CN100437745C - Organic EL display luminance control method and luminance control circuit - Google Patents

Abstract

A luminance control method for an organic EL display comprises 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.

Description

Brightness control method and brightness control circuit of organic EL display

technical field

The invention relates to a brightness control method of an organic electroluminescent (organic EL) display, a brightness control circuit and a portable phone with an organic EL display.

Background technique

Organic EL displays include a passive type with a simple matrix structure and an active type using TFTs.

FIG. 1 shows the basic pixel configuration of an active-type organic EL display.

A circuit of one pixel of an active organic EL display is constituted by a switching TFT 101 , a capacitor 102 , a driving TFT 103 , and an organic EL element 104 .

A display signal Data (Vin) is applied to the drain of the switching TFT 101 via a display signal line 111 . A selection signal SCAN is applied to the base of the switching TFT 101 via the selection signal line 112 . The source of the switching TFT 101 is connected to the base of the driving TFT 103 and grounded via the capacitor 102 .

A driving power supply voltage Vdd is applied to the drain of the driving TFT 103 via the power supply line 113 . The source of the driving TFT 103 is connected to the anode of the organic EL element 104 . The cathode of the organic EL element 104 is grounded.

The switching TFT 101 is controlled on and off by a selection signal SCAN. The capacitor 102 is charged by the display signal Data (Vin) supplied through the switching TYFT 101 when the switching TYFT 101 is turned on. And, when the switching TYFT 101 is turned off, the charging voltage is held. The driving TFT 103 supplies the organic EL element 104 with a current corresponding to the voltage held by the capacitor 102 applied to the base thereof.

FIG. 2 shows the relationship between the display signal Data (Vin) and the emission luminance (drive current) of the organic EL element 104 in the basic pixel configuration shown in FIG. 1 .

In FIG. 2 , RefW denotes a white reference voltage for specifying emission luminance corresponding to a white level of an input signal, and RefB denotes a black reference voltage for specifying emission luminance corresponding to a black level of an input signal.

In the active type organic EL display as described above, a large current flows through the organic EL element 104 for a bright image on the entire screen. When a large current flows through the organic EL element 104, power consumption is large. In addition, if a large current continues to flow through the organic EL element 104, its performance deteriorates rapidly.

Therefore, by detecting the current flowing through the cathode of the organic EL element 104 and controlling the power supply voltage Vdd of the organic EL element 104 corresponding to the detected current value, a technique has been developed such that when the entire screen is bright, the power supply voltage decrease, thereby reducing the drive current (JP-A-2000-267628).

The power supply voltage control in the above-mentioned prior art is a feedback control in which the power supply voltage Vdd of the organic EL element 104 is controlled in accordance with the detected current value. In the case of feedback control, when the brightness of the image changes sharply, over-control is prone to occur, and the brightness will change in a short period at this time, that is, the so-called parasitic oscillation occurs.

Contents of the invention

An object of the present invention is to provide a brightness control method and a brightness control circuit for an organic EL display capable of suppressing performance degradation of an organic EL element while saving power and preventing spurious oscillation.

An object of the present invention is to provide a cellular phone capable of changing the display luminance of an organic EL display in accordance with the luminance of ambient light.

An object of the present invention is to provide a mobile phone capable of changing the display luminance of an organic EL display according to the direction of the mobile phone.

The luminance control method of the organic EL display of the present invention is characterized in that it has a first step of calculating the luminance integral value of each screen from the image input signal; and controlling the amplitude of the image input signal based on the luminance integral value calculated in the first step and In the second step of supplying the amplitude-controlled image signal to the organic EL display, in the second step, when the luminance integral value calculated in the first step exceeds a predetermined value, the amplitude of the image input signal is controlled so that the luminance integral value and The larger the difference of the predetermined value, the smaller the amplitude of the image input signal.

When the image input signal is a digital image signal, in the second step, the image is controlled by controlling the reference voltage supplied to the DA converter for converting the digital image signal into an analog image signal based on the luminance integral value calculated in the first step. The amplitude of the input signal.

The reference voltage supplied to the DA converter is divided into a black reference voltage for specifying the luminance corresponding to the black level of the input signal and a white reference voltage for specifying the luminance corresponding to the white level of the input signal. The step controls the white reference voltage value based on the luminance integral value calculated in the first step.

