JP5004386B2 - Display device and driving method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a display device and a driving method thereof, and particularly to a display device suitable for use in a portable display device and a driving method thereof.
[0002]
[Prior art]
In recent years, portable display devices such as mobile TVs and mobile phones have been required as market needs. In response to such demands, research and development has been actively conducted in order to cope with the reduction in size, weight, and power consumption of display devices.
[0003]
FIG. 7 shows a circuit configuration diagram of one display pixel of a liquid crystal display device according to a conventional example. A gate signal line 51 and a drain signal line 61 intersect with each other on an insulating substrate (not shown), and a pixel selection TFT 65 connected to both signal lines 51 and 61 is provided in the vicinity of the intersection. ing. The source 11 s of the TFT 65 is connected to the display electrode 80 of the liquid crystal 21.
[0004]
Further, an auxiliary capacitor 85 for holding the voltage of the display electrode 80 for one field period is provided. One terminal 86 of the auxiliary capacitor 85 is connected to the source 11 s of the TFT 65, and the other electrode 87 is connected to each electrode 87. A common potential is applied to the display pixels.
[0005]
Here, when a scanning signal is applied to the gate signal line 51, the TFT 65 is turned on, and an analog video signal is transmitted from the drain signal line 61 to the display electrode 80 and held in the auxiliary capacitor 85. A video signal voltage applied to the display electrode 80 is applied to the liquid crystal 21, and the liquid crystal 21 is aligned according to the voltage, whereby a liquid crystal display can be obtained.
[0006]
Therefore, a display can be obtained regardless of a moving image or a still image. When a still image is displayed on such a liquid crystal display device, for example, an image of a dry cell is displayed as a battery remaining amount display for driving the mobile phone on a part of the liquid crystal display unit of the mobile phone.
[0007]
However, in the liquid crystal display device having the above-described configuration, even when a still image is displayed, the TFT 65 is turned on by a scanning signal and a video signal is sent to each display pixel as in the case of displaying a moving image. There was a need to rewrite.
[0008]
For this reason, driver circuits for generating drive signals such as scanning signals and video signals, and external LSIs for generating various signals for controlling the operation timing of the driver circuits always operate, and thus always consume large power. It was. For this reason, a mobile phone or the like having only a limited power source has a drawback that the usable time is shortened.
[0009]
On the other hand, a liquid crystal display device having a static memory in each display pixel is disclosed in Japanese Patent Laid-Open No. 8-194205. Describing a part of the publication, this liquid crystal display device, as shown in FIG. 8, retains a digital video signal in a memory in which two-stage inverters INV1 and INV2 are positively fed back, that is, a static memory. By using it as a circuit, power consumption is reduced.
[0010]
Here, according to the binary digital video signal held in the static memory, the switch element 24 controls the resistance value between the reference line Vref and the display electrode 80 and adjusts the bias state of the liquid crystal 21. . On the other hand, an AC signal Vcom is input to the common electrode. Ideally, this apparatus does not require a refresh to the memory if there is no change in the display image as in a still image.
[0011]
As described above, a liquid crystal display device including a static memory for holding a digital video signal is suitable for displaying a low-gradation still image and reducing power consumption.
[0012]
[Problems to be solved by the invention]
However, the liquid crystal display device having the above-described configuration has the following problems. This problem will be described with reference to FIG. Now, it is assumed that the source 11s of the pixel selection TFT 65 is at the “L (low)” level and the “H (high)” level is held at the output node of the inverter INV1.
[0013]
In this holding state, when “H” is output from the external circuit to the drain signal line 61 and “H” is written to the static memory, the N-channel TFT of the inverter INV2 is turned on. As shown by, current flows through the path of the drain signal line 61 → the TFT 65 → the N-channel TFT. That is, the “H” level and the “L” level are pulled, and there is a possibility that erroneous writing occurs due to the decrease in “H”.
[0014]
Here, in order to normally write “H” data, the condition that the source 11s of the TFT 65 is higher than the threshold voltage of the inverter INV1 must be satisfied. However, because the current path described above exists. The source 11s of the TFT 65 may be lowered.
[0015]
Therefore, the following measures can be considered to satisfy the above conditions.
(1) The “H” level voltage supplied from the external circuit to the drain line 61 is increased.
(2) Increase the voltage when the gate signal line 51 is selected to reduce the on-resistance of the pixel TFT 65 or increase the channel width of the TFT 65.
