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Fully integrated constant current/constant voltage

Li-ion battery charger

VER 1.4

14

Figure 3. A Circuit to Maximize Thermal Mode Charge Current

Due to their self-resonant and high Q characteristics, some types of ceramic capacitors can cause high

voltage transients under some start-up conditions (i.e connecting the charger input to a live power source).

Adding a small value resistor in series with the ceramic capacitor can minimize start-up voltage transients.

CHARGE Current Soft-Start

To avoid the start-up transients, a soft-start circuit is included to ramp the charge current from zero to

programmed value over a period of time. This has the effect of minimizing the transient current load on

the power supply during start-up.

CHRG Status Output Pin

When the input voltage is larger than the undervoltage lockout threshold, a pull-down current of 20

μA to

the pin indicates that the device is ready to charge. When a discharged battery is connected to the

charger, the constant current portion of the charge cycle begins and the CHRG pin is pulled to ground.

The CHRG pin can sink up to 10mA to drive an LED that indicates that a charge cycle is in progress.

When the battery is close to fully charged, the charger switches to the constant-voltage portion of the

charge cycle and the charge current begins to drop. When the charge current drops below 1/10 of the

programmed current, the charge cycle ends and the strong pull-down is replaced by the 20

μA pull-down

as mentioned before, indicating that the charge cycle has ended.

If the input voltage is removed or drops below the undervoltage lockout threshold, the CHRG pin

becomes high impedance. Figure 4 shows that by using two different value pull-up resistors, a

microprocessor can detect all three states from this pin.

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Figure 4. Using a Microprocessor to Determine CHRG State.

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