MIC79050 datasheet(9/20 Pages) MICREL

9 / 20 page

June 2000

MIC79050

Micrel

Applications Information

Simple Lithium-Ion Battery Charger.

Figure 1A 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 < 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 head-

room needed above 4.2V for the MIC79050 to operate

correctly e.g. For a 500mAhr battery, the output of the semi-

regulated supply should be between 225mA to 500mA ( 0.5C

to 1C ). If it is below 225mA no damage will occur but the

battery will take longer to charge. Figure 1B shows a typical

wall adapter characteristic with an output current of 350mA at

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

max current into the battery, so no external circuitry is needed

to accomplish this.

If extra impedance is needed to achieve the desired loadline,

extra resistance can easily be added in series with the

MIC79050 IN pin.

LM4041

CIM3-1.2

BAT

GND

MIC79050-4.2BM

VREF = 1.225V

Impedence

AC Load-line Wall Adapter

10k

4.7

µF

End of Charge

MIC6270

EOC

REF









Figure 1A. Load-Line Charger With End-Of-Charge Termination Circuit.

0.2

0.4

0.6

0.8

SOURCE CURRENT (A)

Load-Line Source

Characteristics

Figure 1B. Load-Line Characteristics

of AC Wall Adapter

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