MAX8724 Maxim Integrated Products, MAX8724 Datasheet - Page 19

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

can drive a p-channel MOSFET directly at the charger
input, providing a lower dropout voltage than a
Schottky diode (Figure 2). In the MAX1908/MAX8724
the ACOK comparator is enabled after REFIN is ready.
In the MAX8765, the ACOK comparator is independent
of REFIN.
Use ICHG to monitor the battery-charging current being
sensed across CSIP and CSIN. The ICHG voltage is
proportional to the output current by the equation:
where I
the transconductance of ICHG (3µA/mV typ), and R9 is
the resistor connected between ICHG and ground.
Leave ICHG unconnected if not used.
Use IINP to monitor the system input current being
sensed across CSSP and CSSN. The voltage of IINP is
proportional to the input current by the equation:
where I
adapter power, G
(3µA/mV typ), and R10 is the resistor connected between
IINP and ground. ICHG and IINP have a 0 to 3.5V output
voltage range. Leave IINP unconnected if not used.
LDO provides a 5.4V supply derived from DCIN and
can deliver up to 10mA of load current. The MOSFET
drivers are powered by DLOV and BST, which must be
connected to LDO as shown in Figure 1. LDO supplies
the 4.096V reference (REF) and most of the control cir-
cuitry. Bypass LDO with a 1µF capacitor to GND.
The MAX1908/MAX8724/MAX8765 feature a low-power
shutdown mode. Driving SHDN low shuts down the
MAX1908/MAX8724/MAX8765. In shutdown, the DC-
DC converter is disabled and CCI, CCS, and CCV are
pulled to ground. The IINP and ACOK outputs continue
to function.
SHDN can be driven by a thermistor to allow automatic
shutdown of the MAX1908/MAX8724/MAX8765 when
the battery pack is hot. The shutdown falling threshold
is 23.5% (typ) of V
provide smooth shutdown when driven by a thermistor.
The MAX1908/MAX8724/MAX8765 employ a buck reg-
ulator with a bootstrapped nMOS high-side switch and
a low-side nMOS synchronous rectifier.
INPUT
CHG
V
V
ICHG
IINP
Low-Cost Multichemistry Battery Chargers
is the battery-charging current, G
is the DC current being supplied by the AC
= I
= ICHG x RS2 x G
IINP
______________________________________________________________________________________
INPUT
REFIN
is the transconductance of IINP
x RS1 x G
with 1% V
Current Measurement
DC-DC Converter
IINP
ICHG
REFIN
LDO Regulator
x R10
x R9
Shutdown
hysteresis to
ICHG
is
The MAX1908/MAX8724/MAX8765 control input current
(CCS control loop), charge current (CCI control loop),
or charge voltage (CCV control loop), depending on
the operating condition.
The three control loops, CCV, CCI, and CCS are brought
together internally at the LVC amplifier (lowest voltage
clamp). The output of the LVC amplifier is the feedback
control signal for the DC-DC controller. The output of the
G
amplifier and also clamps the other two control loops to
within 0.3V above the control point. Clamping the other
two control loops close to the lowest control loop ensures
fast transition with minimal overshoot when switching
between different control loops.
The MAX1908/MAX8724/MAX8765 feature a variable off-
time, cycle-by-cycle current-mode control scheme.
Depending upon the conditions, the MAX1908/MAX8724/
MAX8765 work in continuous or discontinuous-conduc-
tion mode.
With sufficient charger loading, the MAX1908/MAX8724/
MAX8765 operate in continuous-conduction mode
(inductor current never reaches zero) switching at
400kHz if the BATT voltage is within the following range:
The operation of the DC-DC controller is controlled by
the following four comparators as shown in Figure 4:
• IMIN—Compares the control point (LVC) against
• CCMP—Compares the control point (LVC) against the
• IMAX—Compares the charging current (CSI) to 6A
• ZCMP—Compares the charging current (CSI) to
M
333mA (RS2 = 0.015Ω). If ZCMP output is high, then
0.15V (typ). If IMIN output is low, then a new cycle
cannot begin.
charging current (CSI). The high-side MOSFET on-
time is terminated if the CCMP output is high.
(RS2 = 0.015Ω). The high-side MOSFET on-time is
cannot begin until IMAX goes low.
both MOSFETs are turned off.
terminated if the IMAX output is high and a new cycle
3.1V x (number of cells) < V
amplifier that is the lowest sets the output of the LVC
CCV, CCI, CCS, and LVC Control Blocks
Continuous-Conduction Mode
BATT
< (0.88 x V
DC-DC Controller
DCIN
)
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