MIC79050 Datasheet, PDF (9/20 Pages) Micrel Semiconductor – Simple Lithium-Ion Battery Charger Preliminary Information

MIC79050
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.
Micrel
Impedence
V_S ^{MIC79050-4.2BM}
IN BAT
EN FB
GND
1k
MIC6270
4.7µF
10k R1
R2
AC Load-line Wall Adapter
V_EOC = V_REF ^{}1+ _RR21^{}
V_REF = 1.225V
End of Charge
LM4041
CIM3-1.2
Figure 1A. Load-Line Charger With End-Of-Charge Termination Circuit.
Load-Line Source
Characteristics
8
6
4
2
00
0.2 0.4 0.6 0.8
SOURCE CURRENT (A)
Figure 1B. Load-Line Characteristics
of AC Wall Adapter
June 2000
9
MIC79050