The luminance control circuit of the organic EL display of the present invention is characterized in that it has a DA converter that converts a digital image signal into an analog image signal and supplies it to the organic EL display according to the input-output characteristics specified by a given reference voltage, and a digital image input signal based on the digital image. The signal controls a reference voltage control circuit for supplying a reference voltage to the DA converter. The reference voltage control circuit has a luminance integral value calculation circuit for calculating a luminance integral value for each screen from a digital image input signal, and a luminance integral value calculation circuit based on the luminance integral value calculated by the luminance integral value calculation circuit. A voltage adjustment circuit for controlling a reference voltage supplied to the DA converter by the luminance integral value. The reference voltage supplied to the DA converter is divided into a black reference voltage for specifying the luminance corresponding to the black level of the input signal and a black reference voltage for specifying the luminance corresponding to the black level of the input signal. The white reference voltage of the luminous luminance corresponding to the white level of the input signal, when the luminance integral value calculated by the luminance integral value calculation circuit exceeds the specified value, the voltage adjustment circuit controls the white reference voltage, so that when the difference between the luminance integral value and the specified value The larger the value, the lower the luminance corresponding to the white level of the input signal.

The voltage adjustment circuit has a gain calculation circuit for calculating a gain for controlling the white reference voltage based on the luminance integral value calculated by the luminance integral value calculation circuit, and a control circuit for controlling the white reference voltage based on the gain calculated by the gain calculation circuit.

The gain calculation circuit has an input-output characteristic that the output gain is constant when the input luminance integral value is below the specified value, and the output gain is smaller when the input luminance integral value exceeds the specified value. The circuit controls the white reference voltage so that the smaller the gain, the lower the luminous brightness corresponding to the white level of the input signal.

The voltage adjustment circuit has a gain calculation circuit for calculating a first gain for controlling the white reference voltage from the luminance integral value calculated by the luminance integral value calculation circuit, and a gain calculation circuit for multiplying the gain calculated by the gain calculation circuit by a second gain supplied from outside. A multiplication circuit and a control circuit for controlling a white reference voltage based on a third gain obtained as a result of multiplication by the multiplication circuit.

The gain calculation circuit has an input-output characteristic that the output gain is constant when the input luminance integral value is below the specified value, and the output gain is smaller when the input luminance integral value exceeds the specified value. The circuit controls the white reference voltage so that the smaller the third gain is, the lower the luminous brightness corresponding to the white level of the input signal is.

The first mobile phone of the present invention is a mobile phone including a camera with an automatic exposure control function and an organic EL display, and is characterized in that: there is a judging means for judging the brightness of ambient light based on the exposure control information of the camera; The brightness of the ambient light judged by the judging means controls the display brightness of the organic EL display.

The display luminance control means controls the display luminance of the organic EL display so that when the luminance of the ambient light determined by the determination means is dark, the display luminance of the organic EL display becomes low, and when the luminance of the ambient light determined by the determination means is bright , so that the display brightness of the organic EL display becomes higher.

The camera exposure control information is one type of information selected from exposure time information and AGC gain information.

The second mobile phone of the present invention is a mobile phone including an organic EL display, and is characterized in that it has detection means for detecting the direction of the display surface of the organic EL display, and based on the direction of the display surface of the organic EL display detected by the detection means. A display brightness control device that controls the display brightness of an organic EL display in a direction.

The display luminance control device controls the display luminance of the organic EL display so that when the direction of the display surface of the organic EL display is upward, the display luminance of the organic EL display becomes high, and when the direction of the display surface of the organic EL display is downward, the display luminance of the organic EL display becomes high. The display brightness of the EL display becomes low.

A brief description of the drawings

FIG. 1 is a circuit diagram showing a basic pixel configuration of an active organic EL display.

FIG. 2 is a graph showing the relationship between the display signal Data (Vin) and the light emission luminance (drive current) of the organic EL element in the basic pixel configuration shown in FIG. 1 .

FIG. 3 shows the configuration of the brightness control circuit of the organic EL display according to the first embodiment of the present invention.

FIG. 4 is a graph showing an example of the input/output characteristics of the gain calculation circuit 14 .

FIG. 5 is a circuit diagram showing a reference voltage adjustment circuit for R. FIG.

FIG. 6 is a graph showing the input/output characteristics of DAC2.

