BQ24721 Datasheet, PDF (31/48 Pages) Texas Instruments – ADVANCED MULTI-CHEMISTRY AND MULTI-CELL SYNCHRONOUS SWITCH-MODE CHARGER AND SYSTEM POWER SELECTOR

bq24721
www.ti.com
SLUS683 – NOVEMBER 2005
BATTERY CHARGE CURRENT
The battery charge current, I_O(CHARGE), is established by setting the external sense resistor, R_(SRSNS), and the
SMBus Charge current DAC (0 × 14). In order to set the current, first R_(SRSNS) should be chosen based on the
regulation threshold V_(IREG), across this resistor. The listed SBS current corresponds to a 10 mΩ sense resistor.
R_(SRSNS) = V_(IREG) / I_O(CHARGE)
CURRENT REGULATION REFERENCE RAMP UP/DOWN REFERENCE
The bq24721 current regulation loop reference steps-up whenever charge is enabled, and when returning from
fault/suspend mode into charge where the current regulator is turned on. The loop should take control within a
few hundred micro-seconds with very little overshoot due to the LC output filter and the high compensation loop
bandwidth with 300 kHz or 500 kHz operating frequency; therefore, the reference could ramp up from precharge
to fast-charge within 50 µs to 500 ms. Going into fault/suspend mode, short circuit (V_(BAT) < V_(UVT)), Sleepmode
(V_(ACP) < V_(BAT)), or UVLO (VCC < 3.7 V) initiates an immediate shut-off of the high-side PWM FET by setting its
gate to V_(PH). The output inductor and battery load determines the ramp-down rate as it freewheels through the
Schottky diode.
BATTERY VOLTAGE REGULATION
The voltage regulation feedback is through the BAT pin. This input is tied directly to the positive side of the
battery pack. The bq24721 monitors the battery-pack voltage between the BAT and VSS pins. The regulation
voltage is programmed through the SBS-like SMBus interface.
The voltage regulation DAC register input is decoded into an internal 7-bit DAC that programs the voltage on a
per-cell basis, then is multiplied by the number of cells. There are a total of 128 voltage steps with a 6.25 mV
step, giving a per cell range of [4 V – (4.8 V–6.25 mV)]. There are 128 steps in the 3-cell voltage range of
[12 V – (14.4 V–18.75 mV)]. There are 128 steps in the 4-cell voltage range of [16 V – (19.2 V–25 mV)].
Valid voltage values are 9 V, 12 V–14.381 V, and 16 V–19.175 V. The internal voltage DAC allows programming
to 9 V which could be used for waking-up or closing the battery pack. A 9 V programmed voltage is interpreted
as a 2-cell voltage by the BATDEP and BATSHORT thresholds. Step size for 3-cell battery programming voltage
is 18.75 mV. The Charger interprets a 3-cell battery for any voltage between 12 V–14.4 V. Step size for 4-cell
battery programming voltage is 25 mV. The charger interprets a 4-cell battery for any voltage programmed
between 16 V–19.2 V.
Invalid DAC voltages are indicated by the VOR (voltage out of range) bit of the status register. Voltages below
12 V (except for 9 V) are out of range and keep the converter disabled. Voltages between 14.4 V–16 V (including
14.4 V, but not including 16 V) are out of range and keep the converter disabled. Voltages above 19.2 V
(including 19.2 V) are out of range and allow the converter to charge, but the voltage is always set to the
maximum allowable voltage of 19.175 V = (19.2 V–25 mV).
BATTERY SHORT-CIRCUIT OR LOW CONDITION
The number of cells determines the value for the BATDEP threshold and for the BATSHORT threshold, as these
values are programmed on a per-cell basis. The programmed regulation voltage determines the number of cells.
The battery depleted (BATDEP) threshold is programmed by the control register bits b9-b11. The three bit dac
sets the voltage between 2.2 V to 2.9 V per cell at 100 mV increments. The BATSHORT threshold is 1.7 V/Cell
falling entering a shorted condition, and 1.9 V/Cell rising leaving shorted condition, and entering normal
condition. BATSHORT has a 1 second deglitch on both edge directions to protect from transient conditions, and
to allow closing deeply discharge battery packs.
The PWM duty-cycle immediately resets to zero percent when the battery voltage is sensed to drop below 2 V,
then the regulation loop allows the duty-cycle to settle in the current regulation value, C. After a 1 second
deglitch, the converter regulates battery current to C/8 when the battery voltage falls below 1.7 V/cell. The
converter regulates back at C after a 1 second deglitch from the time the battery voltage rises above 1.9 V/cell.
Charge Termination for Li-Ion or Li-Polymer
The primary termination method for Li-Ion and Li-Polymer is minimum current. Secondary temperature
termination methods is also provided for additional safety. The host controls the charge initiation and the
termination. A battery pack gas gauge assists the hosts on setting the voltages and determining when to
terminate based on the battery pack state of charge.
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