[0016]
However, (1) has a drawback that the power consumption increases because the power supply voltage of the external circuit increases. {Circle over (2)} has the disadvantage that the power supply voltage of the gate driver increases, the TFT size increases, and it becomes difficult to lay out the pixel at a fine pitch.
[0017]
In a display device having a static memory for holding digital video data in display pixels, the present invention prevents erroneous writing of data to the static memory, and reduces power consumption and fine pixel layout. The present invention provides a display device that has been made possible.
[0018]
[Means for Solving the Problems]
Of the inventions disclosed in this application, the outline of the main ones will be described as follows.
[0019]
That is, the first configuration of the present invention includes a plurality of gate signal lines arranged in one direction on the substrate, a plurality of drain signal lines arranged in a direction intersecting the gate line, and the gate signal lines. In the display device in which display pixels to which video signals are supplied from the drain signal lines are selected in accordance with the scanning signal, the signals are arranged in the display pixels and input from the gate signal lines. Accordingly, a holding circuit for writing the digital video signal from the drain signal line and holding the digital video signal, and a boosting circuit for boosting the power supply voltage supplied to the holding circuit after the writing of the digital video signal is completed. And displaying according to the output of the holding circuit.
[0020]
According to such a configuration, the power supply voltage supplied to the holding circuit can be set low enough to hold the digital video signal at the time of writing, and high enough to obtain a good image at the time of display after writing. It is possible to prevent erroneous writing of the digital video signal to the holding circuit and to reduce power consumption.
[0021]
Further, according to the above configuration, the pixel selection element can be made small, so that a fine layout of the pixels can be performed.
[0022]
In the above configuration, the number of circuit elements can be minimized by configuring the holding circuit from a two-stage inverter circuit that is positively fed back. The inverter circuit is preferably a CMOS inverter for reducing power consumption.
[0023]
Further, in the above configuration, a signal selection circuit that selects a signal to be supplied to the display electrode according to an output of the holding circuit is provided, and the signal selection circuit includes a plurality of thin film transistors in which a signal from the holding circuit is applied to the gate. . This is to enable display according to the output of the holding circuit and to reduce the number of circuit elements of the signal selection circuit.
[0024]
Further, in the above structure, the power supply voltage boosted by the booster circuit is higher than a voltage obtained by adding a threshold voltage of the thin film transistor to a signal supplied to the display electrode. This prevents a signal supplied to the display electrode from being lowered, and thus a high-quality image display with good contrast can be obtained.
[0025]
The display device driving method of the present invention includes a plurality of gate signal lines arranged in one direction on a substrate, a plurality of drain signal lines arranged in a direction intersecting the gate line, and the gate signal lines. Display pixels that are selected by a scanning signal from and supplied with video signals from the drain signal lines and arranged in a matrix, and the drain signals according to signals input from the gate signal lines into the display pixels In a method for driving a display device, comprising: a holding circuit for writing a digital video signal from a line and holding the digital video signal; and a boosting circuit for boosting a power supply voltage supplied to the holding circuit. After the digital video signal from the drain signal line is written to the holding circuit according to the input signal, the holding circuit performs the holding circuit. Boosts the power supply voltage supplied to the one in which an image is displayed in accordance with the digital video signal held by said holding circuit.
[0026]
According to this configuration, it is possible to prevent erroneous writing of the digital video signal to the holding circuit and to reduce power consumption.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
Next, a display device according to an embodiment of the present invention will be described. FIG. 1 shows a circuit configuration diagram of the liquid crystal display device according to the first embodiment.
[0028]
On the insulating substrate 10, a plurality of gate signal lines 51 connected to a gate driver 50 for supplying a scanning signal are arranged in one direction, and a plurality of drain signal lines 61 are arranged in a direction intersecting with the gate signal lines 51. Is arranged.
[0029]
Sampling transistors SP1, SP2,..., SPn are turned on to the drain signal line 61 in accordance with the timing of the sampling pulse output from the drain driver 60, and the data signal (analog video signal or digital video signal) of the data signal line 62 is turned on. ) Is supplied.
[0030]
The liquid crystal display panel 100 is configured by a plurality of display pixels 200 which are selected by a scanning signal from the gate signal line 51 and are supplied with a data signal from the drain signal line 61 arranged in a matrix.
