Patent Publication Number: US-8531356-B2

Title: Method of driving a plasma display panel to compensate for the increase in the discharge delay time as the number of sustain pulses increases

Description:
This Application is a U.S. National Phase Application of PCT International Application PCT/JP2009/001687 
     TECHNICAL FIELD 
     The present invention relates to a plasma display device as an image display device using a plasma display panel. 
     BACKGROUND ART 
     Among thin-type image display elements, a plasma display panel (hereinafter simply referred to as a panel) has become practical as a large-screen display device from the advantage of high-speed display performance and easy upsizing. 
     A panel is formed of a front plate and a back plate attached with each other. The front plate has a glass substrate, display electrode pairs of scan electrodes and sustain electrodes disposed on the glass substrate, a dielectric layer formed so as to cover the display electrode pairs, and a protective layer disposed on the dielectric layer. The protective layer not only protects the dielectric layer from ion collision but also promotes generation of a discharge. 
     The back plate has a glass substrate, data electrodes formed on the glass substrate, a dielectric layer that covers the data electrodes, barrier ribs formed on the dielectric layer, and phosphor layers that emit light in red, green, and blue. The front plate and the back plate are oppositely disposed in a manner that the display electrode pairs and the data electrodes cross each other via a discharge space. The two plates are sealed at the peripheries with low-melting glass. The discharge space is filled with discharge gas including xenon. Discharge cells are formed at positions where the display electrode pairs face the data electrodes. 
     With a panel structured above, a plasma display device generates a gas discharge selectively in each discharge cell of the panel. Ultraviolet light generated at the discharge excites phosphors to emit light in red, green, and blue. Color image display is thus attained. 
     In a typical method for driving a panel, one field period is divided into a plurality of subfields, which is known as a subfield method. According to the subfield method, gradation display is attained by combination of the subfields to be lit. Each subfield has an initializing period, an address period and a sustain period. In the initializing period, a voltage is applied to the scan electrodes and the sustain electrodes to generate an initializing discharge. The initializing discharge generates wall charge on each electrode, which is necessary for an address operation in the subsequent address period. In the address period, scan pulses are sequentially applied to the scan electrodes, at the same time, address pulses are applied selectively to the data electrodes to generate an address discharge and to form wall charge. In the sustain period, sustain pulses are applied alternately to the display electrode pairs to generate a sustain discharge selectively in a discharge cell, by which the phosphor layers disposed in the discharge cells emit light for image display. 
     For obtaining higher quality of image, light-emitting control in discharge cells, i.e., which cells should be lit and which cells should not be lit, has to be done with reliability. That is, address operations should be properly completed within a predetermined period. To address above, manufacturers have been working on the development of a panel driven at a high-speed and seeking of improved driving method and driving circuits for providing high quality image so as to get best performance from the panel. 
     Discharge characteristics of a panel largely depend on the characteristics of a protective layer. In particular, the performance of electron emission and charge retention greatly affect the high-speed driving of a panel. To improve above, many studies on the material, structure, and manufacturing method for the protective layer have been made. For example, Patent Literature 1 discloses a plasma display panel with improvements in the panel and the electrode driving circuit. According to the disclosure, the panel has a magnesium oxide layer that exhibits a cathode luminescence emission peak at 200 to 300 nm. The magnesium oxide layer is generated through gas-phase oxidation of magnesium vapor. Besides, according to the electrode driving circuit above, scan pulses are sequentially applied to one of the display electrode pairs that constitute entire display lines, and at the same time, address pulses suitable for the display lines that undergo the application of scan pulses are applied to the data electrodes. 
     Recently, in addition to upsizing the screen, there has been growing demand for a high-definition plasma display device, such as a high-definition plasma display device with 1920 pixels×1080 lines and an extremely high-definition plasma display device with increased lines, for example, 2160 lines or 4320 lines; meanwhile, a sufficient number of subfields is necessary for smooth gradation display. Such a demanding situation requires the period for address operations per line to be further shortened. To complete address operations with reliability in a limited period, manufacturers are searching for an advanced panel with more reliable address operations at higher speed than before, a driving method thereof, and a plasma display device with driving circuits controllable the panel and suitable for the method.
