BQ24725A_17 Datasheet, PDF (26/47 Pages) Texas Instruments – 1-4 Cell Li+ Battery SMBus Charge Controller with N-Channel Power MOSFET Selector and Advanced Circuit Protection

bq24725A
SLUSAL0A – SEPTEMBER 2011 – REVISED AUGUST 2014
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8.5.5 Setting Input Current
System current normally fluctuates as portions of the system are powered up or put to sleep. With the input
current limit, the output current requirement of the AC wall adapter can be lowered, reducing system cost.
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 bq24725A
decreases the charge current to provide priority to system load current. As the system current rises, the available
charge current drops linearly to zero. Thereafter, all input current goes to system load and input current
increases.
During DPM regulation, the total input current is the sum of the device supply current I_BIAS, the charger input
current, and the system load current I_LOAD, and can be estimated as follows:
I_INPUT
= I_LOAD
+
é
ê
ë
I_BATTERY ´
V_IN
V_BATTERY
´η
ù
ú
û
+ I_BIAS
(3)
where η is the efficiency of the charger buck converter (typically 85% to 95%).
To set the input current limit, write a 16-bit InputCurrent() command (0x3FH or 0b00111111) using the data
format listed in Table 6. When using a 10mΩ sense resistor, the bq24725A provides an input-current limit range
of 128mA to 8.064A, with 128mA resolution. The suggested input current limit is set to no less than 512mA.
Sending InputCurrent() below 128mA or above 8.064A clears the register and terminates charging. Upon POR,
the default input current limit is 4096mA.
The ACP and ACN pins are used to sense R_AC with default value of 10mΩ. However, resistors of other values
can also be used. For a larger sense resistor, larger sense voltage is given, and a higher regulation accuracy;
but, at the expense of higher conduction loss.
If input current rises above FAST_DPM threshold, the charger will reduce charging current to allow the input
current drop. After a typical 260-µs delay time, if input current is still above FAST_DPM threshold, the charger
will shut down. The charger will soft restart to charge the battery if the adapter still has power to charge the
battery. This prevents a crash if the adapter is overloaded when the system has a high and fast loading transient.
The waiting time between shut down and restart charging is a natural response time of the input current limit
loop.
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