Abstract:
A single-segment drive scheme for an electronic paper display (EPD) is replaced by a multiplexed drive scheme that reduces the number of driver pins to the number of display segments per digit or alphanumeric character plus one input/output (I/O) line per digit or alphanumeric character. In accordance with the invention, a passive digit selection mechanism enables a multiplex display drive scheme when the EPD material used typically has a stable threshold combined with a small hysteresis. Typically, display operation is better the smaller the hysteresis and the more stable the threshold of the EPD material that is used.

Description:
BACKGROUND 
       [0001]    Typical implementations of alphanumeric electronic paper displays (EPD) are based on a single segment drive scheme, where every display segment is driven by one input/output (I/O) line. More complex displays require a large number of I/O lines, resulting in high-pin-count driver ICs. Typical high pin-count display driver devices are not typically suitable for low-cost implementations. Typically, costs are driven by expensive display drivers that require a large silicon area due to the large number of I/O lines; expensive flex-substrate due to the low-pitch of display drivers; a die-attach process requiring expensive pick and place equipment because high positioning accuracy is required due to the low-pitch display drivers; and reliability losses caused by the large number of I/O lines combined with the low-pitch, especially under mechanical stress. Alphanumeric electronic paper displays may be used, for example, in embedded smart cards, intelligent labels with display functionality, security tokens, bar code displays and electronic documents. 
       BRIEF SUMMARY OF INVENTION 
       [0002]    The single-segment drive scheme for EPD is replaced by a multiplexed drive scheme that reduces the number of driver pins to the number of display segments per digit or alphanumeric character plus one input/output (I/O) line per digit or alphanumeric character. In accordance with the invention, a passive digit selection mechanism enables a multiplex display drive scheme when the EPD material used typically has a stable threshold combined with a small hysteresis. Typically, display operation is better the smaller the hysteresis and the more stable the threshold of the EPD material that is used. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1   a  shows an embodiment in accordance with the invention. 
           [0004]      FIG. 1   b  shows an embodiment in accordance with the invention. 
           [0005]      FIG. 2   a  shows the non-linear flipping behavior of electronic paper display film. 
           [0006]      FIG. 2   b  shows applied voltage and displacement as a function of time for an embodiment in accordance with the invention. 
           [0007]      FIG. 2   c  shows the steps for calibration of the electronic paper display film in accordance with the invention. 
           [0008]      FIG. 2   d  shows the reflectivity transition from the light state (white) to the dark state (black) and the reflectivity transition from the dark state (black) to the light state (white) as a function of time for an exemplary electronic paper display film in accordance with the invention. 
           [0009]      FIG. 3  shows a cross-section of an electronic paper display in accordance with an embodiment of the invention. 
           [0010]      FIG. 4  shows the relevant capacitances and parasitic capacitances of an electronic paper display in accordance with an embodiment of the invention. 
           [0011]      FIG. 5  shows a three digit-seven segment display in an embodiment in accordance with the invention. 
           [0012]      FIG. 6  shows a typical exemplary display drive sequence for the implementation of an active black on white display for an embodiment in accordance with the invention 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Typically, the microcapsules of an electronic paper display (EPD) contain colored and pre-charged particles within a carrier fluid. These pigment particles move within the fluid under the influence of an electric field. The velocity of the particles within the carrier fluid is given by: 
         [0000]        v=μ·E   (1)
 
         [0000]    where v is the velocity of the particles in the carrier fluid, μ is the electrophoretic mobility and E is the applied electric field. For example, black and white particles typically have opposite electrophoretic mobility. Assuming that the electric field is proportional to the applied voltage across the cell gap and the electrophoretic mobilities of the black and white particles are constant: 
         [0000]        s   part   =∫vdt   (2)
 
         [0014]    where s part  is the particle displacement of particle  163  and dt is the change in time. Combining Eq. (1) and Eq. (2), gives: 
         [0000]        s   part   =μ·∫Edt   (3)
 
         [0015]    With reference to  FIG. 1   a:    
         [0000]        E=u   epd   /s   (4)
 
         [0000]    where u epd  is the voltage difference between bottom electrode  160  and top electrode  162  which is typically ITO, and s (cell thickness) is the distance between bottom electrode  160  and top electrode  162 . Defining: 
         [0000]        k=μ/s   (5)
 
         [0000]    where k is a constant dependent on the electrophoretic mobility μ and the cell thickness s allows the particle displacement to be written as: 
         [0000]        s   part   =k·∫u   epd ( t ) dt   (6)
 
