BQ24750_07 Datasheet, PDF (18/38 Pages) Texas Instruments – Host-controlled Multi-chemistry Battery Charger with Integrated System Power Selector and AC Over-Power Protection

bq24750
SLUS735 – DECEMBER 2006
DETAILED DESCRIPTION
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Battery Voltage Regulation
The bq24750 uses a high-accuracy voltage regulator for the charging voltage. The internal default
battery-voltage setting is V_BATT = 4.2 V × cell count. The regulation voltage is ratiometric with respect to VDAC.
The ratio of VADJ and VDAC provides an extra 12.5% adjustment range on the V_BATT regulation voltage. By
limiting the adjustment 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, even while using
1%-mismatched resistors. Ratiometric 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)
REGN – V_t = REGN – 0.5 V
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 charge-termination voltage is a 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 charge 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 ratiometric 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 a default value of 10 mΩ. However, resistors of
other values can also be used. A larger sense-resistor value yields a larger sense voltage, and a higher
regulation accuracy. However, this is at the expense of a higher conduction loss.
Input Adapter Current Regulation
The total input current 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
capacity 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 ratiometric with respect to VDAC, using Equation 3.
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