MAX8731 Maxim Integrated Products, MAX8731 Datasheet - Page 18

• Manufacturer Part Number: MAX8731
• Description: SMBus Level 2 Battery Charger
• Manufacturer: Maxim Integrated Products

where η is the efficiency of the DC-DC converter (typi-
cally 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 3. The InputCurrent() command uses the
Write-Word protocol (see Figure 3). The command
code for InputCurrent() is 0x3F (0b00111111). When
RS1 = 10mΩ, the MAX8731 provides an input-current-
limit range of 256mA to 11.004A, with 256mA resolu-
tion. InputCurrent() settings from 1mA to 256mA result
in a current limit of 256mA. Upon reset the input current
limit is 256mA.
The MAX8731 includes a timer to terminate charging if
the charger does not receive a ChargeVoltage() or
ChargeCurrent() command within 175s. If a timeout
occurs, both ChargeVoltage() and ChargeCurrent()
commands must be resent to reenable charging.
SMBus Level 2 Battery Charger with
Remote Sense
Table 3. InputCurrent() (0x3F) (10mΩ Sense Resistor, RS1)
18
I
INPUT
______________________________________________________________________________________
BIT
10
11
12
13
14
15
0
1
2
3
4
5
6
7
8
9
=
I
LOAD
Input Current, DACS 0
Input Current, DACS 1
Input Current, DACS 2
Input Current, DACS 3
Input Current, DACS 4
Input Current, DACS 5
+
⎡
⎢
⎢
⎣
(
I
CHARGE
BIT NAME
—
—
—
—
—
—
—
—
—
—
(
V
IN
×
Charger Timeout
×
V
BATTERY
η
)
)
⎤
⎥
⎥
⎦
+
I
BIAS
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.
Not used. Normally a 128mA weight.
0 = Adds 0mA of input current compliance.
1 = Adds 256mA of input current compliance.
0 = Adds 0mA of input current compliance.
1 = Adds 512mA of input current compliance.
0 = Adds 0mA of input current compliance.
1 = Adds 1024mA of input current compliance.
0 = Adds 0mA of input current compliance.
1 = Adds 2048mA of input current compliance.
0 = Adds 0mA of input current compliance.
1 = Adds 4096mA of input current compliance.
0 = Adds 0mA of input current compliance.
1 = Adds 8192mA of input current compliance, 11,004mA max.
Not used. Normally a 16,384mA weight.
Not used. Normally a 32,768mA weight.
Not used. Normally a 65,536mA weight.
The MAX8731 features dual remote sense, which allows
the rejection of board resistance and selector resistance
when used in either single- or dual-battery systems. To
fully utilize remote sensing, connect FBS_ directly to the
battery interface through an unshared battery sense
trace in series with a 100Ω resistor, and 10nF capacitor
(see Figure 1). In single-battery systems, connect
BATSEL directly to GND and use only FBSA.
Remote sensing cancels the effect of impedance in
series with the battery. This impedance normally caus-
es the battery charger to prematurely enter constant-
voltage mode with reducing charge current. The result
is that the last 20% of charging takes longer than nec-
essary. When in constant-voltage mode, the remaining
charge time is proportional to the total resistance in
series with the battery. Remote sensing reduces
charge time according to the following equation:
t
CVRS
DESCRIPTION
=
t
CV
0
×
R
Pack
R
Pack
+
R
Board
Remote Sense