Display device

Abstract

To prevent a change in the brightness of a display panel when a frame frequency is changed.SOLUTION: A display device 10 has a backlight 30, a display panel 20 that transmits light from the backlight 30 and displays an image, and a control unit 40. The control unit 40 causes the backlight 30 to emit light at a first light emission quantity Q1 when a frame frequency FR of an image displayed on the display panel 20 is a first frequency FR1, and causes the backlight 30 to emit light at a second light emission quantity Q2 larger than the first light emission quantity Q1 when the frame frequency FR is a second frequency FR2 lower than the first frequency FR1.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The technology disclosed in this specification relates to a display device. [Background technology]

[0002] 2. Description of the Related Art Techniques for switching the frame frequency (drive frequency) of a display panel depending on an image to be displayed are known for the purpose of reducing the power consumption of a display device.

[0003] When the frame frequency is changed, the brightness of the display panel also changes. Specifically, when the frame frequency is lowered, the pixel refresh frequency decreases, which increases the leakage current from the pixel electrode within one frame and reduces the voltage of the pixel electrode. As the voltage decreases, the transmittance of light passing through the pixel decreases, and as a result, the brightness of the display panel decreases. Conversely, when the frame frequency is increased, the brightness of the display panel increases. Changes in the brightness of the display panel cause image flicker.

[0004] Known technology for suppressing changes in luminance is the liquid crystal display device described in Patent Document 1. According to the description in Patent Document 1, changes in luminance are suppressed by minimizing black data inserted as a measure against afterimages. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2007-171948 A Summary of the Invention [Problem to be solved by the invention]

[0006] While reducing the black data can suppress the decrease in luminance, it may increase afterimages and degrade the display quality. There has been a demand for a method for suppressing changes in the luminance of a display panel other than by increasing or decreasing the black data. The present invention has been made in consideration of the above problems, and aims to provide a display device that can suppress changes in the luminance of a display panel even when the frame frequency is changed. [Means for solving the problem]

[0007] (1) A display device according to the technology described in this specification includes a backlight, a display panel that displays an image by transmitting light from the backlight, and a controller, the controller causing the backlight to emit light with a first emitted light amount when a frame frequency of an image displayed on the display panel is a first frequency, and causing the backlight to emit light with a second emitted light amount greater than the first emitted light amount when the frame frequency is a second frequency lower than the first frequency.

[0008] (2) In the above display device, the control unit may cause the display panel to display an image by inserting black data, which is displayed at the lowest gradation for a predetermined period of a frame, in at least some of the frames, and the control unit may cause the backlight to emit light at the first emission light amount during a period in which a frame in which the black data is not inserted is displayed, and may cause the backlight to emit light at a third emission light amount greater than the first emission light amount during a period in which a frame in which the black data is inserted is displayed. Effect of the Invention

[0009] According to the present invention, it is possible to suppress a change in the luminance of a display panel when the frame frequency is changed. [Brief description of the drawings]

[0010] [Figure 1] A side view showing the structure of a liquid crystal display device. [Diagram 2] Electrical block diagram of a liquid crystal display device [Diagram 3] Voltage graph for PWM dimming [Figure 4] Transmittance graph [Diagram 5] Graph showing the amount of light emitted from the backlight [Figure 6] Graph showing the brightness of the display panel [Figure 7] Graph showing transmittance (embodiment 2) [Figure 8] Graph showing the amount of light emitted from the backlight (Embodiment 2) [Figure 9] Graph showing the luminance of the display panel (embodiment 2) DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] <Embodiment 1> 1. Liquid crystal display device configuration A first embodiment will be described with reference to Fig. 1 to Fig. 6. In this embodiment, a liquid crystal display device 10 will be illustrated. The liquid crystal display device 10 is an example of a "display device". The liquid crystal display device 10 is a device that displays an image in response to an input signal input from the outside. The image includes a still image and a moving image.