[0031]
Hereinafter, a detailed configuration of the display pixel 200 will be described. In the vicinity of the intersection of the gate signal line 51 and the drain signal line 61, a circuit selection circuit 40 including a P-channel TFT 41 and an N-channel type 42 is provided. Both drains of the TFTs 41 and 42 are connected to the drain signal line 61, and both gates thereof are connected to the circuit selection signal line 88. One of the TFTs 41 and 42 is turned on in response to a circuit selection signal from the circuit selection signal line 88. Further, as will be described later, a circuit selection circuit 43 including a P-channel TFT 44 and an N-channel TFT 45 is provided in a pair with the circuit selection circuit 40.
[0032]
As a result, it is possible to select and switch between an analog display mode (corresponding to a full-color moving image) and a digital display mode (corresponding to low power consumption and still images) described later. In addition, a pixel selection circuit 70 including an N-channel TFT 71 and an N-channel TFT 72 is disposed adjacent to the circuit selection circuit 40. The TFTs 71 and 72 are connected in series with the TFTs 41 and 42 of the circuit selection circuit 40, respectively, and a gate signal line 51 is connected to both gates thereof. The TFTs 71 and 72 are configured such that both are turned on simultaneously in accordance with the scanning signal from the gate signal line 51.
[0033]
In addition, an auxiliary capacitor 85 for holding an analog video signal is provided. One electrode 86 of the auxiliary capacitor 85 is connected to the source 11 s of the TFT 71. The other electrode 87 is connected to a common auxiliary capacitance line 81 and supplied with a bias voltage Vsc. When the gate of the TFT 71 is opened and an analog video signal is applied to the liquid crystal 21, the signal must be held for one field period. However, with only the liquid crystal 21, the voltage of the signal gradually decreases with time. If it does so, it will appear as display unevenness and a good display cannot be obtained. Therefore, an auxiliary capacitor 85 is provided to hold the voltage for one field period.
[0034]
A P-channel TFT 44 of the circuit selection circuit 43 is provided between the auxiliary capacitor 85 and the liquid crystal 21, and is configured to be turned on / off simultaneously with the TFT 41 of the circuit selection circuit 40. A holding circuit 110 and a signal selection circuit 120 are provided between the TFT 72 of the pixel selection circuit 70 and the display electrode 80 of the liquid crystal 21.
[0035]
The holding circuit 110 includes two inverter circuits that are positively fed back, and constitutes a static memory that holds a digital binary value. Here, the inverter circuit is preferably a CMOS type inverter circuit with low static current consumption for low power consumption.
[0036]
The signal selection circuit 120 is a circuit that selects a signal in accordance with a signal from the holding circuit 110, and includes two N-channel TFTs 121 and 122. Since complementary output signals from the holding circuit 110 are applied to the gates of the TFTs 121 and 122, the TFTs 121 and 122 are turned on and off in a complementary manner.
[0037]
Here, when the TFT 122 is turned on, the AC drive signal (signal B) is selected, and when the TFT 121 is turned on, the counter electrode signal VCOM (signal A) is selected, and a voltage is applied to the liquid crystal 21 via the TFT 45 of the circuit selection circuit 43. It is supplied to the display electrode 80 to be applied.
[0038]
In the digital display mode, all dot scans are performed during one vertical period, and digital video data from the drain signal line 61 is written into the holding circuit 110. Here, the power supply voltage VDD supplied to the two inverter circuits constituting the holding circuit 110 is set to a minimum voltage (for example, 3 V) necessary for the holding circuit 110 to hold data during the data writing period. After the data writing period, the voltage is boosted to a higher voltage during the period for displaying based on the data held in the holding circuit 110 (display of a still image).
[0039]
At this time, the power supply voltage VDD is preferably boosted to a voltage higher than a voltage obtained by adding the threshold voltages (Vt) of the TFTs 121 and 122 to the highest voltage of the signals A and B. That is, the relationship VDD> Vt + max (signal A, signal B) is satisfied. As this VDD, about 8V is appropriate. If this relationship is not satisfied, the signals A and B cannot be supplied and charged to the display electrode 80 without lowering the level by the TFTs 121 and 122, and the contrast of the liquid crystal display deteriorates.
[0040]
Next, a peripheral circuit of the liquid crystal panel 100 will be described. A panel driving LSI 91 is provided on an external circuit board 90 different from the insulating substrate 10 of the liquid crystal panel 100. A vertical start signal STV is input to the gate driver 50 from the panel driving LSI 91 of the external circuit board 90, and a horizontal start signal STH is input to the drain driver 60. A video signal is input to the data line 62.
