BQ24747 Datasheet, PDF (26/36 Pages) Texas Instruments – SMBus-Controlled Level 2 Multi-Chemistry Battery Charger With Input Current Detect Comparator and Charge Enable Pin

bq24747
SLUS988 – OCTOBER 2009 ............................................................................................................................................................................................. www.ti.com
SETTING THE INPUT CURRENT
System current normally fluctuates as portions of the system are powered up or down, or enter low-power mode.
By using the input-current limit circuit, the output-current requirement of the AC wall adapter can be lowered,
reducing system cost.
The total input current, from a power-line wall adapter 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 programmed input current
limit, the bq24747 decreases the charge current to provide priority to the system load current. As the system
supply current rises, the available charge current drops linearly to zero. Thereafter, the total input current can
increase without limit.
The internal amplifier compares the differential voltage between CSSP and CSSN to a scaled voltage set by the
InputCurrent() command (see Table 5). The total input current is the sum of the device supply current, the
charger input current, and the system load current. The total input current can be estimated as follows:
Table 5. Input Current Register (0x3F), Using 10mΩ Sense Resistor
BIT
BIT NAME
DESCRIPTION
0
–
Not used.
1
–
Not used.
2
–
Not used.
3
–
Not used.
4
–
Not used.
5
–
Not used.
6
–
Not used.
7
Input Current, DACS 0
0 = Adds 0 mA of charger current
1 = Adds 256 mA of charger current.
8
Input Current, DACS 1
0 = Adds 0 mA of charger current
1 = Adds 512 mA of charger current
9
Input Current, DACS 2
0 = Adds 0 mA of charger current.
1 = Adds 1024 mA of charger current.
10
Input Current, DACS 3
0 = Adds 0 mA of charger current.
1 = Adds 2048 mA of charger current.
11
Input Current, DACS 4
0 = Adds 0 mA of charger current.
1 = Adds 4096 mA of charger current
12
Input Current, DACS 5
0 = Adds 0 mA of charger current.
1 = Adds 8192 mA of charger current, 11008 mA max.
13
–
Not used.
14
–
Not used.
15
–
Not used.
ƪ ƫ I_INPUT + I_LOAD )
I_LOAD V_BATTERY
V_IN h
) I_BIAS
(2)
where η is the efficiency of the DC-DC converter (typically 85% to 95%).
To set the input current limit, write a 16-bit InputCurrent() command using the data format listed in Table 5. The
InputCurrent() command uses the Write-Word protocol (see Figure 34). The command code for InputCurrent() is
0x3F (0b00111111). When using a 10-mΩ sense resistor, the bq24747 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 result in a current limit
of 256 mA. Upon reset the input current limit is 256 mA.
CHARGER TIMEOUT
The bq24747 includes a timer to terminate charging if the charger does not receive a ChargeVoltage() or
ChargeCurrent() command within 175 s. If a timeout occurs, both ChargeVoltage() and ChargeCurrent()
commands must be resent to re-enable charging.
26
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