Datasheet

bq24270
bq24271
SLUSB10 –JUNE 2012 www.ti.com
Battery Discharge FET (BGATE)
The bq24270 and bq24271 contain a MOSFET driver to drive an external discharge FET between the battery
and the system output. This external FET provides a low impedance path when supplying the system from the
battery. Connect BGATE to the gate of the external discharge MOSFET. BGATE is on under the following
conditions:
1. No valid input supply connected.
2. 2. HZ_MODE = 1
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)
< 3.6 V 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 devices 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 1 A. The
input current limit for the USB input is determined by the D+ and D- detection (bq24270) or PSEL (bq24271).
Default mode is exited by programming the I
2
C interface. Once I
2
C communication is established, PSEL has no
effect on the USB input. 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 bq24270 and bq24271 start 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 128 μs
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 devices contain 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 27 minutes 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 5.
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Product Folder Link(s): bq24270 bq24271