MIC79050 datasheet(11/28 Pages) MICROCHIP

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DS20005771B-page 11

MIC79050

5.0

APPLICATIONS INFORMATION

5.1

Simple Lithium-Ion Battery

Charger

Figure 5-1 shows a simple, complete lithium-ion

battery charger. The charging circuit comprises of a

cheap wall adapter, with a load-line characteristic. This

characteristic is always present with cheap adapters

due to the internal impedance of the transformer

windings. The load-line of the unregulated output

should be less than 4.4V to 4.6V at somewhere

between 0.5C to 1C of the battery under charge. This

4.4 to 4.6V value is an approximate number based on

the headroom needed above 4.2V for the MIC79050 to

operate correctly. In other words, for a 500 mAh

battery, the output of the semi-regulated supply should

be between 225 mA to 500 mA (0.5C to 1C). If it is

below 225 mA no damage will occur but the battery will

take longer to charge. Figure 5-2 shows a typical wall

adapter characteristic with an output current of 350 mA

at 4.5V. This natural impedance of the wall adapter will

limit the maximum current into the battery, so no

external circuitry is needed to accomplish this.

If extra impedance is needed to achieve the desired

load-line, extra resistance can easily be added in series

with the MIC79050 IN pin.

FIGURE 5-1:

Load-Line Charger with End-of-Charge Termination Circuit.

FIGURE 5-2:

Load-Line Characteristics of

AC Wall Adapter.

5.2

The Charging Cycle

See Figure 5-3.

State A: Initial charge. Here the battery’s charging

current is limited by the wall adapter’s natural

impedance. The battery voltage approaches 4.2V.

State B: Constant voltage charge. Here the battery

voltage is at 4.2V ±0.75% and the current is decaying

in the battery. When the battery has reached

approximately 1/10th of its 1C rating, the battery is

considered to have reached full charge. Because of the

natural characteristic impedance of the cheap wall

adapters, as the battery current decreases so the input

voltage increases. The MIC6270 and the LM4041 are

configured as a simple voltage monitor, indicating when

the input voltage has reached such a level so the

current in the battery is low, indicating full charge.

State C: End of charge cycle. When the input voltage,

VS reaches VEOC, an end of charge signal is indicated.

State D: Top up charge. As soon as enough current is

drawn out of the input source, which pulls the voltage

lower than the VEOC, the end of charge flag will be

pulled low and charging will initiate.

Variations on this scheme can be implemented, such

as the circuit shown in Figure 5-4.

For those designs that have a zero impedance source,

see Figure 5-5.

LM4041

CIM3-1.2

BAT

GND

MIC79050-4.2YM

VEOC = VREF (1+

)

Impedance

AC Load-line Wall Adapter

10k

4.7μF

End of Charge

MIC6270

VREF = 1.225V

0.2

0.4

0.6

0.8

SOURCE CURRENT (A)

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