         [Patent Literature 1]Unexamined Japanese Patent Publication No. 2006-54158       

     SUMMARY OF THE INVENTION 
     The plasma display device of the present invention has a panel and a panel driving circuit. The panel contains a front plate, a back plate disposed opposite to the front plate, and discharge cells formed therebetween. The front plate has a first glass substrate, display electrode pairs formed on the first glass substrate, a dielectric layer formed so as to cover the display electrode pairs, and a protective layer formed on the dielectric layer. The back plate has a second glass substrate and data electrodes formed on the second glass substrate. The discharge cells are formed at which the display electrode pairs face the data electrodes. The panel driving circuit drives the panel in a manner that one field period is temporally divided into a plurality of subfields. Each of the subfields has an initializing period for generating an initializing discharge in the discharge cell, an address period for generating an address discharge, and a sustain period for generating a sustain discharge. The protective layer is formed of a base protective layer and a particle layer. The base protective layer is a thin film of metallic oxide including at least any one of magnesium oxide, strontium oxide, calcium oxide, and barium oxide. The particle layer is formed in a manner that single-crystal particles of magnesium oxide, which exhibit the peak of emission intensity of emission spectrum at 200-300 nm more than twice the peak of emission intensity at 300-550 nm, are stuck to the base protective layer. The panel driving circuit of the present invention drives the panel as follows. In an initializing period, the panel driving circuit carries out either one of the following two: an all-cell initializing operation in which an initializing discharge is generated in all of the discharge cells, and a selective initializing operation in which an initializing discharge is generated in a discharge cell having undergone a sustain discharge before the all-cell initializing operation. Besides, the subfields are temporally disposed in a manner that magnitude of luminance weight has monotonous decrease from a subfield where an all-cell initializing operation is carried out to a subfield disposed immediately before a subfield where the next all-cell initializing operation is carried out. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the structure of a panel in accordance with an exemplary embodiment of the present invention. 
         FIG. 2  is a sectional view of the structure of the front plate of the panel. 
         FIG. 3  shows emission spectrum of a single-crystal particle used for the panel. 
         FIG. 4  shows the relation between a discharge delay time and a peak ratio of the emission spectrum of the single-crystal particle used for the panel. 
         FIG. 5  shows an electrode array of the panel. 
         FIG. 6  is a waveform chart of driving voltage applied to each electrode of the panel. 
         FIG. 7  shows the structure of the subfields in accordance with the exemplary embodiment of the present invention. 
         FIG. 8A  shows the relation between the discharge delay time of the panel and a lapse of time since the completion of an all-cell initializing operation. 
         FIG. 8B  shows the relation between the discharge delay time and the number of sustain pulses of the panel. 
         FIG. 9  shows in minimum voltage applied to the data electrodes when a panel is driven with a subfield structure of descending coding and when a panel is driven with a subfield structure of ascending coding. 
         FIG. 10  is a circuit block diagram of a plasma display device in accordance with the exemplary embodiment of the present invention. 
         FIG. 11  is a circuit diagram showing the scan electrode driving circuit and the sustain electrode driving circuit of the plasma display device. 
         FIG. 12  shows a subfield structure in accordance with another exemplary embodiment of the present invention. 
     
    
    
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           10  panel 
           20  front plate 
           21  (first) glass substrate 
           22  scan electrode 
           22   a ,  23   a  transparent electrode 
           22   b ,  23   b  bus electrode 
           23  sustain electrode 
           24  display electrode pair 
           25  dielectric layer 
           26  protective layer 
           26   a  base protective layer 
           26   b  particle layer 
           27  single-crystal particle 
           30  back plate 
           31  (second) glass substrate 
           32  data electrode 
           34  barrier rib 
           35  phosphor layer 
           41  image signal processing circuit 
           42  data electrode driving circuit 
           43  scan electrode driving circuit 
           44  sustain electrode driving circuit 
           45  timing generating circuit 
           50 ,  80  sustain pulse generating circuit 
           60  initializing waveform generating circuit 
           70  scan pulse generating circuit 
           100  plasma display device 
       
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a plasma display device of an exemplary embodiment of the present invention will be described with reference to the accompanying drawings. 
