Provided a voltage generating circuits including assist circuits and operating methods thereof. The voltage generating circuit which includes an assist circuit that generates an assist signal indicating an enable mode or a disable mode. When a first power supply voltage is lower than an assist reference voltage, the assist signal indicates the enable mode, and a compensation circuit generates a compensation signal based on the first power supply voltage. An internal voltage converter generates a regulated voltage based on the first power supply voltage, and a charge pump circuit generates a pump voltage based on the regulated voltage. The compensation signal compensates for the regulated voltage.