ALLPCB
Introduction
When designing wearable applications, engineers often encounter recurring charger-related issues. The following addresses several frequently asked questions and practical considerations for selecting and troubleshooting linear battery chargers.
Which linear charger is best for my application?
Choosing an appropriate charger requires considering factors such as power level, size, and battery chemistry.
For example, in the TI charger portfolio, the bq24232 is a linear charger that provides a 500 mA charge current and includes power-path features. The solution footprint is approximately 3.5 mm × 4.5 mm, and it includes the necessary resistors and capacitors. This option is suitable for applications that require the system to power on immediately and where board area is not constrained.
If PCB space is limited, the bq24040 offers a smaller solution in a 2.5 mm × 3.5 mm footprint. This charger supports charge currents from 10 mA to 1 A and provides charge-status indication and programmable precharge and termination rates. Because of its flexibility, it is widely used in low-power applications. However, the bq24040 has a minimum termination current of 6 mA, which may be too high for very small cells. For applications with very small size and cell capacity (for example, hearing aids), the bq25100 is a better choice. The IC package itself measures only 1.6 mm × 0.9 mm, and the total solution footprint is about 2.1 mm × 2.2 mm. Additionally, the IC can terminate charging at currents below 1 mA, extending runtime for small cells.
Battery voltage is another deciding factor when selecting a charger. The bq24232 and bq24040 families offer 4.2 V and 4.35 V options. The bq25100 provides two options (4.3 V and 4.06 V) to address specific requirements of wearable applications.
Why does my battery stop charging before it is full?
Several conditions can cause premature termination of charging.
First, check whether the input voltage at the VIN pin is stable and remains above VBAT + VIN_DT. Many TI chargers include a power-good detection threshold (VIN_DT), defined as the difference between VIN and VBAT. When VBAT rises and that difference falls below the threshold, charging will terminate. The typical value for this threshold is about 80 mV.
Second, verify whether the battery-trace resistance is large. Sometimes the leads themselves have substantial resistance (up to 1 Ω), which can cause a 300 mV drop at a 300 mA charge current. In such cases, even if the battery voltage is only 3.9 V, the voltage at the charger VBAT pin can reach 4.2 V, causing the charger to terminate charging.
Third, ensure the safety timer is configured correctly. For the bq24232, the safety timer can be set from two to eight hours; if the timer expires, charging will stop. If the charge current is very small and the safety timer is set too short, charging may terminate before the battery is fully charged.
How can I eliminate oscillation at low charge currents?
In most cases, input and output capacitors help stabilize currents. However, under certain conditions—especially when the charge current is very small—parasitic capacitance at the current-programming pin (for example, the ISET pin) can cause oscillation. In those situations, simply increasing input or output capacitance may not resolve the instability.
For the bq25100, when the charge current is below 50 mA, it is recommended to add an RC compensation network in parallel with the ISET resistor. This RC network compensates for the current-regulation loop instability introduced by the parasitic capacitance at the ISET pin.
