SMBus Level 2 Battery Charger
with Remote Sense
Setting Charge Current
To set the charge current, use the SMBus to write a 16-
bit ChargeCurrent() command using the data format
listed in Table 2. The ChargeCurrent() command uses
the Write-Word protocol (see Figure 3). The command
code for ChargeCurrent() is 0x14 (0b00010100). When
RS2 =10mΩ, the MAX8731A provides a charge current
range of 128mA to 8.064A, with 128mA resolution. Set
ChargeCurrent() to 0 to terminate charging. Upon reset,
the ChargeVoltage() and ChargeCurrent() values are
cleared and the charger remains off until both the
ChargeVoltage() and the ChargeCurrent() commands
are sent. Both DHI and DLO remain low until the charger
is restarted.
Setting Input-Current Limit
System current normally fluctuates as portions of the
system are powered up or put to sleep. By using the
input-current-limit circuit, the output-current require-
ment 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 MAX8731A
decreases the charge current to provide priority to sys-
tem load current. As the system supply rises, the avail-
able charge current drops linearly to zero. Thereafter,
the total input current can increase without limit.
The MAX8731A includes a foldback current limit when
the battery voltage is low. If the battery voltage is less
than 2.5V, the charge current is temporarily set to
128mA. The ChargeCurrent() register is preserved and
becomes active again when the battery voltage is high-
er than 2.5V. This function effectively provides a fold-
back current limit, which protects the charger during
short circuit and overload.
The internal amplifier compares the differential voltage
between CSSP and CSSN to a scaled voltage set by
the InputCurrent() command (see Table 3). 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 2. ChargeCurrent() (0x14) (10mΩ Sense Resistor, RS2)
BIT
BIT NAME
DESCRIPTION
0
—
—
—
—
—
—
—
Not used. Normally a 1mA weight.
1
Not used. Normally a 2mA weight.
Not used. Normally a 4mA weight.
Not used. Normally an 8mA weight.
Not used. Normally a 16mA weight.
Not used. Normally a 32mA weight.
Not used. Normally a 64mA weight.
2
3
4
5
6
0 = Adds 0mA of charger current compliance.
1 = Adds 128mA of charger current compliance.
7
8
Charge Current, DACI 0
Charge Current, DACI 1
Charge Current, DACI 2
Charge Current, DACI 3
Charge Current, DACI 4
Charge Current, DACI 5
0 = Adds 0mA of charger current compliance.
1 = Adds 256mA of charger current compliance.
0 = Adds 0mA of charger current compliance.
1 = Adds 512mA of charger current compliance.
9
0 = Adds 0mA of charger current compliance.
1 = Adds 1024mA of charger current compliance.
10
11
12
0 = Adds 0mA of charger current compliance.
1 = Adds 2048mA of charger current compliance.
0 = Adds 0mA of charger current compliance.
1 = Adds 4096mA of charger current compliance, 8064mA max.
13
14
15
—
—
—
Not used. Normally a 8192mA weight.
Not used. Normally a 16,386mA weight.
Not used. Normally a 32,772mA weight.
______________________________________________________________________________________ 17
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
