MIC79050 datasheet(10/20 Pages) MICREL

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MIC79050

Micrel

MIC79050

June 2000

VEOC

Open Circuit

Charger Voltage

Battery Current (IB)

Battery Voltage (VB)

Unregulated Input

Voltage(VB)

79050 Programmed

Output Voltage

(No Load Voltage)

End of Charge (VEOC)

State A

Initial Charge

State B

Voltage Charge

State C

End of Charge

State D

Charge Top

State C

Figure 1C. Charging Cycles

Application Information

BAT

GND

MIC79050-4.2BM

LM4041

CIM3-1.2

±5%@

400mA

±5%

4.7

µF

8.06M

47k

0.050

Ω

10k

MIC6270

MIC7300

10k

Figure 2. Protected Constant-Current Charger

The Charging Cycle (See Figure 1C.)

1. State A: Initial charge. Here the battery’s charging

current is limited by the wall adapter’s natural imped-

ance. The battery voltage approaches 4.2V.

2. State B: Constant voltage charge. Here the battery

voltage is at 4.2V

± 0.75% and the current is decaying in

the battery. When the battery has reached approxi-

mately 1/10th of its 1C rating, the battery is considered

to have reached full charge. Because of the natural

characteristic impedance of the cheap wall adapters, as

the battery voltage decreases so the input voltage

increases. The MIC6270 and the LM4041 are config-

ured as a simple voltage monitor, indicating when the

input voltage has reached such a level so the current in

the battery is low, indicating full charge.

3. State C: End of charge cycle. When the input voltage,

S reaches VEOC, an end of charge signal is indicated.

4. State D: Top up charge. As soon as enough current is

drawn out of the input source, which pulls the voltage

lower than the V

EOC, the end of charge flag will be

pulled low and charging will initiate.

Variations on this scheme can be implemented, such as the

circuit shown in Figure 2.

For those designs that have a zero impedance source , see

Figure 3.

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