FIG. 7 shows the configuration of a luminance control circuit of an organic EL display according to a second embodiment of the present invention.

FIG. 8 is a graph showing a setting example of the input/output characteristics of the gain correction circuits 61 , 62 , and 63 .

Fig. 9 is a block diagram showing a schematic configuration of a mobile phone according to a third embodiment of the present invention.

FIG. 10 is a block diagram showing the configuration of the brightness control circuit provided in the timing control IC 213 of FIG. 9 and the MPU 209 and its peripherals for controlling the brightness of the entire screen.

Fig. 11 is a block diagram showing a schematic configuration of a cellular phone according to a fourth embodiment of the present invention.

FIG. 12 is a block diagram showing the configuration of the brightness control circuit provided in the timing control IC 213 of FIG. 11 and the MPU 209 for controlling the brightness of the entire screen and its peripherals.

Detailed ways

Next, an embodiment of the present invention will be described with reference to FIGS. 3 to 10 .

[1] Explanation of the first embodiment

FIG. 3 shows the configuration of the brightness control circuit of the organic EL display according to the first embodiment of the present invention.

The luminance control circuit of an organic EL display has a reference voltage control circuit 1 and a DAC2. The digital image input signals R_in, G_in, and B_in are sent to the reference voltage control circuit 1 and at the same time sent to the DAC2. The reference voltage control circuit 1 controls the reference voltage supplied to the DAC2. The reference voltages supplied to DAC2 include black reference voltages R_RefB, G_RefB, B_RefB (they are collectively referred to as RefB) and white reference voltages R_RefW, G_RefW, B_RefW (they are collectively referred to as RefW) for R, G, and B, respectively.

The black reference voltage RefB is a reference voltage for specifying the emission luminance corresponding to the black level of the input signal, and is fixed in this embodiment. The white reference voltage RefW is a reference voltage for defining the light emission luminance corresponding to the white level of the input signal, and is controlled by the reference voltage control circuit 1 in this embodiment.

DAC2 converts digital image input signals R_in, G_in, B_in into analog image output signals R_out, G_out, B_out according to input/output characteristics defined by black reference voltage RefB and white reference voltage RefW supplied from reference voltage control circuit 1 . The analog image output signals R_out, G_out, and B_out obtained by DAC2 are supplied to organic EL display 3 . The analog image output signals R_out, G_out, and B_out correspond to the display signal Data(Vin) in FIG. 1 .

The reference voltage control circuit 1 includes a luminance signal generation circuit (Y generation circuit) 11, a luminance integration circuit 12, an LPF 13, a gain calculation circuit 14, a reference voltage adjustment circuit (Ref voltage adjustment circuit) 15, and a plurality of DACs 16-21.

The luminance signal generating circuit 11 generates a luminance signal Y based on digital image input signals R_in, G_in, and B_in. The luminance integration circuit 12 calculates a luminance integral value for each frame from the luminance signal generated by the luminance signal generation circuit 11 . The LPF 13 smoothes the luminance integral value calculated by the luminance integration circuit 12 in units of one frame in the direction of the time axis. This LPF 3 is provided in order to gradually change the gain Gain, which will be described later, in response to a sudden luminance change, but it may be omitted.

The gain calculation circuit 14 calculates a gain Gain for controlling the white reference voltage RefW in accordance with the magnitude of the luminance integral value per frame obtained by the LPF 13 . FIG. 4( a ) and FIG. 4( b ) respectively show examples of the input/output characteristics of the gain calculation circuit 14 , that is, the gain characteristics with respect to luminance integral values in units of one frame.

In the characteristic of FIG. 4( a ), the gain is 1.00 for the luminance integral value of 1 frame unit from 0 to a, and when the luminance integral value of 1 frame unit exceeds a, the gain gradually decreases. In the characteristics of Fig. 4(b), the gain is 1.00 for luminance integral values 0 to b in one frame unit, and the gain decreases slowly when the luminance integral values in one frame unit are b to c. When the integral value exceeds c, the gain drops faster.

The reference voltage adjustment circuit 15 is based on the black reference voltage (hereinafter referred to as the reference black reference voltage) R_RefB, G_RefB, B_RefB preset for each RGB, and the white reference voltage (hereinafter referred to as the reference white reference voltage) preset for each RGB. R_RefW, G_RefW, B_RefW and the gain Gain calculated by the gain calculation circuit 14 generate adjusted white reference voltages R_RefW′, G_RefW′, B_RefW′ for each RGB.

