BQ24747 Datasheet, PDF (17/36 Pages) Texas Instruments – SMBus-Controlled Level 2 Multi-Chemistry Battery Charger With Input Current Detect Comparator and Charge Enable Pin

bq24747
www.ti.com ............................................................................................................................................................................................. SLUS988 – OCTOBER 2009
DETAILED DESCRIPTION
BATTERY VOLTAGE REGULATION
The bq24747 uses a high-accuracy voltage regulator to supply charging voltage. The battery voltage regulation
setting is programmed by the host microcontroller (μC), through the SMBus interface that sets an 11-bit DAC.
The input voltage range of VFB is between 1.024 V and 19.2 V. The per-cell battery termination voltage is a
function of the battery chemistry. (Consult the battery manufacturer to determine this voltage.) The programmed
value should be the per-cell voltage times the number of series cells.
The VFB pin is used to sense the battery voltage for voltage regulation and should be connected as close to the
battery as possible, or directly on the output capacitor. A 0.1-μF ceramic capacitor from VFB to GND is
recommended to be as close to the VFB pin as possible to decouple high frequency noise.
BATTERY CURRENT REGULATION
The ChargeCurrent() SMBus 6-bit DAC register sets the maximum charging current. Battery current is sensed by
resistor R_SR connected between the CSOP and CSON pins. The maximum full-scale differential voltage between
CSOP and CSON is 80.64 mV. Thus, for a 0.010-Ω sense resistor, the maximum charging current is 8.064 A.
The CSOP and CSON pins are used to measure the voltage across R_SR, which has a default value of 10 mΩ.
However, resistors of other values can also be used. A larger sense resistor gives a larger sense voltage and
higher regulation accuracy, but at the expense of higher conduction loss.
INPUT ADAPTER CURRENT REGULATION
The total input current from an AC adapter or other DC source is a function of the system supply current and the
battery charging current. System current normally fluctuates as portions of the systems are powered up or down.
Without Dynamic Power Management (DPM), the source must be able to supply the maximum system current
and the maximum charger input current simultaneously. By using DPM, the input current regulator reduces the
charging current when the input current exceeds the limit set by the InputCurrent() SMBus 6 bit DAC register.
With the high-accuracy limiting, the current capability of the AC adaptor can be lowered, reducing system cost.
In a manner similar to battery-current regulation, adaptor current is sensed by resistor R_AC connected between
the CSSP and CSSN pins. The maximum full-scale differential voltage between CSSP and CSSN is 110.08 mV.
Thus, for a 0.010Ω sense resistor, the maximum input current is 11.008 A.
The CSSP and CSSN pins are used to sense R_AC with default value of 10 mΩ. However, resistors of other
values can also be used. A larger sense resistor gives a larger sense voltage and a higher regulation accuracy,
but at the expense of higher conduction loss.
ADAPTER DETECT AND POWER UP
An external resistor voltage divider attenuates the adapter voltage before it goes to ACIN. The adapter-detect
threshold should typically be programmed to a value greater than the maximum battery voltage and lower than
the minimum allowed adapter voltage.
If DCIN is below 4 V, the charger is disabled and ACOK goes low.
If ACIN is below 0.6 V but DCIN is above 4 V, part of the bias is enabled, including a crude bandgap reference,
ACFET drive and BATFET drive. VICM is disabled and pulled down to GND. The total quiescent current is less
than 10μA.
When ACIN rises above 0.6 V and DCIN is above 4 V, all the bias circuits are enabled, the VDDP output goes to
6 V, and VREF goes to 3.3 V. VICM becomes valid to proportionally reflect the adapter current.
When ACIN keeps rising and passes 2.4 V, it indicates that a valid AC adapter is present. 200μs later, and the
following occurs:
• ACOK is pulled high through an external pull-up resistor to the host digital voltage rail;
• The charger turns on if all the conditions are satisfied after an additional 2-ms deglitch time. (refer to Enable
and Disable Charging )
Copyright © 2009, Texas Instruments Incorporated
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