Patent Publication Number: US-2006017682-A1

Title: Display panel driving device and flat display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-211266, filed Jul. 20, 2004, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to a display panel driving device and flat display device, particularly to a display panel driving device and flat display device which are suitably applicable to a liquid crystal display panel using an OCB technique for realizing a wide viewing angle and high-speed response.  
      2. Description of the Related Art  
      Currently, liquid crystal display panels having characteristics such as lightness, thinness, and small power consumption are used as displays for television receivers, personal computers and car navigation systems.  
      A twisted nematic (TN) mode liquid crystal display panel widely utilized as such a liquid crystal display panel is configured such that a liquid crystal material having optically positive refractive anisotropy is set to a twisted alignment of substantially 90° between glass substrates opposed to each other, and optical rotary power of incident light is adjusted by controlling its twisted alignment. Although this TN-mode liquid crystal display panel can be comparatively easily manufactured, its viewing angle is narrow, and its response speed is low. Thus, this panel has been unsuitable to display a moving image such as a television image, in particular.  
      On the other hand, an optically compensated birefringence (OCB) mode liquid crystal display panel is attracting attention as a liquid crystal display panel which improves the viewing angle and response speed. The OCB-mode liquid crystal display panel contains a liquid crystal material sealed between the opposed glass substrates and capable of providing a bend alignment. The response speed is improved by one digit as compared with the TN-mode liquid crystal display panel. Further, there is an advantage that the viewing angle is wide because optically self compensation is made from an alignment state of the liquid crystal material.  
      In the OCB-mode liquid crystal display panel, as shown in (a) of  FIG. 5 , liquid crystal molecules  65  of a liquid crystal layer are set to a splay alignment when no voltage is applied between a pixel electrode  62  disposed on a glass based array substrate  61  and a counter electrode  64  disposed similarly on a glass based counter substrate  63  which is opposed to the array substrate  61 . Thus, when a high voltage of the order of some tens of voltages is applied between the pixel electrode  62  and the counter electrode  64  upon supply of power, the liquid crystal molecules  65  are transferred to the bend alignment.  
      To reliably transfer the alignment state upon high voltage application, voltages opposite in polarity are applied to adjacent horizontal lines of the pixels to create a nucleus by a laterally twisted potential difference between the adjacent pixel electrode  62  and transfer pixel electrode. The alignment state is transferred around the nucleus. Such an operation is carried out for substantially one second, whereby the splay alignment is transferred to the bend alignment. Thereafter, a difference in potential between the pixel electrode  62  and the counter electrode  64  is temporarily eliminated by equalization to cancel an undesired record.  
      After the liquid crystal molecules  65  have been thus transferred to the bend alignment, a voltage exceeding a low OFF voltage, at which the liquid crystal molecules  65  are maintained in the bend alignment as shown in (b) of  FIG. 5 , is applied from a drive power supply  66  during operation. The OFF voltage or an ON voltage which is higher than the OFF voltage is applicable from the drive power supply  66  as shown in (c) of  FIG. 5 . Thus, the drive voltage between the electrodes  62  and  64  changes in the range of the OFF voltage to the ON voltage. Consequently, the alignment state of the liquid crystal molecules  65  is transferred between the bend alignment shown in (b) of  FIG. 5  and the bend alignment shown in (c) of  FIG. 5  to change a retardation value of the liquid crystal layer, thereby controlling transmittance.  
      In the case where the OCB-mode liquid crystal display panel is used for displaying an image, birefringence is controlled in association with polarizing plates. The liquid crystal panel is driven by a driver circuit such that light is shielded (for a black display) upon application of a high voltage and is transmitted (for a white display) upon application of a low voltage, for example.  
      As shown in  FIG. 6 , this driving circuit has: n×n pixel electrodes  62  arrayed in a matrix on the array substrate  61 ; n scanning lines (gate lines) G 1  to Gn formed along rows of the pixel electrodes  62 ; n signal lines (source lines) S 1  to Sn formed along columns of the pixel electrodes  62 ; and n×n thin-film transistors (TFTs)  67  which are disposed near intersections between the scanning lines G 1  to Gn and the signal lines S 1  to Sn as switching elements for the n×n pixel electrodes  62 .  
      Each TFT  67  has a gate electrode  62  connected to one scanning line G, a source electrode connected to one source line S. When the TFT  67  is made conductive by a drive voltage that is applied from a gate driver (scanning line driving circuit)  68  via the scanning line G, a signal voltage from a source driver (signal line driving circuit)  69  is applied via a source-drain path of the TFT  67  to one pixel electrode  62 . The TFTs  67  operate in the manner described above.  
