ISL6251 INTERSIL [Intersil Corporation], ISL6251 Datasheet - Page 13

• Manufacturer Part Number: ISL6251
• Description: Low Cost Multi-Chemistry Battery Charger Controller
• Manufacturer: INTERSIL [Intersil Corporation]

regulation and current-regulation loops allows for optimal
compensation.
Setting the Battery Charge Current Limit
The CHLIM input sets the maximum charging current. The
current set by the current sense-resistor connects between
CSOP and CSON. The full-scale differential voltage between
CSOP and CSON is 165mV for CHLIM=3.3V, so the
maximum charging current is 4.125A for a 40m Ω sensing
resistor. Other battery charge current-sense threshold
values can be set by connecting a resistor divider from
VREF or 3.3V to ground, or by connecting a low impedance
voltage source like a D/A converter in the micro-controller.
The charge current limit threshold is given by:
To set the trickle charge current for the dumb charger, a
resistor in series with a switch Q3 (Figure 12) controlled by
the micro-controller is connected from CHLIM pin to ground.
The trickle charge current is determined by:
When the CHLIM voltage is below 88mV (typical), it will
disable the battery charger. When choosing the current
sensing resistor, note that the voltage drop across the
sensing resistor causes further power dissipation, reducing
efficiency. However, adjusting CHLIM voltage to reduce the
voltage across the current sense resistor R1 will degrade
accuracy due to the smaller signal to the input of the current
sense amplifier. There is a trade-off between accuracy and
power dissipation. A low pass filter is recommended to
eliminate switching noise. Connect the resistor to the CSOP
pin instead of the CSON pin, as the CSOP pin has lower
bias current and less influence on current-sense accuracy
and voltage regulation accuracy.
Setting the Input Current Limit
The total input current from an AC adapter, or other DC
source, is a function of the system supply current and the
battery-charging current. The input current regulator limits
the input current by reducing the charging current, when the
input current exceeds the input current limit set point.
System current normally fluctuates as portions of the system
are powered up or down. Without input current regulation,
the source must be able to supply the maximum system
I
I
CHG
CHG
=
=
165mV
-------------------
165mV
-------------------
TABLE 1. CELL NUMBER PROGRAMMING
R
R
CELLS
1
1
GND
Float
VDD
V
--------------------- -
V
--------------------------------------- -
CHLIM
CHLIM trickle
3.3V
3.3V
,
13
CELL NUMBER
4
3
2
ISL6251, ISL6251A
current and the maximum charger input current
simultaneously. By using the input current limiter, the current
capability of the AC adapter can be lowered, reducing
system cost.
The ISL6251, ISL6251A limits the battery charge current
when the input current-limit threshold is exceeded, ensuring
the battery charger does not load down the AC adapter
voltage. This constant input current regulation allows the
adapter to fully power the system and prevent the AC
adapter from overloading and crashing the system bus.
An internal amplifier gm3 compares the voltage between
CSIP and CSIN to the input current limit threshold voltage
set by ACLIM. Connect ACLIM to REF, Float and GND for
the full-scale input current limit threshold voltage of 100mV,
75mV and 50mV, respectively, or use a resistor divider from
VREF to ground to set the input current limit as the following
equation:
When choosing the current sense resistor, note that the
voltage drop across this resistor causes further power
dissipation, reducing efficiency. The AC adapter current
sense accuracy is very important. Use a 1% tolerance
current-sense resistor. The highest accuracy of ±3% is
achieved with 100mV current-sense threshold voltage for
ACLIM=VREF, but it has the highest power dissipation. For
example, it has 400mW power dissipation for rated 4A AC
adapter and 1W sensing resistor may have to be used. ±4%
and ±6% accuracy can be achieved with 75mV and 50mV
current-sense threshold voltage for ACLIM=Floating and
ACLIM=GND, respectively.
A low pass filter is suggested to eliminate the switching
noise. Connect the resistor to CSIN pin instead of CSIP pin
because CSIN pin has lower bias current and less influence
on the current-sense accuracy.
AC Adapter Detection
Connect the AC adapter voltage through a resistor divider to
ACSET to detect when AC power is available, as shown in
Figure 12. ACPRN is an open-drain output and is high when
ACSET is less than V
above V
Where I
V
hysteresis is I
and 4.4µA (max.).
V
V
I
INPUT
ACSET
th
th
,
,
rise
fall
=
=
=
hys
th,fall








= 1.24V (min), 1.26V (typ.) and 1.28V (max.). The
R
R
R
R
R
1
2
8
9
8
9
is the ACSET input bias current hysteresis and



+
. V
+
hys
VREF
1
1
. 0

•  


•  

th,rise
05
R
V
V
8
ACSET
ACSET
, where I
V
th,rise
and V
ACLIM
−
, and active low when ACSET is
I
th,fall
hys
hys
+
. 0
R
=2.2µA (min.), 3.4µA (typ.)
050
8
are given by:



June 17, 2005
FN9202.1