MP2681B – CC/CV CONTROLLER WITH FULL PROTECTION

MP2681B Rev.1.0

www.MonolithicPower.com

13

12/22/2014

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Charge-Current Setting

sensing the voltage across the sense resistor

(RS) and comparing it to the charge-current

limitation loop operational amplifier decreases.

The opto-coupler current increases and tends to

reduce the output voltage by the way of the PWM

controller (see Figure 3).

Choose the sense resistor RS according to the

charge current and the following formula:

CHG

IREF

I

V

RS

=

The charge-current reference is controlled by

CR to set the charge rate. If CR is logic high,

current can be programmed only by changing the

sense resistor. Pull CR low to set the IC in slow-

charge current without changing the external

resistor. With CR, the MP2681B can set the

charger in fast or slow charge automatically.

Floating CR sets the charger in fast-charge as

per the default setting.

To avoid large currents flowing into the battery,

the

MP2681B

implements

an

over-current

protection scheme. When voltage over 500mV is

detected on IFB, the MP2681B cuts off the

charge-current path to the battery.

Pre-Charge Mode

Before the charging cycle starts, the MP2681B

senses the battery voltage.

If the battery voltage is lower than 1.0V/cell, the

MP2681B acts as if there is a dead battery, and it

never starts charging. If the battery voltage is

lower than 3.0V/cell, the MP2681B goes into pre-

charge mode. The external MOSFET is off in this

mode, and the output-voltage control of the

flyback is set at a default value of 3.2V/cell. The

MP2681B acts as a constant current source,

supplying a constant 150mA current to charge

the battery pack through BATT.

This period lasts until the battery voltage

increases over the 3.0V/cell threshold or during

time-out. A fixed timer starts when pre-charge

mode starts. The MP2681B sets the time period

for pre-charge at 60min with a 0.1uF timer

will change the time period only during charge

mode but not during pre-charge mode

The pre-charge mode ensures that any deeply

discharged battery will be charged safely.

Charge Cycle (Mode Change: CC

CV)

At the end of the pre-charge cycle (and when the

battery voltage is at least equal to 3.0V/cell), the

external MOSFET is turned on, and the

MP2681B enters constant-current charge mode.

This mode of operation will stay as long as the

battery voltage does not cross the full, pre-set

charge threshold.

After the battery voltage is over 3.0V/cell, the IC

begins to charge at the programmed constant

current in CC mode. Once the output voltage

reaches the battery-regulation voltage, the

charger operates in constant-voltage (CV) mode

until the battery is fully charged. During this time,

the charge current starts to decrease down to the

constant-current value).