13

Figure 5. Using a Microprocessor to Determine CHRG State

CHRG Status Output Pin

The CHRG pin can provide an indication that the input

voltage is greater than the undervoltage lockout threshold

level. A weak pull-down current of approximately 20

µA

charging. When a discharged battery is connected to the

charger, the constant current portion of the charge cycle

begins and the CHRG pin pulls 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 nearing full charge, the charger enters

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, 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 5 shows that by using two

different value pull-up resistors, a microprocessor can

detect all three states from this pin.

Figure 6. Low Loss Input Reverse Polarity Protection

KB4540

DRAIN-BULK

DIODE OF FET

CHRG

OUT

IN

2k

800k

KB4540

µPROCESSOR

To detect when the KB4540 is in charge mode, force the

digital output pin (OUT) high and measure the voltage at

the CHRG pin. The N-channel MOSFET will pull the pin

voltage low even with the 2k pull-up resistor. Once the

charge cycle terminates, the N-channel MOSFET is turned

off and a 20

µA current source is connected to the CHRG

pin. The IN pin will then be pulled high by the 2k pull-up

resistor. To determine if there is a weak pull-down current,

the OUT pin should be forced to a high impedance state.

The weak current source will pull the IN pin low through

the 800k resistor; if CHRG is high impedance, the IN pin

will be pulled high, indicating that the part is in a UVLO

state.

Reverse Polarity Input Voltage Protection

In some applications, protection from reverse polarity

enough, a series blocking diode can be used. In other

cases, where the voltage drop must be kept low a P-

channel MOSFET can be used (as shown in Figure 6).

KB4540

Kingbor Technology Co.,Ltd

TEL:(86)0755-26508846 FAX:(86)0755-26509052

APPLICATIONS INFORMATION