     First Exemplary Embodiment 
       FIG. 1  is a perspective view showing the structure of panel  10  in accordance with the exemplary embodiment of the present invention. Panel  10  has a structure in which front plate  20  is disposed opposite to back plate  30  and the two plates are sealed at the outer peripheries with sealing material of low-melting glass. Discharge space  15  inside panel  10  is filled with discharge gas of, for example, xenon, with a charged pressure of 400 to 600 Torr. 
     On glass substrate (first glass substrate)  21  of front plate  20 , display electrode pairs formed of scan electrodes  22  and sustain electrodes  23  are disposed in parallel, and over which, dielectric layer  25  is formed so as to cover display electrode pairs  24 . Protective layer  26  having magnesium oxide as a major component is formed on dielectric layer  25 . 
     On glass substrate (second glass substrate)  31  of back plate  30 , a plurality of data electrodes  32  are disposed in parallel in a direction orthogonal to display electrode pairs  24 . Data electrodes  32  are covered with dielectric layer  33 . Barrier ribs  34  are formed on dielectric layer  33 . Phosphor layers  35 , which emit light in red, green, and blue by ultraviolet light, are formed on dielectric layer  33  and on the side surface of barrier ribs  34 . The discharge cells are formed at intersections of display electrode pairs  24  and data electrodes  32 . A set of discharge cells having red, green, and blue phosphor layers  35  forms a pixel for color display. Dielectric layer  33  is not necessarily needed for the panel, and may be omitted from the structure of the panel. 
       FIG. 2  is a section view showing the structure of front plate  20  of panel  10  in accordance with the exemplary embodiment of the present invention.  FIG. 2  is an upside-down view of front plate  20  of  FIG. 1 . Display electrode pairs  24  formed of scan electrodes  22  and sustain electrodes  23  are formed on glass substrate  21 . Each scan electrode  22  is formed of transparent electrode  22   a  and bus electrode  22   b  disposed on transparent electrode  22   a . Transparent electrodes  22   a  are made of indium tin oxide, tin oxide, and the like. Similarly, each sustain electrode  23  is formed of transparent electrode  23   a  and bus electrode  23   b  disposed on transparent electrode  23   a . Bus electrodes  22   b ,  23   b  are made of conductive material containing silver as a major component, which allows transparent electrodes  22   a ,  23   a  to have conductivity in its lengthwise direction. 
     Dielectric layer  25  is formed in a manner that low-melting glass containing lead oxide, bismuth oxide, or phosphorus oxide as a major component is applied by, for example, screen printing, die-coating and then fired. Protective layer  26  is formed on dielectric layer  25 . 
     Protective layer  26  is formed on dielectric layer  25 . Details on protective layer  26  will be described below. Protective layer  26  protects dielectric layer  25  from ion collision, at the same time, it enhances performance of electron emission and charge retention, which have a great influence on the driving speed of a panel. Protective layer  26  is formed of base protective layer  26   a  disposed on dielectric layer  25  and particle layer  26   b  on base protective layer  26   a.    
     Base protective layer  26   a  is a magnesium oxide thin-film layer with a thickness of 0.3 to 1 μm formed by, for example, sputtering, ion-plating, and electron-beam deposition. 
     Particle layer  26   b , which is formed by firing magnesium oxide precursor, has a structure where single-crystal particles  27  of magnesium oxide are stuck on base protective layer  26   a . Single-crystal particle  27  has a relatively uniform particle-size distribution with an average particle diameter of 0.3 to 4 μm. Single-crystal particles  27  are not necessarily disposed over the entire surface of base protective layer  26   a ; island-shaped distribution with a covering ratio of 1% to 30% is effective enough. Although single-crystal particle  27  is basically shaped into a regular hexahedron or regular octahedron, some differences from variations caused in the manufacturing process are allowable. Besides, particle  27  may have a shape, with a vertex of the hexahedron or the octahedron truncated, or may have a shape with an rhombic plane appeared on a ridge line as a result of cutting off the ridge line. 
