This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-304723 filed Sep. 28, 2001; and No. 2001-375002, filed Sep. 29, 2001, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a display device having a plurality of display pixels arrayed in a matrix form to display an image, to a driving method thereof and, for example, to a self-luminous display device in which each display pixel is configured by a self-luminous element such as an organic EL (Electro Luminescence) element.
2. Description of the Related Art
In recent years, much attention has been focused on organic EL display devices as monitor displays for portable information terminals since the devices have such characteristics as lightness, thinness, and high luminance. A typical organic EL display device includes organic EL elements as self-luminous elements incorporated in display pixels which are arrayed in a matrix form to display an image. In this organic EL display device, a plurality of scanning lines are disposed along rows of the display pixels, a plurality of signal lines are disposed along columns of the display pixels, and a plurality of pixel switches are disposed near intersections of the scanning and signal lines.
Each display pixel includes a pixel switch, driving element and organic EL element. The pixel switch is connected to receive a video signal from a corresponding signal lines in response to a scanning signal from a corresponding scanning line. The driving element is connected in series with the organic EL element between a pair of power lines to supply a driving current corresponding to the video signal from the pixel switch. The driving element and pixel switch are formed of thin-film transistors disposed on a glass or synthetic resin substrate, a conductive substrate, or a semiconductor substrate having an insulating film of SiO2 or SiN, for example.
The organic EL element has a structure in which a luminous layer is formed of a thin film containing fluorescent organic compounds of red, green or blue, and is held between the cathode and the anode so that holes and electrons are supplied and recombined in the luminous layer to produce excitons. The organic EL element outputs light radiated upon deactivation of the excitons. The anode is a transparent electrode formed of ITO or the like and the cathode is a reflective electrode formed of a metal such as aluminum. With the this structure, the organic EL element can provide a luminance of about 100 to 100000 cd/m2 with an applied voltage of just 10 V or less.
The driving current of the organic EL element is controlled by utilizing the constant current character-istic of a driving thin-film transistor serving as the driving element.
Further, if the preset potential from the signal line X is applied to the gate of the driving thin-film transistor as the voltage Vgs by turning ON the pixel switch, the operating point of the organic EL element which is an intersection between the I-V characteristic curve and the equi-Vgs line of
In a normal liquid crystal display device, the brightness of the backlight is generally adjusted to optimize the power consumption and ease of observation of an image depending on the service environment. For example, when the user carries around a portable information terminal which is battery-driven, electricity of the battery is saved by causing the user to select a low-power consumption operation in which the backlight is made dark or automatically changing the operation mode into the above operation when it is battery-driven. The brightness of the backlight can be made dark by lowering the power-supply voltage applied from the exterior.
On the other hand, the organic EL element is a self-luminous element whose luminance depends on a driving current thereof. Therefore, the luminance of the organic EL element cannot be adjusted by changing the power-supply voltage.
In a gradation display system in which a driving thin-film transistor turned ON/OFF in the non-saturation region is used, it is considered to adjust the ON-time of the thin-film transistor in order to attain desired luminance and gradation. However, extremely slight time adjustment is required and, as a result, it becomes difficult to adequately set either the luminance or gradation.
Further, it has been considered to change the video signal level in order to attain desired luminance which is half the maximum luminance, for example. However, if the currents flowing through all of the organic EL elements in the luminance adjusting system are equally reduced, the white balance cannot be maintained due to a difference in the luminance characteristics of the organic EL elements which depend on the luminescent colors of red, green and blue. If a correction circuit which corrects variation amounts of video signal levels for respective luminescent colors is used in order to solve the above problem, it cannot be avoided that the circuit configuration is complicated in comparison with that of the brightness adjusting system of the liquid crystal display device.
An object of the present invention is to provide, as a solution to the above problem, a self-luminous display device which can adjust the luminance irrespective of gradation control.
According to the present invention, there is provided a self-luminous display device comprising a plurality of display pixels forming a display screen, a plurality of scanning lines disposed along rows of the display pixels, a plurality of signal lines disposed along columns of the display pixels, and a power-supply section which supplies a power-supply voltage to the display pixels, each of the display pixels including a luminous element, a pixel switch which receives a video signal from a corresponding signal lines in response to a scanning signal from a corresponding scanning line and a driving element which is connected between the luminous element and the power supply section to supply a driving current corresponding to the video signal from the pixel switch to the luminous element, and each luminous element being connected to the power supply section via a dimmer switch portion.
