UB2017

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

10 of 13

www.unisonic.com.tw

QW-R502-868.a

APPLICATION INFORMATION(Cont.)

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 20mA indicates that sufficient voltage is applied to VCC to begin

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 20mA 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 3 shows that by using two different value pull-up resistors, a

microprocessor can detect all three states from this pin. To detect when the UTC UB2017 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 2kΩ pull-up resistor. Once the charge cycle terminates, the N-channel MOSFET is turned

off and a 20mA current source is connected to the CHRG pin. The IN pin will then be pulled high by the 2kΩ 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 800kΩ resistor; if CHRG is high impedance, the IN pin

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

Fig. 3 Using a Microprocessor to Determine CHRG State

Reverse Polarity Input Voltage Protection

In some applications, protection from reverse polarity voltage on VCC is desired. If the supply voltage is high

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.

Fig. 4 Low Loss Input Reverse Polarity Protection