     In this way, protective layer  26  is formed of base protective layer  26   a  and particle layer  26   b  disposed on base protective layer  26   a . The structure allows panel  10  to have excellent protective layer  26  with high performance of electron emission and charge retention. 
     From a study on cathode luminescence emission of a single-crystal particle, the inventors have found that an evaluation of characteristics of the single-crystal particle, in particular, electron emission performance is evaluated by an emission spectrum.  FIG. 3  shows an emission spectrum of single-crystal particle  27  used for the panel of the embodiment of the present invention. For comparison purposes,  FIG. 3  also shows an emission spectrum of single-crystal particles of magnesium oxide formed on a base protective layer by gas-phase oxidation. The emission spectrum of single-crystal particle  27  exhibits a large peak of emission intensity at 200-300 nm and a small peak thereof at 300-550 nm. On the other hand, according to the emission spectrum of the single-crystal particle formed by gas-phase oxidation, the peaks at 200-300 nm and 300-550 nm are both small. 
     Focusing attention on the two peaks of emission intensity, the inventors have looked at the relation between electron emission performance and the ratio of a peak of emission intensity (hereinafter, simply as peak ratio PK) at 200-300 nm to another peak of emission intensity at 300-550 nm. Specifically, the inventors have prepared panels with different values of peak ratio PK as a prototype and measured discharge delay time of them.  FIG. 4  shows the relation between discharge delay time Td and peak ratio PK of the emission spectrum of single-crystal particle  27  employed for the panel of the embodiment of the present invention. The horizontal axis represents peak ratio PK. Peak ratio PK is determined by calculating the ratio of the integration value of an emission spectrum in the range of 200 nm or greater and less than 300 nm to the integration value of an emission spectrum at 300 nm or greater and less than 550 nm. The vertical axis represents discharge delay time as value TS normalized with respect to the discharge delay time calculated when peak ratio PK takes nearly zero. That is, a panel having smaller TS exhibits excellent electron emission. When the value of peak ratio PK of the emission spectrum takes 2 or more, i.e., the peak of emission intensity at 200-300 nm is at least twice the peak of emission intensity at 300-550 nm, normalized discharge delay time TS is constantly kept at below 0.2, which shows excellent electron emission. 
     Although the measurement by the inventors does not establish an obvious correlation between peak ratio PK of the emission spectrum and electron emission performance, it leads to the consideration below. That is, the peak of the emission spectrum at 200-300 nm shows the presence of a relaxation process of energy of an amount of approx. 5 eV. This indicates a high probability of occurrence of Auger electron emission that accompanies with the large amount of energy relaxation. On the other hand, the peak of the emission spectrum at 300-550 nm shows that there are many trap levels between bandgaps, which are caused, for example, by oxygen deficiency. It seems hard to generate the relaxation process of a large amount of energy and therefore Auger electron emission has low probability of occurrence. That is, having a higher peak at 200-300 nm and a lower peak at 300-550 nm allows a single-crystal particle to promote electron emission. Therefore, employing single-crystal particle  27  with characteristic above for particle layer  26   b  contributes to a panel with high electron emission. 
     Single-crystal particle  27  with a high peak in the emission spectrum at 200-300 nm and with a low peak at 300-550 nm can be created by a liquid phase method. Specifically, magnesium hydroxide, which is a precursor of magnesium oxide, is evenly fired in an oxygen-containing atmosphere at high temperatures, as described below. 
     (Liquid Phase Method  1 ) 
     An aqueous solution of magnesium alkoxide or magnesium acetylacetone of a purity greater than 99.95% is prepared. A little amount of acid is added to the solution and the solution is hydrolyzed. Through the hydrolysis, magnesium hydroxide gel is obtained. The gel is dehydrated by firing in air, by which powder of single-crystal particle  27  is obtained. 