The respective reference black reference voltages R_RefB, G_RefB, B_RefB and the respective reference white reference voltages R_RefW, G_RefW, B_RefW are given as digital signals.

The reference voltage adjustment circuit 15 includes reference voltage adjustment circuits for R, G, and B respectively, but since the configuration is the same, only the reference voltage adjustment circuit for R will be described here.

Figure 5 shows the reference voltage adjustment circuit for R.

The reference voltage adjustment circuit includes a subtractor 31 , a multiplier 32 and a subtractor 33 .

The subtracter 31 calculates the difference (R_RefB−R_RefW) between the reference black reference voltage R_RefB for R and the reference white reference voltage R_RefW for R. The multiplier 32 multiplies the gain Gain and the output (R_RefB−R_RefW) of the subtracter 31 . The subtracter 33 calculates the adjusted white reference voltage R_RefW' by subtracting the output of the multiplier 32 (Gain×(R_RefB−R_RefW)) from the reference black reference voltage R_RefB.

When the gain Gain is 1.00, the adjusted white reference voltage R_RefW' is equal to the reference white reference voltage R_RefW. Moreover, the smaller the gain Gain is, that is, the larger the luminance integral value of one frame unit is, the larger the adjusted white reference voltage R_RefW' is, and it is close to the reference black reference voltage R_RefB. That is, the larger the luminance integral value per frame unit, the lower the emission luminance (drive current) of the organic EL element corresponding to the white level of the input signal.

The reference black reference voltages R_RefB, G_RefB, and B_RefB are respectively converted into analog signals by DAC16, 17, and 18, and then supplied to DAC2. The adjusted white reference voltages R_RefW', G_RefW', B_RefW' are respectively converted into analog signals by DAC19, 20, 21, and then supplied to DAC2.

Figure 6 shows the input and output characteristics of DAC2.

In FIG. 6 , RefW'1 represents a white reference voltage (=reference white reference voltage RefW) supplied to DAC2 when the luminance integral value is small (when the image is dark). RefW'3 represents a white reference voltage supplied to DAC2 when the luminance integral value is large (when the image is bright). RefW'2 represents the white reference voltage supplied to DAC2 when the luminance integral value is an intermediate value (when the image is of medium luminance).

When the white reference voltage supplied to DAC2 is RefW'1, the input/output characteristics of DAC2 become those shown by straight line L1. At this time, if the input signal changing from black level to white level is periodically input to DAC2, the output waveform shown in curve S1 can be obtained.

When the white reference voltage supplied to DAC2 is RefW'3, the input/output characteristics of DAC2 become those shown by straight line L3. At this time, if the input signal changing from black level to white level is periodically input to DAC2, the output waveform shown in curve S3 can be obtained.

When the white reference voltage supplied to DAC2 is RefW'2, the input/output characteristics of DAC2 become those shown by straight line L2. At this time, if the input signal changing from black level to white level is periodically input to DAC2, the output waveform shown in curve S2 can be obtained.

That is, the amplitude of the output signal of DAC2 can be controlled by controlling the white reference voltage corresponding to the luminance integral value in units of frames.

In the above-mentioned embodiment, when the input image is a bright image, the amplitude of the image input signal (display signal) is reduced to reduce the driving current of the organic EL element. By controlling the reference voltage at the time of DA conversion, the amplitude of the image input signal is controlled, so the gradation does not decrease.

In addition, since the amplitude control of the image input signal (display signal) is performed by feedforward control, spurious oscillation does not occur.

[2] Explanation of the second embodiment

FIG. 7 shows the configuration of a luminance control circuit of an organic EL display according to a second embodiment of the present invention. In FIG. 7 , the same reference numerals are attached to the same components as those in FIG. 3 , and their descriptions are omitted.

The luminance control circuit of the organic EL display in the second embodiment differs from the luminance control circuit of the organic EL display in the first embodiment in the following points.

