An organic light emitting diode (OLED) display, comprising: an illuminance sensing unit configured to sense an external illuminance; a driving voltage determination unit; a brightness determination unit configured to determine a brightness of the OLED display according to an illuminance sensed by the illuminance sensing unit and to generate a first control signal supplied to the driving voltage determination unit; a voltage conversion unit; a display unit configured to receive a first voltage (ELVDD) and a second voltage (ELVSS) from the voltage conversion unit and display an image; wherein the display unit includes a plurality of pixels, each pixel having: a driving transistor (Td) having a gate electrode and a first electrode, the gate electrode configured to receive a data voltage (D(n)) and the first electrode configured to receive the first voltage (ELVDD) and an OLED having an anode connected to a second electrode of the driving transistor (Td) and a cathode configured to receive the second voltage (ELVSS); the driving voltage determination unit configured to generate a second control signal supplied to the voltage conversion unit to determine a driving voltage (Vds) corresponding with a current saturation point of the OLED display, the driving voltage (Vds) being determined based at least in part on a driving current (Ids) and the brightness determined by the brightness determination unit; characterised in that the voltage conversion unit is configured to receive an input voltage (Vi), generate the first voltage (ELVDD) higher than the input voltage (Vi), and generate the second voltage (ELVSS) lower than the input voltage (Vi), and wherein only the second voltage (ELVSS) is adjusted by said second control signal generated by the driving voltage determination unit to control the driving voltage (Vds) of the display unit. an illuminance sensing unit configured to sense an external illuminance; a driving voltage determination unit; a brightness determination unit configured to determine a brightness of the OLED display according to an illuminance sensed by the illuminance sensing unit and to generate a first control signal supplied to the driving voltage determination unit; a voltage conversion unit; a display unit configured to receive a first voltage (ELVDD) and a second voltage (ELVSS) from the voltage conversion unit and display an image; wherein the display unit includes a plurality of pixels, each pixel having: a driving transistor (Td) having a gate electrode and a first electrode, the gate electrode configured to receive a data voltage (D(n)) and the first electrode configured to receive the first voltage (ELVDD) and an OLED having an anode connected to a second electrode of the driving transistor (Td) and a cathode configured to receive the second voltage (ELVSS); the driving voltage determination unit configured to generate a second control signal supplied to the voltage conversion unit to determine a driving voltage (Vds) corresponding with a current saturation point of the OLED display, the driving voltage (Vds) being determined based at least in part on a driving current (Ids) and the brightness determined by the brightness determination unit; characterised in that the voltage conversion unit is configured to receive an input voltage (Vi), generate the first voltage (ELVDD) higher than the input voltage (Vi), and generate the second voltage (ELVSS) lower than the input voltage (Vi), and wherein only the second voltage (ELVSS) is adjusted by said second control signal generated by the driving voltage determination unit to control the driving voltage (Vds) of the display unit.