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When designing a wearable application, we’ve seen some common questions about chargers. Let’s look at some of the most frequently asked ones here.
Q: Which linear charger is the most suitable one for my application?
A: You should consider multiple factors when choosing the right charger for a specific application: power level, size, battery type, etc.
Take the example of the different chargers in TI’s charger portfolio. The bq24232 is a linear charger with a 500mA charge current and power-path feature. The solution size is about 3.5mm by 4.5mm2, including the necessary resistors and caps. It is a great choice for applications that require a system instant ON feature and are not space limited.
If board space is limited, the bq24040 provides a 2.5mm by 3.5mm2 solution. This charger supports a charge current from 10mA to 1A and has a charge status indication and programmable pre-charge and termination rate. Because of its flexibility, this device is one of the most widely used linear chargers in low-power applications. However, the minimum termination current of the bq24040 is 6mA, which may be too high for ultra-small batteries. Therefore, for applications like hearing aids, which are extremely small in both size and battery capacity, the bq25100 is a good option. The package size of the integrated circuit (IC) itself is only 1.6mm by 0.9mm and the total solution size is as small as 2.1mm by 2.2 mm2. Moreover, this IC can terminate charging below 1mA and extend the operating time for small batteries.
Battery voltage is another determining factor when selecting a charger. Both the bq24232 and bq24040 families have 4.2V and 4.35V options. The bq25100 offers two more options at 4.3V and 4.06V to cater to the special needs of wearable applications.
Q: Why does my battery terminate charging before it is fully charged?
A: Several things could lead to early termination. First, check if the input voltage at the input voltage (VIN) pin is stable and above VBAT+ VIN_DT. For most TI chargers, there is a power-good detection threshold (VIN_DT), which is the difference between VIN and VBAT. Once the VBAT is increased and the difference is below the threshold, charging will terminate. The typical value of the threshold is about 80mV.
Second, confirm that the battery-trace resistance is small. Sometimes, the wire itself has a resistance as high as 1Ω, which will cause a 300mV voltage drop with a 300mA charging current. In this case, even if the battery is only 3.9V, the VBAT pin of the charger will see 4.2V, and therefore terminate charging.
Third, make sure that the safety timer is programmed to the correct value. For the bq24232, the safety timer is programmable from two to eight hours; charging will terminate once the timer expires. If the charging current is too small and the safety timer is too short, it is possible that charging will stop before the battery is fully charged.
Q: How do I remove the oscillation of the small charging current?
A: Most of the time, input and output capacitance can help stabilize the input and output current. In some cases – especially when the charging current is very small – the parasitic capacitance at the current program pin (for example, the ISET pin) will cause oscillations, and the input and output capacitance is no longer the right solution here.
For the bq25100, if the charging current is less than 50mA, I recommend adding a resistor/capacitor (RC) compensation circuit (Figure 1) in parallel with the ISET resistor. This can effectively compensate for the instability of the current-regulation loop that is brought by the parasitic capacitance present at the ISET pin.
Figure 1: Compensation circuit for bq25100
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