The present invention relates to a liquid-crystal display device using an OCB mode and driving device thereof.
The liquid-crystal display device is thin and lightweight, whose application has been further expanded in recent years as a substitute for a conventional cathode ray tube.
The TFT 411a is opened or closed by a gate voltage Vg applied from the gate driver 430 and a video signal Vs is supplied from the source driver 420 to the pixel electrode 411b. Moreover, a voltage Vcom is applied to the counter electrode 411c and the common electrode 411e. Thereby, a predetermined gray scale voltage corresponding to the video signal Vs is held by a liquid-crystal capacity CLC and a storage capacitor Cs constituting each pixel 411. Moreover, an image is displayed by receiving light-source light from a backlight 450 set to the back of the liquid-crystal panel 410.
In
A TN(Twisted Nematic)-mode liquid-crystal panel widely used for the liquid-crystal layer 411d of the liquid-crystal panel 410 is inferior in image quality to a self-light-emitting display such as a cathode ray tube because the panel has a narrow angle of visibility and a slow response speed and a holding-type liquid-crystal element causes trails to be seen in a dynamic image.
However, an OCB (Optically Compensated Bend)-mode liquid crystal using a bend alignment state has been proposed in recent years (for example, refer to Patent Document 1).
The OCB-mode liquid crystal can sufficiently accommodate dynamic image display and large screen because it has a high response speed and a wide viewing angle compared with the TN-mode liquid crystal, and has an advantage that it can provide a large-screen display which is thinner in size and lower in power consumption than that of a cathode ray tube.
However, the OCB-mode liquid crystal has two alignment states such as a splay alignment state and a bend alignment state. The splay alignment state is an initial-state of liquid-crystal alignment in which no voltage is applied to the OCB-mode liquid crystal as shown in
Moreover, as shown in
[Patent Document 1] Japanese Patent Laid-Open No. 61-116329
[Problems to Be Solved by the Invention]
In the case of a liquid-crystal display device using OCB-mode liquid crystal, it takes a lot of time for the entire liquid-crystal layer surface to migrate to a uniform splay alignment state after turning off the power supply of a liquid-crystal display panel.
According to this power-off sequence, because a voltage held by the liquid-crystal layer 411d constituting each pixel 411 varies depending on a display state displayed just before turning off a power supply, a portion to be quickly transited to the splay alignment and a portion to be slowly transited to the splay alignment occur when a display screen is transited to the splay alignment state after turning off the power supply. For example, at room temperature, approximately 5 sec is required to transit every liquid-crystal layer 411d to the splay alignment. More specifically, reverse transition from the bend alignment state to the splay alignment progresses in the following steps. First, when the voltage applied to OCB-mode liquid crystal becomes 0 V, the bend alignment becomes unstable and 180° twist occurs in all regions. In this case, 180° twist is liquid-crystal alignment in which the arrangement direction of liquid-crystal molecules is twisted between upper substrate and lower substrate and the twist angle is 180°. This alignment state is recognized as transparent, bright yellow. This twist alignment state may be referred to as second spry alignment.
However, in a state in which no voltage is applied to OCB-mode liquid crystal, the splay alignment is more stable than twist alignment. Therefore, there grows a splay alignment region remaining on a display surface or a splay alignment region accidentally generated by using foreign matter or protruded portion on the display surface as a core and finally, the entire display surface turned into the splay alignment and is stabilized. This splay alignment is, for example, transparent blue.
A problem is that it takes a lot of time for the entire surface of a liquid-crystal layer to transit to the splay alignment and a state in which twist alignment (yellow) and splay alignment (blue) are mixed after turning off the power supply is present for a predetermined time ununiformly or depending on the pattern at the time of display and thereby, when outside light is strong, difference between alignment states of various portions of the liquid-crystal layer 411d are seen on a screen as irregularity or afterimage.
Moreover, in the time until the present state is completely transited to the splay alignment after turning off the power supply, when turning on the power supply again, a long transfer driving period is necessary compared to a case of turning on the power supply from a uniform splay alignment state and a lot of time is required from the time when the power supply is turned on until the time when a video is displayed.