     (Liquid Phase Method  2 ) 
     An aqueous solution of magnesium nitrate of a purity greater than 99. 95% is prepared. An alkali solution is added to the solution of magnesium nitrate so that magnesium hydroxide is precipitated. After separated from the solution, the precipitate of magnesium hydroxide is dehydrated by firing in air, by which powder of single-crystal particle  27  is obtained. 
     (Liquid Phase Method  3 ) 
     An aqueous solution of magnesium chloride of a purity greater than 99. 95% is prepared. Calcium hydroxide is added so that magnesium hydroxide is precipitated. After separated from the solution, the precipitate of magnesium hydroxide is dehydrated by firing in air, by which powder of single-crystal particle  27  is obtained. 
     Throughout the methods above, the firing temperature should preferably be 700° C. or higher, more preferably, 1000° C. or higher. A single-crystal particle fired at a temperature lower than 700° C. has an immature crystal face, forming a defective structure. 
     The experiment by the inventors has found that when magnesium hydroxide is fired at temperatures of 700° C. or higher and lower than 2000° C., two types of single-crystal particle are formed: one is the single-crystal particle with a peak ratio PK of 1 or more, and the other is the single-crystal particle that has a peak ratio PK of less than 1 and has a noticeable peak in the emission spectrum at 680-900 nm. Further, the firing process at a temperature of 1400° C. or more increases the rate of forming the latter type of the single-crystal particle, i.e., having a peak ratio PK of less than 1 and a noticeable peak in the emission spectrum at 680-900 nm. Therefore, for increasing the rate of forming magnesium-oxide single-crystal with a peak ratio of 1 or more, the firing temperature should preferably be 700° C. or higher and lower than 1400° C. 
     Instead of magnesium hydroxide above, more than one of the followings can be employed for a magnesium-oxide precursor: magnesium alkoxide, magnesium acetylacetone, magnesium nitrate, magnesium chloride, magnesium carbonate, magnesium sulfate, magnesium oxalate, and magnesium acetate. The purity of a magnesium compound as the magnesium-oxide precursor should preferably be greater than 99.95%, more preferably, greater than 99.98%. This is because having a large amount of impurity element, such as alkali metals, boron, silicon, iron, aluminum, in the precursor invites sintering or fusion bonding between the particles in the firing process, resulting in immature growth of crystalline structure. 
     The magnesium-oxide single-crystal particle that has peak ratio PK less than 1 and has a peak in the spectrum at 680-900 nm tends to have a particle diameter smaller than that of magnesium-oxide single-crystal particle with peak ratio PK of 1 or greater. Therefore, the two types of magnesium-oxide single-crystal particle can be separated from each other by classification, by which a desired single-crystal particle with greater peak ratio PK is sorted out. 
     As described above, particle layer  26   b  of the embodiment has a structure where single-crystal particle  27 , which has the peak in the emission spectrum at 200-300 nm being at least twice the peak at 300-550 nm, is stuck on base protective layer  26   a . Such structured particle layer  26   b  offers stable and high performance both in electron emission and charge retention, allowing panel  10  to be driven at a high speed. 
     Next will be described a method for driving panel  10  of the embodiment of the present invention. 
       FIG. 5  shows an electrode array of panel  10  in accordance with the embodiment of the present invention. In a row (line) direction, panel  10  has n long scan electrodes SC 1  through SCn (corresponding to scan electrodes  22  in  FIG. 1 ) and n long sustain electrodes SU 1  through SUn (corresponding to sustain electrodes  23  in  FIG. 1 ). In a column direction, panel  10  has m long data electrodes D 1  through Dm (corresponding to data electrodes  32  in  FIG. 1 ). A discharge cell is formed at an intersection of a pair of scan electrode SCi and sustain electrode SUi (where, i is 1 through n) and data electrode Dj (where, j is 1 through m). That is, panel  10  contains m×n discharge cells in the discharge space. When the panel is used in a high-definition plasma display device, for example, m=1920×3=5760 and n=1080. 
