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Exploring Power Solutions for Charging Wearable Devices

The wearable electronics market is booming. From entertainment to fitness and health care, multi-function wearable devices are helping people perform a multitude of tasks. Such devices are normally powered by batteries, which must be charged efficiently and quickly on a regular basis. Since space is limited, wearable devices like smart watches, fitness wristbands, and headphones require tiny, low-power chargers.

Power semiconductor suppliers like Texas Instruments have released a number of solutions to rapidly charge Lithium-ion (Li-ion) and Lithium-polymer (Li-pol) batteries of different capacities. One such solution is TI’s bq25100, a single-cell Li-ion charger that comes in a 0.9 mm x 1.6 mm WCSP package. According to the supplier, it is the industry’s smallest, lowest-power linear charger that allows accurate control of fast-charge currents as low as 10 mA or as high as 250 mA. In fact, TI claims that bq25100 offers a solution that is half the size of existing charger solutions in the market. By comparison, other charger ICs from TI and competing suppliers measure about 2.0 mm x 2.0 mm in size.

Linear chargers

The recommended input-voltage range for TI’s bq25100 (Figure 1) is 3.5 V to a maximum of 30 V with 6.5 V input overvoltage protection. To support Li-ion coin cells, the device supports minimum 1 mA charge termination current and handles output leakage current down to 75 nA. Consequently, it is tailored for low-power wearable applications that require very low charge currents.

Figure 1: The bq25100 is designed to charge a single-cell Li-ion or Li-pol battery.

The bq25100 offers a single-power output that charges a single-cell Li-ion or Li-pol battery. In essence, the battery is charged in three phases: pre-conditioning, constant current, and constant voltage. While pre-charge or pre-conditioning recovers a fully discharged battery, fast-charge constant current supplies the charge safely and voltage regulation ensures that the battery reaches full capacity safely. However, in all charge phases an internal control loop monitors the IC junction temperature and reduces the charge current if an internal temperature threshold is exceeded. The charger IC’s datasheet suggests that the charger power stage and charge current-sense functions are fully integrated.

Upon application of a 5 VDC power source, the ISET and OUT pins are checked for any shorts to assure that the charge cycle is proper. If the battery voltage is below the VO(LOWV) threshold (Figure 2), the battery is considered discharged and a preconditioning cycle begins, per TI’s description. As the supplier explains, the amount of precharge current can be programmed using the PRETERM pin. It programs a percent of fast charge current (10 to 100%) as the precharge current. According to the manufacturer, the precharge current can be set higher to account for the system loading while allowing the battery to be properly conditioned. In other words, the PRETERM pin is a dual-function pin. It sets the precharge current level as well as the termination threshold level. The complete charging profile depicted in Figure 2 indicates that the termination “current threshold” is about half of the precharge programmed current level.

Figure 2: The battery-charging profile includes thermal-regulation phase.

Once the battery voltage has charged to the VO(LOWV) threshold, fast charge is initiated and the fast charge current is applied. The fast charge constant current, which provides the bulk of the charge, is programmed using the ISET pin. Also, power dissipation in the charger is highest during fast charge with a lower battery voltage. Consequently, if the IC’s junction temperature reaches 125°C, it enters a thermal-regulation phase, which slows the timer clock by half and reduces the charge current as needed to keep the temperature from rising any further, according to the maker. The complete charging profile with thermal regulation is illustrated in Figure 2. Typically, under normal operating conditions, the IC’s junction temperature is less than 125°C and thermal regulation is not initiated.

Once the cell has charged to the regulation voltage, the voltage loop takes control and holds the battery at the regulation voltage until the current tapers to the termination threshold. TI notes that the termination can be disabled if desired.

To evaluate the charger IC bq25100 in a real working system, TI has readied an evaluation board, labeled BQ25100EVM, which is a complete charger solution for single-cell, Li-ion

and Li-polymer batteries used in wearables and low-power portable applications. Key features of this evaluation module include programmable charge current, precharge/termination current, input operating range of 4.45 to 6.45 V, LED indication for status signals and test points for key signals available for testing purposes with easy probe hook-up.

More choices

For higher charge current, TI has other options. A good example is bq24230/32, a fully-compliant USB charger supporting up to 500 mA charge current with current monitoring output. The unit operates from either a USB port or AC adapter with the ability to handle charge currents between 25 mA and 500 mA. The input voltage range for the charger is similarly high with input overvoltage protection. As a result, it supports low-cost, unregulated adapters, according to TI. Furthermore, the USB input current limit accuracy and start-up sequence allow the bq2423x ICs to meet USB-IF inrush current specification. Additionally, the input dynamic power management (VIN-DPM) prevents the charger from crashing poorly designed or incorrectly configured USB sources. Other features include status indication such as Charging/Done and Power Good, dynamic power-path management (DPPM), thermal protection and compact QFN package.

With low-power wearable devices like smart watches, fitness wristbands, and headphones becoming ultra-thin, manufacturers are moving away from traditional micro-USB or plug-and-jack-style connectors for charging. They are instead adopting wireless-inductive charging. And semiconductor devices used for the Qi standard established by the Wireless Power Consortium (WPC) can be easily adapted for this lower-power application. TI’s bq51003 is crafted specifically for use as a receiver in wireless inductive charging. It works in tandem with linear charger ICs like bq25100 or bq24232 to regulate and manage the current flow to a Li-ion battery. Combining such devices, TI has prepared a Qi-compliant wireless-charger-reference design (Figure 3) for low-power wearables. The TIDA-00318 features an ultra-small size (5 x15 mm), and offers low-charging currents down to 10 mA and up to 250 mA with support of termination currents as low as 1 mA.

Figure 3: The TI Qi (WPC)-compliant wireless-charger-reference design employs the company’s bq51003 and its ultra-low-current one-cell Li-ion linear charger IC bq25100.

In summary, a number of design options are available to designers developing charging solutions for Li-ion and Li-pol batteries employed in wearable products. These options are aimed at enabling designers to select the best choice that will deliver optimal price/performance in the shortest amount of time.

For more information on the products discussed in this article, use the links provided to access product pages on the Hotenda website.