[0012] <Liquid crystal panel> Fig. 1 shows a cross-sectional view of a liquid crystal display device 10 according to this embodiment. The liquid crystal display device 10 includes a liquid crystal panel 20 (an example of a "display panel"), a backlight 30 that emits light to the liquid crystal panel 20 with an emission light amount Q (this emission light amount is variably controllable), and a control unit 40 (see Fig. 2). In the liquid crystal display device 10, the liquid crystal panel 20 side is the front side, and the backlight 30 side is the rear side. The upper side of Fig. 1 is the front side of the liquid crystal display device 10, and the lower side is the rear side.

[0013] The liquid crystal panel 20 is formed by bonding together a pair of glass substrates 21 and 22. Of the pair of glass substrates 21 and 22, the outer surface of the glass substrate 21 located on the side farther from the backlight 30 (the front side) is a display surface 21A that displays images. A polarizing plate is attached to each of the main surface on the front side of the glass substrate 21 and the main surface on the rear side of the glass substrate 22.

[0014] A liquid crystal layer 23 containing liquid crystal molecules, which are a substance whose optical properties change when an electric field is applied, is interposed between the pair of glass substrates 21, 22. A seal portion 24 that seals the liquid crystal layer 23 is interposed between the outer periphery ends of the pair of glass substrates 21, 22. The seal portion 24 is formed to surround the liquid crystal layer 23.

[0015] On the inner surfaces of the opposing glass substrates 21, 22, alignment films, electrodes, transistors, color filters, etc. are laminated. When a control unit 40 described later applies a gate voltage VG to the liquid crystal panel 20, the alignment state of the liquid crystal molecules contained in the liquid crystal layer 23 can be arbitrarily changed based on the gradation voltage. Changing the alignment state changes the light transmittance G of the liquid crystal panel 20, and the display gradation of the image displayed on the display surface 21A can be changed.

[0016] <Backlight> The backlight 30 is disposed on the rear side of the liquid crystal panel 20, and includes a plurality of light sources 31 and a plate-shaped support substrate 32 on which the light sources 31 are mounted. The light sources 31 are, for example, white LED elements that emit white light. The light sources 31 are disposed in a planar shape on the rear side of the liquid crystal panel 20. The backlight 30 is a so-called direct type backlight device.

[0017] A wiring pattern made of a conductive material is formed on the support substrate 32, and power is supplied to each light source 31 through the wiring pattern. The power supplied to the light source 31 is PWM (pulse width modulation) controlled by the control unit 40, and the amount of light Q emitted by the backlight 30 can be adjusted as desired.

[0018] An overview of PWM control will be described with reference to Fig. 3. The horizontal axis of the graph shown in Fig. 3 represents time, and the vertical axis represents the drive voltage V input to the light source 31. The light source 31 emits light with a constant brightness when a predetermined voltage Von is applied.

[0019] In order to achieve the amount of emitted light Q required in a predetermined period T (e.g., one frame period), the light source 31 alternates between off periods Toff during which no voltage is applied and on periods Ton during which the driving voltage Von is applied in the form of a pulse wave. The off periods Toff are non-light-emitting periods during which the light source 31 does not emit light. The on periods Ton are light-emitting periods during which the light source 31 emits light. The ratio of the on periods Ton to the predetermined period T is the duty ratio. To increase the amount of emitted light Q, the duty ratio is increased (the ratio of the on periods Ton is increased and the ratio of the off periods Toff is decreased). Conversely, to decrease the amount of emitted light Q, the duty ratio is decreased (the ratio of the on periods Ton is decreased and the ratio of the off periods Toff is increased).

[0020] The control unit 40 can emit light with an arbitrary emission light amount Q from the rear side of the liquid crystal panel 20 by changing the duty ratio.

[0021] <Control Unit> Fig. 2 is a block diagram showing the electrical configuration of the liquid crystal display device 10. The control unit 40 has a timing controller 41, an LED driver 42, a power supply IC 43, a level shifter 44, etc. The elements of the control unit 40 are connected to each other so that they can communicate with each other, as shown by the arrows in Fig. 2. The timing controller 41 and the power supply IC 43 are connected by a serial bus of an I2C (Inter-Integrated Circuit) interface.

