1 PF

1k

EN

ISEL

Mode

SCL

SDA

TS

VT

RS

RT

VBSense

BATT

10 PF

Li-Ion

CHG

EOC

GND

CHG-IN

LP3947

Wall

Adaptor

To

System

Supply

USB

Port

P-Ch

MOSFET

Diff-Amp

10k

LP3947

www.ti.com

SNVS298B – NOVEMBER 2004 – REVISED APRIL 2013

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 (VUVLO-TSHD), over-voltage (VOVLO-TSHD) requirements, and the Adaptor OK signal is

detected, then pre-qualification cycle begins (see Figure 2). 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 I2C interface can be used to program this 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 terminal 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 VBATT voltage across the battery terminals. During this cycle, the charge current, ICHG,

continues to decrease with time 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 initiated 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.

The LP3947 with I2C interface allows maximum flexibility in selecting the charge current, battery regulation

voltage and EOC current. The LP3947 operates in default mode during power up. See I2C INTERFACE for more

detail.

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.

Figure 3. LP3947 with External Switch

Copyright © 2004–2013, Texas Instruments Incorporated

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