MIC79050_05 Datasheet, PDF (7/14 Pages) Micrel Semiconductor – Simple Lithium-Ion Battery Charger

MIC79050
Applications Information
Simple Lithium-Ion Battery Charger.
Figure 1A shows a simple, complete lithium-ion battery char-
ger. 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 headroom needed
above 4.2V for the MIC79050 to operate correctly e.g. For
Micrel, Inc.
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 cur-
rent 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.
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+
R1 ₎
R2
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
August 2005
7
MIC79050