BQ24745_14 Datasheet, PDF (22/39 Pages) Texas Instruments – SMBus-Controlled Multi-Chemistry Battery

bq24745
SLUS761D – DECEMBER 2007 – REVISED OCTOBER 2011
Table 4. Charge Current Register (0x14), Using 10-mΩ Sense Resistor (continued)
BIT
BIT NAME
12
Charge current, DACI 5
13
–
14
–
15
–
DESCRIPTION
0 = Adds 0 mA of charger current
1 = Adds 4,096 mA of charger current
Not used
Not used
Not used
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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 system 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 to keep the input current from exceeding the limit set by the Input Current SMBus 6-bit DAC
register. With high-accuracy limiting, the current capability of the ac adaptor can be lowered, reducing system
cost.
The CSSP and CSSN pins are used to sense R_AC with a default value of 10 mΩ. However, resistors of other
values can also be used. A larger a sense resistor results in a larger sense voltage and a higher regulation
accuracy, but at the expense of higher conduction loss.
The total input current, from a wall cube or other dc source, is the sum of the system supply current and the
current required by the charger. When the input current exceeds the set input current limit, the bq24745
decreases the charge current to provide priority to system load current. As the system supply rises, the available
charge current drops linearly to zero.
I_INPUT
=
I_SYST
EM
+
é
ê
ë
I_LO
AD
´
V_IN
V_{BATT ER Y}
´h
ù
ú
û
+ I_BIAS
(1)
where η is the efficiency of the dc-dc converter (typically 85% to 95%).
To set the input current limit, use the SMBus to write a 16-bit InputCurrent() command using the data format
listed in Table 5. The InputCurrent() command uses the Write-Word protocol (see Figure 33). The command
code for InputCurrent() is 0x3F (0b0011 1111). When using a 10-mΩ sense resistor, the bq24745 provides an
input-current limit range of 256 mA to 11.008 A, with 256-mA resolution. InputCurrent() settings from 1 mA to
256 mA clears DAC and terminates charge. On reset the input current limit is 256 mA.
Table 5. Input Current Register (0x3F), Using 10-mΩ Sense Resistor.
BIT
BIT NAME
0
–
1
–
2
–
3
–
4
–
5
–
6
–
7
Charge current, DACS 0
8
Charge current, DACS 1
9
Charge current, DACS 2
10
Charge current, DACS 3
11
Charge current, DACS 4
DESCRIPTION
Not used
Not used
Not used
Not used
Not used
Not used
Not used
0 = Adds 0 mA of charger current
1 = Adds 256 mA of charger current
0 = Adds 0 mA of charger current
1 = Adds 512 mA of charger current
0 = Adds 0 mA of charger current
1 = Adds 1,024 mA of charger current
0 = Adds 0 mA of charger current
1 = Adds 2,048 mA of charger current
0 = Adds 0 mA of charger current
1 = Adds 4,096 mA of charger current
22
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