Disclosed is a user system which includes a first device and a second device, which share a shared voltage, and a power management integrated circuit (PMIC) generating the shared voltage. An operation method of the user system includes performing a first operation of the first device, determining whether a second operation of the second device is to be performed while the first device performs the first operation, based on an operation profile, and when it is determined that the second operation of the second device is to be performed while the first device performs the first operation, changing a power mode of the PMIC from a first power mode to a second power mode, before the second device performs the second operation. The PMIC generates the shared voltage based on the first power mode or the second power mode.