MAX8724 Maxim Integrated Products, MAX8724 Datasheet - Page 18

• Manufacturer Part Number: MAX8724
• Description: (MAX1908 - MAX87xx) Low-Cost Multichemistry Battery Chargers
• Manufacturer: Maxim Integrated Products

The ICTL input sets the maximum charging current. The
current is set by current-sense resistor RS2, connected
between CSIP and CSIN. The full-scale differential
voltage between CSIP and CSIN is 75mV; thus, for a
0.015Ω sense resistor, the maximum charging current
is 5A. Battery-charging current is programmed with
ICTL using the equation:
The input voltage range for ICTL is V
V
forced below V
does not.
Connect ICTL to LDO to select the internal default full-
scale, charge-current sense voltage of 45mV. The
charge current when ICTL = LDO is:
where RS2 is 0.015Ω, providing a charge-current set
point of 3A.
The current at the ICHG output is a scaled-down replica
of the battery output current being sensed across CSIP
and CSIN (see the Current Measurement section).
When choosing the current-sense resistor, note that the
voltage drop across this resistor causes further power
loss, reducing efficiency. However, adjusting ICTL to
reduce the voltage across the current-sense resistor
can degrade accuracy due to the smaller signal to the
input of the current-sense amplifier. The charging-
current-error amplifier (GMI) is compensated at CCI
(see the Compensation section).
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 fluc-
tuates 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 MAX1908/MAX8724/MAX8765 limit the battery
charge current when the input current-limit threshold is
Low-Cost Multichemistry Battery Chargers
18
REFIN
______________________________________________________________________________________
. The MAX1908/MAX8724 shut down if ICTL is
Setting the Charging-Current Limit
I
REFIN
Setting the Input Current Limit
CHG
I
CHG
=
/ 100 (min), while the MAX8765
V
V
REFIN
ICTL
=
0 045
.
RS
×
2
0 075
V
RS
.
2
REFIN
/ 32 to
exceeded, ensuring the battery charger does not load
down the AC adapter voltage. An internal amplifier
compares the voltage between CSSP and CSSN to the
voltage at CLS. V
between REF and GND. Connect CLS to REF for the
full-scale input current limit. The CLS voltage range for
the MAX1908/MAX8724 is from 1.6V to REF, while the
MAX8765 CLS voltage is from 1.1V to REF.
The input current is the sum of the device current, the
charger input current, and the load current. The device
current is minimal (3.8mA) in comparison to the charge
and load currents. Determine the actual input current
required as follows:
where η is the efficiency of the DC-DC converter.
V
error amplifier. Sense resistor RS1 and V
the maximum allowable input current. Calculate the
input current limit as follows:
Once the input current limit is reached, the charging
current is reduced until the input current is at the
desired threshold.
When choosing the current-sense resistor, note that the
voltage drop across this resistor causes further power
loss, reducing efficiency. Choose the smallest value for
RS1 that achieves the accuracy requirement for the
input current-limit set point.
The MAX1908 includes a battery-voltage comparator
that allows a conditioning charge of overdischarged
Li+ battery packs. If the battery-pack voltage is less
than 3.1V × number of cells programmed by CELLS,
the MAX1908 charges the battery with 300mA current
when using sense resistor RS2 = 0.015Ω. After the
battery voltage exceeds the conditioning charge
threshold, the MAX1908 resumes full-charge mode,
charging to the programmed voltage and current limits.
The MAX8724/MAX8765 do not offer this feature.
Connect the AC adapter voltage through a resistive
divider to ACIN to detect when AC power is available,
as shown in Figure 1. ACIN voltage rising trip point is
V
output and is high impedance when ACIN is less than
V
CLS
REF
REF
/ 2. Since ACOK can withstand 30V (max), ACOK
/ 2 with 20mV hysteresis. ACOK is an open-drain
determines the reference voltage of the GMS
I
INPUT
I
INPUT
=
CLS
I
LOAD
can be set by a resistive divider
=
V
V
+
CLS
REF
AC Adapter Detection
⎛
⎜
⎝
I
Conditioning Charge
CHG
×
V
0 075
IN
RS
.
×
×
V
1
BATT
η
CLS
⎞
⎟
⎠
determine