         [0016]    The particle displacement s part  is a function of the time integral of the applied voltage u epd  (t). In the case of two particle colors, white and gray, a particle displacement s part  greater than zero and less than s results in an intermediate gray level. Introducing an initial displacement s part     —     ini  for particle  163  as an integration constant yields: 
         [0000]        s   part     —     tot   =s   part     —     int   +k·∫μ   epd   d ( t ) dt   (7)
 
         [0017]    The total particle displacement s part     —     tot  is a function of the time integral of the applied voltage u epd  (t) and the initial displacement s part     —     ini  which means that the gray level being represented by the initial displacement s part     —     ini  can be changed by further applying a voltage u epd  (t) to particle  163 . 
         [0018]    In the typical operation of an EPD display, a voltage is applied to an EPD segment in order to change its color or produce a flip. Once the desired color change or flipping has been accomplished, the applied voltage is removed and the color state is maintained without further power consumption. 
         [0019]    In accordance with an embodiment of the invention, capacitive voltage divider  110  is made up of two intrinsic capacitors  121  and  131 . Capacitor  121  is formed by electrode  120  and floating collector  140 . Capacitor  131  is formed by electrode  130  and floating collector  140  as shown in  FIG. 1   b . Capacitive divider  110  has two voltage inputs  110  and  112 . Electronic paper display (EPD) film  170  is directly driven by the resulting voltage on floating collector  140 . Capacitive voltage divider  110  achieves a selective reduction of the E field that controls the nonlinear EPD color change or flipping behavior. Note that capacitive voltage divider  110  can be typically implemented by silver printing or by copper in a standard board process. 
         [0020]      FIG. 2   a  shows the non-linear flipping behavior of EPD film  170 . The nonlinear drive behavior shown by curve  120  arises primarily from the friction in the carrier fluid in EPD film  170 . Once the voltage on floating collector  140  reaches v th , curve  120  shows that flipping or color change occurs in EPD film  170 . Typically, the applied voltage exceeds v th  by a “safety” margin. Below v th  (and at negative voltages related to v th ), EPD film 170  does not change state due to the low mobility. 
         [0021]    The integrating reaction of an electrophoretic display on a driving voltage u epd  applied for a specific period of time as expressed by Eq. (7) is shown in  FIG. 2   b  in a simplified way. For the example shown in  FIG. 2   b , where curve  212  shows displacement s port  as a function of time t, the initial displacement s part     —     ini  is 50 μm which represents an intermediate color level. Curve  215  shows the applied voltage as a function of time t. When at a time t=0.4 seconds, a positive voltage u epd  of 15 volts is applied, the particle moves towards bottom electrode  160  (see  FIG. 1   a ). After 0.2 seconds the particle&#39;s displacement s port     —     tot  is 0 μm (the particle has reached its bottom position). When at 0.9 seconds a negative voltage u epd  of −15 volts is applied the particle moves towards top electrode  162  (see  FIG. 1   a ). After 0.4 seconds the particle&#39;s displacement s port     —     tot  is 100 μm (the particle has reached its top position). 
         [0022]    Because of the bi-stable nature of the electrophoretic material, a voltage of 0 volts on bottom electrode  160  will maintain the current state with no applied power. Intermediate gray levels can be obtained either by applying a voltage less than the drive voltage for a constant time or by applying the drive voltage for a time less than the 1 bit setup time (see Table 1). 
         [0023]    In order to prepare electronic paper film in accordance with the invention, the electronic paper display film needs to be calibrated as shown in  FIG. 2   c . The calibration sequence is as follows: 
         [0024]    In step  220 , measure the required constant drive voltage (±) V a  that needs to be applied to the electronic paper film to obtain the desired contrast ratio CR for both color states (e.g. dark and light). 
         [0025]    In step  222 , apply voltage pulses with the amplitude (±) V a  and varying pulse duration to the electronic paper film. 
         [0026]    In step  224 , measure the 1 bit (minimum) setup time t set  required to flip from one color state at the desired contrast ratio CR to the other color state at the desired contrast ratio CR as shown in  FIG. 2   d  for BRIDGESTONE QR-LPD.  FIG. 2   d  shows curve  240  which gives the reflectivity transition from the light state (white) to the dark state (black) and curve  245  which gives the reflectivity transition from the dark state (black) to the light state (white) as a function of time for an exemplary electronic paper film in accordance with the invention. t set  in  FIG. 2   d  is 0.2 msec. 
