Datasheet

bq24272
www.ti.com SLUSB09 –JUNE 2012
DEFAULT Mode
DEFAULT mode is used when I
2
C communication is not available. DEFAULT mode is entered in the following
situations:
1. When the charger is enabled and V
BAT
<3.6V before I
2
C communication is established
2. When the watchdog timer expires without a reset from the I
2
C interface and the safety timer has not expired.
3. When the device comes out of any fault condition (sleep mode, OVP, faulty adapter mode, etc.) before I
2
C
communication is established
In default mode, the I
2
C registers are reset to the default values. The 27 min safety timer is reset and starts when
DEFAULT mode is entered. The default value for V
BATREG
is 3.6V, and the default value for I
CHARGE
is 1A. The
input current limit is 1.5A by default. DEFAULT mode is exited by programming the I
2
C interface. Note that if
termination is enabled and charging has terminated, a new charge cycle is NOT initiated when entering
DEFAULT mode.
Safety Timer and Watchdog Timer
At the beginning of charging process, the bq24272 starts the safety timer. This timer is active during the entire
charging process. If charging has not terminated before the safety timer expires, charging is halted and the CE
bit is written to a “1”. The length of the safety timer is selectable using the I
2
C interface. A single 128μs pulse is
sent on the STAT and INT outputs and the STATx bits of the status registers are updated in the I
2
C. The CE bit
must be toggled in order to clear the safety timer fault. The safety timer duration is selectable using the TMR_X
bits in the Safety Timer Register/NTC Monitor register. Changing the safety timer duration resets the safety timer.
If the safety timer expires, charging is disabled (CE changed to a “1”). This function prevents continuous charging
of a defective battery if the host fails to reset the safety timer.
In addition to the safety timer, the bq24272 contains a watchdog timer that monitors the host through the I
2
C
interface. Once a read/write is performed on the I
2
C interface, a 30-second timer (t
WATCHDOG)
is started. The 30-
second timer is reset by the host using the I
2
C interface. This is done by writing a “1” to the reset bit (TMR_RST)
in the control register. The TMR_RST bit is automatically set to “0” when the 30-second timer is reset. This
process continues until battery is fully charged or the safety timer expires. If the 30-second timer expires, the IC
enters DEFAULT mode where the default register values are loaded, the safety timer restarts at 27minutes and
charging continues. The I
2
C may be accessed again to reinitialize the desired values and restart the watchdog
timer as long as the 27 minute safety timer has not expired. The watchdog timer flow chart is shown in Figure 4.
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