[0041]
The external circuit board 90 is provided with a booster circuit 95 for boosting the power supply voltage VDD supplied to the two inverter circuits constituting the holding circuit 110 described above. The booster circuit 95 starts boosting based on a write period end signal Vend from a timing controller (not shown).
[0042]
A timing controller (not shown) generates this signal Vend based on an external vertical synchronization signal Vsync, but the vertical synchronization signal Vsync itself may be used. The booster circuit 95 can be appropriately selected. For example, a charge pump type circuit can be used.
[0043]
FIG. 2 shows a circuit configuration example of the booster circuit 95. In FIG. 2, reference numeral 160 denotes a ring oscillator (Ring) that starts an oscillation operation in response to a write period end signal Vend.
Oscillator). The oscillation clock of the ring oscillator 160 is applied to one end of the capacitors C1 and C2 through an inverter. Here, the number of stages of the inverters is determined so that the clock PCLK2 applied to the capacitor C1 and the clock PCLK1 applied to the capacitor C2 are in opposite phases.
[0044]
Further, it is assumed that the power supply voltage of the ring oscillator 160 and the inverter is Vdd. Therefore, the amplitudes of the clock PCLK1 and the clock PCLK2 are also Vdd. The other end of the capacitor C1 is coupled to a connection point N1 between the TFT 161 and the TFT 162.
[0045]
The other end of the capacitor C2 is coupled to a connection point N2 between the TFT 163 and the TFT 164. Here, the TFT 161 and the TFT 163 are N-channel type, and a power supply voltage Vdd (for example, 3 V) is supplied to their sources. The TFTs 162 and 164 are P-channel type, and their sources are connected to each other. A boosted voltage VPP is obtained from this common source.
[0046]
In the initial state, an initial setting TFT 165 for setting the voltage at the connection point N1 to the power supply voltage Vdd is provided. Similarly, an initial setting TFT 166 is provided for setting the voltage at the node N2 to the power supply voltage Vdd in the initial state. These TFTs 165 and 166 are both N-channel type, and a power supply voltage Vdd is supplied to their gates and sources.
[0047]
The operation of the booster circuit configured as described above will be described as follows. When the ring oscillator 160 starts an oscillation operation in response to the end signal Vend, the clock PCLK2 is applied to the capacitor C1, and the clock PCLK1 having an opposite phase is applied to the capacitor C2. When the clock PCLK2 is at a high level, the voltage at the node N1 rises due to capacitive coupling. If the capacitance value of the capacitor C1 is sufficiently larger than the capacitance value of the parasitic capacitance associated with the connection point N1, the voltage at the connection point N1 is 2Vdd. For example, if Vdd is 3V, the voltage at the connection point N1 is 6V. At this time, since the TFTs 162 and 163 are turned on, the voltage 6V boosted through the TFT 162 is output as the voltage VPP.
[0048]
Next, when the clock PCLK2 falls to a low level and the clock PCLK1 rises to a high level, the voltage at the node N2 rises due to capacitive coupling. If the capacitance value of the capacitor C2 is sufficiently larger than the capacitance value of the parasitic capacitance associated with the connection point N2, the voltage at the connection point N2 is 2Vdd. For example, if Vdd is 3V, the voltage at the connection point N1 is 6V. Thereby, the TFTs 162 and 163 are turned off, and the TFTs 161 and 164 are turned on. Then, the voltage at the connection point N1 returns to Vdd (3V) again. At the same time, the voltage 6V boosted through the TFT 164 is output as the voltage VPP. By repeating the above operation, the power supply voltage Vdd is boosted and output as the voltage VPP.
[0049]
FIG. 3 is a circuit configuration diagram of a video signal switching circuit. When the switch SW1 is connected to the terminal P2 side, the n-bit digital video signal input from the input terminal Din is converted into an analog video signal by the DA converter 130 and then output to the data line 62.
[0050]
On the other hand, when the switch SW1 is switched to the terminal P1 side, for example, the most significant bit of the n-bit digital video signal is output to the data line 62. The switch SW1 is switched according to a mode switching signal MD for controlling switching between the analog display mode and the low power consumption compatible digital display mode.
[0051]
Next, a method for driving the display device having the above-described configuration will be described with reference to FIGS. FIG. 4 is a timing chart when the liquid crystal display device is selected in the digital display mode.
