Circuits and methods for optimizing operating performance of an integrated circuit device within a maximum allowed current by varying a period of a clock signal based on an amount of current consumed by the integrated circuit device. In one aspect, an integrated circuit device includes a plurality of functional blocks, a power supply line which supplies an internal power supply voltage to the functional blocks, a voltage converter circuit which controls an amount of current supplied to the power supply line by comparing a reference voltage with the internal power supply voltage, and a clock generator circuit which generates a clock signal that is applied to the functional blocks. The clock generator circuit adjusts a period of the clock signal according to the amount of current supplied to the power supply line.