(1) The multiplier 41 that is used to control the brightness of the whole picture from the outside is set in the reference voltage control circuit 1;

(2) The multipliers 51, 52, 53 that may adjust the white balance are set in the reference voltage control circuit 1;

(3) Since the luminance characteristics of each R, G, and B display signal are different, the gain correction circuits 61, 62, and 63 used to correct the gain Gain of each R, G, and B are provided in the reference voltage control circuit 1 ;

Hereinafter, these differences will be described in detail.

The gain Gain calculated by the gain calculation circuit 14 is input to the multiplier 41 . The overall brightness control signal W_Gain for controlling the brightness of the entire screen is externally applied to the multiplier 41 . By controlling the signal W_Gain supplied to the multiplier 41, for example, the screen can be brightened when the display is used in a bright place, and the screen can be dimmed after a certain period of time.

The output of multiplier 41 is supplied to multipliers 51, 52, 53, respectively. For each of R, G, and B, arbitrary gains R_Gain, G_Gain, and B_Gain can be added to these multipliers 51, 52, and 53, respectively. Since the gains R_Gain, G_Gain, and B_Gain respectively applied to the multipliers 51, 52, and 53 can be individually controlled, white balance adjustment can be performed.

The output of each multiplier 51, 52, 53 is sent to the corresponding gain correction circuit 61, 62, 63 respectively. Each gain correction circuit 61, 62, 63 corrects the gain of the input by setting the input-output characteristics, for example, like the straight lines K1, K2 in FIG. 8 .

In the reference voltage adjustment circuit 15, the white reference voltage is adjusted for each of R, G, and B using the gains supplied from the correction circuits 61, 62, and 63 corresponding to each of R, G, and B.

[3] Explanation of the third embodiment

Fig. 9 shows a schematic configuration of a mobile phone.

MPU209 performs overall control of the mobile phone. The antenna 201 transmits and receives radio waves. The transceiver unit 202 receives radio waves and transmits the received content to the MPU 209 . In addition, the transmission and reception unit 202 applies the transmission signal output from the MPU 209 to a radio wave and transmits it.

Microphone 203 transmits audio signals to MPU 209 . Speaker 204 outputs the audio signal output from MPU 209 as audio. The first camera 205 is a camera mounted on the front of a mobile phone main body provided with an organic EL display 214, and sends captured images to the MPU 209. The second camera 206 is a camera mounted on the back of the main body of the mobile phone, and sends captured images to the MPU 209 . In the imaging mode, the organic EL display 214 displays an image captured by the camera 205 or 206 instead of the display image in the normal mode.

The operation unit 208 is provided in the main body of the mobile phone, and includes various buttons 221 and various switches 222 as shown in FIG. 10 . The timer 211 is used for brightness control as will be described later.

The flash memory 210 stores data to be saved when the power is turned off. Graphics memory 212 stores image data displayed on the display. Based on the image data output from the MPU 209 and the write control signal, the image data is written to a specified address in the image memory 212 . Further, pixel data of corresponding pixels is output from the image memory 212 in synchronization with the scanning timing in accordance with the display cycle of the organic EL display 214 .

The timing control IC 213 supplies image data and driving signals to the organic EL display 214 and displays an image on the organic EL display 214 . The timing control IC 213 includes a brightness control circuit.

FIG. 10 shows the configuration of the luminance control circuit provided in the timing control IC 213 and the MPU 209 and its peripherals for controlling the luminance of the entire screen.

In FIG. 10 , the same reference numerals are assigned to the same components as those in FIG. 3 , and description thereof will be omitted. The brightness control circuit in FIG. 10 is substantially the same as the brightness control circuit in FIG. 3 except that a multiplier 41 for controlling the brightness of the entire screen (display brightness) is provided in the reference voltage control circuit 1. The overall brightness control signal W_Gain supplied to the multiplier 41 is generated by the MPU 209 .

Various buttons 221 and various switches 222 provided in the operation unit 208 are connected to the MPU 209 . MPU 209 has timer 211 . MPU209 is connected to cameras 205 and 206 . Each camera 205, 206 has an automatic exposure control function. In this example, the exposure time information is sent from the first camera 205 mounted on the front of the main body of the mobile phone to the MPU 209 .

Based on the exposure time information from the first camera 205, the MPU 209 estimates the brightness of ambient light in the current usage environment of the mobile phone, and regenerates the overall brightness control signal W_Gain. The overall brightness control signal W_Gain takes a value between 2.0 and 0.5, for example.

