Micrel, Inc.

MIC79110

October 2007

8

M9999-102507

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:

current limit for the MIC79110 at 1.2 amperes.

End of Charge

resistor is used to set the end of charge current for the

lithium ion battery as follows:

current at 50mA.

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

•

•

output goes high impedance indicating end-of-

charge.

Analog End-Of-Charge Output

to the charge current. The ratio is set at 1/1000th 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

temperature.

Reverse Polarity Protection

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

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

I Current

500mA

0.7V

I

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.