An operating method of a display driver integrated circuit (DDI) includes determining whether a change to a level of a first logic voltage supplied to a logic circuit of the DDI is required, based on determining that a change to the level of the first logic voltage is required, transmitting a logic voltage setting command to a power management integrated circuit (PMIC), and receiving, from the PMIC, a second logic voltage having a level different from the first logic voltage and corresponding to the logic voltage setting command.