Using the UC3909 charging controller, it is possible to form a quick charger for switching lead-acid batteries. The average current PWM control circuit in the chip can generate charging state logic level. The charging state logic level controls the output voltage and current of the charger according to the charging state. The undervoltage blocking circuit in the chip ensures that enough power supply voltage is added. In addition, the chip also contains a differential current sampling amplifier, a reference voltage with an accuracy of 1%, a -3.9mV/℃ thermistor linearized circuit, a voltage and current error amplifier, a PWM oscillator, a PWM comparator, a PWM latch, a charging state decoder and a 100mA open-collector output driver.