[0022] The input signal 50 input to the control unit 40 includes a power supply voltage 51, an eDP (embedded Display Port) video signal 52, a PWM input signal 53, etc. The power supply voltage 51 is a voltage for driving the liquid crystal panel 20. The eDP video signal 52 is a signal standardized for image input / output, and includes a frame frequency FR and an STV signal 54, which will be described later.

[0023] The power supply voltage 51 is a drive voltage for the liquid crystal panel 20. The power supply IC 43 converts the input power supply voltage 51 into a gate voltage VG (VGH, VGL) and outputs it to the level shifter .

[0024] When the timing controller 41 receives the eDP video signal 52, it recognizes the frame frequency FR included in the eDP video signal 52. In addition, it generates an STV signal 54 from the eDP video signal 52. The generated STV signal 54 is input to the power supply IC 43 and the level shifter 44.

[0025] The level shifter 44 converts the gate voltage VG received from the power supply IC 43 and the STV signal 54 into a gate clock GCK and a gate start pulse GSP. The liquid crystal panel 20 is driven at a timing based on the gate clock GCK and the gate start pulse GSP input from the level shifter 44. The gate voltage VG is applied to each pixel included in the liquid crystal panel 20 at a predetermined timing, and the light transmittance G is changed for each pixel.

[0026] Furthermore, the timing controller 41 receives a PWM input signal 53 included in the input signal 50 at the same timing as the generation of the STV signal 54. The timing controller 41 corrects the duty ratio of the PWM input signal 53 by referring to the frame frequency FR, and generates a PWM output signal 56. The PWM output signal 56 is transmitted from the timing controller 41 to the LED driver 42.

[0027] The LED driver 42 causes the light source 31 of the backlight 30 to emit light at the corrected duty ratio in accordance with the received PWM output signal 56. As shown in Fig. 1, the emitted light (emitted light amount Q) generated by the emission of the light source 31 passes through the liquid crystal panel 20 driven by the gate voltage VG, and an image is displayed on the display surface 21A of the liquid crystal panel 20 at a luminance N.

[0028] 2. Relationship between frame frequency and LCD panel brightness 4 to 6, the relationship between the frame frequency FR and the luminance N of the liquid crystal panel 20 will be described. The luminance N of the liquid crystal panel 20 in one frame period is determined by two parameters: the amount of light emitted Q of the backlight 30 and the average transmittance in one frame period.

[0029] The gate voltage VG is applied for one frame period, which is the reciprocal of the frame frequency FR. In the following explanation, for simplicity, one pixel will be explained, and the amount of light Q emitted from the backlight 30 is assumed to be maintained at the same amount for one frame period.

[0030] 4A is a graph showing a change in transmittance G over time when the frame frequency FR is a first frequency FR1. The first frequency FR1 is, for example, 60 Hz.

[0031] When a gate voltage VGH is applied to a pixel at time t0, the transmittance G of the pixel rises to G0. When a gate voltage VGL of the next frame is applied to the pixel at time t1 after one frame period (1 / FR1) has elapsed, the pixel is refreshed and its transmittance becomes G (for example, 0) of the next frame.

[0032] 4B is a graph showing the change in transmittance G over time when the frame frequency FR is a second frequency FR2 lower than the first frequency FR1. The second frequency FR2 is, for example, 1 Hz.

[0033] When the same gate voltage VGH as in Figure 4(A) is applied at time t0, the transmittance G rises to G0. When the gate voltage VGL of the next frame is applied to the pixel at time t2 after one frame period (1 / FR2) has elapsed, the transmittance of the next frame becomes G (for example, 0).

[0034] It is desirable that the transmittance G during one frame period (1 / FR1 or 1 / FR2) be constant at the transmittance G0 at the time of voltage application, as shown by the dashed lines in Figures 4(A) and 4(B). In reality, however, the gate voltage VG decreases over time due to leakage current from pixels, etc.

