9.3.5.4 Dynamic Charge Timers (TMR Input)

The bq2407x devices contain internal safety timers for the pre-charge and fast-charge phases to prevent potential damage to the battery and the system. The timers begin at the start of the respective charge cycles. The timer values are programmed by connecting a resistor from TMR to VSS. The resistor value is calculated using the following equation:

Leave TMR unconnected to select the internal default timers. Disable the timers by connecting TMR to VSS.

Reset the timers by toggling the CE pin, or by toggling EN1, EN2 pin to put the device in and out of USB suspend mode (EN1 = HI, EN2 = HI).

Note that timers are suspended when the device is in thermal shutdown, and the timers are slowed proportionally to the charge current when the device enters thermal regulation.

During the fast charge phase, several events increase the timer durations.

• The system load current activates the DPPM loop which reduces the available charging current
• The input current is reduced because the input voltage has fallen to V_IN(LOW)
• The device has entered thermal regulation because the IC junction temperature has exceeded T_J(REG)

During each of these events, the internal timers are slowed down proportionately to the reduction in charging current. For example, if the charging current is reduced by half for two minutes, the timer clock is reduced to half the frequency and the counter counts half as fast resulting in only one minute of "counting" time.

If the pre charge timer expires before the battery voltage reaches V_LOWV, the bq2407x indicates a fault condition. Additionally, if the battery current does not fall to I_TERM before the fast charge timer expires, a fault is indicated. The CHG output flashes at approximately 2 Hz to indicate a fault condition. The fault condition is cleared by toggling CE or the input power, entering/ exiting USB suspend mode, or an OVP event.