Abstract:
An electrophoretic display and a related driving method are provided, the electrophoretic display and related driving method for causing voltage level switching of a common signal of the electrophoretic display, which induces colored electrophoretic particles to be arranged in a more compact way during a power-off period, thereby improving the quality of a standby image of the electrophoretic display.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates generally to electrophoretic display technology, and more particularly to an electrophoretic display and a related driving method that improves the quality of a standby image during a power-off period by switching the voltage level of a common signal. 
         [0003]    2. Description of the Related Art 
         [0004]    Electrophoretic display technology is a major display technology used by electronic reading devices. The thickness of an electrophoretic display is very close to the thickness of paper, and has the additional advantages of low power consumption, high contrast, wide viewing angle and extreme elasticity. Electrophoretic display technology uses voltages to control charged pigment particles spread in a liquid dielectric material. The charged pigment particles will move within the liquid dielectric material, due to these driving voltages and, depending on the movement of the charged pigment particles, pixels will become lighter or darker, thereby achieving different visual effects. 
         [0005]    Please refer to  FIG. 1 , which illustrates a constructional drawing of an electrophoretic display. A displaying area of the electrophoretic display  100  consists of a plurality of pixels  5 , each of which includes an electrophoretic element  10  consisting of dielectric material  11  and charged pigment particles P. A transparent common electrode  12  is disposed above the electrophoretic elements  10  and an adhesive layer  13  is disposed below the electrophoretic elements  10 . A data electrode  14  is disposed below each electrophoretic element  10 . The common electrode  12  is employed for applying a common signal V COM  generated by the common signal generation device  16  to the electrophoretic element  10 . The data electrode  14  is employed for applying a data signal V DATA  generated by the data signal generation device  18  to the electrophoretic element  10 . A voltage potential difference between the common electrode  12  and the data electrode  14  will form an electric field of a specific direction surrounding the electrophoretic element  10  which causes the charged pigment particles P in the electrophoretic element  10  to move. This allows images displayed on the electrophoretic display  100  to change. 
         [0006]    During a power-off period of the electrophoretic display  100 , a default standby image will be shown (e.g. a white image or an image including a trademark). It is required to drive the electrophoretic element  10  during a period prior to the power-off period such that the arrangement of the charged pigment particles P visually emerges as the standby image. When the internal power supply of the electrophoretic display  100  is removed, the common electrode  12  and the data electrode  14  both enter a high impedance state (hi-Z state) to maintain the arrangement of the charged pigment particles P. As there is no voltage potential difference between the common electrode  12  and the data electrode  14  at this moment, the electric field surrounding the electrophoretic element  10  disappears. The arrangement of the charged pigment particles P will therefore be easily affected or destructed by gravity (as shown in  FIG. 2 ). 
       SUMMARY 
       [0007]    With this in mind, it is one objective of the present invention to provide an electrophoretic display and a driving method. The concept of the present invention is to switch voltage levels of the common signal to make a compact arrangement of charged pigment particles instead of switching voltage levels of the data signal. 
         [0008]    According to one exemplary embodiment of the present invention, an electrophoretic display is provided. The electrophoretic display comprises: a data electrode, a common electrode, an electrophoretic element, a data signal generation device and a common signal generation device. The electrophoretic element is disposed between the data electrode and the common electrode. The data signal generation device is coupled to the data electrode, and employed for outputting a data signal to the data electrode. The common signal generation device is coupled to the common electrode, and employed for outputting a common signal to the common electrode, wherein the common signal has a plurality of voltage levels. The controller is respectively coupled to the data signal generation device and the common signal generation device. During a specific period, the controller controls the data signal generation device to maintain the data signal at a specific voltage level, and controls the common signal generation device to make the common signal alternately switch among a plurality of first specific voltage levels of the voltage levels. 
         [0009]    According to another exemplary embodiment of the present invention, a driving method of driving an electrophoretic display is provided. The electrophoretic display includes an electrophoretic element. The electrophoretic element is disposed between a data electrode and a common electrode. The method comprises: providing a data signal to the data electrode; providing a common signal to the common electrode, wherein the common signal has a plurality of voltage levels; and during a specific period, controlling the data signal to maintain at a specific voltage level, and controlling the common signal to alternately switch among first specific voltage levels of the voltage levels. 
         [0010]    The inventive driving method and display can reduce potential risks of damaging circuits of the display due to voltage level switching on the data electrode. The present invention also reduces the power consumption of signal generation circuits of the display. This is because the common electrodes required in the electrophoretic display are fewer than the data electrodes, so utilizing the common electrode to perform voltage switching will lead to reduced power consumption and reduced circuit complexity. 
         [0011]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a constructional diagram of an electrophoretic display in the conventional art. 
           [0013]      FIG. 2  illustrates the changing arrangement of charged pigment particles of an electrophoretic display. 
