There are provided a method of operating a voltage generator. The method includes providing a reference voltage, sensing a magnitude of a charge current for increasing voltages of a plurality of word lines based on the reference voltage, determining whether the sensed magnitude of the charge current is greater than a peak current value, increasing the reference voltage in accordance with a first slope when the sensed magnitude of the charge current is less than or equal to the peak current value, and increasing the reference voltage in accordance with a second slope less than the first slope when the detected magnitude of the charge current is greater than the peak current value.