Application Notes

LP3947 CHARGER OPERATION

The LP3947 charge cycle is initiated with AC adaptor or USB

power source insertion. If the voltage on the CHG-IN pin

meets under-voltage (V

over-voltage (V

OVLO-

TSHD

) requirements, and the Adaptor OK signal is detected,

then pre-qualification cycle begins (see Figure 1). In this

cycle, a safe current level, less than 70mA, is pumped into

the battery while the voltage across the battery terminals is

measured. Once this voltage exceeds 3.0V, the controller

will initiate constant current fast charge cycle. If the CHG-IN

pin is connected to an AC adaptor, the default charge current

is 500 mA and I

parameter. If the CHG-IN pin is connected to the USB port,

constant current cycle will start with a default of 100 mA.

During this cycle, the 5.6 hr safety timer starts counting.

If the 5.6 hr safety timers times out during constant current

cycle, charging is terminated. As the battery is charged

during constant current mode, the voltage across pack ter-

minal increases until it reaches 4.2V (or 4.1V). As soon as

pack terminal reaches 4.2V (or 4.1V), the controller starts

operating in constant voltage mode by applying regulated

V

the charge current, I

and when it drops below 0.1C (default value), the EOC

signal is activated indicating successful completion of the

charge cycle. The EOC current can be programmed to 0.1C,

0.15C, or 0.2C. The default value is 0.1C. After completing

the full charge cycle, the controller will start the maintenance

cycle where battery pack voltage is monitored continuously.

During the maintenance cycle, if the pack voltage drops 200

mV below the termination voltage, charge cycle will be initi-

ated providing that the wall adaptor is plugged in and is alive.

Charging terminates when the battery temperature is out of

range. For more explanation, please refer to “Ts Pin” section.

The LP3947 with I

selecting the charge current, battery regulation voltage and

EOC current. The LP3947 operates in default mode during

power up. See the “I

When charging source comes from the USB port, charging

starts with 100 mA (low power mode, ISEL = high). The USB

controller can set the ISEL pin low to charge the battery at

500 mA. A simple external circuit selects between an AC

adaptor or the USB port. The circuit is designed with priority

given to the AC adaptor.

20111006

FIGURE 2. LP3947 with External Switch

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