     Next will be described waveforms of driving voltage applied to each electrode for driving panel  10 . 
     Panel  10  employs a subfield method to provide gradation display. In the subfield method, one field period is divided into a plurality of subfields. Light-emitting control of the discharge cells is carried out on a subfield basis. Each subfield has an initializing period, an address period, and a sustain period. 
     In the initializing period, an initializing discharge is generated to form wall charge on each electrode required for a subsequent address discharge. Initializing operations in the initializing period have two types: one is for generating the initializing discharge in all of the discharge cells (hereinafter, an all-cell initializing operation) and the other is for generating the initializing discharge selectively in a discharge cell having undergone a sustain discharge in the sustain period in the immediately preceding subfield (hereinafter, a selective initializing operation). 
     In the address period, an address discharge is generated selectively in a discharge cell to be lit to form wall charge. In the sustain period, sustain pulses corresponding in number to each luminance weight are alternately applied to the display electrode pairs so that a sustain discharge is generated in the discharge cell having undergone an address discharge. Detailed description on the subfield structure will be given later, and the waveforms of driving voltage and the workings thereof are described hereinafter. 
       FIG. 6  shows a waveform chart of driving voltage applied to each electrode of panel  10  of the embodiment of the present invention.  FIG. 6  shows two subfields; one carries out the all-cell initializing operation, and the other carries out the selective initializing operation. 
     First will be described the subfield (the all-cell initializing subfield) that carries out the all-cell initializing operation. 
     In the first half of the initializing period, 0 (V) is applied to data electrodes D 1  through Dm and sustain electrodes SU 1  through SUn, and a ramp waveform voltage is applied to scan electrodes SC 1  through SCn. The ramp waveform voltage gradually increases, starting from voltage Vi 1 —that is lower than the discharge start voltage with respect to sustain electrodes SU 1  through SUn—toward voltage V 12  that exceeds the discharge start voltage. 
     During the application of the up-ramp voltage, a weak initializing discharge occurs between scan electrodes SC 1  through SCn and sustain electrodes SU 1  through SUn, and between scan electrodes SC 1  through SCn and data electrodes D 1  through Dm. Through the initializing discharge, negative wall voltage is accumulated on scan electrodes SC 1  through SCn, and positive wall voltage is accumulated on data electrodes D 1  through Dm and sustain electrodes SU 1  through SUn. Here, the wall voltage on each electrode represents the voltage generated by wall charge accumulated, for example, on the dielectric layer, the protective layer, and the phosphor layer disposed over the electrodes. In the initializing discharge above, an excessive amount of wall charge is accumulated prior to the subsequent latter half of the initializing period where wall voltage is optimized to a proper value. 
     In the latter half of the initializing period, voltage Ve 1  is applied to sustain electrodes SU 1  through SUn, and a ramp waveform voltage is applied to scan electrodes SC 1  through SCn. The ramp waveform voltage gradually decreases, starting from voltage Vi 3 —that is lower the discharge start voltage with respect to sustain electrodes SU 1  through SUn—toward voltage V 14  that exceeds the discharge start voltage. During the application of the down-ramp voltage, a weak initializing discharge between scan electrodes SC 1  through SCn and sustain electrodes SU 1  through SUn, and between scan electrodes SC 1  through SCn and data electrodes D 1  through Dm. Through the discharge, negative wall voltage on scan electrodes SC 1  through SCn and positive wall voltage on sustain electrodes SU 1  through SUn are weakened, on the other hand, positive wall voltage on data electrodes D 1  through Dm is adjusted to a value suitable for the address operation. In this way, the all-cell initializing operation for generating an initializing discharge in all the discharge cells is completed. 
     In the subsequent address period, voltage Vet is applied to sustain electrodes SU 1  through SUn, and voltage Vc is applied to scan electrodes SC 1  through SCn. 