(1) Analog display mode
When the analog display mode is selected according to the mode switching signal MD, an analog video signal is set to be output to the data signal line 62, and the circuit selection signal line 88 is set to “L”. The TFTs 41 and 44 of 40 and 43 are turned on.
[0052]
Further, the sampling transistor SP is turned on according to the sampling signal based on the horizontal start signal STH, and the analog video signal of the data signal line 62 is supplied to the drain signal line 61.
[0053]
A scanning signal is supplied to the gate signal line 51 based on the vertical start signal STV. When the TFT 71 is turned on according to the scanning signal, the analog video signal Sig is transmitted from the drain signal line 61 to the display electrode 80 and held in the auxiliary capacitor 85. A video signal voltage applied to the display electrode 80 is applied to the liquid crystal 21, and the liquid crystal 21 is aligned according to the voltage, whereby a liquid crystal display can be obtained.
[0054]
This analog display mode is suitable for displaying a full-color moving image. However, the LSI 91 and the drivers 50 and 60 of the external circuit board 90 are constantly consuming power to drive them.
(2) Digital display mode
When the digital display mode is selected in accordance with the mode switching signal MD, the digital video signal is set to be output to the data signal line 62, and the potential of the circuit selection signal line 88 becomes “H” and held. The circuit 110 becomes operable. Further, the TFTs 41 and 44 of the circuit selection circuits 40 and 43 are turned off, and the TFTs 42 and 45 are turned on.
[0055]
Further, start signals STV and STH are input to the gate driver 50 and the drain driver 60 from the panel driving LSI 91 of the external circuit board 90. In response to this, sampling signals are generated in sequence, and the sampling transistors SP1, SP2,.
[0056]
Here, the first row, that is, the gate signal line 51 to which the scanning signal G1 is applied will be described. First, the TFTs of the display pixels P11, P12,... P1n connected to the gate signal line 51 by the scanning signal G1 are turned on for one horizontal scanning period.
[0057]
When attention is paid to the display pixel P11 in the first row and first column, the digital video signal S11 sampled by the sampling signal SP1 is inputted to the drain signal line 61. When the TFT 72 is turned on by the scanning signal G1, the drain signal D1 is written to the holding circuit 110 of the display pixel P11.
[0058]
At the time of writing, the power supply voltage VDD supplied to the two inverter circuits of the holding circuit 110 is set to the minimum voltage (for example, 3 V) necessary for the holding circuit 110 to hold data. As a result, the ON resistance of the N-channel TFT of the inverter INV2 shown in FIG. 1 is increased and the threshold value of the inverter INV1 is lowered. When the “H” level of (= digital video signal S11) is written, the writing margin is improved.
[0059]
That is, since the “H” level voltage of the drain signal D1 (= digital video signal S11) can be lowered, the power supply voltage of the drive circuit such as the drain driver 60 can be lowered. In addition, the size of the TFT 72 constituting the pixel selection circuit 70 can be reduced.
[0060]
The signal held by the holding circuit 110 is input to the signal selection circuit 120, the signal selection circuit 120 selects the signal A or the signal B, the selected signal is applied to the display electrode 80, and the voltage Is applied to the liquid crystal 21. By scanning from the gate signal line 51 to the gate signal line 51 in the last row in this way, writing for one screen (one field period) is completed.
[0061]
Thereafter, display based on the data held in the holding circuit 110 (display of a still image) is performed. Then, the booster circuit 95 operates in response to the end signal Vend of the writing period, and the power supply voltage VDD supplied to the holding circuit 110 is boosted. At this time, the power supply voltage VDD is preferably boosted to a voltage higher than a voltage obtained by adding the threshold voltages (Vt) of the TFTs 121 and 122 to the highest voltage of the signals A and B.
[0062]
Thereby, the signals A and B are supplied to the display electrode 80 by the TFTs 121 and 122 without lowering the level, so that a display with good image quality can be obtained.
[0063]
In the digital display mode, voltage supply to the gate driver 50, the drain driver 60, and the external panel driving LSI 91 is stopped to stop driving them. The holding circuit 110 is always driven by supplying the voltages VDD and VSS, the counter electrode voltage is supplied to the counter electrode 32, and the signals A and B are supplied to the selection circuit 120.
