The BQ2405x is a highly integrated family of 2-mm × 2-mm single-cell Li-Ion and Li-Pol chargers. The charger can be used to charge a battery, power a system or both. The charger has three phases of charging: precharge to recover a fully discharged battery, fast-charge constant current to supply the buck charge safely and voltage regulation to safely reach full capacity. The charger is flexible, allowing programming of the fast-charge current, precharge current and termination. This charger is designed to work with a USB connection or adaptor (DC out). The charger also checks to see if a battery is present.

The charger also comes with a full set of safety features: JEITA Temperature Standard, Overvoltage Protection, DPM-IN, Safety Timers, and ISET short protection. All of these features and more are described in detail below.

The charger is designed for a single power path from the input to the output to charge a single cell Li-Ion or
Li-Pol battery pack. Upon application of a 5VDC power source the D+, D– detection routine is run to determine if the source is an Adaptor or a USB port. This feature is useful, when the battery is discharged (USB transceiver dead) or there is no transceiver, by early detection of an adaptor, thus allowing initial charging at the adaptor level. ISET and OUT short checks are performed in parallel with the detection routine to assure a proper charge cycle.

If the battery voltage is below the LOWV threshold, the battery is considered discharged and a preconditioning cycle begins. The amount of precharge current can be programmed using the PRE-TERM pin which programs a percent of fast charge current (10 to 100%) as the precharge current. This feature is useful when the system load is connected across the battery stealing the battery current. The precharge current can be set higher to account for the system loading while allowing the battery to be properly conditioned. The PRE-TERM pin is a dual-function pin which sets the precharge current level and the termination threshold level. The termination "current threshold" is always half of the precharge programmed current level.

Once the battery voltage has charged to the V_LOWV threshold, fast charge is initiated and the fast charge current is applied. The fast charge constant current is programmed using the ISET pin. The constant current provides the bulk of the charge. Power dissipation in the IC is greatest in fast charge with a lower battery voltage. If the IC reaches 125°C, the IC enters thermal regulation. Slow the timer clock by half and reduce the charge current as needed to keep the temperature from rising any further. Figure 7-1 shows the charging profile with thermal regulation. Typically under normal operating conditions, the IC’s junction temperature is less than 125°C and thermal regulation is not entered.

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. The charge termination can be disabled if desired. The CHG pin is low (LED on) during the first charge cycle only and turns off once the charge termination threshold is reached, regardless if termination is enabled or disabled.

The TS pin monitors the voltage across the pack thermistor and implements the JEITA standard. This allows for reduced voltage regulation at hot temperatures and reduced charge currents at low temperatures. The TS pin incorporates a chip disable feature when pulled low and an Termination and Timer Disable Mode (TTDM) feature when left floating or pulled high.