MP2618 – 2A, 24V INPUT, 600kHz 2-3CELL SWITCHING LI-ION BATTERY CHARGER

MP2618 Rev. 0.92

www.MonolithicPower.com

11

7/14/2010

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© 2010 MPS. All Rights Reserved.

OPERATION

The MP2618 is a peak current mode controlled

switching charger for use with Li-Ion batteries.

Figure 2 shows the block diagram. At the

beginning of a cycle, M1 is off. The COMP

voltage is higher than the current sense result

from amplifier A1’s output and the PWM

comparator’s output is low. The rising edge of the

600 kHz CLK signal sets the RS Flip-Flop. Its

output turns on M1 thus connecting the SW pin

and inductor to the input supply.

The increasing inductor current is sensed and

amplified by the Current Sense Amplifier A1.

Ramp compensation is summed to the output of

A1 and compared to COMP by the PWM

comparator.

When the sum of A1’s output and the Slope

Compensation

signal

exceeds

the

COMP

voltage, the RS Flip-Flop is reset and M1 turns

off. The external switching diode D1 then

conducts the inductor current.

If the sum of A1’s output and the Slope

Compensation signal does not exceed the COMP

voltage, then the falling edge of the CLK resets

the Flip-Flop.

The MP2618 have two internal linear regulators

power internal circuit, VREF33 and VREF25. The

output of 3.3V reference voltage can also power

external circuitry as long as the maximum current

(50mA) is not exceeded. A 1µF bypass capacitor

is required from VREF33 to GND to ensure

stability.

Charge Cycle (Mode change: Trickle

CC

CV)

The battery current is sensed via RS1 (Figure 2)

and amplified by A2. The charge will start in

reaches 3V/cell. If the charge stays in the “trickle

charging mode” till “timer out” condition triggered,

and the charge is terminated. Otherwise, the

output of A2 is then regulated to the level set by

RS1. The charger is operating at “constant

current charging mode.” The duty cycle of the

switcher is determined by the COMPI voltage

that is regulated by the amplifier GMI.

When the battery voltage reaches the “constant

voltage mode” threshold, the amplifier GMV will

regulate the COMP pin, and then the duty cycle.

The charger will then operate in “constant voltage

mode.”

Automatic Recharge

Once the battery charging current drops below

the termination threshold, the charger will cease

drain. If for some reason, the battery voltage is

lowered to 4.0V/Cell, recharge will automatically

kick in.

Charger Status Indication

MP2618 has two open-drain status outputs:

CHGOK

and

ACOK . The ACOK pin pulls low

when an input voltage is greater than battery

voltage 300mV and over the under voltage

status of the charge cycle. Table 1 describes the

status of the charge cycle based on the

CHGOK

and

ACOK outputs.

Table 1―Charging Status Indication

ACOK

CHGOK

Charger status

low

low

In charging

low

high

End of charge, NTC fault,

timer out, thermal

shutdown

high

high

Timer Operation

MP2618 uses internal timer to terminate the

charge if the timer times out. The timer duration

is programmed by an external capacitor at the

TMR pin.

The trickle mode charge time is:

0.1uF

C

30mins

T

TMR

TICKLE_TMR

×

=

The total charge time is:

0.1uF

C

3hours

T

TMR

TOTAL_TMR

×

=