[0064]
That is, VDD and VSS for driving the holding circuit are supplied to the holding circuit 110, the counter electrode voltage VCOM (signal A) is applied to the counter electrode, and the liquid crystal display panel 100 is normally white (NW). For the signal A, a voltage having the same potential as that of the counter electrode 32 is applied to the signal A, and an AC voltage (for example, 60 Hz) for driving the liquid crystal is only applied to the signal B. By doing so, one screen can be held and displayed as a still image. In addition, no voltage is applied to the other gate driver 50, drain driver 60, and external LSI 91.
[0065]
At this time, when “H (high)” is input to the drain signal line 61 as a digital video signal to the holding circuit 110, “L” is input to the first TFT 121 in the signal selection circuit 120. Therefore, the first TFT 121 is turned off, and “H” is input to the other second TFT 122, so that the second TFT 122 is turned on.
[0066]
Then, the signal B is selected and the voltage of the signal B is applied to the liquid crystal. That is, since the alternating voltage of signal B is applied and the liquid crystal rises by an electric field, the display can be observed as a black display on the NW display panel.
[0067]
When “L” is input to the drain signal line 61 as a digital video signal to the holding circuit 110, “H” is input to the first TFT 121 in the signal selection circuit 120. Is turned on, and “L” is input to the other second TFT 122, so that the second TFT 122 is turned off.
[0068]
Then, the signal A is selected and the voltage of the signal A is applied to the liquid crystal. That is, since the same voltage as that of the counter electrode 32 is applied, no electric field is generated and the liquid crystal does not stand up, so that the display can be observed as white display on the NW display panel.
[0069]
In this way, it is possible to display a still image by writing one screen and holding it, but in this case, the driving of each driver 50, 60 and LSI 91 is stopped, so that the power consumption is reduced accordingly. Can do.
[0070]
As described above, according to the embodiment of the present invention, two types of display, that is, full-color moving image display (in the case of the analog display mode) and digital gradation display (in the case of the digital display mode) are performed on one liquid crystal display panel 100. Can correspond to the display. Further, malfunction during writing of the holding circuit 110 can be prevented, and low power consumption and fine pixel layout are possible.
[0071]
In the above-described embodiment, the display device capable of selecting the analog display mode and the digital display mode has been described. However, the present invention includes a circuit 110 that writes and holds a digital video signal, and displays an image according to the held signal. The present invention can be widely applied to display devices.
[0072]
Further, the display device of the present invention is preferably applied to a reflective liquid crystal display device among liquid crystal display devices. The device structure of this reflective liquid crystal display device will be described with reference to FIG.
[0073]
As shown in FIG. 5, on one insulating substrate 10, a gate insulating film 12 is formed on a semiconductor layer 11 made of polycrystalline silicon and formed into an island, and above the semiconductor layer 11, the gate insulating film 12 is formed. A gate electrode 13 is formed thereon.
[0074]
A source 11 s and a drain 11 d are formed in the lower semiconductor layer 11 located on both sides of the gate electrode 13. An interlayer insulating film 14 is deposited on the gate electrode 13 and the gate insulating film 12, and a contact hole 15 is formed at a position corresponding to the drain 11d and a position corresponding to the source 11s. The drain 11 d is connected to the drain electrode 16, and the source 11 s is connected to the display electrode 19 through a contact hole 18 provided in the planarization insulating film 17 provided on the interlayer insulating film 14.
[0075]
Each display electrode 19 formed on the planarization insulating film 17 is made of a reflective material such as aluminum (Al). An alignment film 20 made of polyimide or the like for aligning the liquid crystal 21 is formed on each display electrode 19 and the planarization insulating film 17.
[0076]
On the other insulating substrate 30, a color filter 31 exhibiting each color of red (R), green (G), and blue (B), a counter electrode 32 made of a transparent conductive film such as ITO (Indium Tin Oxide), And the alignment film 33 which orientates the liquid crystal 21 is formed in order. When color display is not used, the color filter 31 is not necessary.
[0077]
The periphery of the pair of insulating substrates 10 and 30 thus formed is adhered with an adhesive sealing material, and the liquid crystal 21 is filled in the gap formed thereby, thereby completing the reflective liquid crystal display device.
[0078]
As indicated by the dotted arrows in the figure, the external light incident from the viewer 1 side enters from the counter electrode substrate 30 in order, is reflected by the display electrode 19, is emitted to the viewer 1 side, and the display is viewed by the viewer 1. Can be observed.