Specifically, when the exposure time is long, that is, when the ambient light is dark, the overall brightness control signal W_Gain is reduced. As a result, the gain output from the multiplier 41 is smaller than the gain calculated by the gain calculation circuit 14, and the adjusted white reference voltage R_Refw' becomes larger, so that the display brightness decreases. On the contrary, when the exposure time is short, that is, when the ambient light is bright, the overall brightness control signal W_Gain is increased. As a result, the gain output from the multiplier 41 is larger than the gain calculated by the gain calculation circuit 14, and the adjusted white reference voltage R_Refw' becomes smaller, so that the display brightness is improved.

In addition, instead of the exposure time information of the first camera 205, the above control may be performed using AGC gain information of the first camera 205. At this time, when the AGC gain is large, it is determined that the ambient light is dark, and the overall brightness control signal W_Gain is reduced. Conversely, when the AGC gain is small, it is determined that the ambient light is bright, and the overall brightness control signal W_Gain is increased.

Also, when various buttons 221 or various switches 222 provided in operation unit 208 are operated, MPU 209 increases display brightness by reducing overall brightness control signal W_Gain. Then, after a certain time elapses, the display brightness is reduced by increasing the overall brightness control signal W_Gain.

The timer 211 is used to determine whether or not a certain period of time has elapsed. Specifically, the timer 211 is reset when various buttons 221 or various switches 222 are operated, and automatic timing starts. In addition, the brightness of the screen is controlled corresponding to the time counted by the timer 211 . For example, if a time not lower than a predetermined time elapses, the display brightness is halved.

[4] Explanation of the fourth embodiment

Fig. 11 shows a schematic configuration of a mobile phone. In FIG. 11 , the same components as those in FIG. 9 are denoted by the same reference numerals and their descriptions are omitted.

This mobile phone differs from the mobile phone of FIG. 9 in that a direction sensor 207 for detecting the direction (upward, downward, horizontal, etc.) of the display surface of the organic EL display 214 is provided. In addition, this mobile phone does not perform display brightness control based on the exposure time information from the first camera 205 .

FIG. 12 shows the configuration of the brightness control circuit provided in the timing control IC 213 and the MPU 209 and its peripherals for controlling the brightness of the entire screen.

In FIG. 12 , the same reference numerals are assigned to the same components as those in FIG. 3 , and description thereof will be omitted. The luminance control circuit of FIG. 12 is substantially the same as the luminance control circuit of FIG. 3 except that a multiplier 41 for controlling the luminance of the entire screen (display luminance) is provided in the reference voltage control circuit 1. The overall brightness control signal W_Gain supplied to the multiplier 41 is generated by the MPU 209 .

Various buttons 221 and various switches 222 provided in the operation unit 208 are connected to the MPU 209 . MPU 209 has timer 211 . The MPU 209 is connected to the direction sensor 207 .

The MPU 209 estimates the orientation (upward, downward, horizontal, etc.) of the display surface of the organic EL display 214 based on the detection signal of the orientation sensor 207, and generates the overall brightness control signal W_Gain. The overall brightness control signal W_Gain takes a value between 2.0 and 0.5, for example.

Specifically, as the display surface of the organic EL display 214 goes upward, the overall brightness control signal W_Gain becomes smaller, thereby increasing the display brightness. The overall brightness control signal W_Gain is controlled so that when the display surface of the organic EL display 214 is facing upward, it is set to a small value, and when the display surface of the organic EL display 214 is facing downward, it is set to a large value. When the display surface of the EL display 214 is in the horizontal direction, the value is set to be an intermediate value.

In addition, MPU 209 increases display brightness by reducing overall brightness control signal W_Gain when various buttons 221 or various switches 222 provided in operation unit 208 are operated, as in the third embodiment described above. Then, after a certain time elapses, the display brightness is reduced by increasing the overall brightness control signal W_Gain.

Furthermore, the direction (upward, downward, horizontal, etc.) of the display surface of the organic EL display 214 may be detected based on the exposure time of the two cameras 205 and 206 and the AGC gain.

That is, when the direction of the display surface of the organic EL display 214 is upward, since the front of the main body of the mobile phone is more likely to be brighter than the back, the exposure time of the first camera 205 mounted on the front of the main body of the mobile phone is longer than that of the first camera 205 mounted on the front of the main body of the mobile phone. The exposure time of the second camera 206 on the back of the main body of the mobile phone is short (when the exposure time is the same, the AGC gain is small).

