Patent Publication Number: US-7710379-B2

Title: Display device and method thereof

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a display device having light emitting elements, and a driving method thereof. 
   2. Description of the Related Art 
   In recent years, a flat panel display device which is widely used for a display portion of a portable information terminal as well as a medium-size or a large-size display device is shifting in its driving method from a passive matrix method to the main stream of an active matrix method in which writing of video signals to pixels are carried out rapidly, in accordance with the increase in the number of pixels with the higher resolution. 
   According to the active matrix method, there are a dot sequential drive in which pixels are sequentially driven on a dot-by-dot basis and a line sequential drive in which pixels are driven on a line-by-line basis. Circuit configurations of the both are shown in  FIGS. 5A and 5B . 
     FIG. 5A  shows an example of a circuit configuration of an active matrix type display device using a dot sequential drive. Around a pixel portion  501 , a source signal line driver circuit  502  including a shift register  504 , a sampling switch  505  and a level shifting buffer  506 , and a gate signal line driver circuit  503  including a shift register  507  and a level shifting buffer  508  are disposed. 
   The shift register  507  outputs a row selection pulse in accordance with a clock pulse (GCK) and a start pulse (GSP) from the first stage in sequence. The outputted pulse undergoes an amplitude modulation and the like in the level shifting buffer  508 , whereby gate signal lines are selected from the first row in sequence. 
   In the rows where gate signal lines are selected, the shift register  504  outputs a sampling pulse in accordance with a clock signal (SCK) and a start pulse (SSP) from the first stage in sequence. The sampling switch  505  samples a video signal (Video) in accordance with a timing at which the sampling pulse is inputted, and charges or discharges source signal lines. 
   The above operation is performed from the first row to the last row in sequence, thus writing for one frame is completed. A similar operation is repeated thereafter to display images. 
     FIG. 5B  shows an example of a circuit configuration of an active matrix display device using a line sequential drive. Around a pixel portion  511 , a source signal line driver circuit  512  including a shift register  514 , a first latch circuit  515 , a second latch circuit  516  and a level shifting buffer  517 , and a gate signal line driver circuit  513  including a shift register  518  and a level shifting buffer  519  are disposed. 
   The shift register  518  outputs a row selection pulse in accordance with a clock pulse (GCK) and a start pulse (GSP) from the first stage in sequence. The outputted pulse undergoes an amplitude modulation and the like in the level shifting buffer  519 , whereby gate signal lines are selected from the first row in sequence. 
   In the rows where gate signal lines are selected, the shift register  514  outputs a sampling pulse in accordance with a clock signal (SCK) and a start pulse (SSP) from the first stage in sequence. The first latch circuit  515  samples a video signal in accordance with a timing at which the sampling pulse is inputted, and the video signal that is sampled on each stage is held in the first latch circuit  515 . 
   After video signals for one row are sampled and a latch pulse (LAT) is inputted, the video signals held in the first latch circuit  515  are transferred to the second latch circuit  516  all at once, whereby all the source signal lines are charged or discharged at a time. 
   The above operation is performed from the first row to the last row in sequence, thus writing for one frame is completed. A similar operation is repeated thereafter to display images. 
   According to the dot sequential drive shown in  FIG. 5A , circuit configuration is relatively simple, which leads to small-scale driver circuits while it takes a long time to charge and discharge one source signal line. On the other hand, according to the line sequential drive shown in  FIG. 5B , circuit configuration is relatively complex, which leads to large-scale driver circuits. However, as the charge and discharge for all the source signal lines are performed in parallel, writing can be performed with a sufficient time. 
   SUMMARY OF THE INVENTION 
   A source signal line is a load to a buffer due to a plurality of TFTs provided in the pixel portion and parasitic capacitance. In the line sequential drive, all the source lines are charged or discharged at a time when a latch signal (LAT) is inputted, therefore, a large instantaneous current flows through the buffer. When a current supply capacity of a power supply line is not high enough for the instantaneous current, a circuit may malfunction due to a voltage drop of the power supply line itself. In addition, since a large current supply capacity is required for an external circuit as well, it involves quite a large load. 
   In particular, while the display device used for a portable information terminal is required to have a higher resolution in order to enhance image quality, compactness and low consumption are regarded as vital, which makes the above problem unavoidable. That is, a method for extending a width of a wiring in order to give enough capacity to a power supply line, or a method for adopting a high-capacity power supply IC for an external circuit is not a realistic solution to the above problem since it requires a size enlargement of a driver circuit and increase in cost. 