[0035] Therefore, as shown by the solid lines in Figures 4(A) and (B), after the gate voltage VGH is applied, the transmittance G gradually decreases over time. When the frame frequency FR is the first frequency FR1, as shown in Figure 4(A), the transmittance G decreases from G0 to G1 in one frame period. The average transmittance GX in one frame period has a value between G0 and G1.

[0036] In contrast, at the second frequency FR2, as shown in FIG. 4B, the transmittance G gradually decreases from G0 to G2 during one frame period. At the second frequency FR2, one frame period is longer than at the first frequency FR1, and there is more leakage current during one frame period. Therefore, the decrease in the transmittance G at the second frequency FR2 is greater than the decrease at the first frequency FR1, and G1>G2. Therefore, the average transmittance GY during one frame period is lower than GX (GX>GY).

[0037] Even when the same gate voltage VGH is applied, the average transmittance GY at the second frequency FR2 is lower than the average transmittance GX at the first frequency FR1 (GX>GY). When the frame frequency of the liquid crystal panel 20 is lowered from the first frequency FR1 to the second frequency FR2, the average transmittance decreases from GX to GY.

[0038] Conversely, when the frame frequency FR increases from the second frequency FR2 to the first frequency FR1, the average transmittance increases from GY to GX.

[0039] 3.Backlight output light intensity Since a change in the luminance N of the liquid crystal panel 20 causes flickering of the displayed image, it is necessary to suppress the change in the luminance N.

[0040] Fig. 5(A) is a graph of the emitted light quantity Q corresponding to the voltage application timing of Fig. 4(A). When the frame frequency FR is the first frequency FR1, the control unit 40 causes the backlight 30 to emit light with the first emitted light quantity Q1 from time t0 to t1. At this time, as shown on the left side of the graph in Fig. 6, the liquid crystal panel 20 displays an image with a luminance N1.

[0041] As described above, the average transmittance GY at the second frequency FR2 is lower than the average transmittance GX at the first frequency FR1. Therefore, when the frame frequency FR decreases from the first frequency FR1 to the second frequency FR2, the average transmittance decreases from GX to GY. At this time, if the emission light amount Q of the backlight 30 remains constant at the first emission light amount Q1 before and after the change in the frame frequency FR, the amount of light that passes through the liquid crystal panel 20 and reaches the front side decreases. As a result, as shown by the two-dot chain line in FIG. 6, the luminance of the liquid crystal panel 20 decreases from N1 to Na.

[0042] On the other hand, Fig. 5(B) is a graph of the emitted light amount Q corresponding to the voltage application timing of the second frequency FR2 shown in Fig. 4(B). In the liquid crystal display device 10 of this embodiment, when the frame frequency FR is the second frequency FR2, the control unit 40 causes the backlight 30 to emit light with a second emitted light amount Q2 larger than the first emitted light amount Q1 from time t0 to t2.

[0043] In this way, more light is incident on the liquid crystal panel 20 from the rear side of the liquid crystal panel 20, and this makes it possible to compensate for the decrease in transmitted light due to the low average transmittance GY.

[0044] 6, when the backlight 30 is caused to emit light with a second amount of emitted light Q2 at the second frequency FR2, the luminance of the liquid crystal panel 20 is set to N2. The luminance N2 is higher than the luminance Na because the amount of emitted light Q of the backlight 30 is larger (Q2>Q1).

[0045] The decrease in luminance N due to the decrease in average transmittance is compensated for by increasing the amount of emitted light Q, so the difference between luminance N1 and luminance N2 is smaller than the difference between luminance N1 and luminance Na. This makes it possible to suppress the decrease in luminance N caused by the decrease in frame frequency FR and reduce the occurrence of flickering.