      A liquid crystal layer  70  containing the liquid crystal molecules  65  exists between the pixel electrode  62  and the counter electrode  64 , and is further connected in parallel with a storage capacitance  71  that stores a potential equal to that of the pixel electrode  62 . The counter electrode  64  is configured to receive a driving voltage supplied from a counter electrode driving circuit (not shown).  
      In such an OCB-mode liquid crystal display panel, the alignment state can be transferred from the splay alignment unusable for a display to the bend alignment usable for a display, by means of a voltage applied between the pixel electrode  62  and the counter electrode  64 . Further in the OCB-mode liquid crystal display panel, a countermeasure is employed to insert black (black signal) into a signal in order to prevent an inverse transfer phenomenon in which the bend alignment is inverse-transferred to the splay alignment.  
      As shown in  FIG. 7 , a power source voltage from a power supply circuit  72  is supplied to a voltage dividing resistor unit  73 . The voltage dividing resistor unit divides the power source voltage into reference voltages representing gradations for video and black signals. The reference voltages are supplied to the source driver  69 .  
      In this OCB-mode liquid crystal display, when a temperature of a liquid crystal display panel  74  (see  FIG. 6 ) or an external environment temperature changes, a voltage-transmittance relation (VT) also shifts following the temperature change. Therefore, the power supply circuit  72  for the source driver  69  is controlled by a thermistor or the like to cancel dependence on the temperature. Moreover, a black display tends to be reversed especially at a high temperature. Therefore, when the power supply voltage is lowered to prevent the black reverse phenomenon, a black insertion voltage necessarily becomes small which has been derived from the voltage dividing resistor unit  73  connected to the power supply circuit  72 . Therefore, an increase of a black insertion ratio is required to prevent the inverse transfer. However, when the black insertion ratio is increased, there has occurred a problem that luminance and contrast drop.  
     BRIEF SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a display panel driving device and flat display device which do not require an increase of a black insertion ratio in order to reliably prevent inverse transfer at a high temperature.  
      According to a first aspect of the present invention, there is provided a display panel driving device for an OCB-mode liquid crystal display panel, comprising: gate and source drivers which are connected to the liquid crystal display panel; and a voltage supply circuit which supplies video signal reference voltages and black insertion reference voltages to the source driver, the voltage supply circuit being configured to independently generate the video signal reference voltages and the black insertion voltages.  
      According to a second aspect of the present invention, in the display panel driving device, the above-mentioned voltage supply circuit includes a video signal voltage control circuit which generates the video signal reference voltages and a black insertion voltage control circuit which generates the black insertion voltages, and the video signal reference voltages and the black insertion voltages being independently output to the source driver from the video signal voltage control circuit and the black insertion voltage control circuit, respectively.  
      According to a third aspect of the present invention, in the display panel driving device, the above-mentioned voltage supply circuit further includes: a switching circuit which switches the video signal reference voltages from the video signal voltage control circuit and the black insertion voltages from the black insertion voltage control circuit; and a voltage dividing resistor unit which divides the voltage from the switching circuit into voltages to be output to the source driver.  
      According to a fourth aspect of the present invention, there is provided a display panel driving device for an OCB-mode liquid crystal panel which displays an image by a matrix array of pixels each having liquid crystal molecules set in a bend alignment, comprising: a driver circuit which sequentially performs writing for a video signal and writing for a non-video signal that maintains the bend alignment into different rows of pixels in one vertical scanning period; a video signal voltage control circuit which controls a voltage to be written as the video signal by the driver circuit; and a non-video signal voltage control circuit which controls a voltage to be written as the non-video signal by the driver circuit, independently from the video signal voltage control circuit.  
      According to a fifth aspect of the present invention, there is provided a flat display device comprising: an OCB-mode liquid crystal panel which displays an image by a matrix array of pixels each having liquid crystal molecules set in a bend alignment; a driver circuit which sequentially performs writing for a video signal and writing for a non-video signal for maintaining the bend alignment into different rows of pixels in one vertical scanning period; a video signal voltage control circuit which controls a voltage to be written as the video signal by the driver circuit; and a non-video signal voltage control circuit which controls a voltage to be written as the non-video signal by the driver circuit, independently from the video signal voltage control circuit.  
