isl88731 Intersil Corporation, isl88731 Datasheet - Page 14

• Manufacturer Part Number: isl88731
• Description: Smbus Level 2 Battery Charger
• Manufacturer: Intersil Corporation

Setting Input-Current Limit
The total power from an AC adapter is the sum of the power
supplied to the system and the power into the charger and
battery. When the input current exceeds the set input current
limit, the ISL88731 decreases the charge current to provide
priority to system load current. As the system load rises, the
available charge current drops linearly to zero. Thereafter, the
total input current can increase to the limit of the AC adapter.
The internal amplifier compares the differential voltage
between CSSP and CSSN to a scaled voltage set by the
InputCurrent register. 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:
I
Where η is the efficiency of the DC/DC converter (typically
85% to 95%).
The ISL88731 has a 16-bit InputCurrent register that
translates to a 2mA LSB and a 131.071A full scale current
using a 10mΩ current-sense resistor (RS1 in Figure 2).
Equivalently, the 16-bit InputCurrent number sets the voltage
across CSSP and CSSN inputs in 20µV per LSB increments.
INPUT
BIT
10
12
13
14
15
11
0
1
2
3
4
5
6
7
8
9
=
I
SYSTEM
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
TABLE 4. INPUT CURRENT (REGISTER 0x3F) (10mΩ SENSE RESISTOR, RS1)
BIT NAME
14
×
V
BATTERY
)
⁄
(
V
IN
×
Not used.
Not used.
Not used.
Not used.
Not used.
Not used.
Not used.
0 = Adds 0mA of input current.
1 = Adds 256mA of input current.
0 = Adds 0mA of input current.
1 = Adds 512mA of input current.
0 = Adds 0mA of input current.
1 = Adds 1024mA of input current.
0 = Adds 0mA of input current.
1 = Adds 2048mA of input current.
0 = Adds 0mA of input current.
1 = Adds 4096mA of input current.
0 = Adds 0mA of input current.
1 = Adds 8192mA of input current, 11004mA max.
Not used.
Not used.
Not used.
η
)
]
(EQ. 2)
ISL88731
To set the input current limit use the SMBus to write a 16-bit
InputCurrent register using the data format listed in Table 4.
The InputCurrent register uses the Write-Word protocol (see
Figure 18). The register code for InputCurrent is 0x3F
(0b00111111). The InputCurrent register can be read back to
verify its contents.
The ISL88731 ignores the first 7 LSBs and uses the next
6 bits to control the input-current DAC. The input-current
range of the ISL88731 is from 256mA to 11.004A. All 16-bit
numbers requesting input current above 11.004A result in an
input-current setting of 11.004A. All 16-bit numbers
requesting input current between 0mA to 256mA result in an
input-current setting of 0mA. The default input-current-limit
setting at POR is 256mA. When choosing the current-sense
resistor RS1, carefully calculate its power rating. Take into
account variations in the system’s load current and the
overall accuracy of the sense amplifier. Note that the voltage
drop across RS1 contributes additional power loss, which
reduces efficiency. System currents normally fluctuate as
portions of the system are powered up or put to sleep.
Without input current regulation, the input source must be
able to deliver the maximum system current and the
maximum charger-input current. By using the input-current-
limit circuit, the output-current capability of the AC wall
adapter can be lowered, reducing system cost.
DESCRIPTION
November 20, 2006
FN9258.0