Wearable devices are integrating more and more features, which poses a challenge for power management. The battery in a wearable device is usually very small, and the device needs to work for a long time without charging, so power consumption is a key design consideration. To ensure the wearability of wearable devices, the circuit needs to be kept within a very small size, which makes devices such as power management more inclined to adopt a single-chip solution. In order to cope with the difficulty in solving the battery life of battery technology, it is possible for wearable devices to adopt new power sources, such as energy harvesting for solar energy or thermal energy.
The safety of the wearable device power supply cannot be ignored, and the charger needs to take into account various protective measures. Wearable device power supplies also need to consider a range of issues such as charging accuracy, display drivers, and platform compatibility. In addition, the wearable device market is still in its infancy, and many wearable devices are still being developed using existing portable device power management solutions.
TI: High precision, low power consumption is the key to extending battery life
Dr. Wen Sihua, Marketing and Applications Manager, Battery Management Products, High Performance Analog Semiconductor Products Division, Texas Instruments, believes that in wearable devices, the battery is usually very small (eg 100 mAh) and the device needs to last for a few days or even a few Power consumption is a key design consideration for weeks without charging. Therefore, high power conversion efficiency will be a key design element. Stylish wrist-worn wearable devices need to be kept to a minimum size in order to maintain their cool features. This drives a high level of full device integration including power management ICs. In addition, the footprint and package of the power device should be as small as possible. In addition, wearable devices may use new power sources, such as energy harvesting for solar or thermal energy. At the same time, some products may prefer non-contact charging. Wireless charging is included in these contactless charging. (Please refer to: More Power Management, Data Conversion Design.)
For the current design challenges of wearable power, he summarized the following five points.
â— If the lithium battery is overcharged or the temperature is too high, it will cause fire. The safety of the lithium battery is still worrying. At the same time, the charger should have almost all types of safety design, including overvoltage protection, overcurrent protection and overtemperature protection. Short-circuit protection and low-temperature charging, even when the charger IC and solution size must be kept very small.
â— Because the battery in the wearable device is small, the need for charging accuracy is increased, making it difficult to charge these small batteries. The charger must be able to provide a smaller charge cut-off current. In other words, the accuracy of the charger should be higher, up to mA level. For example, in a smartphone system, the normal charging current of a 2,000 mAh battery is 1.2 A (0.6 C), and the charging cut-off current should be 1/10 to 1/20 of the normal charging current, that is, 120 mA to 60 mA. However, in the bracelet, since the battery capacity may be 100 mAh, the normal charging current will be 60 mA, and the charging cut-off current should be 6 mA to 3 mA. Charger devices that meet this requirement are difficult to find.
â— Wearables should have a long battery life. Often, consumers are not happy if they charge their devices every day. In the current situation where many of the best smartphones have to be recharged in a day or two, end users are clearly looking forward to improvements. The entire power management should be able to provide a highly efficient power conversion system, including: regulator efficiency, and regulators and battery chargers should provide low quiescent current, low standby current and low leakage current. Power management devices with very low leakage current and standby current as well as low quiescent current are more difficult to design.
â— Even the best battery technology available today does not fully address battery runtime requirements. We need to develop new power supplies while using wearables. The bottleneck of the new power supply is that its power density and conversion efficiency are extremely low when it is converted to usable power.
â— In wearable devices, the failure point of many products is the charging/signal connector due to humidity, corrosion, etc.
In response to these design challenges, TI's response is as follows.
â— TI's charger ICs and fuel gauge ICs provide the widest range of protection and monitoring for working batteries. TI has succeeded in reducing the size of semiconductors while maintaining the same level of protection. For example, both the bq24040 and bq25100 are compatible with the JEITA standard. Both have a Ts input for monitoring the battery thermistor to ensure that the charging temperature is within the proper range. The bq24040 measures 2mm x 2mm and was the smallest charger before the bq25100 was introduced in mid-2014. The bq25100 measures 1.6 mm by 0.9 mm and is equivalent to a 0603 size capacitor (Figure 1).
Figure 1: The smallest lithium battery linear charger bq25100, quiescent current only 75nA, off current 1mA
â— The mainstream wearable device brands use TIbq24040/45 or bq24232 (with power path) charger ICs because of their small size (2mm Ã— 2mm and 3mm Ã— 3mm respectively) and high precision. And when the adapter is connected, even when the battery is exhausted, the bq24232 allows the system to power up immediately. Accuracy and power path characteristics are the most popular in advanced wearable designs. In addition, the bq25100 allows a normal charge current of at least 10 mA and the ability to set the charge cut-off current or pre-charge current to 1 mA, which is the industry's highest precision for wearable devices.
â— To reduce power consumption in power management systems, TI has adapted these devices for wearable devices with very low current consumption. For example, the bq25100 has a quiescent current of less than 1Î¼A in active mode and a battery leakage current as low as 75 nA when the bq25100 is in shutdown mode. At the same time, the â€œNano-Buckâ€ TPS62736 buck converter has a very low quiescent current of only 380nA, which can help achieve over 90% efficiency at 15nA low load. This can extend battery run time by 30% to 50% compared to conventional DC/DC conversion under low load conditions.
â— Energy harvesting technology has spurred new applications for wearable devices. The TPS25504 is capable of extracting energy from low energy sources such as a single solar cell (about 0.3 to 0.6 V) or TEG (thermoelectric generator, <0.3 V). It has a low starting voltage requirement of only 0.33V and is capable of operating down to 80 mV. Its quiescent current is also very low, only 330nA.
â— With the great success of applications such as Moto360, wireless charging has become a replacement for wearable devices, especially smart watches. Using wireless charging allows wearables to be better sealed without the need for connectors; wireless charging facilitates the promotion of easy-to-charge use cases with custom mounts; wireless charging Rx recommendations are Qi-compatible so that wearables can Charge on any standard Qi charger. Tx can be Qi-compatible, but some designs are limited to size or special shape requirements, and TX coils require customization (non-Qi specifications).
In the future planning of wearable device power products, Wen Sihua said: "We will continue to provide highly integrated, low power, high precision power management devices, chargers, energy harvesters and fuel gauges for wearable devices. And wireless charging products that may cover inductive charging and magnetic resonance charging."
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