      According to a sixth aspect of the present invention, in the flat display device, the above-mentioned video signal voltage control circuit is configured to change the range of the voltage to be written as the video signal, in accordance with the environment where the device is used, and the non-video signal voltage control circuit is configured to set the voltage to be written as the non-video signal into a level equal to or more than a predetermined level at which the bend alignment is maintained.  
      With the display panel driving device and the flat display device, the video signal reference voltages and the black insertion voltages are generated independently. In this case, an increase in the black insertion voltage is attainable irrespective of the video signal voltage. Further, the voltage to be written as the video signal and the voltage to be written as the non-video signal voltage are controlled independently. In this case, an increase in the voltage to be written as the non-video signal is attainable irrespective of the voltage to be written as the video signal. That is, the black (non-video signal) insertion ratio does not have to be increased in order to reliably prevent the inverse transfer at high temperature, and as a result, it is possible to display a high-quality image whose luminance and contrast are prevented from being lowered.  
      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. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
      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.  
       FIG. 1  is a diagram showing the circuit configuration of a liquid crystal display panel according to one embodiment of the present invention;  
       FIG. 2  is a diagram showing an example of the configuration of a circuit for supplying reference voltages to a source driver shown in  FIG. 1 ;  
       FIG. 3  is a diagram showing a first modification of the circuit shown in  FIG. 2 ;  
       FIG. 4  is a diagram showing a second modification of the circuit shown in  FIG. 2 ;  
       FIG. 5  is a diagram for schematically explaining a display operation of a typical OCB-mode liquid crystal display panel;  
       FIG. 6  is a diagram showing the liquid crystal display panel shown in  FIG. 5  together with a driving circuit; and  
       FIG. 7  is a diagram showing the configuration of a reference voltage generating circuit connected to the driving circuit shown in  FIG. 6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A liquid crystal display device according to one embodiment of the present invention will be described with reference to the accompanying drawings.  
      As shown in  FIG. 1 , in this liquid crystal display, input signals, such as a vertical sync signal defining one vertical scanning period, a horizontal sync signal defining one horizontal scanning period, and a video signal, are input from an input port  11  and supplied to a controller  12 . The controller  12  includes a black signal insertion timing setting unit that serves as a black insertion timing determination circuit and a driver control circuit. The driver control circuit generates a timing pulse to insert a black signal on conditions set at the black signal insertion timing setting unit.  
      In the OCB mode, continuous application of a low voltage allows the alignment state of liquid crystal molecules to be inverse-transferred from the bend alignment to the splay alignment. The black signal is a signal for preventing the inverse transfer phenomenon, and is used as an example of the non-video signal in this embodiment. A write operation for the black signal is called black insertion, and the black signal is inserted at a desired black insertion ratio for each field. The black insertion ratio is controlled as a time difference between the write timing for writing the video signal into a row (line) of the pixels and the write timing for writing the black signal into these pixels, in one field period (one vertical scanning period).  
      The black insertion timing setting unit sets an appropriate timing for writing or inserting a black signal in one field to effectively prevent occurrence of the inverse transfer phenomenon. The black signal is written at a timing when the predetermined number of the horizontal sync signal pulses have been supplied after a video signal write timing. The black insertion ratio is changeable to arbitrarily shift a black signal write timing for black insertion.  
      The controller  12  supplies driving signals to a gate driver  13  and a source driver  14 . The gate driver  13  and the source driver  14  supply signals such as a gate pulse and the video signal to an OCB-mode liquid crystal display panel  15 , respectively.  
      A power supply control circuit  16  is connected to the gate driver  13 , the source driver  14 , and the controller  12  to supply predetermined power source voltages. A driving voltage from the power supply control circuit  16  and the gate pulse, the video signal and the like from the controller  12  are used to display a desired image on the liquid crystal display panel  15 .  
      As shown in  FIG. 2 , the power supply control circuit  16  supplies video signal reference voltages VrefS and black insertion reference voltages VrefB to the source driver  14 . The black insertion reference voltages VrefB and the video signal reference voltages VrefS are separately supplied to the source driver  14  from a black insertion voltage control circuit  17  and a video signal voltage control circuit  18 , both of which are provided in the power supply control circuit  16 .  
