Datasheet

Low-Cost Multichemistry Battery Chargers
Detailed Description
The MAX1908/MAX8724/MAX8765/MAX8765A include
all the functions necessary to charge Li+ batteries. A
high-efficiency synchronous-rectified step-down DC-DC
converter controls charging voltage and current. The
device also includes input-source current limiting and
analog inputs for setting the charge current and charge
voltage. Control charge current and voltage using the
ICTL and VCTL inputs, respectively. Both ICTL and
VCTL are ratiometric with respect to REFIN, allowing
compatibility with DACs or microcontrollers (µCs).
Ratiometric ICTL and VCTL improve the accuracy of the
charge current and voltage set point by matching V
RE-
FIN
to the reference of the host. For standard applica-
tions, internal set points for ICTL and VCTL provide 3A
charge current (with 0.015Ω sense resistor), and 4.2V
(per cell) charge voltage. Connect ICTL and VCTL to
LDO to select the internal set points. The MAX1908
safely conditions overdischarged cells with 300mA (with
0.015Ω sense resistor) until the battery-pack voltage
exceeds 3.1V × number of series-connected cells. The
SHDN input allows shutdown from a microcontroller or
thermistor.
The DC-DC converter 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 1 uses a µC to control charging cur-
rent, while Figure 2 shows a typical application 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 current set by the VCTL,
ICTL, and CELLS inputs.
The MAX1908/MAX8724/MAX8765/MAX8765A feature
a voltage regulation loop (CCV) and two current regula-
tion loops (CCI and CCS). The CCV voltage regulation
loop monitors BATT to ensure that its voltage does not
exceed the voltage set by VCTL. The CCI battery cur-
rent regulation loop monitors current delivered to BATT
to ensure that it does not exceed 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 input current exceeds
the input current limit set by CLS.
Setting the Battery-Regulation Voltage
The MAX1908/MAX8724/MAX8765/MAX8765A use a
high-accuracy voltage regulator for charging voltage.
The VCTL input adjusts the charger output voltage.
VCTL control voltage can vary from 0 to V
REFIN
, provid-
ing a 10% adjustment range on the V
BATT
regulation
voltage. By limiting the adjust range to 10% of the regu-
lation voltage, the external resistor mismatch error is
reduced from 1% to 0.05% of the regulation voltage.
Therefore, an overall voltage accuracy of better than
0.7% is maintained while using 1% resistors. The per-
cell battery termination voltage is a function of the bat-
tery chemistry. Consult the battery manufacturer to
determine this voltage. Connect VCTL to LDO to select
the internal default setting V
BATT
= 4.2V × number of
cells, or program the battery voltage with the following
equation:
CELLS is the programming input for selecting cell count.
Connect CELLS as shown in Table 2 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 (GMV) maintains voltage
regulation (Figure 3). The voltage error amplifier is
compensated at CCV. The component values shown in
Figures 1 and 2 provide suitable performance for most
applications. Individual compensation of the voltage reg-
ulation and current regulation loops allows for optimal
compensation (see the Compensation section).
V CELLS V
V
V
BATT
VCTL
REFIN
=×+×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
404.
Table 2. Cell-Count Programming
DESCRIPTION MAX1908 MAX8724
MAX8765/
MAX8765A
Conditioning
Charge Feature
Yes No No
ICTL Shutdown
Mode
Yes Yes No
ACOK Enable
Condition
REFIN must
be ready
REFIN must
be ready
Independent
of REFIN
Table 1. Versions Comparison
CELLS CELL COUNT
GND 2
Open 3
V
REFIN
4
MAX1908/MAX8724/MAX8765/MAX8765A
14
Maxim Integrated