In the display device, the luminous element is connected to the power supply section via the dimmer switch portion which is independent of the driving element. Therefore, if the dimmer switch portion is turned ON at the rate of half a preset period, for example, the luminance of the luminous element can be equivalently reduced by half. That is, the luminance of the luminous element (or the luminance level for maximum gradation) can be set to a desired level irrespective of gradation control by adjusting the light emission rate of the luminous element for each unit time based on the ON time of the dimmer switch portion.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.
There will now be described an organic EL display device according to a first embodiment of the present invention with reference to the accompanying drawings.
The scanning line driver YD sequentially supplies a scanning signal to the scanning lines in one frame period (1F) under control of the controller 13. That is, each scanning line Y is driven by the scanning signal in a different one of horizontal scanning periods. The signal line driver XD sequentially converts a digital video signal into gradation voltages in each horizontal scanning period under control of the controller 13 and outputs the gradation voltages to the signal lines X as analog video signals.
The pixel switches 15 on each row are turned ON by a scanning signal supplied from a corresponding one of the scanning lines Y for one horizontal scanning period and are then kept turned OFF until the scanning signal is supplied again after one frame period. The driving elements 17 respectively supply those driving currents to the organic EL elements 16, which correspond to the analog video signals supplied via the respective pixel switches 15 and held by wiring capacitances.
The scanning line driver YD and signal line driver XD are connected to the driver power supply 11 so as to receive the driver power-supply voltage, and the display pixels PX are connected to the EL power supply 12 via the power lines VDD and VSS so as to receive the EL power-supply voltage.
The organic EL display device further includes a dimmer switch 14 which is inserted into the driving power line VDD between the EL power supply 12 and the display pixels PX. The dimmer switch 14 is controlled to be turned ON and OFF by a luminance adjusting switch control signal SC from the controller 13 in a preset cycle or at pseudo random. In this case, “pseudo random” indicates a state in which the ON time is equivalently set to a preset rate of a constant time period. The organic EL element 16 emits light when the dimmer switch 14 is turned ON. In the case where the organic EL element 16 is observed for a preset period of time and the total ON time period of the dimmer switch 14 is half the preset period of time, the luminance of the organic EL element 16 becomes equivalent to half the maximum level obtained when the dimmer switch 14 is kept ON for the preset period of time.
In the organic EL display device of the present embodiment, the organic EL element 16 is connected to the EL power supply 12 via the dimmer switch 14 which is independent of the driving element 17. Therefore, if the dimmer switch 14 is kept ON for half a preset period in each preset period, for example, the luminance of the organic EL element 16 can be equivalently reduced by half. That is, the luminance of the organic EL element 16 can be set to a desired level irrespective of gradation control, by adjusting the light emission rate for each unit time based on the ON time of the dimmer switch 14.
In the present embodiment, the dimmer switch 14 is disposed outside the organic EL panel 10, but it can be formed on a glass plate used as a circuit board of the organic EL panel 10. However, if a large amount of current flows in the dimmer switch 14, it is preferable that a plurality of thin-film transistors are formed as the dimmer switch 14 so as to prevent the current from being concentrated in one portion.
Next, an organic EL display device according to a second embodiment of the present invention is explained.
Also, in the present embodiment, the luminance of the organic EL element can be set to a desired level irrespective of gradation control by adjusting the light emission rate for each unit time based on the ON time of the dimmer switch 19, as in the first embodiment.
The video signal is supplied to the gate of the driving element 17 from the signal line X via the pixel switch 15 when the scanning line Y is set at the high level. The memory control switch 21 supplies the video signal to the static memory section 20 under control of the memory control signal. The switch element 20C is set in the OFF state when the scanning line Y is set at the high level and it is set into the ON state if the scanning line Y is set at the low level. As a result, the video signal is held in the static memory section 20 in a digital form of a high or low potential.
Luminance adjustment made by use of the dimmer switch 14 shown in
The static memory sections 20 shown in
Next, an organic EL display device according to a third embodiment of the present invention is explained.