           [0014]      FIG. 3  is a constructional diagram of an electrophoretic display according to one exemplary embodiment of the present invention. 
           [0015]      FIG. 4  illustrates waveforms of a common signal and a data signal according to one exemplary embodiment of the present invention. 
           [0016]      FIG. 5  illustrates waveforms of the common signal and the data signal according to another exemplary embodiment of the present invention. 
           [0017]      FIGS. 6 and 7  illustrate waveforms of the common signal and the data signal according to other exemplary embodiments of the present invention. 
           [0018]      FIG. 8  is a circuit diagram of a common signal generation device according to one exemplary embodiment of the present invention. 
           [0019]      FIG. 9  is a flow chart of a driving method according to one exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Certain terms are used throughout the following descriptions and claims to refer to particular system components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not differ in functionality. In the following discussion and in the claims, the terms “include”, “including”, “comprise”, and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ” The terms “couple” and “coupled” are intended to mean either an indirect or a direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
         [0021]    In the specification, the invention will be described with reference to specific exemplary embodiments thereof; however, it will be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the following claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. 
         [0022]    With reference to  FIG. 3 , a constructional diagram of an inventive electrophoretic display is schematically according to one exemplary embodiment. Please note that only part of the structure of the electrophoretic display is illustrated. The display area of the electrophoretic display  200  includes a plurality of pixels  5 ′, and each pixel  5 ′ has an electrophoretic element  20 , wherein the electrophoretic element  20  comprises at least dielectric material  21  and charged pigment particles P′  20 . Please note that, although the charged pigment particles P′ are represented by white positively charged particles, in various embodiments of the present invention, the charged pigment particles P′ may comprise particles having different colors or be oppositely charged (e.g. black negatively charged particles). Furthermore, although only structures and components related to the spirit of the invention are mentioned and explained in the specification, this should not be considered as limitations of the invention. The electrophoretic element  20  may comprise other components. 
         [0023]    A transparent common electrode  22  is disposed above the upper part of the electrophoretic element  20  and an adhesive layer  23  is disposed below the electrophoretic element  20 . Below the adhesive layer  23 , a data electrode  24  is disposed at each electrophoretic element  20 . The common electrode  22  is employed for applying a common signal V COM  that is generated by a common signal generation device  26  to the electrophoretic element  20 . The data electrode  24  is employed for applying the data signal V DATA  that is generated by a data signal generation device  28  to the electrophoretic element  20 . Please note that the process of applying the data signal V DATA  also involves scan-line driving technology and related circuits in order to correctly control the timing when the pixel  5 ′ is driven. As scan-line driving technology is well-known to those of ordinary skill in the art, detailed descriptions are omitted here for the sake of brevity. 
         [0024]    A voltage potential difference between the common electrode  22  and the data electrode  24  can cause an electric field having a specific direction to be formed surrounding the electrophoretic element  20 , thereby allowing the charged pigment particles P′ to move, for different visual effects. The controller  30  is respectively coupled to the data signal generation device  28  and the common signal generation device  26 . During a specific period Period_X, the controller  30  controls the data signal generation device  28  to maintain the data signal V DATA  at a specific voltage level, and simultaneously controls the common signal generation device  26 , to make the common signal V COM  alternate between a plurality of voltage levels VL_ 1 ˜VL_M of a plurality of voltage levels VL_ 1 ˜VL_N, wherein N is greater than or equal to M. The switching of the voltage levels of the common signal V COM  and relationship between the voltage levels of the common signal V COM  and the data signal V DATA  are explained in detail as below. 
         [0025]    The electrophoretic display may be driven in an alternate current (AC) manner or a direct current (DC) manner. Depending on the driving types of the electrophoretic display, the switching of the voltage levels of the common signal V COM  and the data signal V DATA  will also be different. The following paragraphs will respectively illustrate switching of the voltage levels for different driving types. 
         [0026]    Please refer to  FIG. 4 , which illustrates waveforms of the common signal V COM  and the data signal V DATA  in accordance with one exemplary embodiment of the invention. This embodiment is related to the AC driving type. As shown, when the display  200  is operated during a normal display period Period_D, in order to generate an image having specific grey levels (e.g. a standby image), the common signal V COM  will be switched between a higher voltage level H A1  and a lower voltage level L A1 , and the data signal V DATA  will be switched between a higher voltage level H B1  and a lower voltage level L B1 , such that an image including specific grey levels will be shown on the display  200 . When a power-off instruction is acknowledged, the electrophoretic display  200  will enter the specific period Period_X. At the same time, the controller  30  controls the data signal generation device  28  to maintain the data signal V DATA  at a voltage level (e.g. 0V), and also controls the common signal generation device  26  to make the common signal V COM  frequently switch between a higher voltage level H A2  and a lower voltage level L A2 . Afterwards, when the specific period Period_X ends, the electrophoretic display  200  will actually enter the power-off period. The common electrode  22  and the data electrode  24  will be controlled by the common signal generation device  26  and the data signal generation device  28 , respectively, to enter the hi-Z state. During the power-off period, the common signal generation device  26  and the data signal generation device  28  will not provide voltage to the electrophoretic element  20 . As a consequence, the image having the specific grey levels generated during the normal display period Period_D will last for the power-off period. Furthermore, because the switching of the voltage levels that is performed during the specific period Period_X causes the charged pigment particles P′ to be arranged more compactly, the arrangement of the charged pigment particles P′ has better persistence, guaranteeing the quality of the standby image. 