For the above inconvenience, an afterimage prevention circuit when turning off the power supply of a liquid-crystal display using conventional OCB-mode liquid crystal shown in
As shown in
As shown by the timing chart in
In this case, when the power supply is turned on again in the period until the transition to the splay alignment is completed after the power supply is turned off as described above, disorder of second splay alignment is added in addition to the disorder of the splay alignment when the power supply is turned on as described above. Therefore, a lot of time is required for the time from t0 to t1. For example, the time from t0 to t1 is approximately 0.2 sec when turning on the power supply from a state not the second splay alignment. However, the time from t0 to t1 requires approximately 0.4 sec when the second splay alignment is present and the power supply is turned on. Thus, when the second splay alignment is present, it is necessary to previously set the time until an image is displayed after the power supply is turned on to a large value or a display trouble appears.
For the above trouble, the transfer circuit shown in
As shown in
The transfer circuit having the above configuration holds a transfer state of eliminating splay alignment by a drastic potential difference in the period from power on to normal display state as shown by the timing chart in
More specifically, immediately after power on, each pixel electrode is reset while applying the voltage Vsc from the source driver, the selection switches 920a and 920b are set to LOW state and the selection switch 920c is set to HIGH state so that the voltage Vsc is supplied to a counter electrode, then in the transfer period, the selection switch 920c is set to LOW state while keeping the selection switches 920a and 920b as they are so that the voltage V+ is applied, and the selection switch 920a is set to HIGH state and then the voltage is collapsed from V+ to V−. In this case, a large potential of |V+−Vsc| or |V−−Vsc| in absolute value is applied to the liquid-crystal layer of the liquid-crystal panel as a transfer voltage to transit the liquid-crystal layer of the liquid-crystal panel from a splay alignment to a bend alignment.
When the transfer period is completed, the selection switches 920b and 920c are set to HIGH, that is, all the selection switches are set to HIGH state and the voltage Vcom is applied to transfer to normal display.
Thus, there are proposed techniques which eliminate respective display troubles when the power supply of the liquid-crystal display is turned off or on.
However, it is necessary to independently constitute respective circuits having the above actions for power on and power off and increase of a liquid-crystal display in size is brought about.
The present invention is made to solve the above problems and its object is to provide a drive apparatus of liquid-crystal panel and the like of a liquid-crystal display using OCB-mode liquid crystal and capable of preventing irregularity of a display screen after a power supply is turned on and quickly transferring the liquid-crystal layer of a liquid-crystal panel to bend alignment when the power supply is turned on.
The 1st aspect of the present invention is a driving device for applying a voltage to a liquid-crystal panel having a liquid-crystal layer capable of being a splay alignment state or a bent alignment state, comprising:
The 2nd aspect of the present invention is a driving device for applying a voltage to a liquid-crystal panel having a liquid-crystal layer capable of being a splay alignment state or a bent alignment state, comprising:
The 3rd aspect of the present invention is a driving device for applying a voltage to a liquid-crystal panel having a liquid-crystal capable of being a splay alignment or a bent alignment comprising:
The 4th aspect of the present invention is the liquid-crystal panel driving device according to any one of the 1st or the 3rd aspect of the present invention, wherein
The 5th aspect of the present invention is the liquid-crystal panel driving device according to the 1st aspect of the present invention, wherein
The 6th aspect of the present invention is the liquid-crystal panel driving device according to the 3rd aspect of the present invention, wherein
The 7th aspect of the present invention is the liquid-crystal pane driving device according to the 5th or the 6rd aspect of the present invention, wherein
The 8th aspect of the present invention is the liquid-crystal panel driving device according to the 7th aspect of the present invention, wherein
The 9th aspect of the present invention is the liquid-crystal panel driving device according to the 7th aspect of the present invention, wherein
The 10th aspect of the present invention is the liquid-crystal panel driving device according to the 1st aspect of the present invention, wherein
The 11th aspect of the present invention is the liquid-crystal panel driving device according to the 2nd aspect of the present invention, wherein
The 12th aspect of the present invention is the liquid-crystal panel driving device according to the 1st aspect of the present invention, wherein the voltage output means comprises;
The 13th aspect of the present invention is a liquid-crystal display device comprising:
According to the above present invention, it is possible to provide a liquid-crystal-panel driving circuit capable of preventing irregularity of a display screen after a power supply is turned off and quickly eliminate disorder of the screen when the power supply is turned on.
Moreover, according to the present invention, it is possible to downsize a liquid-crystal display by realizing the sequence for power on/off by a simple circuit configuration.
Embodiments of the present invention are described below by referring to the accompanying drawings.