In the whole portion of the organic EL panel 10, the dimmer switches 22 are arranged in one column as shown in
In the organic EL display device of the present embodiment, the organic EL elements 16 are connected to an EL power supply 12 via the respective dimmer switches 22 which are provided independently of the driving elements 17. Therefore, if the dimmer switch 22 is kept in the ON state for a half preset period, in each preset period, for example, the luminance of the organic EL element 16 can be equivalently reduced by half. That is, the luminance of the organic EL element 16 can be set to a desired level irrespective of gradation control, by adjusting the light emission rate for each unit time based on the ON time of the dimmer switch 22.
Next, an organic EL display device according to a fourth embodiment of the present invention is explained.
The dimmer switch 22 is controlled to be turned ON and OFF in a preset cycle or at pseudo random under control of the switch control signals SC1 and SC2 from the controller 13. Each dimmer switch 22 equally switches driving currents flowing in organic EL elements 16 of the display pixels PX of one block connected to a corresponding one of the power lines VDD. The luminance of the organic EL element 16 becomes equivalent to half the maximum level which is obtained when the dimmer switch 22 is kept ON for a preset period of time in a case where the total ON time period of the dimmer switch 22 is half the preset period of time, for example.
In the organic EL display device of the present embodiment, the organic EL elements 16 are connected to the EL power supply 12 via the respective dimmer switches 22 which are commonly used for each block. Therefore, if the dimmer switch 22 is kept in the ON state for half a preset period, in each preset period, for example, the luminance of the organic EL element 16 can be equivalently reduced by half. That is, the luminance of the organic EL element 16 can be set to desired luminance irrespective of gradation control by adjusting the light emission rate for each unit time based on the ON time of the dimmer switch 22. Further, since the luminance of the EL display areas 1 and 2 can be set to different luminance levels, the range of Application thereof can be expanded.
In each of the above embodiments, the switch control signal SC, SC1 or SC2 is generated from the controller 13 and supplied to the dimmer switch 14, 19 or 22, but the switch control signal can be supplied from a host processing unit or the like disposed outside the organic EL display device to the dimmer switch 14, 19 or 22. Further, the controller 13 can be designed to generate a switch control signal SC which lowers the luminance of the organic EL element 16 in a dark place by referring to an output signal of a sensor which is provided to sense outside light. In addition, the controller 13 can be designed to refer to an output signal of a sensor which detects the remaining battery power, and generate a switch control signal SC for lowering the luminance of the organic EL element 16 when the remaining battery power is reduced to a predetermined amount.
Next, an organic EL display device according to a fifth embodiment of the present invention is explained with reference to the accompanying drawings.
The organic EL panel 10 includes a plurality of display pixels PX arrayed in a matrix form on an insulating substrate GL such as a glass plate to display an image, a plurality of scanning lines Y disposed along respective rows of the display pixels PX, a plurality of signal lines X disposed along respective columns of the display pixels PX, a plurality of pixel switches 15 disposed near the intersections between the scanning lines Y and the signal lines X, a scanning line driver YD which drives the scanning lines Y, and a signal line driver XD which drives the signal lines X. Each of the display pixels PX includes an organic EL element 16, a driving transistor 17 connected in series with the organic EL element 16 between paired power lines VDD and VSS and a capacitor 18 which holds the gate voltage of the driving transistor 17. The pixel switch 15 is formed of an N-channel thin-film transistor having a semiconductor layer of polycrystalline silicon, for example. The pixel switch permits the capacitor 18 to hold the voltage of a video signal supplied from a corresponding one of the signal lines X when it is driven by a scanning signal from a corresponding scanning line Y and supplies the thus held voltage to the driving transistor 17 as the gate voltage. The driving transistor 17 is formed of a P-channel thin-film transistor having a semiconductor layer of polycrystalline silicon, for example, and causes a driving current corresponding to the gate voltage to flow in the organic EL element 16. The organic EL element 16 has a structure in which a luminous layer is formed of a thin film containing fluorescent organic compounds of red, green or blue, and is held between the cathode and the anode so that holes and electrons are supplied and recombined in the luminous layer to produce excitons. The organic EL element 16 outputs light radiated upon deactivation of the excitons. In the organic EL panel 10, columns of organic EL elements 16 whose luminescent color is red, columns of organic EL elements 16 whose luminescent color is green and columns of organic EL elements 16 whose luminescent color is blue are repeatedly arranged in order of red, green and blue in the row direction.