On the contrary, when the direction of the display surface of the organic EL display 214 is downward, the possibility that the back of the main body of the mobile phone is brighter than the front is high, so the exposure time of the second camera 206 installed on the back of the main body of the mobile phone is longer than the exposure time. The exposure time of the first camera 205 mounted on the front of the main body of the mobile phone is short (when the exposure time is the same, the AGC gain is small).

Therefore, the orientation of the display surface of the organic EL display 214 can be determined from the exposure time of the two cameras 205 and 206 and the AGC gain.

Claims (6)

1. A brightness control method of an organic EL display, characterized in that: It has a first step of calculating the luminance integral value of each frame based on the digital image input signal; and a second step of controlling the amplitude of the image input signal based on the luminance integral value calculated in the first step and supplying the amplitude-controlled image signal to the organic EL display. 2 steps, In the second step, the amplitude of the image input signal is controlled by controlling the reference voltage supplied to the DA converter for converting the digital image input signal into an analog image signal based on the luminance integral value calculated in the first step, The reference voltage supplied to the DA converter is divided into a black reference voltage for specifying the luminance corresponding to the black level of the input signal and a white reference voltage for specifying the luminance corresponding to the white level of the input signal. In step 2, the white reference voltage value is controlled according to the luminance integral value calculated in the first step. When the luminance integral value exceeds a specified value, the white reference voltage is controlled so that when the difference between the luminance integral value and the specified value is larger, the input The white level of the signal corresponds to the lower the luminous brightness. 2. A brightness control circuit of an organic EL display, characterized in that: It has a DA converter that converts a digital image input signal into an analog image output signal and supplies it to an organic EL display according to the input and output characteristics specified by a given reference voltage; and controls the reference voltage supplied to the DA converter based on the digital image input signal. reference voltage control circuit, The reference voltage control circuit has a luminance integral value calculation circuit for calculating a luminance integral value for each frame from the digital image input signal; and a voltage adjustment for controlling a reference voltage supplied to the DA converter based on the luminance integral value calculated by the luminance integral value calculation circuit. circuit, The reference voltage supplied to the DA converter is divided into a black reference voltage for specifying the luminance of light emission corresponding to the black level of the input signal and a white reference voltage for specifying the luminance of light emission corresponding to the white level of the input signal. When the luminance integral value calculated by the luminance integral value calculation circuit exceeds a specified value, the voltage adjustment circuit controls the white reference voltage so that the greater the difference between the luminance integral value and the specified value, the lower the luminance corresponding to the white level of the input signal. . 3. The brightness control circuit of the organic EL display of claim 2 record, it is characterized in that: The voltage adjustment circuit has a gain calculation circuit for calculating a gain for controlling the white reference voltage based on the luminance integral value calculated by the luminance integral value calculation circuit; and a control circuit for controlling the white reference voltage based on the gain calculated by the gain calculation circuit. 4. The brightness control circuit of the organic EL display of claim 3 record, it is characterized in that: The gain calculation circuit has an input-output characteristic that the output gain is constant when the input luminance integral value is below the specified value, and the output gain is smaller when the input luminance integral value exceeds the specified value. The control circuit of the white reference voltage is used to control the white reference voltage so that the smaller the gain is, the lower the luminous brightness corresponding to the white level of the input signal is. 5. The brightness control circuit of the organic EL display of claim 2 record, it is characterized in that: The voltage adjustment circuit has a gain calculation circuit that calculates a first gain for controlling the white reference voltage based on the luminance integral value calculated by the luminance integral value calculation circuit, and multiplies the gain calculated by the gain calculation circuit by a second gain supplied from the outside. The multiplication circuit and the control circuit for controlling the white reference voltage according to the third gain which is the multiplication result of the multiplication circuit. 6. The brightness control circuit of the organic EL display of claim 5 record, it is characterized in that: The gain calculation circuit has an input-output characteristic that the output gain is constant when the input luminance integral value is below the specified value, and the output gain is smaller when the input luminance integral value exceeds the specified value. The white reference voltage control circuit is used to control the white reference voltage so that the smaller the third gain is, the lower the luminance corresponding to the white level of the input signal is.

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