[0046] The value of the second emitted light amount Q2 that can suppress the decrease in luminance N associated with the decrease in frame frequency FR may be obtained in advance by experiment or simulation. A data table that associates the frame frequencies FR1 and FR2 before and after the change with the values ​​of the emitted light amounts Q1 and Q2 that can suppress the change in luminance N before and after the change in frame frequency FR is created and stored in the control unit 40 or a storage medium. When the frame frequency is changed from FR1 to FR2, the amount of emitted light Q2 that corresponds to the changed frame frequency FR2 can be quickly called up by referring to the stored data table.

[0047] <Embodiment 2> Next, a liquid crystal display device 11 according to a second embodiment of the present invention will be described with reference to Fig. 7 to Fig. 9. In this embodiment, the same components as those in the first embodiment are designated by the same reference numerals, and redundant descriptions of the structure, operation, and effects will be omitted.

[0048] The control unit 40 inserts black data into at least some of the frames. The black data is data that causes the transmittance G of the liquid crystal panel 20 to be set to a minimum value (e.g., zero) for a predetermined period of one frame period, and displays the image at the minimum gradation. By inserting the black data, an image at the minimum gradation is displayed between frames, reducing afterimages between frames and improving the display quality of the image.

[0049] Fig. 7(A) shows the time change of the transmittance G for five frames when black data is not inserted, and Fig. 7(B) shows the time change of the transmittance G for five frames when black data is inserted. The frame frequency FR is the first frequency FR1 in both Fig. 7(A) and Fig. 7(B).

[0050] As shown in Figure 7(A), when black data is not inserted, the transmittance G rises to G0 when the gate voltage VG is applied at time t0, and then drops to G1 at time t1, one frame period (1 / FR1) after the gate voltage VG is applied again after one frame period has passed, and the drop is repeated. The average transmittance GX during one frame period is a value between the transmittance G0 when the gate voltage VG is applied and the transmittance G1 immediately before the next gate voltage VG is applied.

[0051] 7B, when black data is inserted, the gate voltage VG is applied at time t0, the transmittance rises to G0, and then a black data period 55 is inserted during one frame period until the gate voltage VG is next applied, during which the transmittance G becomes the minimum value (zero). The average transmittance GZ during one frame period when black data is inserted is lower than the average transmittance GX when black data is not inserted.

[0052] Fig. 8(A) shows that the emitted light amount Q when black data is not inserted, corresponding to the voltage application timing of Fig. 7(A), is the first emitted light amount Q1. When black data is not inserted and the emitted light amount is the first emitted light amount Q1, as shown on the left side of the graph in Fig. 9, the liquid crystal panel 20 displays an image with a luminance N1 when the emitted light amount is the first emitted light amount Q1.

[0053] In contrast, Fig. 8(B) shows the third amount of emitted light Q3 when black data is inserted, which corresponds to the voltage application timing of Fig. 7(B). In the liquid crystal display device 11 according to the second embodiment, the third amount of emitted light Q3 when black data is inserted is set to a value larger than the first amount of emitted light Q1 when black data is not inserted.

[0054] Even if the same gate voltage VG is applied and the transmittance at the rising edge is the same at G0, the average transmittance drops from GX to GZ due to the insertion of black data. Therefore, if the amount of light Q emitted from the backlight 30 remains unchanged at Q1 before and after the insertion of black data, the amount of light passing through the liquid crystal panel 20 drops due to the insertion of black data, and the luminance N drops.

[0055] 9, the luminance Nb indicated by the two-dot chain line is the luminance of the liquid crystal panel 20 when black data is inserted and the first emitted light amount Q1 is used. The luminance Nb is lower than the luminance N1 when the first emitted light amount Q1 is used due to the effect of the average transmittance (GX to GZ) that is reduced by the insertion of the black data.

[0056] In the liquid crystal display device 11 of this embodiment, the amount of emitted light when displaying a frame into which black data is inserted is increased from the first amount of emitted light Q1 when displaying a frame without black data inserted to a third amount of emitted light Q3. When the amount of emitted light is increased, more light is irradiated toward the liquid crystal panel 20. The right side of Fig. 9 shows the luminance N3 when black data is inserted and the third amount of emitted light Q3 is used.

