SMBus Level 2 Battery Charger
with Remote Sense
Remote Sense
The MAX8731A 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.
⎡
⎤
I
(
× V
)
CHARGE
BATTERY
I
= I
+
+I
BIAS
⎢
⎥
INPUT
LOAD
V
× η
⎢
⎣
⎥
⎦
IN
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 MAX8731A 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.
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:
Charger Timeout
The MAX8731A 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.
R
Pack
t
= t
×
CV0
CVRS
R
+R
Pack
Board
Table 3. InputCurrent() (0x3F) (10mΩ Sense Resistor, RS1)
BIT
BIT NAME
DESCRIPTION
0
—
—
—
—
—
—
—
Not used. Normally a 2mA weight.
1
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.
2
3
4
5
6
0 = Adds 0mA of input current compliance.
1 = Adds 256mA of input current compliance.
7
8
Input Current, DACS 0
Input Current, DACS 1
Input Current, DACS 2
Input Current, DACS 3
Input Current, DACS 4
Input Current, DACS 5
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.
9
0 = Adds 0mA of input current compliance.
1 = Adds 2048mA of input current compliance.
10
11
12
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.
13
14
15
—
—
—
Not used. Normally a 16,384mA weight.
Not used. Normally a 32,768mA weight.
Not used. Normally a 65,536mA weight.
18 ______________________________________________________________________________________
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