[0079]
Thus, the reflective liquid crystal display device is a method of observing the display by reflecting external light, and unlike the transmissive liquid crystal display device, it is not necessary to use a so-called backlight on the side opposite to the viewer side. Does not require power to turn on its backlight. Therefore, the display device of the present invention is preferably a reflective liquid crystal display device that does not require a backlight and is suitable for power saving.
[0080]
In the above-described embodiment, the case where the counter electrode voltage and the voltages of the signals A and B are applied during the entire dot scan period of one screen has been described, but the present invention is not limited thereto. It is not necessary to apply these voltages during this period.
[0081]
In the above-described embodiment, the case where a 1-bit digital data signal is input in the digital display mode has been described. However, the present invention is not limited thereto, and even in the case of a multi-bit digital data signal. It is possible to apply.
[0082]
By doing so, multi-gradation display can be performed. At that time, it is necessary to set the number of holding circuits and signal selection circuits according to the number of input bits.
[0083]
In the above-described embodiment, the case where a still image is displayed on a part of a liquid crystal display panel has been described. However, the present application is not limited thereto, and a still image can be displayed on all display pixels. Thus, the present invention has a characteristic effect of the present invention.
[0084]
In the above-described embodiment, the case of the reflective liquid crystal display device has been described. However, by disposing a transparent electrode in a region excluding the TFT, the holding circuit, the signal selection circuit, and the signal wiring in one pixel, the transmissive liquid crystal It can also be used for a display device. Also, when used in a transmissive liquid crystal display device, after displaying one screen, the supply of voltage to the gate driver 50, the drain driver 60, and the external panel driving LSI 91 is stopped, thereby consuming the corresponding amount. Electric power can be reduced.
[0085]
Next, a display device according to a second embodiment of the present invention will be described. FIG. 6 shows a circuit configuration diagram when the display device of the present invention is applied to an EL (electroluminescence) display device. A pixel selection TFT 72 is disposed near the intersection of the gate signal line 51 and the drain signal line 61, and the source of the TFT 72 is connected to the holding circuit 110. The holding circuit 110 includes two inverter circuits INV1 and INV2 that are positively fed back.
[0086]
The output of the holding circuit 110 is applied to the gate of the N-channel EL driving TFT 125. The source of the EL driving TFT is connected to the voltage source VA, and the drain is connected to the anode of the organic EL element 22. The cathode 33 of the organic EL element 22 is biased to the common voltage VCOM.
[0087]
Here, the digital video data from the drain signal line 61 is written into the holding circuit 110 in the same manner as in the above-described embodiment. Here, the power supply voltage VDD supplied to the two inverter circuits constituting the holding circuit 110 is set to a minimum voltage (for example, 3 V) necessary for the holding circuit 110 to hold data during the data writing period.
[0088]
Considering the case where “H” is output from the holding circuit 110, a relatively low voltage (eg, 3 V) is applied to the gate of the EL driving TFT 125. Here, it is assumed that the organic EL element 22 is in the off state or the high resistance state and is extinguished by adjusting the threshold value of the EL driving TFT 125.
[0089]
Then, after the end of the data writing period, the power supply voltage VDD is boosted to a high voltage for a period in which display based on the data held in the holding circuit 110 (display of a still image) is performed. Then, the gate voltage of the EL driving TFT 125 also increases. Therefore, when the bias of VF or higher is applied to the anode of the organic EL element 22, the light is turned on.
[0090]
Therefore, according to the EL display device having the above-described configuration, the power supply voltage VDD is set low during the data writing period, so that the power consumption can be reduced as in the above-described embodiment, and after the writing is completed. When the power supply voltage VDD is boosted, the organic EL element is lit and a good light emission display is obtained.
[0091]
【Effect of the invention】
According to the display device of the present invention, in a display device having a holding circuit for holding digital video data in each display pixel, the power supply voltage supplied to the holding circuit is set low during writing and high during display after writing. Accordingly, erroneous writing of data to the holding circuit can be prevented, and power consumption can be reduced.
[0092]
Further, according to the display device of the present invention, the pixel selection element can be made small, so that a fine layout of the pixels can be performed.
[Brief description of the drawings]
FIG. 1 is a circuit configuration diagram of a liquid crystal display device according to a first embodiment of the present invention.
FIG. 2 is a circuit configuration diagram of a booster circuit according to the first embodiment of the present invention.
FIG. 3 is a circuit configuration diagram of a video signal switching circuit according to the first embodiment of the present invention;
FIG. 4 is a timing chart of the liquid crystal display device according to the first embodiment of the present invention.