Moreover,
Operations of the driving circuit of this embodiment having the above configuration are described below by referring to
First, operations when the power supply of a liquid-crystal display is turned off are described by referring to the timing chart in
Then, when a power-off signal is input to the source/gate driving means 440, the control means 103 performs the control of changing the selection switches 102a and 104c to LOW state from a state in which the voltage Vcom is applied and therefore the liquid-crystal panel 410 displays in normal display, and thereby, outputs the voltage V+ to the counter electrode 411c only for a predetermined period. Moreover, a voltage of AVDD/2 is simultaneously applied to the respective pixel electrodes 411b through the source driver 420. The AVDD/2 is set to, for example, 5 V. Moreover, at the same time, the control means 103 also performs the control of turning off the backlight 450. Furthermore, the control means 103 performs the control of changing the selection switch 102a to HIGH state and outputs the voltage V− to the counter electrode 411c for a predetermined period. Thereby, in the case of each pixel of the liquid-crystal panel 410, by applying a transfer voltage serving as an alternating voltage of an absolute value |V+−AVDD/2| or |V−−AVDD/2|, which is higher than the voltage applied to liquid-crystal layer during normal driving, the arrangement of liquid crystal in the liquid-crystal layer 411d more quickly transfers to the uniform bend alignment. The applying period of the transfer voltage corresponds to the third period of the second present invention or the fourth period of the third present invention and is set to 150 ms in this case. It is preferable that the applying period is 100 msec or more and it is possible to change the period by monitoring a circumferential environmental temperature and corresponding to the temperature. For example, it is possible to lengthen the period under low-temperature environment or shorten the period under high-temperature environment.
Then, the control means 103 performs control so as to apply the voltage Vcom to the counter electrode 411c. In this case, voltages Vs (black) such as +10 V and 0 V of substantially performing uniform black display are supplied to each pixel electrode 411b through the source driver 420. Thereby, voltages of 5 V are applied to the liquid-crystal layer for a predetermined period and uniform black display is performed.
It is preferable that in previous operations, the transfer voltage |V+−AVDD/2| or |V−−AVDD/2| to be applied to a liquid-crystal layer is 1.5 times or more higher than the voltage |Vcom−Vs (black)| to be applied to a liquid-crystal layer when performing black display or more preferable that the former voltage is 2 times or more higher than the latter voltage and the applying period is 100 msec or more. It is also possible to change the period by monitoring a circumferential environmental temperature and changing the period by corresponding to the temperature. For example, it is possible to lengthen the period under low-temperature environment and shorten the period under a high-temperature environment. Particularly, when use under a low-temperature environment such as 0° C. is considered, it is preferable that the period is 300 nsec or more. The applying period of the black display voltage corresponds to the fourth period of the second present invention or the sixth period of the third present invention.
When the black display period is completed, the selection switch 102b is changed to LOW, the reset voltage Vsc is output to the counter electrode 411c, the voltage Vs (white) of substantially performing uniform white display such as a voltage of +7 V is applied to each pixel electrode 411b through the source driver 420, a voltage of 0 V is substantially applied to the liquid-crystal layer, and thereafter supply of voltage is cut off to complete power off. It is preferable that the applying period is 2,000 msec or more and it is possible to change the period by monitoring a circumferential environmental temperature and corresponding to the temperature. For example, it is possible to lengthen the period under low-temperature environment and shorten the period under high-temperature environment. The applying period of the reset voltage Vsc corresponds to the fourth period of the second present invention or the fifth period of the third present invention.
Thus, in the case of the above off operations, by applying a transfer voltage having a potential sufficiently higher than the normal liquid-crystal driving voltage, the liquid-crystal layer 411d quickly transfers to the uniform bend alignment state. Therefore, it is possible to prevent a portion to be quickly transited to the spry alignment and a portion to be slowly transited to the splay alignment from mixing like a conventional example. Thereafter, it is possible to stabilize flickers by applying a black display voltage to each pixel. By continuously applying a reset voltage, it is possible to transfer the whole liquid-crystal layer 411d after a power supply is turned off from uniform bend alignment to uniform splay alignment, effectively remove irregularity and afterimage, and obtain a stable image quality even if outside light comes in after turning off the backlight 450.