   In view of the foregoing problems, the invention provides a display device and a driving method thereof which can provide enough time for source signal lines to be charged or discharged and a reduction of a load to a power supply line and external circuits, which are the advantages of the line sequential drive. 
   In order to solve the foregoing problems, the invention takes the following measures. 
   As set forth above, in the line sequential drive, source signal lines are charged or discharged all at once after the input of a latch pulse (LAT). Therefore, a large current flows in the early stages of the charge or the discharge period, and the amount of current is decreased with the change in potential of the source signal lines. Thus, the current flow stops at the completion of the charge and discharge. 
   Hereupon, the source signal lines are divided into a plurality of groups. By inputting a latch pulse to each of the groups at different timing, the start timing of charge and discharge for each source signal line becomes different. Accordingly, the number of the source signal lines which start to be charged or discharged at the same time is reduced, which reduces a load to the power supply line. Although the start timing of the charge and discharge is made different in each line, the total amount of current remains the same as a result while the influence such as a voltage drop in the power supply line is mitigated. Thus, all the source signal lines can be charged or discharged normally as a whole. 
   The structure of the invention is described below. 
   According to the invention, a display device and a driving method thereof are provided, which can provide enough time for source signal lines to be charged or discharged and a reduction of a load to a power supply line and external circuits, which are the advantages of the line sequential drive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are diagrams showing an embodiment mode of the invention. 
       FIGS. 2A and 2B  are diagrams showing an embodiment mode of the invention. 
       FIGS. 3A and 3B  are diagrams showing an embodiment mode of the invention. 
       FIGS. 4A and 4B  are diagrams showing the measurement result on an instantaneous current of a display device using a conventional method and the display device of the invention respectively. 
       FIGS. 5A and 5B  are configuration diagrams of a display device using a dot sequential drive and a line sequential drive respectively. 
       FIGS. 6A to 6F  are views showing examples of electronic apparatuses to which the invention is applied. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   Embodiment Mode 1 
     FIG. 1A  is a block diagram of a source signal line driver circuit using a line sequential drive which is used in the display device of the invention. As in the conventional driver circuit using a line sequential drive shown in  FIG. 5B , the source signal line driver circuit in  FIG. 1A  includes a shift register  101 , a first latch circuit  102 , second latch circuits  103  and  104 , and a level shifting buffer  105 . The second latch circuit is divided into a plurality of groups. In  FIG. 1A , it is divided into two groups: the second latch circuit (first group)  104  and the second latch circuit (second group)  105 . 
   The operation thereof is described now with reference to  FIG. 1B . The shift register  101  outputs sampling pulses (SR 1 , SR 2 , SR 3 , . . . and SRn) from the first stage to the last stage in sequence in accordance with a clock pulse (SCK) and a start pulse (SSP). The first latch circuit  102  samples a video signal (Video) from the stage to which the sampling pulse is outputted in sequence. The video signal sampled here is held in the first latch circuit  102  until a latch pulse (LAT) is inputted. 
   In a data sampling period, when video signals from the first stage to the last stage (n-th stage), namely for one row are sampled, a latch pulse is inputted in a fly-back period. At this time, two kinds of latch pulses, LATa and LATb are inputted at different timing. 
   In accordance with the input of the latch pulse (LATa), the video signal is transferred to the second latch circuits (first group)  104 , and source signal lines (first group)  106  start to be charged or discharged. Subsequently, in accordance with the input of the latch pulse (LATb), the video signal is transferred to the second latch circuits (second group)  105 , thus source signal lines (second group)  107  start to be charged or discharged. 
   The above operation is performed from the first row to the last row in sequence, thus writing for one frame is completed. A similar operation is repeated thereafter to display images. Seeing the timing of the charge or discharge of the source signal lines (first group)  106  and the source signal lines (second group)  107 , the rising edge of each potential differs from each other according to the input timing of the latch pulse (LATa or LATb). Accordingly, an instantaneous current generated due to the charge or discharge of the source signal lines can be suppressed to around a half of the conventional one. 
   When the input timing of the latch pulses is different, time required to charge or discharge all the source lines becomes long in some measure. However, in the line sequential drive, the source signal lines may be charged or discharged between the period in which the latch pulses (LATa and LATb) are inputted once and the period in which the next latch pulses (LATa and LATb) are inputted, which will set off the above problem. 
   In this embodiment mode, the charge and discharge of the source signal lines are performed by dividing the source signal lines into two groups, however, they may be divided into three, four, or more groups. For example, in a display device capable of displaying a color image, the charge and discharge timing of source signal lines can be performed by dividing the source signal lines into R, G and B groups. 