      That is, for example, the black insertion voltage control circuit  17  directly supplies to the source driver  14  as a high reference voltage, voltages Vref 0  and Vref 9  (corresponding to VrefB), which are of 15 V and 0V, for example. The video signal voltage control circuit  18  supplies voltages Vref 1  and Vref 8  to ends of a voltage dividing resistor unit  19 , which are connected to the source driver  14  in parallel. The voltages Vref 1  and Vref 8  are set at intermediate levels between the voltages Vref 0  and Vref 9 . In addition to the voltages Vref 1  and Vref 8 , required voltages Vref 2  to Vref 7  obtained from intermediate points of the voltage dividing resistor unit  19  are supplied to the source driver  14 .  
      As described above, individual configuration and individual connection are employed to obtain the black insertion voltage control circuit  17  and the video signal voltage control circuit  18  which are independent from each other. Thus, a black insertion voltage is controllable by the black insertion voltage control circuit  17  irrespective of the setting of the video signal voltage control circuit  18 . Therefore, the black insertion voltage can be increased by the black insertion voltage control circuit  17  even at the high temperature. Accordingly, the black insertion ratio can be set to a small value. Therefore, it is possible to inhibit a problem of drop of luminance or contrast from being caused by an increase of the black insertion ratio.  
      Furthermore, a temperature of the liquid crystal display panel  15  itself or an external environment surrounding the panel is detected, for example, by a thermistor or the like. When the black insertion voltage control circuit  17  is controlled in accordance with the detected temperature, and the temperature is high, the timing of the black insertion is controlled, or the black insertion voltage is controlled to be low. Accordingly, it is possible to suppress the drop of the contrast of the liquid crystal display panel  15 . With this configuration, when the thermistor or the like detects a temperature change by a change of the temperature of the liquid crystal display panel  15  itself or an ambient temperature, it is possible to change the black insertion ratio in conjunction with the temperature change. Therefore, the black signal insertion timing can be set to be optimum in accordance with a use state.  
      In the present embodiment, it has been described that the voltage dividing resistor unit  19  is connected to the video signal voltage control circuit  18 , and the black insertion voltage control circuit  17  is directly and independently connected to the source driver  14 . However, as shown in  FIG. 3 , a resistor circuit  19 ′ for the black insertion voltage control circuit  17  may be provided separately from the voltage dividing resistor unit  19  for the video signal voltage control circuit  18  to obtain video signal reference voltages VrefS 0  to VrefS 9  and black insertion reference voltages VrefB 0  and VrefB 1  from voltages supplied from a single power supply control circuit  16  and supply these voltages to the source driver  14 . Even with this configuration, the source driver  14  can be similarly driven.  
      In any case, there has been described the circuit configuration in which the black insertion reference voltages and the video signal reference voltages are supplied to the source driver  14  via different routes, but it is also possible to provide another display panel driving device in which the black insertion reference voltages and the video signal reference voltages are supplied commonly via a single voltage dividing resistor unit.  
      That is, as shown in  FIG. 4 , the black insertion voltage control circuit  17  supplies voltages VrefB 0  and VrefB 9  to a Vref-switching circuit  20 , and the video signal voltage control circuit  18  supplies voltages VrefS 0  and VrefS 9  to the Vref switching circuit  20 . The Vref switching circuit  20  is controlled to perform switching between the voltages from the black insertion voltage control circuit  17  and the voltages from the video signal voltage control circuit  18  in accordance with a switching signal supplied from the controller  12  to an input terminal  21 . Output voltages from the Vref-switching circuit  20  are supplied to ends of the voltage dividing resistor unit  19  which are connected to the source driver in parallel, so as to obtain the voltages Vref 0  to Vref 9  and supply these voltages to the source driver  14 .  
      In the above-described configuration, the voltages from the black insertion voltage control circuit  17  and the voltages from the video signal voltage control circuit  18  are switched by the Vref-switching circuit  20  and supplied to the voltage dividing resistor unit  19  for a black insertion period and for a video signal period, respectively. Accordingly, the reference voltages supplied to the source driver  14  can be optimized for the video signal or the black signal. Since the outputs from the black insertion voltage control circuit  17  and the voltage control circuit  18  for the video signal are supplied commonly via the voltage dividing resistor unit  19  to the source driver  14 , not via a separate route, it is possible to simplify circuit wiring that includes the voltage dividing resistor unit  19  on a source driver  14  side.  
      It is to be noted that it has been described above in the embodiment that video signal reference voltages are provided, but the number of the reference voltages can appropriately be set to attain required gradations. Moreover, a configuration of the voltage dividing resistor unit is not limited to a shown configuration, and replaced by another configuration of the voltage dividing resistor unit that has a combination of parallel resistors, or an active element, or a switching element.  
      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 invention concept as defined by the appended claims and their equivalents.