The external drive circuit 30 is formed on an external drive circuit board disposed outside the organic EL panel 10. The external drive circuit 30 comprises a DA converter circuit (DAC) 31 which has DA converters for converting a digital signal into analog signals, and supplies analog video signals for the signal lines to the signal line driver XD based on the analog signals. It further comprises a controller 32 which controls the scanning line driver YD, signal line driver XD and DAC 31, and a DC/DC converter 33 which produces power-supply voltages such as a pixel power-supply voltage VEL and a circuit power-supply voltage VCR, based on a DC power-supply voltage supplied from the exterior and outputs the power-supply voltages to drive the organic EL panel 10. Among the above power-supply voltages, the pixel power-supply voltage VEL is applied between the paired power lines VDD and VSS to operate the display pixels PX. The controller 32 receives a digital video signal and sync signal which are supplied from the exterior, and produces a vertical scanning control signal for controlling the vertical scanning timing, a horizontal scanning control signal for controlling the horizontal scanning timing, and a DAC control signal synchronized with the horizontal and vertical scanning timings, based on the sync signal. Further, the controller 32 respectively supplies the vertical scanning control signal, horizontal scanning control signal and DAC control signal to the scanning line driver YD, signal line driver XD and DAC 31, and also supplies a digital video signal to the DAC 31 in synchronism with the horizontal and vertical scanning timings.
The DAC 31 is a converter IC disposed on the external drive circuit board and sequentially converts the digital video signal into an analog form under control of the DAC control signal. The signal line driver XD samples the analog video signals derived from the DAC 31 in each horizontal scanning period under control of the horizontal scanning control signal to supply the sampled signals to the signal lines X in parallel. Further, the scanning line driver YD sequentially supplies a scanning signal to the scanning lines Y in each vertical scanning period under control of the vertical scanning control signal. The pixel switches 15 on each row are turned ON for one horizontal scanning period by a scanning signal commonly supplied thereto from a corresponding one of the scanning lines Y and are then kept in the OFF state until the scanning signal is supplied again after one vertical scanning period. The driving transistors 17 of one row cause driving currents corresponding to voltages of video signals supplied as gate voltages from the signal lines X by turn-ON of the pixel switches 15 to flow into the organic EL elements 16.
Further, the organic EL display device includes a dimmer switch portion 34 which is formed on the insulating substrate GL of the organic EL panel 10 and controlled by a switch control signal SC from the controller 32. The dimmer switch portion 34 is inserted between a node of the DC/DC converter 33 side and a node of the display pixel PX side serving as the power line VDD on the insulating substrate GL. It is set into a first state in which the display pixel PX side node is connected to the DC/DC converter 33 side node to cause all of the organic EL elements 16 to emit light when the switch control signal SC is set at the high level and set into a second state in which the display pixel PX side node is connected to the power line VSS to interrupt light emission of all of the organic EL elements 16 when the switch control signal SC is set at the low level. The controller 32 determines the luminance level based on a dimmer signal supplied from the exterior and changes the ratio of the high-level period to the low-level period of the switch control signal SC according to the luminance level in each vertical scanning period. The dimmer switch portion 34 equally switches currents which respectively flow in all of the organic EL elements 16 under control of the switch control signal SC to equally control the ratios of the luminous time to the non-luminous time of each of the organic EL elements 16. In this case, the dimmer signal is a signal obtained as the result that desired luminance is selected by use of a luminance selection switch or the like which is operated by a user or an external computer.
In the luminance adjusting operation of setting the luminance of the screen to 50%, a switch control signal SC(A) as shown in
Further, in the luminance adjusting operation of setting the luminance of the screen to 60%, a switch control signal SC(A) as shown in
In the above embodiment, the switch control signal SC(A) which causes the dimmer switch portion 34 to perform the switching operation once in one vertical scanning period is explained. However, as shown in
In the organic EL display device of the embodiment, the dimmer switch portion 34 switches driving currents flowing in all of the organic EL elements 16 in each vertical scanning period under control of the switch control signal SC to control the ratio of the luminous time to the non-luminous time of the organic EL elements 16 to adjust the luminance of the screen. This system can avoid degradation in the white balance that occurs due to a difference between the luminance characteristics of the organic EL elements 16 depending on the luminescent colors when the driving currents flowing in all of the organic EL elements 16 are not interrupted and changed in each vertical scanning period to adjust the luminance of each organic EL element 16. That is, the luminance of the display screen can be adjusted without degradation in the white balance. Further, since the controller 32 controls the switching operation of the single dimmer switch portion 34 to determine the ratio of the luminous time to the non-luminous time of all the organic EL elements 16, the complicated structure such as a correction circuit for correcting a variation amount of a video signal level is not required to maintain the white balance.