   Embodiment Mode 2 
     FIG. 2A  is a block diagram of a source signal line driver circuit using a line sequential drive which is used in the display device of the invention, which has a different configuration from that in Embodiment Mode 1. As a primary configuration, the source signal line driver circuit includes a shift register  201 , a first latch circuit  202 , a second latch circuit  203 , and a level shifting buffer  204  as in the conventional circuit and Embodiment Mode 1. In this embodiment mode, the second latch circuit  203  is divided into three groups of R, G and B. A latch pulse for controlling the operation of the second latch circuit  203  and controlling the charge and discharge timing of the source signal lines is generated internally in dummy stages shown by a dotted frame  205  in  FIG. 2A  by using a clock signal (SCK) and a start pulse (SSP). 
   The operation thereof is described now with reference to  FIG. 2B . In accordance with the clock signal (SCK) and the start pulse (SSP), the shift register  201  outputs sampling pulses (SR 1 , SR 2 , SR 3 , . . . and SRn) from the first stage to the last stage (n-th stage) in sequence. In  FIG. 2A , the first stage to the fourth stage of the shift register  201  are the dummy stages. Thus, the sampling pulses which are actually used for sampling video signals correspond to the outputs of the fifth stage to the last stage of the shift register  201 . 
   In a data latch sampling period, the first latch circuit  202  samples and stores a video signal from the first stage in sequence. After the completion of the sampling of the video signal on the last stage, the shift register  201  starts to output a sampling pulse once again in accordance with the clock signal (SCK) and the start pulse (SSP). Here, among the sampling pulses outputted from the dummy stages, those from the first stage to the third stage are used as latch pulses so as to drive the second latch circuit  203 . 
   In the second latch circuit  203 , when the latch pulse using the sampling pulse (SR 1 ) from the first stage is inputted, source signal lines which belong to the R group start to be charged or discharged. Then, when the latch pulse using the sampling pulse (SR 2 ) from the second stage is inputted, source signal lines which belong to the G group start to be charged or discharged. Further, when the latch pulse using the sampling pulse (SR 3 ) from the third stage is inputted, source signal lines which belong to the B group start to be charged or discharged. 
   The subsequent operation from the sampling of the video signals to the charge and discharge of the source signal lines are performed to the last row in sequence, thus writing for one frame is completed. A similar operation is repeated thereafter to display images. 
   According to the configuration of this embodiment mode, the latch pulse is not required to be inputted externally, and operation from the sampling of the video signals to the charge and discharge of the source signal lines are performed automatically in synchronism with the operation of the shift register, which contributes to the reduction in the number of input pins to a panel. Such reduction in the number of input pins is quite effective for reducing a panel size of a display device used for a portable information terminal, in particular. 
   Here, the dummy stages are provided in the forward ends of the shift register and sampling pulses from the first stage to the third stages are utilized as a means for generating a latch pulse internally. However, as shown in  FIG. 3A , the dummy stages may be provided in the tail ends of the shift register and a sampling pulse around the last stage may be used as a latch pulse as well. In this case, the first stage to the n-th stages are used for sampling video signals, and the (n+1)-th stage to the (n+4)-th stage are used as dummy stages. Sampling pulses from the (n+2)-th stage to the (n+4)-th stage are utilized as latch pulses for controlling the charge and discharge timing of the source signal lines for R, G and B. According to the invention, a means for generating a latch pulse internally is not limited to the above. 
   Embodiment 1 
   The invention is applied to a display device fabricated for the use of a portable information terminal, in which organic electroluminescence (EL) elements are arranged in a light emitting portion, whereby a current consumption thereof is compared to that of a display device using a conventional method. The result is shown in  FIGS. 4A and 4B . 
   A display device used for the experiment has a pixel density of 240×3 (RGB) columns×320 rows, and source signal lines thereof are charged or discharged using a line sequential drive. According to the conventional method, 720 source signal lines are charged or discharged at the same time. On the other hand, according to the display device to which the invention is applied, 240 source signal lines are charged or discharged at the same time. 
     FIG. 4A  shows a screen of an oscilloscope which shows a potential change of each of a latch pulse inputted to the panel, and positive and negative power supplies connected to the last buffer portion which charges or discharges source signal lines. In  FIG. 4A , reference numeral  401  denotes a potential change of a latch pulse,  402  denotes a potential change of a negative power supply, and  403  denotes a potential change of a positive power supply. The potential change of a power supply line is measured by connecting a resistor having a resistance of 100   to the power supply line in series, and measuring the potential change in that portion. According to an input of a latch pulse, source signal lines are charged or discharged. Here, the experiment is carried out by alternately writing a High level signal to all the source signal lines as video signals (charge) and writing a Low signal to all the source signal lines (discharge) per line period. It can be seen that the potentials at the negative power supply and the positive power supply change alternately at substantially the same timing as the input of a latch pulse. 
