Disclosed herein is an apparatus that includes a first semiconductor chip including a memory cell array having a volatile memory cell and an access control circuit configured to perform a refresh operation on the volatile memory cell, and a second semiconductor chip including a power generator configured to supply a first power supply voltage to the first semiconductor chip. The access control circuit is configured to activate a first enable signal during the refresh operation. The second semiconductor chip is configured to change a capability of the power generator based on the first enable signal.