Rainbow Electronics MAX8724 - Manual (Page 16)

MAX1908/MAX8724
Setting the Charging-Current Limit
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 REFIN / 32 to
V
REFIN
. The device shuts down if ICTL is forced below
V REFIN / 100 (min).
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 cur-
rent-error amplifier (GMI) is compensated at CCI (see
the Compensation section).
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 MAX1908/MAX8724 limit 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 compares the
voltage between CSSP and CSSN to the voltage at
CLS. V
CLS can be set by a resistive divider between
REF and GND. Connect CLS to REF for the full-scale
input current limit.
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
CLS determines the reference voltage of the GMS
error amplifier. Sense resistor RS1 and V
CLS
determine
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.
Conditioning Charge
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 does not offer this feature.
AC Adapter Detection
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
REF / 2 with 20mV hysteresis. ACOK is an open-drain
output and is high impedance when ACIN is less than
V
REF / 2. Since ACOK can withstand 30V (max), ACOK
can drive a P-channel MOSFET directly at the charger
input, providing a lower dropout voltage than a
Schottky diode (Figure 2).
I
V
V RS
INPUT
CLS
REF
= ×
0 075
1
.
I I
I V
V
INPUT LOAD
CHG BATT
IN
= +
Ч
Ч






η
I
V
RS
CHG
=
0 045
2
.
I
V
V RS
CHG
ICTL
REFIN
= ×
0 075
2
.
Low-Cost Multichemistry Battery Chargers
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