This application relates to and incorporates herein by reference Japanese Patent Application No. 11-264156 filed on Sept. 17, 1999.
The present invention relates to an organic electroluminescent (EL) display device having an organic EL display panel in which a plurality of organic EL elements are provided.
Organic EL display devices are widely used recently, particularly in the field of a compact-sized display. The organic EL device is advantageous in that it requires no back-light, has a quick response characteristics, provides a wide viewing angle, and the like. It also provides a higher luminance and requires a lower drive voltage than an inorganic EL display device.
Organic EL elements of the organic EL device degrade with time much faster than inorganic EL elements. The EL elements are driven differently from one another with respect to the number of elements being driven at a particular time. The luminance of each EL element will decrease as it is driven more frequently. Thus, the organic EL display device will have some EL elements that are degraded more than others as the use of the organic EL display device proceeds. Thus, the organic EL display device will resultantly have variations in luminance.
It is therefore an object of the present invention to provide an organic EL display device that has less variations in the luminance of display.
According to the present invention, an EL display device has a plurality of organic EL elements and a drive circuit which applies a drive voltage to EL elements to be driven for a required image display. The drive circuit is so constructed to equalize a degree of degradation in luminance among the EL elements.
In one aspect of the present invention, the circuit periodically applies a dummy voltage to the EL element that is not driven for the required image display operation, while applying the drive voltage to the EL elements that are driven. The dummy voltage promotes degradation of the non-driven EL elements. Preferably, the dummy voltage has a wave form different from that of the drive voltage and a period shorter than that of the drive voltage.
In another aspect of the present invention, the circuit modifies the drive voltage applied to the EL elements to be driven. For instance, the circuit may apply an operation characteristics recovery voltage following the drive voltage to the EL element to be driven that is more degraded than the other. The circuit may apply an initial drive voltage to the EL element to be driven that is less degraded than the other, only when a power supply is started for starting the required image display operation. Preferably, the degree of degradation of each EL element is determined base on an accumulated drive period, the number of drives or the like.
In a further aspect of the present invention, the circuit applies the drive voltage for a screen saver operation to all the EL elements or only EL elements that are less degraded.