A voltage generating apparatus is provided. The voltage generating apparatus includes a reference voltage generator and an output voltage generator. The reference voltage generator is used for generating a reference voltage, and the reference voltage generator decides to generate the reference voltage or not according to a control signal. The output voltage generator includes a comparator, a variable resistor and a current source. The comparator compares the reference voltage and an output voltage to generator a calibrating signal. A resistance of the variable resistor is decided by the calibrating signal. The current source provides an output current to flow through the variable resistor for generating the output voltage. Wherein, the reference voltage is generated during an initial timing period, and the generator is turned off after the initial timing period. The initial timing period is determined according to the control signal.