MIC79110

Micrel

M9999-072004

8

July 2004

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

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

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

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

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 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

I Current

500mA

0.7V

Standard V-I Curve

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

Charge Current

Voltage/Cell

1.00/4

0.75/3

0.50/2

0.25/1

12

TIME (hrs)

Stage 1

Max. charge current

is applied until the

cell voltage limit is

reached

Stage 2

Max. cell voltage is reached

Charge current starts to drop

as full charge is approached

Stage 3

Occasional

topping charge

about 1 per

500h

3

Terminate

charge when

current < 3% of

rated current

....

....

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%