An internal voltage generator includes a control section and a switchable internal voltage generating circuit. The control section generates a control signal in response to a bank activation command and a bank activation signal for enabling memory banks. The internal voltage generating circuit receives a reference voltage, and responds to the control signal to output an internal voltage equal to the reference voltage. The control signal is enabled when the bank activation command and the bank activation signal are concurrently enabled. The bank activation signal is generated in response to a partial array self refresh (PASR) signal. The internal voltage may be supplied to banks selected by the bank PASR signal, thereby enabling refresh operations in the entire bank, or an internal voltage adequate to partially enable refresh operations in all the banks may be supplied. Thus, unnecessary power consumption may be effectively controlled.