         [0027]    One advantage of this embodiment is that the switching of the common signal V COM  is accomplished by a higher voltage level H A2  and a lower voltage level L A2  that are both smaller than the voltage levels used during the normal display period Period_D. As the power consumption is related to the voltage levels, compared to the switching of data signal V DATA  in the conventional art (i.e. the switching is performed between voltage levels that are identical to the voltage levels used in the normal display period Period_D), the present invention significantly reduces the power consumption. In addition, the common electrode  22  is generally a single electrode with a large area that provides the common voltage to many electrophoretic elements  20  of the electrophoretic display  200  simultaneously, meaning this embodiment, under certain circumstances, can use only one common signal generation device  26 . Since each electrophoretic element  20  has a respective data electrode  24 , the electrophoretic display  200  also needs to include many data signal generation devices  28  if each data signal generation device  28  is designed to provide the voltage levels for switching. In doing so, both the circuit complexity and the power consumption will be increased. 
         [0028]    Please continue to refer to  FIG. 5 , which illustrates waveforms of the common signal V COM  and the data signal V DATA  according to one exemplary embodiment, which is related to a DC driving type. As shown, when the display  200  operates during the normal display period Period_D, the common signal V COM  is maintained at a specific voltage level while the data signal V DATA  switches between a higher voltage level H D1  and a voltage level L D1 . As there is a voltage potential difference between the common signal V COM  and the data signal V DATA , colors of different grey levels can be formed by the electric field. When the power-off instruction is acknowledged, the display  200  enters the specific period Period_X. At the same time, the controller  300  controls the data signal generation device  28  to maintain the data signal V DATA  at a fixed voltage level (e.g. 0V) and simultaneously controls the common signal generation device  26 , to make the common signal V COM  rapidly and frequently switch between a higher voltage level H C2  and a lower voltage level L C2 . Afterwards, when the specific period Period_X ends, the display  200  will enter the power-off period. At this time, the common electrode  22  and the data electrode  24  are both under the control of the common signal generation device  26  and the data signal generation device  28  when entering the hi-Z state. In this period, the common electrode  22  and the data electrode  24  will not provide any voltage to the electrophoretic element  20 . Since the switching of the voltage performed during the specific period Period_X causes the charged pigment particles P′ to be arranged more compactly, the arrangement of the charged pigment particles P′ will have better persistence during the power-off period, which guarantees the quality of the standby image. 
         [0029]    In addition to the driving types mentioned above, there are other driving types for the common signal V COM  and the data signal V DATA  according to other embodiments of the present invention. Please refer to  FIG. 6  and  FIG. 7 . The two driving types illustrated in the top half of  FIG. 6  are both intended to achieve the switching of the common signal V COM  for assuring the image quality. The difference between these two is DC balance. The first driving type does not reach DC balance while the second driving type does. In other words, for the first driving type, during the specific period Period_X, the higher voltage level HE 2  and the lower voltage level L E2  may have only one polarity (both have the same polarity or one voltage level is zero), or have opposite polarities with different respective absolute values. For the second driving type, the higher voltage level H F2  and the lower voltage level L F2  have two different polarities (one being positive and the other being negative), and the absolute values of the voltage levels are the same. 
         [0030]    Additionally, driving types illustrated in the bottom half of  FIG. 6  can eliminate the DC offset generated during the normal display period Period_D. Taking the third driving type illustrated in  FIG. 6  as an example, if during the normal display period Period_D, an electric field of a fixed direction is constantly applied to the electrophoretic element  20  for a long time, it will cause the characteristics of electrophoretic element  20  to be changed or even deteriorated. In order to avoid these influences, the common electrode V COM provides a bias voltage in an opposite direction (e.g. a higher voltage level H G2 ) for a certain period, to cancel the effect of the electric field. After the certain period ends, the common signal V COM  switches to the lower voltage level L G2 . In this embodiment, the higher voltage level H G2  and the lower voltage levelL G2  have only one polarity, or the higher voltage level H G2  and the lower voltage level L G2  have two different polarities but different absolute values: the common signal V COM  does not reach DC balance. The fourth driving type does reach DC balance, and the higher voltage level H H2  and the lower voltage level L H2  have two respective different polarities and have the same absolute values.  FIG. 7  illustrates the relationship between waveforms of the common signal V COM  and the data signal V DATA  in accordance with various embodiments of the present invention. These embodiments can be in conjunction with either the AC driving type or the DC driving type. As illustrated, the higher voltage level H I2  and the voltage level L I2 , the higher voltage level H K2  and the voltage level L K2  do not reach DC balance. The higher voltage level H J2  and the lower voltage level L J2 , the higher voltage level H L2  and the voltage level L L2  do reach DC balance. 
