BQ24747 Datasheet, PDF (20/36 Pages) Texas Instruments – SMBus-Controlled Level 2 Multi-Chemistry Battery Charger With Input Current Detect Comparator and Charge Enable Pin

bq24747
SLUS988 – OCTOBER 2009 ............................................................................................................................................................................................. www.ti.com
The ISYNSET comparator, or charge undercurrent comparator, compares the voltage between CSOP-CSON and
the 13-mV internal threshold. The threshold is set internally to 13 mV on the falling edge and 8 mV on the rising
edge (with built-in hysteresis) with 10% variation.
HIGH ACCURACY VICM USING CURRENT SENSE AMPLIFIER (CSA)
An industry-standard, high-accuracy current-sense amplifier (CSA) provides an analog output voltage at the
VICM pin that can be used by a host system to monitor the input current. The CSA amplifies the input sensed
voltage of CSSP-CSSN by 20× through the VICM pin. The VICM output is a voltage source 20× the input
differential voltage. When DCIN is above 4 V and ACIN is above 0.6 V, VICM no longer stays at ground, but
becomes active. A lower voltage can be used by connecting a resistor divider from VICM to GND, while still
achieving good accuracy over temperature if the resistors are matched by their thermal coefficients.
A 0.1μF capacitor connected on the output is recommended for decoupling high-frequency noise. An additional
RC filter is optional, after the 0.1μF capacitor, if additional filtering is desired. Note that adding filtering also adds
additional response delay.
VDDSMB INPUT SUPPLY
The VDDSMB input provides bias power to the SMBus interface which is active when: 1) DCIN > DCIN_UVLO,
2) ACIN > 0.6 V, and 3) VDDSBM > VDDSBM_UVLO. Connect VDDSMB to an external 3.3 V or 5 V supply rail
to keep the SMBus interface active while the supply to DCIN is connected. Under this condition, the internal
registers are maintained, and SMBus communication can occur between the host and the charger. Bypass
VDDSMB to GND with a 0.1-μF or greater ceramic capacitor. The VDDSMB UVLO threshold is 2.7 V rising and
250 mV falling (with hysteresis). The SMBus is always active and can be written to or read from whenever
VDDSMB is above the VDDSMB UVLO threshold.
INPUT UNDER VOLTAGE LOCK OUT (UVLO)
The system must have a minimum 4 V DCIN voltage from the input adapter to allow proper charger operation.
When the DCIN voltage is below 4 V, the bias circuits VDDP and VREF stay inactive, even with ACIN above
0.6 V.
BATTERY OVERVOLTAGE PROTECTION
The converter will not allow the high-side FET to turn-on when the battery voltage at VFB exceeds 104% of the
regulation voltage set-point, until the VFB voltage returns below 101% of the regulation voltage. This allows quick
response to an overvoltage condition – such as occurs when the load is removed or the battery is disconnected.
A 10-mA current sink from VFB to PGND is on only during charge and allows discharging the stored output
inductor energy that is transferred to the output capacitors.
BATTERY SHORTED (Battery Undervoltage) PROTECTION AND BATTERY TRICKLE CHARGING
The bq24747 has a VFB SHORT comparator monitoring the output battery VFB voltage. If the voltage falls below
2.5 V (absolute, fixed), a battery-short status is detected. The charger continues charging at the value
programmed on the ChargeCurrent(0x14) register down to 2.5 V falling, and 2.7 V rising on the VFB pin.
The bq24747 automatically reduces the charge current limit to a fixed 128 mA to trickle charge the battery, when
the voltage on the VFB pin falls below 2.5 V. The charge current returns to the value programmed on the
ChargeCurrent(0x14) register, when the VFB pin voltage rises above 2.7 V.
This function provides short circuit protection from the battery node, and it also provides a safe trickle charge to
close deeply discharged open packs.
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