MIC79110_07 Datasheet, PDF (8/14 Pages) Micrel Semiconductor – Simple 1.2A Linear Li-Ion Battery Charger

Micrel, Inc.
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.
active low, there is reverse battery current protection
built in. The current is limited to less than 10µA over
temperature.
Constant Output Voltage/Current Charging
The MIC79110 features constant voltage and constant
current output to correctly charge lithium-ion batteries.
The constant voltage is either 4.2V or adjustable. The
constant current is set by the R_set pin and is constant
down to around 300mV. Since R_set 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.
End of Charge
R_EOC 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.
I_REOC 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 I_BAT.
• Low state indicates that the I_BAT current is higher
than the programmed current set by R_EOC.
• High state indicates that the I_BAT current is lower
than the programmed current set by R_EOC. The
output goes high impedance indicating end-of-
charge.
Analog End-Of-Charge Output
The A_CHG pin provides a small current that is proportional
to the charge current. The ratio is set at 1/1000th of the
output current.
I Current
I_MAX = (0.2/R_SET )×1000
0.7V
I Current
500mA
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.
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
temperature.
Reverse Polarity Protection
In the event that V_BAT > V_IN and the shutdown pin is
October 2007
8
M9999-102507