FIG. 5 is a cross-sectional view of a reflective liquid crystal display device.
FIG. 6 is a circuit configuration diagram of an EL display device according to a second embodiment of the present invention.
FIG. 7 is a circuit configuration diagram of a liquid crystal display device according to a conventional example.
FIG. 8 is another circuit configuration diagram of a liquid crystal display device according to a conventional example.
FIG. 9 is a circuit diagram for explaining a problem of a liquid crystal display device according to a conventional example.
[Explanation of symbols]
10 Insulating substrate
13 Gate electrode
21 liquid crystal
40 Circuit selection circuit
43 Circuit selection circuit
50 Gate driver
51 Gate signal line
60 Drain driver
61 Drain signal line
70 pixel selection circuit
85 Auxiliary capacity
95 Booster circuit
110 holding circuit
120 signal selection circuit

Claims (6)

A plurality of gate signal lines arranged in one direction on the substrate; a plurality of drain signal lines arranged in a direction crossing the gate signal line; and a gate driver for supplying a scanning signal to the plurality of gate signal lines; A display pixel that includes a drain driver that supplies a video signal to the plurality of drain signal lines and a pixel selection element that selects a display pixel by a scanning signal from the gate signal line, and the video signal is supplied from the drain signal line In a display device arranged in a matrix,
A holding circuit that includes first and second inverter circuits that are positively fed back, and in which a digital video signal is written from the drain signal line in response to a scanning signal input from the gate signal line and holds the digital video signal When,
A booster circuit that boosts the power supply voltage supplied to the first and second inverter circuits of the holding circuit after the digital video signal has been written to the holding circuit, and boosts the power supply voltage by the booster circuit. In this state, it is configured to display a still image according to the outputs of the first and second inverter circuits of the holding circuit and to stop the operation of the gate driver and the drain driver. A display device comprising an oscillation circuit that starts an oscillation operation in response to a vertical synchronization signal or a signal created based on the vertical synchronization signal.   A signal selection circuit for selectively supplying a first signal and a second signal to the display electrode in accordance with the output of the holding circuit, wherein the signal selection circuit applies the output of the first inverter circuit to the gate; And a second thin film transistor that is turned on and off complementarily with the first thin film transistor, the power source boosted by the boost circuit. The display device according to claim 1, wherein the voltage is higher than a voltage obtained by adding a threshold voltage of the first and second thin film transistors to the highest voltage of the first and second signals. The display device according to claim 1 or 2, wherein the first and second inverter circuits are CMOS inverter circuits. A plurality of gate signal lines arranged in one direction on the substrate; a plurality of drain signal lines arranged in a direction crossing the gate signal line; and a gate driver for supplying a scanning signal to the plurality of gate signal lines; A drain driver for supplying a video signal to the plurality of drain signal lines, and a pixel selection element for selecting a display pixel by a scanning signal from the gate signal line, and a video signal is supplied from the drain signal line together with a matrix Digital image from the drain signal line in response to a scanning signal input from the gate signal line into the display pixel. A signal is written and the digital video signal is held and supplied to the holding circuit and the first and second inverter circuits of the holding circuit Includes a booster circuit for boosting the power supply voltage, a method of driving a display device which performs display in accordance with the output of said holding circuit,
The booster circuit includes an oscillation circuit that starts an oscillation operation according to a vertical synchronization signal or a signal created based on the vertical synchronization signal, and the drain circuit according to a scanning signal input from the gate signal line After writing the digital video signal from the signal line to the holding circuit, the booster circuit boosts the power supply voltage supplied to the first and second inverter circuits of the holding circuit, and the booster circuit supplies the power supply voltage. Driving a display device, wherein a still image is displayed in accordance with the outputs of the first and second inverter circuits of the holding circuit in a boosted state, and the operations of the gate driver and the drain driver are stopped. Method. A signal selection circuit for selectively supplying a first signal and a second signal to the display electrode in accordance with the output of the holding circuit, wherein the signal selection circuit applies the output of the first inverter circuit to the gate; And a second thin film transistor that is turned on and off complementarily with the first thin film transistor, the power source boosted by the boost circuit. 5. The display device drive according to claim 4, wherein the voltage is higher than a voltage obtained by adding a threshold voltage of the first and second thin film transistors to the highest voltage of the first and second signals. Method.   6. The display device driving method according to claim 4, wherein the first and second inverter circuits are CMOS type inverter circuits.

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