BQ24273 Datasheet, PDF (10/27 Pages) Texas Instruments – 2.5A, Single Input, Single Cell Switchmode Li-Ion Battery Charger with Integrated Current Sense

bq24273
SLUSB08 – JUNE 2012
www.ti.com
Battery Charging Process
When the battery is deeply discharged or shorted (V_BAT<V_BATSHRT), the bq24273 applies a 50mA current to close
the pack protector switch and bring the battery voltage up to acceptable charging levels. Once the battery rises
above V_BATSHRT, the charge current is regulated to the value set in the I²C register. If the die temperature does
heat up, the thermal regulation circuit reduces the charge current to maintain a die temperature less than 125°C.
The slew rate for fast charge current is controlled to minimize the current and voltage over-shoot during transient.
The charge current is regulated to I_OCHARGE until the battery is charged to the regulation voltage. Once the
battery voltage is close to the regulation voltage, V_BATREG, the charge current is tapered down as shown in
Figure 2. The voltage regulation feedback occurs by monitoring the battery-pack voltage between the BAT and
PGND pins. The bq24273 is a fixed single-cell voltage version, with adjustable regulation voltage (3.5V to 4.44V)
programmed using the I²C interface.
The bq24273 monitors the charging current during the voltage regulation phase. Once the termination threshold,
I_TERM, is detected and the battery voltage is above the recharge threshold, the bq24273 terminates the charge
cycle and enters battery detection. The termination current level is programmable. To disable the charge current
termination, the host sets the charge termination bit (TE) of charge control register to 0, refer to I²C section for
details.
A new charge cycle is initiated when one of the following conditions is detected:
1. The battery voltage falls below the V_BATREG-V_RCH threshold.
2. CE bit toggle or RESET bit is set
3. HI-Z bit toggle
Battery Detection
When termination conditions are met, a battery detection cycle is started. During battery detection, I_DETECT is
pulled from V_BAT for t_DETECT to verify there is a battery. If the battery voltage remains above V_DETECT for the full
duration of t_DETECT, a battery is determined to present and the IC enters “Charge Done”. If V_BAT falls below
V_DETECT, a “Battery Not Present” fault is signaled and battery detection continues. The next cycle of battery
detection, the bq2427x turns on I_BATSHORT for t_DETECT. If V_BAT rises to V_DETECT, the current source is turned off and
after t_DETECT, the battery detection continues through another current sink cycle. Battery detection continues until
charge is disabled or a battery is detected. Once a battery is detected, the fault status clears and a new charge
cycle begins. Battery detection is not run when termination is disabled.
DEFAULT Mode
DEFAULT mode is used when I²C communication is not available. DEFAULT mode is entered in the following
situations:
1. When the charger is enabled and V_BAT<3.6V before I²C communication is established
2. When the watchdog timer expires without a reset from the I²C 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 I²C
communication is established
In default mode, the I²C 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 for the IN input is set to 1.5A. Default mode is exited by programming the I²C 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 bq24273 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 I²C 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 I²C. 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.
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