         [0027]    In step  226 , apply pulses with a duration of a safe setup time t ssetup  which is typically twice t setup  and vary the amplitude of the pulses from −V a  to +V a  and measure the corresponding contrast ratio CR. 
         [0028]    In step  228 , identify threshold voltage +V p  in the light state when CR starts to increase which is equal to the voltage in the dark state when CR starts to decrease. 
         [0029]    In step  230 , identify the voltage +V a  when the CR in the light state has reached the maximum, CR light . 
         [0030]    In step  232 , identify threshold voltage −V p  in the dark state when CR starts to increase which is equal to the voltage in the light state when CR starts to decrease. 
         [0031]    In step  234 , identify the voltage −V a  when the CR in the dark state has reached the maximum, CR dark . 
         [0032]      FIG. 3  shows a cross-section of electronic paper display with multiplexing  300  in accordance with an embodiment of the invention. EPD film  170  is covered by transparent indium tin oxide (ITO) or PEDOT common electrode  310 . Common electrode  310  is not structured. Shield electrodes  315  are situated beneath EPD film  170  along with segment collector electrode  320 . Note that segment collector electrode is completely isolated so that no junctions exist which could cause temperature dependent leakage currents. Polymer  350  provides the isolation between segment select electrode  365 , digit select electrode  360  and segment collector electrode  320 . Polymer  350  typically needs to have a relatively high dielectric constants in order to maintain a typical ratio of at least 15 between the capacitance of EPD film  170  and the selection capacitances (see  FIG. 4 ). Polymer  350  may typically have a relative dielectric constant K that is as high as 20. Suitable polymer materials include poly(vinylidene fluoride-trifluoroethylene), barium titanate/benzocyclobutene, barium strontium titanate (e.g. Ba 0.6 Sr 0.4 TiO 3 ) and cyanoethylpullulan. 
         [0033]    Segment select contact  335  is electrically coupled to segment select electrode  365 , digit select contact  340  is electrically coupled to digit select electrode  360  and shield contact  345  is electrically coupled to shield electrode  315 . Substrate  355  provides structural support for electronic paper display with multiplexing  300 . 
         [0034]      FIG. 4  shows the relevant capacitances and parasitic capacitances of electronic paper display with multiplexing  300  in accordance with an embodiment of the invention from  FIG. 3  as well as an illustration of electronic ink microcapsule  480  that is comprised in EPD film  170 . Capacitance  410  is the capacitance associated with EPD film  170 . Capacitances  420  and  430  are the capacitances associated with the shields  315 . Capacitance  440  is the capacitance associated with segment select electrode  365  and capacitance  450  is the capacitance associated with digit select electrode  360 . Note that capacitances  410 ,  440  and  450  form capacitive voltage divider  110  shown in  FIG. 1   b . Finally, capacitance  460  is the capacitance between segment select electrode  365  and digit select electrode  360 . 
         [0035]    In an embodiment in accordance with the invention, electronic ink microcapsule  480  is comprised of negatively charged black pigment particles  485  and positively charged white pigment particles  490  separated by clear fluid  495 . Electronic ink microcapsule  480  is shown in its dark state. In the light state, the polarity is reversed and positively charged white pigment particles  490  occupy the positions of black pigment particles  485  while negatively charged black pigment particles  485  occupy the positions of positively charged white pigment particles  490  in  FIG. 4 . 
         [0036]    Three digit-seven segment display  500  is shown in  FIG. 5  to provide an exemplary embodiment in accordance with the invention. Segments  501 ,  502 ,  503 ,  504 ,  505 ,  506  and  507  each require input/output (I/O) lines  501   a ,  502   a ,  503   a ,  504   a ,  505   a ,  506   a  and  507   a  to create a digit, respectively. Three I/O lines  510 ,  520  and  530  are used to control access to digits  560 ,  570  and  580 , respectively. I/O line  550  controls shield electrodes  315  (see  FIG. 3 ) to set the background color (e.g., black or white). Top ITO common electrode  310  (not shown in  FIG. 5 ) requires an additional I/O line (not shown). 
         [0037]    As shown in  FIG. 1   b , capacitive voltage divider  110  has two voltage inputs  110  and  112  and controls the flipping of EPD film  170 . If V segment  is the voltage input on voltage input  110  and V digit  is the voltage input on voltage input  112  then: 
         [0000]        V   out =( C   121   ·V   segment   +C   131   ·V   digit )/( C   121   +C   131 )  (8)
 
