isl88731c Intersil Corporation, isl88731c Datasheet - Page 16

• Manufacturer Part Number: isl88731c
• 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 ISL88731C 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 shown in Equation 2.
Where η is the efficiency of the DC/DC converter
(typically 85% to 95%).
The ISL88731C 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 4). Equivalently, the 16-bit InputCurrent number
sets the voltage across CSSP and CSSN inputs in 20µV
per LSB increments. To set the input current limit use the
I
INPUT
BIT
10
11
12
13
14
15
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
+
[
(
TABLE 4. INPUT CURRENT (REGISTER 0x3F) (10mΩ SENSE RESISTOR, RS1)
I
CHARGE
BIT NAME
16
×
V
BATTERY
)
⁄
(
V
IN
Not used.
Not used.
Not used.
Not used.
Not used.
Not used.
Not used.
1 = Adds 256mA of input current.
1 = Adds 512mA of input current.
1 = Adds 1024mA of input current.
1 = Adds 2048mA of input current.
1 = Adds 4096mA of input current.
1 = Adds 8192mA of input current, 11004mA max.
Not used.
Not used.
Not used.
×
0 = Adds 0mA of input current.
0 = Adds 0mA of input current.
0 = Adds 0mA of input current.
0 = Adds 0mA of input current.
0 = Adds 0mA of input current.
0 = Adds 0mA of input current.
η
)
]
(EQ. 2)
ISL88731C
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 21). The
register code for InputCurrent is 0x3F (0b00111111).
The InputCurrent register can be read back to verify its
contents.
The ISL88731C ignores the first 7 LSBs and uses the
next 6 bits to control the input-current DAC. The
input-current range of the ISL88731C 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
September 9, 2010
FN6978.1