9.3.5.6 Thermal Regulation and Thermal Shutdown

The bq2407x contain a thermal regulation loop that monitors the die temperature. If the temperature exceeds T_J(REG), the device automatically reduces the charging current to prevent the die temperature from increasing further. In some cases, the die temperature continues to rise despite the operation of the thermal loop, particularly under high VIN and heavy OUT system load conditions. Under these conditions, if the die temperature increases to T_J(OFF), the input FET Q1 is turned OFF. FET Q2 is turned ON to ensure that the battery still powers the load on OUT. Once the device die temperature cools by T_J(OFF-HYS), the input FET Q1 is turned on and the device returns to thermal regulation. Continuous overtemperature conditions result in a "hiccup" mode. During thermal regulation, the safety timers are slowed down proportionately to the reduction in current limit.

Note that this feature monitors the die temperature of the bq2407x. This is not synonymous with ambient temperature. Self heating exists due to the power dissipated in the IC because of the linear nature of the battery charging algorithm and the LDO associated with OUT. A modified charge cycle with the thermal loop active is shown in Figure 19. Battery termination is disabled during thermal regulation.

Figure 19. Charge Cycle Modified by Thermal Loop