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Luminance control method and luminance control circuit for organic el displayLuminance control method and luminance control circuit for organic el display description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080084433, Luminance control method and luminance control circuit for organic el display. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001] This application is a Divisional of U.S. application Ser. No. 10/500,374, filed Oct. 28, 2004, which is a 371 of PCT/JP02/13728, filed Dec. 26, 2002, claiming priority of Japanese Application Nos. 2001-400238, filed Dec. 28, 2001, and 2002-91796, filed Mar. 28, 2002, the entire contents of each of which are hereby incorporated by reference. TECHNICAL FIELD [0002] The present invention relates to a luminance control method and a luminance control circuit for an organic electroluminescence (EL) display and a portable telephone set comprising the organic EL display. BACKGROUND ART [0003] Examples of organic EL displays include passive-type ones having a simple matrix structure and active-type ones using a TFT (Thin Film Transistor). [0004] FIG. 1 illustrates the basic pixel structure of the active type organic EL display. [0005] A circuit corresponding to one pixel in the active type organic EL display comprises a switching TFT 101, a capacitor 102, a driving TFT 103, and an organic EL element 104. [0006] A display signal Data(Vin) is applied to the drain of the switching TFT 101 through a display signal line 111. A selection signal SCAN is applied to the base of the switching TFT 101 through a selection signal line 112. The switching TFT 101 has its source connected to the base of the driving TFT 103 and grounded through the capacitor 102. [0007] A driving power supply voltage Vdd is applied to the drain of the driving TFT 103 through a power supply line 113. The driving TFT 103 has its source connected to the anode of the organic EL element 104. The cathode of the organic EL element 104 is grounded. [0008] The switching TFT 101 is subjected to ON/OFF control by the selection signal SCAN. The capacitor 102 is charged by the display signal Data(Vin) fed through the switching TFT 101 when the switching TFT 101 is turned on. The capacitor 102 holds a charging voltage when the switching TFT 101 is turned off. The driving TFT 103 supplies to the organic EL element 104 a current corresponding to the voltage, held in the capacitor 102, which is applied to the base thereof. [0009] FIG. 2 illustrates the relationship between the display signal Data(Vin) and a light-emitting luminance (a driving current) of the organic EL element 104 in the basic pixel structure shown in FIG. 1. [0010] In FIG. 2, RefW and RefB respectively indicate a white-side reference voltage for defining a light-emitting luminance corresponding to the white level of an input signal and a black-side reference voltage for defining a light-emitting luminance corresponding to the black level of the input signal. [0011] In the above-mentioned active type organic EL display, a large current flows through the organic EL element 104 with respect to an image which is bright on the whole screen. When the large current flows through the organic EL element 104, power consumption is increased. When the large current continues to flow through the organic EL element 104, the degradation of the performance thereof is advanced. [0012] Therefore, a technique for detecting the current flowing into the cathode of the organic EL element 104, and controlling the power supply voltage Vdd of the organic EL element 104 in response to the value of the detected current, thereby reducing the power supply voltage to reduce a driving current when the overall screen is bright, for example, has been developed (see JP-A-2000-267628). [0013] The control of the power supply voltage by the above-mentioned prior art is feedback control for controlling the power supply voltage Vdd of the organic EL element 104 in response to the value of the detected current. In the case of the feedback control, when the brightness of a video is rapidly changed, for example, excessive control easily occurs. In the case, a luminance varies in a short period, which is so-called "hunting". [0014] An object of the present invention is to provide a luminance control method and a luminance control circuit for an organic EL display capable of achieving power saving as well as restraining the degradation in the performance of an organic EL element and capable of preventing "hunting" from occurring. [0015] Another object of the present invention is to provide a portable telephone set capable of changing the display luminance of an organic EL display depending on peripheral brightness. [0016] A further object of the present invention is to provide a portable telephone set capable of changing the display luminance of an organic EL display depending on the direction of a portable telephone set. DISCLOSURE OF INVENTION [0017] A luminance control method for an organic EL display according to the present invention is characterized by comprising a first step of calculating a luminance accumulation value for each screen on the basis of a video input signal; and a second step of controlling the amplitude of the video input signal on the basis of the luminance accumulation value calculated in the first step, and feeding to the organic EL display the video signal whose amplitude has been controlled and in that in the second step, the amplitude of the video input signal is controlled, when the luminance accumulation value calculated in the first step exceeds a predetermined value, such that the larger the difference between the luminance accumulated value and the predetermined value is, the smaller the amplitude of the video input signal becomes. [0018] When the video input signal is a digital video signal, a reference voltage supplied to a digital-to-analog (DA) converter for converting the digital video input signal into an analog video signal is controlled on the basis of the luminance accumulation value calculated in the first step, to control the amplitude of the video input signal in the second step. [0019] The reference voltage supplied to the digital-to-analog converter includes a black-side reference voltage for defining a light-emitting luminance corresponding to the black level of the input signal and a white-side reference voltage for defining a light-emitting luminance corresponding to the white level of the input signal, and in the second step, the white-side reference voltage is controlled on the basis of the luminance accumulation value calculated in the first step. [0020] A luminance control circuit for an organic EL display according to the present invention is characterized by comprising a digital-to-analog converter for converting a digital video input signal into an analog video output signal on the basis of input/output characteristics defined by a given reference voltage, and feeding the analog video output signal to the organic EL display; and a reference voltage control circuit for controlling the reference voltage supplied to the digital-to-analog converter on the basis of the digital video input signal, and in that the reference voltage control circuit comprises a luminance accumulation value calculation circuit for calculating a luminance accumulation value for each screen on the basis of the digital video input signal, and a voltage control circuit for controlling the reference voltage supplied to the digital-to-analog converter on the basis of the luminance accumulation value calculated by the luminance accumulation value calculation circuit the reference voltage supplied to the digital-to-analog converter includes a black-side reference voltage for defining a light-emitting luminance corresponding to the black level of the input signal and a white-side reference voltage for defining a light-emitting luminance corresponding to the white level of the input signal, and the voltage control circuit controls, when the luminance accumulation value calculated by the luminance accumulation value calculation circuit exceeds a predetermined value, the white-side reference voltage such that the larger the difference between the luminance accumulation value and the predetermined value is, the lower the light-emitting luminance corresponding to the white level of the input signal becomes. 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