BQ24705 Datasheet, PDF (15/30 Pages) Texas Instruments – Host-Controlled Multi-Chemistry Battery Charger With Low Input Power Detect

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DETAILED DESCRIPTION
bq24705
SLUS779 – DECEMBER 2007
BATTERY VOLTAGE REGULATION
The bq24705 uses a high-accuracy voltage regulator for charging voltage. The internal default battery voltage
setting V_BATT= 4.2 V × cell count. The regulation voltage is ratio-metric with respect to VREF. The ratio of VADJ
and VREF provides extra 12.5% adjust range on V_BATT regulation voltage. By limiting the adjust range to 12.5%
of the regulation voltage, the external resistor mismatch error is reduced from ±1% to ±0.1%. Therefore, an
overall voltage accuracy as good as 0.5% is maintained, while using 1% mis-match resistors. Ratio-metric
conversion also allows compatibility with D/As or microcontrollers (µC). The battery voltage is programmed
through VADJ and VREF using Equation 1.
ƪ ǒ Ǔƫ V_BATT + cell count
4V ) 0.5
V_VADJ
V_REF
(1)
VADJ is set between 0 and VREF. V_BATT defaults to 4.2 V × cell count when VADJ is connected to REGN.
CELLS pin is the logic input for selecting cell count. Connect CELLS to charge 2,3, or 4 Li+ cells. When charging
other cell chemistries, use CELLS to select an output voltage range for the charger.
CELLS
Float
AGND
VREF
CELL COUNT
2
3
4
The per-cell battery termination voltage is function of the battery chemistry. Consult the battery manufacturer to
determine this voltage.
The BAT 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 BAT to AGND is
recommended to be as close to the BAT pin as possible to decouple high frequency noise.
BATTERY CURRENT REGULATION
The SRSET input sets the maximum charging current. Battery current is sensed by resistor R_SR connected
between SRP and SRN. The full-scale differential voltage between SRP and SRN is 100 mV. Thus, for a 0.010 Ω
sense resistor, the maximum charging current is 10 A. SRSET is ratio-metric with respect to VREF using
Equation 2:
I _CHARGE
+
V_SRSET
V_REF
0.10
R_SR
(2)
The input voltage range of SRSET is between 0 and VREF, up to 3.3 V.
The SRP and SRN pins are used to sense across R_SR with default value of 10 mΩ. However, resistors of other
values can also be used. A larger the sense resistor, gives a larger sense voltage, and a higher regulation
accuracy, but at the expense of higher conduction loss.
INPUT ADAPTER CURRENT REGULATION
The total input from an AC adapter or other DC sources 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 input current limit set by ACSET. The current capability of the AC
adapter can be lowered, which may reduce the system cost.
Similar to setting battery regulation current, adapter current is sensed by resistor R_AC connected between ACP
and ACN. The maximum value is set by ACSET, which is a ratio-metric with respect to VREF, using Equation 3.
Copyright © 2007, Texas Instruments Incorporated
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