Micrel, Inc.

MIC79110

October 2007

13

M9999-102507

Auto Top-Off-Charger - Application Circuit

Lithium-Ion batteries will begin to lose their charge over

time. The MIC79110 can be configured to automatically

recharge the battery if the voltage drops below a certain

voltage. This minimum voltage is set by a resistor divider

at the battery and connected to the SD pin. When the

battery voltage falls below the minimum voltage, the SD

pin is pulled low to start the normal charging process.

4.2 V

Deoc trip

Battery

charging

R1

1

Self discharge

4

1

2

3

Vbat set (4.2V)

Vbat low set

by divider.

Vbat

SD Voltage

to ground. Divider holds SD low so part can

start.

2. SD held low by divider.

open.

5. Divider voltage drops below SD threshold and

charging begins again.

Top-Off-Charger with Internal Reset - Application

Circuit

3M

1M

LI-ion Battery

SD

VIN

RSET

SNS

BAT

VIN

GND

REOC

DEOC

AEOC

This circuit is similar to the auto top off charger circuit

triggered to restart the charging cycle. It still uses the

same resistor divider to set the minimum battery voltage

before the lithium-ion needs to be recharged.

Auto-Shutdown Using Shutdown Pin

The shutdown pin on the MIC79110 can be used to

automatically shutdown the battery charger when the

input voltage rises above a safe operating voltage. To

keep the part from heating up and entering thermal

a resistor divider. Use the following equation to setup the

1

R2

R1

V

(MAX)

V

SD

IN

+

=

The MIC79110 can be connected to a wall wart with a

rectified DC voltage and protected from over voltages at

the input.