ISL6253 Datasheet, PDF (14/22 Pages) Intersil Corporation – Highly Integrated Battery Charger for Notebook Computers

ISL6253
Connect CELLS as shown in Table 1 to charge 2, 3, or 4 Li+
cells. When charging other cell chemistries, use CELLS to
select an output voltage range for the charger. The internal
error amplifier gm1 maintains voltage regulation. The voltage
error amplifier is compensated at VCOMP. The component
values shown in Figure 18 provide suitable performance for
most applications. Individual compensation of the voltage
regulation and current-regulation loops allows for optimal
compensation.
TABLE 1. CELL NUMBER PROGRAMMING
CELLS
CELL NUMBER
VCC
4
GND
3
Float
2
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. There are three default battery charge
current-sense threshold voltages: 127mV for CHLIM = 3.3V,
100mV for CHLIM = VREF, 65mV for Float, and 30mV for
ground. The full-scale differential voltage between CSOP
and CSON is 100mV for CHLIM = VREF, so the maximum
charging current is 4.0A for a 25mΩ sensing resistor. Other
battery charge current-sense threshold values can be set by
connecting a resistor divider from VREF or VDD to ground,
or by connecting a low impedance voltage source like a D/A
converter in the microcontroller. The charge current limit
threshold is given by:
I_CHG=
---¹---
R₁


---⁰----^.-⁰----⁷----
VREF
V_CHLIM
+
0.03_
(EQ. 2)
If the battery voltage is less than 3.0V/cell and the battery
charging voltage is a percentage of the charging current in
constant current charge mode, the trickle charge current limit
threshold is given by:
I_{T R ,C H G} =
---¹---
R₁
(
0.00157
×
V_CHLIM
+
0.0037_
(EQ. 3)
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.
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
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 ISL6253 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. 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 103mV, 78mV,
and 53mV, respectively, or use a resistor divider from VREF to
ground to set the input current limit as the following equation:
I_INPUT
=
---¹---
R₂


---⁰----^.-⁰----⁵----
VREF
V_A
C
LI
M
+
0.053_
(EQ. 4)
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 103mV 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 a 1W sensing resistor may have to be used. ±
4% and ± 6% accuracy can be achieved with 78mV and
53mV current-sense threshold voltage for ACLIM = Floating
and ACLIM = GND, respectively.
A low pass filter is recommended to eliminate the switching
noise. Connect the resistor to the CSIN pin instead of the
CSIP pin because the 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 17. ACPRN is an open-drain output and is high
impedance when ACSET is less than V_th,rise and active low
impedance when ACSET is above V_th,fall . V_th,rise and
V_th,fall are given by:
V_{th, rise}
=



R_-----8-
R₉
+

1

• V_ACSET
(EQ. 5)
V_{th, fall}
=



R_-----8-
R₉
+

1

• V_ACSET–I_hysR₈
Where I_hys is the ACSET input bias current hysteresis and
V_ACSET = 1.235V (min), 1.26V (typ.) and 1.285V (max.). The
hysteresis is I_hys R₈, where I_hys = 2µA (min.), 3.4µA (typ.)
and 4.8µA (max.)
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