         [0031]    A possible implementation of the inventive common signal generation device  26  is illustrated in  FIG. 8 . As can be seen from the top half of  FIG. 8 , a plurality of voltage sources  262 _ 1 ˜ 262 _n are employed for providing different voltage levels and a hi-Z component  263  (for allowing the common electrode  22  to enter the hi-Z state during the power-off period). The output selecting device  264  are employed for selecting one of the voltage sources  262 _ 1 ˜ 262 _n to provide the common signal V COM . The output selecting device  264  can be implemented with a selector, and used to determine the common signal V COM  according to the control signal of the controller  30  during different periods. As can be seen from the bottom half of  FIG. 8 , only two voltage sources  262 ′_ 1 ˜ 262 ′_ 2 , a hi-Z component  263 , and a voltage divider  265  are employed. With the voltage divider  265  (e.g. resistor ladder) dividing the voltage, the combination effect is equivalent to several different voltage sources. The output selecting device  264  accordingly determines the common signal V COM . It should be noted that the actual implementation of the common signal generation device  26  is not restricted in scope to the implementation illustrated in  FIG. 8 . In fact, any signal generation device that is capable of providing a plurality of different voltage levels and selectively outputting one of the voltage levels can be used for implementing the common signal generation device  26 . 
         [0032]    Regarding the inventive driving method, please refer to a flow chart illustrated in  FIG. 9 , which includes the following steps: 
         [0033]    Step  310 : providing a data signal V DATA  to the data electrode  24 ; 
         [0034]    Step  320 : providing a common signal V COM  to the common electrode  22 , wherein the common signal V COM  has a plurality of voltage levels VL_ 1 -VL_N; and 
         [0035]    Step  330 : during a specific period Period_X, controlling the data signal V DATA  to be maintained at a specific voltage level and controlling the common signalV COM  to alternately switch among a plurality of first specific voltage levels VL 1 _ 1  -VL 1 _M of the voltage levels VL_ 1 -VL_N. 
         [0036]    The specific period Period_X follows the normal display period Period_D. In addition, the inventive driving method further comprises: during a normal display period Period_D, controlling the common signal V COM  to alternately switch among a plurality of voltage levels VL 2 _ 1 ˜VL 2 _O of the voltage levels VL_ 1 ˜VL_N. At least one of the first specific voltage levels VL 1 _ 1 ˜VL 1 _M is different from the second specific voltage levels VL 2 _ 1 ˜VL 2 _O. Furthermore, the first specific voltage levels VL 1 _ 1 ˜VL 1 _M have at least one polarity (depending on whether DC balance is reached; if not, the first specific voltage levels may only have one polarity). The present invention uses different ways of switching the voltage levels of the common signal V COM  to obtain the stable standby image and to cancel the DC offset concurrently. In a preferred embodiment, the specific period Period_X is prior to a power-off period. During the power-off period, the inventive driving method allows the data electrode  22  and common electrode  24  to enter the hi-Z state. 
         [0037]    Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an implementation. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. Thus, although embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that claimed subject matter may not be limited to the specific features or acts described. Rather, the specific features and acts are disclosed as sample forms of implementing the claimed subject matter. For example, the first driving method illustrated in  FIG. 6  can be combined with the third driving method therein. As such, during the specific period Period_X, the common signal V COM  will be switched rapidly and frequently. At the same time, it also serves as a bias voltage for cancelled DC offset. In short, any combination of the driving methods illustrated in  FIG. 6  and/or  FIG. 7  may be in various embodiments of the present invention. 
         [0038]    The electrophoretic display and driving method of the present invention can be widely used in any types of displaying electronic devices, especially in electrical reading devices. Therefore, any electronic device which adopts the inventive electrophoretic display and/or the inventive driving method should fall within the scope the present invention. 
         [0039]    In summary, the concept of the present invention is to switch the voltage level of the common signal that is applied to the common electrode. Such changing of the voltage level can cause the charged pigment particles to be arranged more compactly without affecting the standby image previously generated. Also, it is possible for the present invention to provide a stable bias voltage to cancel the DC offset generated during the previous display period. Hence, the standby image can be more stable during the power-off period. In addition, the switching of the common signal can avoid damage to the circuits caused by the switching of the data signal in the conventional manner. 
         [0040]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.