ISL88731CHRTZ Intersil, ISL88731CHRTZ Datasheet - Page 16

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ISL88731CHRTZ

Manufacturer Part Number
ISL88731CHRTZ
Description
IC BATT CHRGR SMBUS LVL2 28TQFN
Manufacturer
Intersil
Datasheet

Specifications of ISL88731CHRTZ

Function
Charge Management
Battery Type
Lithium-Ion (Li-Ion)
Voltage - Supply
8 V ~ 26 V
Operating Temperature
-10°C ~ 100°C
Mounting Type
Surface Mount
Package / Case
28-WFQFN Exposed Pad
Lead Free Status / RoHS Status
Lead free / RoHS Compliant

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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
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
+
[
(
I
CHARGE
BIT NAME
16
×
V
BATTERY
TABLE 4. INPUT CURRENT (REGISTER 0x3F) (10mΩ SENSE RESISTOR, RS1)
)
(
V
IN
×
η
)
]
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.
ISL88731C
(EQ. 2)
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 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
February 8, 2011
FN6978.2

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