A method has been known in which an integrated circuit called an SoC (System on Chip) is designed by integrating a series of necessary functional system into one semiconductor chip. Since various functions are integrated into one semiconductor chip, a high-performance semiconductor device is provided. However, a load current is changed by the functions performed on the semiconductor chip, which results in a variation in power supply voltage. In recent years, a technique has been proposed which selectively supplies a plurality of power supply voltages generated by a DCDC converter according to the operation state of each functional block in a semiconductor device including a semiconductor chip into which various functional blocks are integrated.
When the power supply voltage varies, the operation speed of the circuit of the functional block on the semiconductor chip also varies. As the power supply voltage increases, the operation speed of the circuit increases. As the power supply voltage is reduced, the operation speed of the circuit is reduced. When the power supply voltage is reduced to below a predetermined threshold voltage, the operation of the circuit of the functional block is likely to be disabled. In addition, when the manufacturing conditions of each semiconductor chip vary, the operation speed of the circuit of the functional block formed in the semiconductor chip varies. In some cases, EMI (Electro Magnetic Interference) occurs due to noise from a power circuit, depending on the relationship between the switching frequency of the power circuit which supplies the power supply voltage and the operating frequency of the functional block on the semiconductor chip. Therefore, there is a demand for a power supply system capable of supplying an appropriate power supply voltage to a semiconductor chip according to various environments.