MAX8724 Maxim Integrated Products, MAX8724 Datasheet - Page 14

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

The MAX1908/MAX8724/MAX8765 include all the func-
tions necessary to charge Li+ batteries. A high-efficien-
cy 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
ence of the host. For standard applications, 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
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 feature a voltage
regulation loop (CCV) and two current regulation loops
(CCI and CCS). The CCV voltage regulation loop moni-
tors BATT to ensure that its voltage does not exceed
the voltage set by VCTL. The CCI battery current regu-
lation 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.
The MAX1908/MAX8724/MAX8765 use a high-accuracy
voltage regulator for charging voltage. The VCTL input
adjusts the charger output voltage. VCTL control volt-
age can vary from 0 to V
ment range on the V
the adjust range to 10% of the regulation voltage, the
Low-Cost Multichemistry Battery Chargers
14
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Setting the Battery-Regulation Voltage
BATT
Detailed Description
REFIN
regulation voltage. By limiting
Typical Application Circuit
, providing a 10% adjust-
REFIN
to the refer-
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 termina-
tion voltage is a function of the battery chemistry.
Consult the battery manufacturer to determine this volt-
age. Connect VCTL to LDO to select the internal default
setting V
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).
Table 1. Versions Comparison
Conditioning
Charge Feature
ICTL Shutdown
Mode
ACOK Enable
Condition
Table 2. Cell-Count Programming
DESCRIPTION
V
BATT
BATT
CELLS
V
GND
Float
REFIN
= 4.2V × number of cells, or program the
=
CELLS
REFIN must
MAX1908
be ready
Yes
Yes
×
⎛
⎜
⎝
4
V
+
⎛
⎜
⎝
REFIN must
0 4 .
MAX8724
be ready
Yes
No
×
CELL COUNT
V
V
REFIN
VCTL
2
3
4
Independent
⎞
⎟
⎠
MAX8765
of REFIN
⎞
⎟
⎠
No
No