Then, operations of a liquid-crystal display by the driving circuit of this embodiment when the power supply is turned on are described below. As shown by the timing chart in
Then, the selection switch 102c is set to LOW state while once keeping the selection switches 102a and 102b as they are so that the voltage V+ is applied to the counter electrode 411c, and a voltage of AVDD/2V is simultaneously applied to pixel electrodes through all source lines from the source driver 420. The AVDD/2 is set to, for example, 5 V. Then, the selection switch 102a is set to HIGH state and a voltage to be applied to all source lines is collapsed from V+ to V−. Thereby, a transfer voltage of |V+−AVDD/2| or |V−−AVDD/2| in absolute value which is an alternating voltage is applied to each liquid-crystal layer of the liquid-crystal panel and the liquid-crystal layer of each pixel 411 is transferred from the uniform splay alignment to bend alignment. It is preferable that the magnitude of the transfer voltage in this case is 1.5 or more higher than the voltage |Vcom−Vs (black)| to be applied to the liquid-crystal layer when performing black display similarly to the case of power off or more preferable that the magnitude is 2 or more higher than the voltage. The period of applying the transfer voltage corresponds to the second period of the second or third present invention.
When the transfer period is completed, the control means 103 controls the selection switches 102b and 104c so that they are set to HIGH, that is, all selection switches are set to HIGH state and applies the voltage Vcom to the counter electrode 411c. Moreover, a predetermined video signal is applied to each source line from the source driver 420 and predetermined display is performed.
Thus, by once applying the transfer voltage to the liquid-crystal layers 411d of the liquid-crystal panel 410 and transiting all liquid-crystal layers 411d to the uniform bend alignment and displaying a video signal, it is possible to quickly eliminate disorder of a screen when the power supply is turned on.
As described above, according to the driving circuit of this embodiment 1, a liquid-crystal display device mounting a liquid-crystal panel using OCB-mode liquid crystal makes it possible to effectively remove an after image appearing on a display screen when a power supply is turned off and the same circuit configuration makes it possible to quickly transit the liquid-crystal panel to the bend alignment when the power supply is turned on.
In the case of the description of the embodiment 1, the reset voltage Vsc is set to an almost intermediate potential between voltages V+ and V− in off state. However, it is also allowed that the reset voltage Vsc is lower than the voltage Vcom and an optional potential between transfer voltages V− and V+. Moreover, though black display is performed in the off-state operation, it is allowed to perform white display in the case of normally black. Furthermore, it is allowed to omit the period of black display.
Then,
An embodiment of the present invention is more specifically described below by referring to
In the case of the video display period 301 shown in
When a power-off signal is input to the liquid-crystal panel 410 from the outside, the source/gate driving means 440 completes the video display period 301 and at the same time, turns off the backlight 450 and starts off-sequence periods 302, 303, and 304 in order.
First in the off-sequence period 302, the control means 103 applies a transfer voltage to the counter electrode 411c of the liquid-crystal panel 410. Similarly to the case of
Similarly to the above mentioned, because the transfer voltage in the off-sequence period 302 is set to a voltage higher than the black display voltage the arrangement of liquid crystal in the liquid-crystal layer 411d quickly becomes the uniform bend alignment. Therefore, it is preferable that the off-sequence period 302 is 100 msec or more when a voltage to be applied is approximately 1.5 times higher than the black display voltage.
When the off-sequence period 302 is completed, the control means 103 starts the off-sequence period 303. When a display screen is normally white, an alternating voltage of displaying black gradation on the entire display screen is applied to the liquid-crystal panel 410 in the off-sequence period 303. It is preferable that the black display voltage is applied in the off-sequence period 303 for 100 msec or more.
Thus, by applying a black-display alternating voltage in the off-sequence period 303 after applying a high voltage in the off-sequence period 302, it is possible to stabilize flickers compared with the case of only the off-sequence period 302 and shorten the time until transiting to the splay alignment.
When the off-sequence period 303 is completed, the off-sequence period 304 is started. When the display screen is normally white, the control means 103 applies a voltage of displaying white gradation on the entire display screen to the liquid-crystal panel 410 in the off-sequence period 304. That is, the potential difference between the counter electrode and the pixel electrode is substantially brought to zero by performing white display. Then, the control means 103 performs control so as to bring at least either of the potential difference between the gate line 413 and the pixel electrode 411b and the potential difference between the common electrode 411e (electrode other than pixel electrode) and the pixel electrode to accelerate the transition to the splay alignment.
In this case, because an applied voltage become 0 V when the arrangement of liquid crystal is a uniform state in the liquid-crystal layer 411d, OCB-mode liquid crystal can uniformly transit from the bend alignment to the splay alignment.
After the off-sequence period 304 is completed, the control means 103 starts a power-off period 305. When the power-off period 305 is started, the control means 103 opens the selection switches 102a to 102c to cut off the power supplied from the outside.
