low-power power management technology, and product collaboration to design an ultra-low-power power system that can meet the wearable market is the key to stimulating this market. It is necessary to realize that ultra-low-power devices are not only important for wearable devices. New low-power technologies are also important for applications that rely on battery power and energy harvesting.
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