         [0000]    where C 121  is the capacitance of the intrinsic capacitor  121  and C 131  is the capacitance of the intrinsic capacitor  131 . Each segment  501 ,  502 ,  503 ,  504 ,  505 ,  506 ,  507  of each digit  560 ,  570  and  580  shown in  FIG. 5  is controlled by a capacitive voltage divider similar to capacitive voltage divider  110 . For example, with reference to changing segment  501  of digit  560  from black to white in  FIG. 5 , V segment  is applied to I/O line  501   a  while V digit  is applied to I/O line  510  and passes to voltage input  112  of all 7 capacitive voltage dividers of digit  560 . Segments  501  of digits  560 ,  570  and  580  are connected in parallel so that V segment  is applied to all segments  501  through I/O line  501   a . If flipping to white requires a V th =3 volts (with C 121 =C 131 ), for example, and V segment =4 volts and V digit = 7  volts, then segment  501  of digit  560  starts to turn to white with the application of V out =5.5 volts using Eq. (8). The higher the voltage V out  the faster the white state will be achieved. Assuming segment  501  of digit  570  is also black but is not to be flipped to white, V segment =4 volts as all segments  501  are connected in parallel but V digit =0 volts is applied to I/O line  520  and V out =2 volts so that segment  501  of digit  570  does not start to turn to white. As can be seen, flipping is controlled by the voltage on I/O inputs  510 ,  520  and  530  which provide the voltage to digits  560 ,  570  and  580 , respectively. 
         [0038]      FIG. 6  shows a typical exemplary display drive sequence for the implementation of an active black on white display in accordance with the invention. In step  610 , the background of three digit-seven segment display  500  is set to white by applying voltage, V shield , to I/O line  550 . In step  620 , all segments  501 ,  502 ,  503 ,  504 ,  506 ,  507  are set to black by applying voltage, −V segment  (typically equal to −V shield ) to segment I/O lines  501   a ,  502   a ,  503   a ,  504   a ,  505   a ,  506   a  and  507   a , respectively, and applying voltage, −V digit , (typically equal to −V shield ) to I/O lines  510 ,  520  and  530 . In step  630 , selected segments of segments  501 ,  502 ,  503 ,  504 ,  506 ,  507  are set to white by applying voltage, V segment , to the segment I/O lines of selected segments of segments  501 ,  502 ,  503 ,  504 ,  506 ,  507  and applying voltage, V digit , to digit I/O lines for selected digits of digits  560 ,  570  and  580 . Finally, in step  640 , voltage is set to zero on all I/O lines  501   a ,  502   a ,  503   a ,  504   a ,  505   a ,  506   a ,  507   a ,  510 ,  520 ,  530  and  550 . Due to the nonlinearity of the flipping behavior shown in  FIG. 2   a , the display will be stable with no power being supplied. Note that for a white on black display the voltages in  FIG. 6  are inverted. 
         [0039]    In accordance with the invention, other electrophoretic displays such as SIPIX MICROCUP and BRIDGESTONE QR-LPD may also be used. The use of SIPIX MICROCUP typically requires modification of the required voltage levels to adapt to the different threshold voltages shown in Table 1 and the electrophoretic mobilities represented by the 1 bit setup time of the display material shown in Table 1. BRIDGESTONE QR-LPD operates on an electrostatic principle that is similar to E-Ink Because the charged particles are suspended in air, a higher voltage is typically needed to drive the display. Positive, negative and both intermediate voltage levels need to be adjusted for the threshold voltages shown in Table 1 and electrophoretic mobilities represented by the 1 bit setup time of the display material shown in Table 1. 
         [0040]    Table 1 shows typical values for materials that are suitable for the electrophoretic display in accordance with the invention. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Threshold 
                   
                 1 bit 
               
               
                   
                 Contrast 
                 Voltage 
                 Drive 
                 setup time (t set ) 
               
               
                 Material 
                 Ratio 
                 (±V p ) 
                 Voltage (±V a ) 
                 (ms) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 E Ink Vizplex 
                 7 
                 3-5 
                 10-12 
                 260 
               
               
                 E Ink Pearl 
                 10 
                 5-7 
                 15 
                 120 
               
               
                 Bridgestone 
                 8 
                 35 
                 90 
                 0.3 
               
               
                 QRLPD 
               
               
                 SIPIX 
                 10 
                 10 
                 24 
                 60 
               
               
                 MICROCUP 
               
               
                 PMEPD 1 st  gen 
               
               
                 SIPIX 
                 12 
                 12 
                 75 
                 120 
               
               
                 MICROCUP 
               
               
                 PMEPD 
               
               
                 variant 1 
               
               
                 SIPIX 
                 12 
                 50 
                 110 
                 120 
               
               
                 MICROCUP 
               
               
                 PMEPD 
               
               
                 Variant 2 
               
               
                   
               
             
          
         
       
     
         [0041]    While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.