MIC79110 Datasheet, PDF (8/12 Pages) Micrel Semiconductor – Simple 1.2A Linear Li-Ion Battery Charger

MIC79110
Application Information
Detailed Description
The MIC79110 forms a complete charger for 1-cell Lithium-
ion batteries. It includes precision voltage control (0.75%
over temperature) to optimize both cell performance and
cycle life. All are compatible with common 4.2V Lithium-ion
chemistries. Voltages other than 4.2V can be obtained with
the adjustable version. Other features include current limit,
end-of-charge flag and end-of-charge current limit using an
external resistor. The shutdown pin enables low quiescent
current when not charging.
Current Limit Mode
MIC79110 features an internal current limit that is set by the
Rset pin with a resistor-to-ground. The maximum current is
calculated by the following equation:
IR_set = (0.2/R_set) × 1000
Using a 167Ω R_set resistor will achieve the maximum current
limit for the MIC79110 at 1.2 amperes.
End of Charge
REOC pin is connected to a resistor-to-ground. This resistor
is used to set the end of charge current for the lithium ion
battery as follows:
IR_EOC = (0.2/R_EOC) × 1000
Using a 4kΩ R_EOC resistor will set the end-of-charge
current at 50mA.
IReoc should be set at 10% of the battery’s rated current.
Digital End-of Charge Output
This pin is the output of an open drain. When tied high to the
supply using a resistor, the output will toggle high or low
depending on the output current of Ibat.
- Low state indicates that the Ibat current is higher
than the programmed current set by REOC.
- High state indicates that the Ibat current is lower
than the programmed current set by REOC. The
output goes high impedance indicating end-of-
charge.
Analog-End-Of-Charge Output
The ACHG pin provides a small current that is proportional to
the charge current. The ratio is set at 1/1000^th of the output
current.
Shutdown
The SD pin serves as a logic input (active low) to enable the
charger.
Built-in hysteresis for the shutdown pin is 50mV over tem-
perature.
Reverse Polarity Protection
In the event that V_BAT > V_IN and the shutdown pin is active
low, there is reverse battery current protection built in. The
current is limited to less than 10µA over temperature.
Micrel
Constant Output Voltage/Current Charging
The MIC79110 features constant voltage and constant cur-
rent output to correctly charge lithium-ion batteries. The
constant voltage is either 4.2V or adjustable. The constant
current is set by the Rset pin and is constant down to around
300mV. Since Rset can be set below 500mA, the minimum
output current is set at 500mA for output voltages below
100mV. This minimum voltage starts the charging process in
lithium-ion batteries. If the output current is too low, the
battery will not begin charge.
I Current
Standard V-I Curve
I_MAX = (0.2/R_SET)×1000
0.7V
I Current 500mA
MIC79110 V-I Curve
Lithium Ion Batteries
Lithium-ion batteries are charged in two stages to reach full
capacity. The first stage charges the battery with maximum
charge current until 90% of the battery cell’s voltage limit is
reached. The second stage tops off the charge with constant
voltage charge as the charge current slowly decreases. End
of charge is reached when the current is less than 3% of the
rated current. A third stage will occasionally top off with
charge with constant voltage charge if the battery voltage
drops below a certain threshold.
1.25/5
Stage 1
Max. charge current
is applied until the
cell voltage limit is
reached
1.00/4
Stage 2
Max. cell voltage is reached
Charge current starts to drop
as full charge is approached
Voltage/Cell
Charge Current
Stage 3
Occasional
topping charge
about 1 per
500h
....
0.75/3
0.50/2
0.25/1
Terminate
charge when
current < 3% of
rated current
....
1
2
3
TIME (hrs)
All lithium-ion batteries take approximately 3 hours to charge
with the second stage taking twice as long as the first stage.
Some chargers claim to be fast chargers by skipping the
second stage and just charges the battery until the cell
voltage is reached. This only charges the battery to 70%
M9999-072004
8
July 2004