The dimmer signal can be configured not only to reflect a selection result of a desired luminance but also to reflect the remaining battery power or the illuminance of incident light to the organic EL panel 10 from the exterior, for example. Further, the luminance of the screen can be controlled to be lowered when the video signal is kept unchanged for a preset period of time by interruption of the computer operation. Also, the above dimmer switch portion 34 is formed of a thin-film transistor using a polycrystalline silicon thin film and formed on the same insulating substrate GL at the same process as that of the pixel switches 15, driving transistors 17, and drivers YD, XD.
The dimmer switch portion 34 is inserted between an output node for pixel power-supply voltage VEL and a power line VDD and controlled by a switch control signal SC from a controller 32, as in the fifth embodiment. That is, the dimmer switch portion 34 is set into a first state in which it connects the power line VDD to the output node for pixel power-supply voltage VEL to cause all of the organic EL elements 16 to emit light when the switch control signal SC is set at the high level, and set into a second state in which it connects the power line VDD to the power line VSS to interrupt light emission of all of the organic EL elements 16 when the switch control signal SC is set at the low level. The controller 32 determines the luminance level based on a dimmer signal supplied from the exterior to change the ratio of the high-level period to the low-level period of the switch control signal SC in each vertical scanning period according to the luminance level. The dimmer switch portion 34 switches driving currents which respectively flow in all of the organic EL elements 16 under control of the switch control signal SC to equally control the ratios of the luminous time to the non-luminous time of all the organic EL elements 16 to adjust the luminance of the display screen.
In this case, the controller 32 receives a digital video signal and sync signal supplied from the exterior and produces a scanning line driver control signal for controlling the vertical scanning timing, a signal line driver control signal for controlling the horizontal scanning timing, and a DAC control signal synchronized with the horizontal and vertical scanning timings, based on the sync signal. Further, the controller 32 respectively supplies the scanning line driver control signal, signal line driver control signal and DAC control signal to the scanning line driver YD, signal line driver XD and DAC 31 and also supplies a digital video signal to the DAC 31 in synchronism with the horizontal and vertical scanning timings. The scanning line driver YD is formed on an insulating substrate GL and connected to the scanning lines Y which are integrally formed with the scanning line driver YD on the insulating substrate GL. Further, the signal line driver XD and DAC 31 are formed of driver ICs disposed on a flexible wiring board as a TCP (tape carrier package) and connected to the signal lines X formed on the insulating substrate GL. In the driver ICs, the DAC 31 sequentially converts a digital video signal into an analog form by control of the DAC control signal, and the signal line driver XD samples the analog video signals derived from the DAC 31 in each horizontal scanning period by control of the horizontal scanning control signal and supplies the sampled signals to corresponding ones of the signal lines X in parallel.
In the organic EL display device of the sixth embodiment, the dimmer switch portion 34 is disposed on the external drive circuit board, and switches driving currents flowing in all of the organic EL elements 16 in each vertical scanning period by control of the switch control signal SC, as in the first embodiment, to equally control the ratios of the luminous time to the non-luminous time of all the organic EL elements 16. Therefore, the luminance of the display screen can be adjusted without degrading the white balance. Further, since the controller 32 controls the switching operation of the single dimmer switch portion 34 to determine the ratio of the luminous time to the non-luminous time of all the organic EL elements 16, the complicated structure such as a correction circuit for correcting a variation amount of a video signal level is not required to maintain the white balance.
In this case, the arrangement of the DAC 31, scanning line driver YD and signal line driver XD has no relation with respect to the configuration having the dimmer switch portion 34 formed on the board of the external drive circuit 30, thus the configuration is applicable to the fifth embodiment.
The photosensitive element 35 receives light applied to the organic EL panel 10 from the exterior. The correction circuit 36 corrects a switch control signal SC from a controller 32 based on an output signal of the photosensitive element 35, so that the dimmer switch portion 34 is controlled according to the switch control signal SC obtained as the result of correction.
