A method of controlling an output voltage of an apparatus is provided. The method includes comparing an input voltage of the apparatus with a reference voltage, activating the output voltage when the input voltage is greater than or equal to the reference voltage, comparing the input voltage with a first low limit voltage or a first high limit voltage, comparing a first elapse time with a reference time when the input voltage is less than or equal to the first low limit voltage, comparing a second elapse time with the reference time when the input voltage is greater than or equal to the first high limit voltage, and deactivating the output voltage when the first elapse time or the second elapse time is longer than or equal to the reference time.