MAX1772
Low-Cost, Multichemistry Battery-
Charger Building Block
______________________________________________________________________________________ 11
Detailed Description
The MAX1772 includes all of the functions necessary to
charge Li+, NiMH, and NiCd batteries. A high-efficiency
synchronous-rectified step-down DC-DC converter con-
trols charging voltage and current. It also includes input
source-current limiting and analog inputs for setting the
charge current and charge voltage. The DC-DC con-
verter uses external N-channel MOSFETs as the buck
switch and synchronous rectifier to convert the input
voltage to the required charging current and voltage.
The typical application circuit shown in Figure 1a uses
a microcontroller (µC) to allow control of charging cur-
rent or voltage, while Figure 1b shows a typical appli-
cation with charging voltage and current fixed to
specific values for the application. The voltage at ICTL
and the value of RS2 set the charging current. The DC-
DC converter generates the control signals for the
external MOSFETs to regulate the voltage and the cur-
rent set by the VCTL, ICTL, and CELLS inputs.
The MAX1772 features a voltage-regulation loop (CCV)
and two current-regulation loops (CCI and CCS). The
CCV voltage-regulation loop monitors BATT to ensure
that its voltage never exceeds the voltage set by VCTL.
The CCI battery current-regulation loop monitors cur-
rent delivered to BATT to ensure that it never exceeds
the current limit set by ICTL. A third loop (CCS) takes
control and reduces the battery-charging current when
the sum of the system load and the battery-charging
current exceeds the charging source current limit set
by CLS.
Setting the Battery Regulation Voltage
The MAX1772 uses a high-accuracy voltage regulator
for charging voltage. The VCTL input adjusts the bat-
tery output voltage. VCTL is allowed to vary from 0 to
REFIN (≈ 3.3V). The per-cell battery termination voltage
is a function of the battery chemistry and construction;
thus, consult the battery manufacturer to determine this
voltage. The battery voltage is calculated by the equa-
tion:
CELLS is the programming input for selecting cell
count. Table 1 shows how CELLS is connected to
charge 2, 3, or 4 cells. Use a voltage-divider from LDO
to set the desired voltage at CELLS.
The internal error amplifier (GMV) maintains voltage
regulation (Figure 2). The voltage error amplifier is com-
pensated at CCV. The component values shown in
Figure 1 provide suitable performance for most appli-
cations. Individual compensation of the voltage regula-
tion and current-regulation loops allow for optimal com-
pensation.
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 nominal differential volt-
age between CSIP and CSIN is 204mV; thus, for a
0.05Ω sense resistor, the maximum charging current is
4A. Battery-charging current is programmed with ICTL
using the equation:
The input range for ICTL is REFIN/32 to REFIN (≈ 3.3V).
The device shuts down if ICTL is forced below
REFIN/55 (typical). The current at ICHG is a scaled-
down replica of the battery output current being sensed
across CSIP and CSIN.
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
may degrade accuracy due to the input offset of the
current-sense amplifier. The charging current-error
amplifier (GMI) is compensated at CCI. A 0.01µF
capacitor at CCI provides suitable performance for
most applications.
Setting the Input Current Limit
The total input current (from a wall cube or other DC
source) is a function of the system supply current and
the battery-charging current. The input current regula-
tor limits the source current by reducing the charging
current when the input current exceeds the set input
current limit. System current will 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 supply the maximum system current and the
maximum charger input current. By using the input cur-
rent limiter, the current capability of the AC wall adapter
may be lowered, reducing system cost.
The MAX1772 limits the current drawn by the charger
when the load current becomes high. The device limits
the charging current, so the AC adapter voltage is not
loaded down. An internal amplifier compares the volt-
age between CSSP and CSSN to the voltage at CLS.
V
CLS
can be set by a resistor-divider between REF and
GND. Connect CLS to REF for maximum input current
limiting.