In the organic EL display device of the seventh embodiment, it is possible to prevent a display image from becoming difficult to observe due to the illumination of the service environment of the organic EL panel 10 when the luminance level of the display screen is determined based on the dimmer signal.
In the above example, the correction circuit 36 can be configured to correct the switch control signal SC based on a desired signal supplied from a selection circuit which is operated by a user or an external computer, instead of a signal from the photosensitive element 35 which receives light applied to the organic EL panel 10 from the exterior.
The photosensitive element 35 receives light applied to the organic EL panel 10 from the exterior. The correction circuit 36 corrects a switch control signal SC from a controller 32 based on an output signal of the photosensitive element 35, so that the dimmer switch portion 34 is controlled according to the switch control signal SC obtained as the result of correction.
In the organic EL display device of the eighth embodiment, it is possible to prevent a display image from becoming difficult to observe due to the illumination of the service environment of the organic EL panel 10 when the luminance of the display screen is determined based on the dimmer signal.
In the above embodiment, the dimmer switch portion 34 performs the control operation of setting all of the organic EL elements 16 in one of the luminous and non-luminous states. If a node on the display pixel PX side forming the power line VDD on the insulating substrate GL is connected to a node on the DC/DC converter 33 side via the dimmer switch portion 34, driving currents are supplied to the organic EL elements 16 to set them in the luminous state. On the other hand, if the node on the display pixel PX side is connected to the power line VSS via the dimmer switch portion 34, supply of driving currents is interrupted to set the organic EL elements 16 in the non-luminous state. The dimmer switch portion 34 is not limited to the above configuration and it can be so configured as to connect the node on the display pixel PX side to a second pixel power-supply voltage line which is provided to supply a minute current so as to maintain the organic EL elements 16 in the non-luminous state, for example.
In the organic EL display device, the external drive circuit 30 further includes a luminance selection section 37 and level-shift circuit 38. A controller 32 and the level-shift circuit 38 are formed as an integrated circuit. The level-shift circuit 38 is used to convert the level of a switch control signal SC obtained from the controller 32 to a gate voltage which is required for the switching operation of the dimmer transistors 39. The luminance selection section 37 includes manual switches SW1 to SW3 which are each connected to the power line VSS at one end and respectively connected to a power line VC at the other ends via pull-up resistors R. Nodes of the pull-up resistors R and manual switches SW1 to SW3 are respectively connected to luminance selection terminals B1 to B3 of the controller 32. The manual switches SW1 to SW3 are closed when the luminance selection terminals B1 to B3 are set to a logic value “0” and opened when the luminance selection terminals B1 to B3 are set to a logic value “1”. That is, the manual switches SW1 to SW3 are controlled by combinations of logic values of “000”, “001”, “010”, “011”, “100”, “101”, “110”, “111” to create a switch control signal SC which can be used to select one of eight-step luminance levels. The controller 32 receives the combination of the logic values obtained from the luminance selection section 37 as a dimmer signal instead of the dimmer signal from the exterior. Then, it sets one of the eight-step luminance levels selected by the thus received dimmer signal and changes the ratio of the high-level period to the low-level period of the switch control signal SC in each vertical scanning period according to the thus set luminance level. When the switch control signal SC is received from the controller 32, it is level-shifted by the level-shift circuit 38 and supplied to the gates of the dimmer transistors 39 of the organic EL panel 10.
The plurality of dimmer transistors 39 are provided for the plurality of display pixels PX and commonly controlled by the switch control signal SC obtained from the controller 32 of the external drive circuit 30. As shown in
Next, the operation of each display pixel PX is explained in detail.
Further, in order to attain 0% of the luminance of the display screen, the switch control signal SC is maintained at the high level to always keep the dimmer transistor 39 in the OFF state in a period B, for example. Thus, the dimmer transistor 39 totally interrupts the current IeL flowing in the organic EL element 16 in the period B irrespective of the driving transistor 17.
Also, in order to attain 50% of the luminance of the display screen, the switch control signal SC is set to have a high-level period and low-level period of the ratio which is set to 1:1 to uniformly perform the operation of setting the dimmer transistor 39 in the ON state and the operation of setting the dimmer transistor 39 in the OFF state in a period C, for example. Thus, the dimmer transistor 39 interrupts the current IeL flowing in the organic EL element 16 for a period which is half a period obtained as the sum of the high-level period and low-level period in the period C.