   According to the waveform  402  in  FIG. 4A , a largest instantaneous voltage drop in the resistor portion which is connected to the negative power supply (the potential draws closer to 0 V due to the voltage drop, namely it is on the rise as it is a negative power supply) is 3.6 V. That is, the largest instantaneous current is 3.6 V/100  =36 mA. 
   Similarly, according to the waveform  403  in  FIG. 4A , the largest instantaneous voltage drop in the resistor portion which is connected to the positive power supply is 2.8 V. That is, the largest instantaneous current is 2.8 V/100  =28 mA. 
     FIG. 4B  shows a similar screen of an oscilloscope in the case of applying the invention. The display device of this embodiment mode has the configuration shown in Embodiment Mode 2 ( FIG. 3 ). Reference numerals  404 ,  405  and  406  each denotes a potential change of a latch pulse for controlling the timing at which source signal lines for R, G and B are charged or discharged,  407  denotes a potential change of the negative power supply, and  408  denotes a potential change of the positive power supply. The conventional measuring method is employed here as the above. 
   According to the waveform  407  in  FIG. 4B , the largest instantaneous voltage drop in the resistor portion which is connected to the negative power supply is 2.0 V. That is, the instantaneous largest current is 2.0 V/100  =20 mA. 
   Similarly, according to the waveform  408  in  FIG. 4B , a largest instantaneous voltage drop in the resistor portion which is connected to the positive power supply is 2.4 V. That is, the largest instantaneous current is 2.4 V/100  =24 mA. 
   When comparing the instantaneous largest current between the conventional method and the case of applying the invention, the largest instantaneous current at the negative power supply is decreased by 44% while the largest instantaneous current at the positive power supply is decreased by 29%, which proves the advantageous effect of the invention. The instantaneous current is ideally in proportion to the divided number of source lines to be charged or discharged. According to the timing of this embodiment mode, an input timing of each latch pulse is close to each other: at the moment the source signal lines for G start to be charged or discharged, the source lines for R are not yet charged or discharged completely, and at the moment at which the source signal lines for B start to be charged or discharged, the source signal lines for G are not yet charged or discharged completely. Thus, the number of the source signal lines to be charged or discharged is large in the overlapped period. The instantaneous current is preferably small. Therefore, it is preferable that the charge and discharge timing of each source signal line is set so as to be as far from each other as possible. 
   Embodiment 2 
   Electronic apparatuses using a display device having a pixel region in which light emitting elements are arranged include a television set (TV, TV receiver), a digital camera, a digital video camera, a mobile telephone set (mobile phone), a portable information terminal such as a PDA, a portable game machine, a monitor, a computer, a sound reproducing device such as a car audio set, an image reproducing device provided with a recording medium, such as a home game machine, and the like. Specific examples of these electronic apparatuses are described with reference to  FIGS. 6A to 6F . 
     FIG. 6A  shows a portable phone using the display device of the invention, which includes a main body  9201 , a display portion  9202 , and the like. According to the invention, charge and discharge time of source signal lines and a load to an external circuit can be reduced. 
     FIG. 6B  shows a digital video camera using the display device of the invention, which includes display portions  9701  and  9702 , and the like. According to the invention, charge and discharge time of source signal lines and a load to an external circuit can be reduced. 
     FIG. 6C  shows a portable terminal using the display device of the invention, which includes a main body  9101 , a display portion  9102 , and the like. According to the invention, charge and discharge time of source signal lines and a load to an external circuit can be reduced. 
     FIG. 6D  shows a portable television set using the display device of the invention, which includes a main body  9301 , a display portion  9302 , and the like. According to the invention, charge and discharge time of source signal lines and a load to an external circuit can be reduced. 
     FIG. 6E  shows a portable personal computer using the display device of the invention, which includes a main body  9401 , a display portion  9402 , and the like. According to the invention, charge and discharge time of source signal lines and a load to an external circuit can be reduced. 
     FIG. 6F  shows a television set using the display device of the invention, which includes a main body  9501 , a display portion  9502 , and the like. According to the invention, charge and discharge time of source signal lines and a load to an external circuit can be reduced.