MIC79050_05 Datasheet, PDF (8/14 Pages) Micrel Semiconductor – Simple Lithium-Ion Battery Charger

MIC79050
End of Charge (V_EOC)
Unregulated Input
Voltage(V_B)
79050 Programmed
Output Voltage
(No LoadVoltage)
Battery Voltage (V_B)
Battery Current (I_B)
V_EOC
Open Circuit
Charger Voltage
Micrel, Inc.
State A
Initial Charge
State B
Voltage Charge
State C
End of Charge
State D
Charge Top
Figure 1C. Charging Cycles
State C
The Charging Cycle (See Figure 1C.)
1. State A: Initial charge. Here the battery’s charging cur-
rent is limited by the wall adapter’s natural impedance.
The battery voltage approaches 4.2V.
2. 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 approxi-
mately 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 voltage decreases so the input voltage in-
creases. 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.
3. State C: End of charge cycle. When the input voltage,
V_S reaches V_EOC, an end of charge signal is indicated.
4. State D: Top up charge. As soon as enough current
is drawn out of the input source, which pulls the volt-
age lower than the V_EOC, 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 2.
For those designs that have a zero impedance source , see
Figure 3.
MIC79050
5V 5%@
400mA 5%
0.050Ω
1k
MIC79050-4.2BM
IN BAT
EN FB
4.7µF
GND
8.06M
10k
R2
47k
1k
MIC7300
Q1
10k _MIC6270
47k
LM4041
CIM3-1.2
Figure 2. Protected Constant-Current Charger
8
August 2005