As shown in
Further, as shown in
In the ninth embodiment described above, the luminance of the display screen can be adjusted to a desired one of the eight-step levels by use of three or a relatively small number of manual switches.
In the organic EL display device, the external drive circuit 30 further includes a level-shift circuit 38, luminance selection section 41, sawtooth waveform generator 43 and comparator 42. Like the ninth embodiment, a controller 32 and the level-shift circuit 38 are formed as an integrated circuit. The level-shift circuit 38 is used to convert the level of a switch control signal SC obtained from the controller 32 to a gate voltage which is required for the switching operation of dimmer transistors 39. The luminance selection section 41 has fixed resistors R1, R2 and a variable resistor VM to configure a voltage dividing circuit which divides power-supply voltage VCR. The variable resistor VM is connected to a power line VC at one end via the fixed resistor R1 and connected to a power line VSS at the other end via the resistor R2 and an intermediate tap of the variable resistor VM is connected to the reference input terminal of the comparator 42. The comparison input terminal of the comparator 42 is connected to the sawtooth waveform generator 43 which generates sawtooth voltage Vsaw. The sawtooth waveform generator 43 generates sawtooth voltage Vsaw in synchronism with at least one of a vertical scanning control signal and horizontal scanning control signal generated by the controller 32. In this case, the period of the sawtooth voltage Vsaw is shorter than the vertical scanning period. The comparator 42 compares the sawtooth voltage Vsaw generated from the sawtooth waveform generator 43 with divided voltage obtained from the intermediate tap of the variable resistor VM as comparison reference voltage Vref to generate a switch control signal SC. The switch control signal SC is set at the low level when Vref>Vsaw and set at the high level when Vref<Vsaw.
For example, as shown in
In the tenth embodiment, the ratio of the high-level period to the low-level period of the switch control signal SC can be changed by continuously changing the reference voltage Vref by the manual operation of the variable resistor VM. Therefore, the luminance of the display screen can be continuously adjusted.
In the organic EL display device, the organic EL panel 10 further includes a plurality of registers RG cascade-connected to configure a shift register 45 which shifts a switch control signal SC supplied from a controller 32 via a level-shift circuit 38. For example, the controller 32 is so configured as to change the level of the switch control signal SC at a timing synchronized with the horizontal scanning period. When the ratio of the high-level period to the low-level period of the switch control signal SC is set to 3:1, the controller 32 continuously sets the switch control signal SC at the high level for a period corresponding to three horizontal scanning periods and continuously sets the switch control signal SC at the low level for one horizontal scanning period following the above period. The shift register 45 shifts the switch control signal SC for each horizontal scanning period by control of the horizontal scanning control signal from the controller 32 and respectively supplies switch control signals SC from the registers RG to the dimmer transistors 39 of the corresponding rows.
In the above eleventh embodiment described above, since all of the dimmer transistors 39 are not simultaneously turned ON, a temporary increase in the power consumption can be prevented and the power supply ability of the DC/DC converter 33 can be lowered.
This invention is not limited to the above embodiments and can be variously modified without departing from the technical scope thereof. For example, a plurality of organic EL elements 16 can be replaced by other luminous elements such as self-luminous LEDs. Further, the present invention uses the dimmer switch portion 34 or dimmer transistors 39, 46 to maintain the relation between the white balances of the display pixels having different luminescent colors, but the configuration is also applicable to a case where the luminescent colors of the display pixels are the same.
In the above embodiments, a case wherein the DAC is formed on the external drive circuit board or formed in the TCP form as the driver IC is explained. However, it can be integrated on the insulating substrate on which the pixel transistors are formed and it can be formed in the same process of forming the pixel transistors and driving transistors.
Further, in the above embodiments, a case wherein the driving current amount is controlled based on the video signal to attain multi-gradation display is explained, but this is not limitative. For example, the present invention is applicable to a case of a pulse width modulation drive system in which a driving current flowing in the organic EL element is kept constant and time of supply of the driving current is controlled to perform gradation display. In the case of the pulse width modulation drive system, the ratio in the switch control signal is set so that the luminance can be adjusted at the time of minimum pulse width.
Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
Number | Date | Country | Kind |
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2001-304723 | Sep 2001 | JP | national |
2001-375002 | Sep 2001 | JP | national |
Number | Date | Country | |
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Parent | 10259478 | Sep 2002 | US |
Child | 11441227 | May 2006 | US |