7.3.1 Battery Preconditioning

During a charge cycle, if the battery voltage is below the V_(LOWV) threshold, the device applies a precharge current, I_O(PRECHG), to the battery. This feature revives deeply discharged cells. Resistor R_SET, connected between the ISET and V_SS, determines the precharge rate. The V_(PRECHG) and K_(SET) parameters are specified in the Electrical Characteristics table.

Equation 1.

The device activates a safety timer, t_(PRECHG), during the conditioning phase. If the V_(LOWV) threshold is not reached within the timer period, the device turns off the charger and enunciates FAULT on the STATx pins. See the Timer Fault and Recovery section for additional details.

7.3.2 Battery Fast-Charge Constant Current

The device offers on-chip current regulation with programmable set point. Resistor R_SET, connected between the ISET and V_SS, determines the charge rate. The V_(SET) and K_(SET) parameters are specified in the specifications table.

Equation 2.

7.3.3 Charge-Current Monitor

When the charge function is enabled internal circuits generate a current proportional to the charge current at the ISET pin. This current, when applied to the external charge current programming resistor R_ISET generates an analog voltage that can be monitored by an external host to calculate the current sourced from the OUT pin.

Equation 3.

7.3.4 Battery Fast-Charge Voltage Regulation

The voltage regulation feedback is through the OUT pin. This input is tied directly to the positive side of the battery pack. The device monitors the battery-pack voltage between the OUT and V_SS pins. When the battery voltage rises to the V_O(REG) threshold, the voltage regulation phase begins and the charging current begins to taper down.

As a safety backup, the device also monitors the charge time in the charge mode. If charge is not terminated within this time period, t_(CHG), the charger is turned off and FAULT is set on the STATx pins. See the Timer Fault and Recovery section for additional details.

7.3.5 Charge Termination Detection and Recharge

The device monitors the charging current during the voltage regulation phase. Once the termination threshold, I_(TERM), is detected, charge is terminated. The V_(TERM) and K_(SET) parameters are specified in the Electrical Characteristics table.

Equation 4.

After charge termination, the device restarts the charge once the voltage on the OUT pin falls below the V_(RCH) threshold. This feature keeps the battery at full capacity at all times.

The device monitors the charging current during the voltage regulation phase. Once the termination threshold, I_(TERM), is detected, the charge is terminated immediately.

Resistor R_SET, connected between the ISET and V_SS, determines the current level at the termination threshold.

7.3.6 Charge Status Outputs

The open-drain STAT1 and STAT2 outputs indicate various charger operations as shown in Table 1. These status pins can be used to drive LEDs or communicate to the host processor. Note that OFF indicates the open-drain transistor is turned off.

7.3.7 PG Output (bq24080-Q1)

The open-drain power-good (PG) output pulls low when a valid input voltage is present. This output is turned off (high-impedance) in sleep mode. The PG pin can be used to drive an LED or communicate to the host processor.

7.3.8 Charge-Enabled (CE) Input (bq24080-Q1)

The CE digital input is used to disable or enable the charge process. A low-level signal on this pin enables the charge and a high-level signal disables the charge and places the device in a low-power mode. A high-to-low transition on this pin also resets all timers and timer fault conditions.

7.3.9 Timer Enabled (TE) Input (bq24081-Q1)

The TE digital input is used to disable or enable the fast-charge timer. A low-level signal on this pin enables the fast-charge timer, and a high-level signal disables this feature.

7.3.10 Temperature Qualification (bq24081-Q1)

The bq24081-Q1 continuously monitors battery temperature by measuring the voltage between the TS and V_SS pins. An internal current source provides the bias for common 10-kΩ negative-temperature-coefficient thermistors (NTC) (see the functional block diagram). The device compares the voltage on the TS pin with the internal V_(TS1) and V_(TS2) thresholds to determine if charging is allowed. If a temperature outside the V_(TS1) and V_(TS2) thresholds is detected, the device immediately suspends the charge by turning off the power FET and holding the timer value (i.e., timers are not reset). Charge is resumed when the temperature returns within the normal range.

The allowed temperature range with a 103AT-type thermistor is 0°C to 45°C. However, the user may modify these thresholds by adding external resistors (see Figure 8 and Figure 9).

Figure 8. Default Temperature Thresholds

Figure 9. Temperature Thresholds Modified by External Resistors

7.3.11 Timer Fault and Recovery

As shown in Figure 7, the device provides a recovery method to deal with timer fault conditions. The following summarizes this method:

7.4.1 Sleep Mode

The device enters the low-power sleep mode if the input power (IN) is removed from the circuit. This feature prevents draining the battery during the absence of input supply.