3 Common Problems with Charging Wearable Products

Q: Which linear charger is best for my application?

A: When choosing the right charger for a specific application, you should consider a variety of factors: power level, size, battery type, etc.

Take the different chargers in TI's charger portfolio as an example. The bq24232 is a linear charger with a 500mA charge current and power path features. The solution is approximately 3.5mm × 4.5mm2, including the necessary resistors and capacitors. This is a great choice for applications that require instant system on functionality and are not space constrained.

If board space is limited, the bq24040 provides a 2.5mm × 3.5mm2 solution. The charger supports charge currents from 10mA to 1A and features charge status indication and programmable precharge and termination rates. Because of its flexibility, the device is one of the most widely used linear chargers in low-power applications. But the bq24040 has a minimum termination current of 6mA, which may be too much for ultra-small batteries. Therefore, for applications where both size and battery capacity are very small, such as hearing aids, the bq25100 is a good choice. The package size of the integrated circuit (IC) itself is only 1.6mm × 0.9mm, and the total solution volume is also small, only 2.1mm × 2.2 mm2. In addition, the IC can terminate charging when the current is less than 1mA and extend the operating time for small batteries.

Battery voltage is another factor when choosing a charger. Both the bq24232 and bq24040 product families have 4.2V and 4.35V options. The bq25100 offers two more options (4.3V and 4.06V options) to meet the special needs of wearable applications.

Q: Why does my battery terminate charging before it is fully charged?

A: There are several conditions that may cause premature termination of charging. First, check that the input voltage at the input voltage (VIN) pin is stable and above VBAT + VIN_DT. Most TI chargers have a power good detection threshold (VIN_DT) that is the difference between VIN and VBAT. Once VBAT increases and the difference falls below this threshold, charging is terminated. The typical value of this threshold is about 80mV.

Second, determine if the battery tracking resistance is small. Sometimes, the lead itself has a large resistance (up to 1Ω), which will cause a voltage drop of 300mV and a charging current of 300mA. In this case, even if the battery voltage is only 3.9V, the voltage of the charger VBAT pin will reach 4.2V, thus terminating the charge.

Third, make sure the safety timer is set to the correct value. For the bq24232, the safety timer can be set from two to eight hours; once the timer expires, charging is terminated. If the charge current is too low and the safety timer is set too short, charging may stop before the battery is fully charged.

Q: How can I eliminate oscillations in small charging currents?

A: Most of the time, input and output capacitors can help stabilize the input and output currents. But in some cases (especially when the charging current is very small), parasitic capacitance at the current programming pin (such as ISET pin) can cause oscillation, and input and output capacitors are no longer a suitable solution.

For the bq25100, if the charging current is less than 50mA, I recommend adding a resistor/capacitor (RC) compensation circuit in parallel with the ISET resistor (Figure 1). This effectively compensates for the current regulation loop instability caused by the parasitic capacitance present at the ISET pin.

Figure 1: Compensation circuit of BQ25100