At the point of time when the power-off period 305 is started, electric potentials of the counter electrode 411c, pixel electrode, gate line 413, and common electrode 411e are equal to each other. Therefore, transition to the splay alignment state is started from the point of time. Reference numerals 503 and 504 shown in
Furthermore, by inserting the off-sequence period 304 of performing white display by the reset voltage Vsc, a potential difference reaching a transfer potential is not generated even if a difference occurs between potentials in the period from the point of time when the power-off period 305 is started until each potentials reaches the ground level (that is, the region A shown by (d) in
For this embodiment, it is described that an alternating voltage is applied in the off-sequence periods 302 and 303. However, it is also allowed to apply a constant voltage. In this case, the advantage that transit to the splay alignment is accelerated is the same as the above described though the advantage that the flicker characteristic is improved cannot be obtained. In this case, by setting the black display period like the case of the off-sequence period 303, it is possible to effectively generate reverse transfer.
Moreover, for this embodiment, it is allowed that a voltage at which white gradation is substantially displayed on a display screen is applied to the liquid-crystal layer 411d in the off-sequence period 304. Also in this case, an advantage same as the above described can be obtained.
Furthermore, it is allowed that the off-sequence period 303 is omitted, the off-sequence period 302 is started after the video display period 301 is completed, and the power-off period 305 is started after the off-sequence period 304 is performed after the off-sequence period 302 is completed. Also in this case, an advantage same as the above described can be obtained. In this case, the off-sequence period 304 corresponds to the fourth period of the second present invention.
Furthermore, it is described that a voltage to be applied to the liquid-crystal layer 411d is uniform. However, when a transfer voltage is applied, it is allowed that the voltage is ununiform. Also in this case, an advantage same as the above described can be obtained.
Though it is described above that the liquid-crystal layer 411d is normally white, it is also allowed that the layer 411d is normally black. In the off-sequence period 303, it is enough that a voltage at which white is substantially displayed on a display screen is applied. Moreover, in the off-sequence period 302, it is enough that a voltage higher than the voltage at which white is displayed on the display screen and equal to or lower than a voltage to be applied to the liquid-crystal layer 411d is applied as a transfer voltage. Furthermore, in the off-sequence period 304, it is enough that a voltage at which black is substantially displayed on a display screen is applied. In this way, it is possible to obtain an advantage same as the above described even if the liquid-crystal layer 411d id normally black.
Furthermore, it is described above that irradiation by the backlight 450 is turned off at the same time as end of the video display period 301. However, it is also allowed that irradiation by the backlight 450 is turned off after end of the off-sequence period 304. Moreover, it is allowed that irradiation by the backlight 450 is turned off before the off-sequence period 304 is started after the video display period 301 is completed. Also in this case, because the liquid-crystal layer 411d can transit from the uniform bend alignment to the splay alignment, irregularity does not occur on the display screen.
Furthermore, it is allowed that irradiation by the backlight 450 is turned off before the video display period 301 is completed.
In the case of the above embodiment, the driving circuit 100 corresponds to a driving circuit of the present invention and the output system 102 and control means 103 correspond to voltage output means of the present invention. Moreover, the output system 102 corresponds to the first driving circuit of the third present invention and the control means 103 corresponds to a control circuit of the third present invention. Furthermore, means of supplying a common potential to the common electrode 411e corresponds to the second driving circuit of the third present invention.
Furthermore, the liquid-crystal panel 416 corresponds to a liquid-crystal panel of the present invention. Furthermore, the voltages V+ and V− or voltages having absolute values of |V+−AVDD/2| and |V−−AVDD/2| correspond to a transfer voltage of the present invention, the voltage Vsc corresponds to a reset voltage of the present invention, the voltage Vcom corresponds to a counter voltage when a predetermined video signal is displayed. Furthermore, the selection switch 102a corresponds to a first selection switch of the present invention, the selection switch 102b corresponds to a second selection switch of the present invention, and the selection switch 102c corresponds to a third selection switch of the present invention. Furthermore, the source driver 420 corresponds to a driver of the present invention.
Furthermore, a liquid-crystal display mounting a driving circuit of the present invention is included in the present invention. The OCB-mode liquid crystal is used as a liquid crystal having a bend alignment and a splay alignment. However, it is allowed to use another liquid crystal as long as the liquid crystal can take these states.
A drive apparatus of liquid-crystal panel of the present invention has advantages capable of preventing irregularity on a display screen after turning off a power supply by a simple circuit configuration and quickly eliminating disorder of the screen when the power supply is turned on and is useful as a liquid-crystal display.
Number | Date | Country | Kind |
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2004-120820 | Apr 2004 | JP | national |