BQ24740_07 Datasheet, PDF (17/32 Pages) Texas Instruments – Host-controlled Multi-chemistry Battery Charger with Low Input Power Detect

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TYPICAL APPLICATIONS
bq24740
SLUS736 – DECEMBER 2006
DETAILED DESCRIPTION
BATTERY VOLTAGE REGULATION
The bq24740 uses a high-accuracy voltage regulator for charging voltage. Internal default battery voltage setting
V_BATT=4.2 V × cell count. The regulation voltage is ratio-metric with respect to VADC. The ratio of VADJ and
VDAC 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 VDAC using Equation 1.
ƪ ǒ Ǔƫ V_BATT + cell count
4V ) 0.5
V_VADJ
V_VDAC
(1)
The input voltage range of VDAC is between 2.6 V and 3.6 V. VADJ is set between 0 and VDAC. 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 VDAC using
Equation 2:
I _CHARGE
+
V_SRSET
V_VDAC
0.10
R_SR
(2)
The input voltage range of SRSET is between 0 and V_DAC, up to 3.6 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. For a larger the sense resistor, you get 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, reducing system cost.
Similar to setting battery regulation current, adapter current is sensed by resistor R_AC connected between ACP
and ACN. Its maximum value is set ACSET, which is ratio-metric with respect to VDAC, using Equation 3.
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