MAX1908_09 datasheet(19/30 Pages) MAXIM

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Low-Cost Multichemistry Battery Chargers

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VREF/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). In the MAX1908/MAX8724

the ACOK comparator is enabled after REFIN is ready.

In the MAX8765/MAX8765A, the ACOK comparator is

independent of REFIN.

Current Measurement

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:

VICHG = ICHG x RS2 x GICHG x R9

where ICHG is the battery-charging current, GICHG is

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:

VIINP = IINPUT x RS1 x GIINP x R10

where IINPUT is the DC current being supplied by the AC

adapter power, GIINP is the transconductance of IINP

(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 Regulator

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.

Shutdown

The MAX1908/MAX8724/MAX8765/MAX8765A feature

a low-power shutdown mode. Driving SHDN low shuts

down the MAX1908/MAX8724/MAX8765/MAX8765A. 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/

MAX8765A when the battery pack is hot. The shutdown

falling threshold is 23.5% (typ) of VREFIN with 1%

VREFIN hysteresis to provide smooth shutdown when

driven by a thermistor.

DC-DC Converter

The MAX1908/MAX8724/MAX8765/MAX8765A employ

a buck regulator with a bootstrapped nMOS high-side

switch and a low-side nMOS synchronous rectifier.

CCV, CCI, CCS, and LVC Control Blocks

The MAX1908/MAX8724/MAX8765/MAX8765A 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

GM amplifier that is the lowest sets the output of the LVC

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.

DC-DC Controller

The MAX1908/MAX8724/MAX8765/MAX8765A feature a

variable off-time, cycle-by-cycle current-mode control

scheme. Depending upon the conditions, the MAX1908/

MAX8724/MAX8765/MAX8765A work in continuous or

discontinuous-conduction mode.

Continuous-Conduction Mode

With sufficient charger loading, the MAX1908/MAX8724/

MAX8765/MAX8765A operate in continuous-conduction

mode (inductor current never reaches zero) switching at

400kHz if the BATT voltage is within the following range:

3.1V x (number of cells) < VBATT < (0.88 x VDCIN )

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

0.15V (typ). If IMIN output is low, then a new cycle

cannot begin.

• CCMP—Compares the control point (LVC) against the

charging current (CSI). The high-side MOSFET on-

time is terminated if the CCMP output is high.

• IMAX—Compares the charging current (CSI) to 6A

(RS2 = 0.015

Ω). The high-side MOSFET on-time is

terminated if the IMAX output is high and a new cycle

cannot begin until IMAX goes low.

• ZCMP—Compares the charging current (CSI) to

333mA (RS2 = 0.015

Ω). If ZCMP output is high, then

both MOSFETs are turned off.

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