BQ24742 Datasheet, PDF (18/35 Pages) Texas Instruments – Li-Ion or Li-Polymer Battery Charger with Low Iq and Accurate Trickle Charge

bq24741, bq24742
SLUS875B – MARCH 2009 – REVISED OCTOBER 2009
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During the DCM mode the loop response automatically changes and has a single pole system at which the pole
is proportional to the load current, because the converter does not sink current, and only the load provides a
current sink. This means at very low currents the loop response is slower, as there is less sinking current
available to discharge the output voltage. At very low currents during non-synchronous operation, there may be a
small amount of negative inductor current during the 80ns recharge pulse. The charge should be low enough to
be absorbed by the input capacitance.
Whenever the converter goes into zero percent duty-cycle, the high-side MOSFET does not turn on, and the
low-side MOSFET does not turn on (no 80ns recharge pulse) either, and there is no discharge from the battery.
Battery Voltage Regulation
The bq24741/2 uses a high-accuracy voltage regulator for charging voltage. The regulation voltage is ratio-metric
with respect to VDAC. 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 the following equation:
V_BATT
=
cell
é
count ´ _ê4
êë
V
æ
+ _ç 0.512 ´
è
V_VADJ
V_VDAC
öù
÷ú
ø úû
(2)
The input voltage range of VDAC is between 2.6V and 3.6V. VADJ is set between 0 and VDAC.
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.
Table 3. Cell-Count Selection
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 0.1μF 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 ISET input sets the maximum charging current. Battery current is sensed by resistor RSR connected
between CSP and CSN. The full-scale differential voltage between CSP and CSN is 100 mV. Thus, for a 0.020 Ω
sense resistor, the maximum charging current is 5 A. ISET is ratio-metric with respect to VDAC using the
following equation:
I_CHARGE
=
V_ISET
V_VDAC
0.10
´ _RSR
(3)
The input voltage range of ISET is between 0 and VDAC, up to 3.6 V.
The CSP and CSN pins are used to sense across R_SR with default value of 20 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.
18
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