9.3.5 Battery Charging

Set CE low to initiate battery charging. First, the device checks for a short-circuit on the BAT pin by sourcing I_BAT(SC) to the battery and monitoring the voltage. When the BAT voltage exceeds V_BAT(SC), the battery charging continues. The battery is charged in three phases: conditioning pre-charge, constant current fast charge (current regulation) and a constant voltage tapering (voltage regulation). 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.

Figure 17 illustrates a normal Li-Ion charge cycle using the bq2407x:

Figure 17. Typical Charge Cycle

In the pre-charge phase, the battery is charged at with the pre-charge current (I_PRECHG). Once the battery voltage crosses the V_LOWV threshold, the battery is charged with the fast-charge current (I_CHG). As the battery voltage reaches V_BAT(REG), the battery is held at a constant voltage of V_BAT(REG) and the charge current tapers off as the battery approaches full charge. When the battery current reaches I_TERM, the CHG pin indicates charging done by going high-impedance.

Note that termination detection is disabled whenever the charge rate is reduced because of the actions of the thermal loop, the DPPM loop or the V_IN(LOW) loop.

The value of the fast-charge current is set by the resistor connected from the ISET pin to VSS, and is given by the equation:

The charge current limit is adjustable up to 1.5 A. The valid resistor range is 590 Ω to 5.9 kΩ. If I_CHG is programmed as greater than the input current limit, the battery will not charge at the rate of I_CHG, but at the slower rate of I_IN(MAX) (minus the load current on the OUT pin, if any). In this case, the charger timers will be proportionately slowed down.