[0057] By setting the emitted light amount to a third emitted light amount Q3, which is greater than the first emitted light amount Q1, when black data is inserted, the brightness of the liquid crystal panel 20 becomes a brightness N3, which is greater than the brightness Nb when the emitted light amount Q1 is maintained. In this way, the decrease in average transmittance (from GX to GZ) due to the insertion of black data is compensated for by the increase in the emitted light amount (from Q1 to Q3), thereby suppressing the decrease in brightness of the liquid crystal panel 20 due to the insertion of black data. At the same time, the occurrence of afterimages is reduced by the insertion of black data.

[0058] <Other embodiments> The technology disclosed in this specification is not limited to the embodiments described above and illustrated in the drawings, and the following embodiments, for example, are also included in the technical scope.

[0059] (1) In the above embodiment, a liquid crystal display device having a liquid crystal panel has been described as an example. The embodiment of the present invention is not limited to a liquid crystal display device, and may be a display device having a display panel other than a liquid crystal panel.

[0060] (2) In the above embodiment, a so-called direct-type backlight in which a light source is disposed directly under a liquid crystal panel is exemplified. The backlight does not have to be a direct-type. An edge-light type in which a light source is disposed on an edge of a display panel and light is diffused by a light guide plate, or another type of backlight may also be used.

[0061] In the above embodiment, the first frequency is 60 Hz and the second frequency is 1 Hz, but the frame frequency is not limited to these values. The first frequency may be higher or lower than 60 Hz, such as 30 Hz or 120 Hz. The present invention is applicable as long as the second frequency is relatively lower than the first frequency.

[0062] In the above embodiment 1, it has been described that when the frame frequency is the second frequency lower than the first frequency, the decrease in luminance can be suppressed by increasing the amount of light emitted from the backlight. In other words, when the frame frequency is increased from the second frequency to the first frequency, the increase in luminance can be suppressed by decreasing the amount of light emitted. Therefore, the present invention can be applied to both cases where a relatively high frequency is changed to a low frequency and where a relatively low frequency is changed to a high frequency.

[0063] In the above-described second embodiment, it has been described that the decrease in luminance can be suppressed by making the amount of emitted light larger during the period in which a frame into which black data is inserted is displayed than during the period in which a frame into which black data is not inserted is displayed. In other words, the increase in luminance can be suppressed by making the amount of emitted light smaller during the period in which a frame into which black data is not inserted is displayed than during the period in which a frame into which black data is inserted is displayed. Therefore, the present invention can be applied to both the case in which black data is inserted into an image into which black data is not inserted and the case in which black data is removed from an image into which black data is inserted. [Explanation of symbols]

[0064] 10: liquid crystal display device (an example of a display device), 20: liquid crystal panel (an example of a display panel), 30: backlight, 40: control unit, 50: input signal, 53: PWM input signal, 56: PWM output signal, FR1: first frequency, FR2: second frequency, Q1: first emitted light amount, Q2: second emitted light amount

Claims

1. A display device, comprising: Backlight and a display panel that displays an image by transmitting light from the backlight; A control unit and The control unit is When a frame frequency of an image to be displayed on the display panel is a first frequency, the backlight is caused to emit light with a first emission amount; When the frame frequency is a second frequency lower than the first frequency, the display device causes the backlight to emit light with a second amount of emitted light greater than the first amount of emitted light.

2. The display device according to claim 1 , the control unit causes the display panel to display an image by inserting black data, which is displayed at a minimum gradation level for a predetermined period of one frame, in at least a portion of the frames; The control unit is causing the backlight to emit light with the first emitted light amount during a period in which a frame in which the black data is not inserted is displayed; the display device causes the backlight to emit light with a third emitted light amount that is greater than the first emitted light amount during a period in which the frame into which the black data is inserted is displayed.

Images