     Next, negative scan pulse voltage Va is applied to scan electrode SC 1  located in the first line, and positive address pulse voltage Vd is applied to data electrode Dk (k is 1 through m), which corresponds to the discharge cell to be lit in the first line. At this time, difference in voltage at the intersection of data electrode Dk and scan electrode SC 1  is calculated by adding the difference in wall voltage between data electrode Dk and scan electrode SC 1  to the difference in voltage applied from outside (i.e., Vd−Va). The calculated value exceeds the discharge start voltage, thereby generating an address discharge between data electrode Dk and scan electrode SC 1 , and between sustain electrode SU 1  and scan electrode SC 1 . Through the address discharge, positive wall voltage is accumulated on scan electrode SC 1  and negative wall voltage is accumulated on sustain electrode SU 1  and data electrode Dk. 
     In the process above, the time that has elapsed since application of scan pulse voltage Va and address pulse voltage Vd before an address discharge is referred to as a “discharge delay time”. For example, if a panel offers poor electron emission and accordingly the discharge delay time of the panel increases, there is a necessity to extend the time required for the application of scan pulse voltage Va and address pulse voltage Vd so as to complete an address operation without failure. That is, the scan pulse and the address pulse need a longer pulse width, increasing the time required for an address operation. Similarly, if a panel offers poor charge retention, there is a necessity to increase each voltage value of scan pulse voltage Va and address pulse voltage Vd so as to compensate for decrease in wall voltage. However, panel  10  of the embodiment offers high electron emission, allowing pulse widths of the scan pulse and the address pulse to be shorter than those in a conventional panel. This contributes to a reliable address operation at high speed. Besides, panel  10  of the embodiment offers high charge retention, allowing voltage values of scan pulse voltage Va and address pulse voltage Vd to be smaller than those in a conventional panel. 
     In an address operation, as described above, an address discharge is generated so as to accumulate wall voltage on each electrode in the discharge cell to be lit in the first line. On the other hand, the voltage at the intersections of scan electrode SC 1  and data electrodes D 1  through Dm with no application of address pulse voltage Vd is lower than the discharge start voltage and therefore no address discharge. After the address operation is repeatedly carried out until the discharge cells located in the n-th line, the address period is completed. 
     In the subsequent sustain period, positive sustain pulse voltage Vs is applied to scan electrodes SC 1  through SCn, and 0 (V) is applied to sustain electrodes SU 1  through SUn. In the discharge cell having undergone an address discharge, difference in voltage between scan electrode SCi and sustain electrode SUi is calculated by adding sustain pulse voltage Vs to the difference between the wall voltage on scan electrode SCi and the wall voltage on sustain electrode SUi. The calculated value exceeds the discharge start voltage, thereby generating a sustain discharge between scan electrode SCi and sustain electrode SUi. The sustain discharge produces ultraviolet light, allowing phosphor layer  35  to emit light. Negative wall voltage is accumulated on scan electrode SCi and positive wall voltage is accumulated on sustain electrode SUi and data electrode Dk. A discharge cell without an address discharge in the address period has no sustain discharge and therefore maintains the wall voltage the same as that at the end of the initializing period. 
     Subsequently, 0 (V) is applied to scan electrodes SC 1  through SCn, and sustain pulse voltage Vs is applied to sustain electrodes SU 1  through SUn. In the discharge cell having undergone a sustain discharge, difference in voltage between sustain electrode SUi and scan electrode SCi exceeds the discharge start voltage, thereby generating a sustain discharge again between sustain electrode SUi and scan electrode SCi. Through the discharge, negative wall voltage is accumulated on sustain electrode SUi and positive wall voltage is accumulated on scan electrode SCi. Similarly, sustain pulses corresponding in number to each luminance weight are applied alternately to scan electrodes SC 1  through SCn and sustain electrodes SU 1  through SUn to apply potential difference between the electrodes of the display electrode pairs. This allows the sustain discharge to repeatedly occur in a discharge cell having undergone an address discharge in the address period. 
     At the end of the sustain period, a voltage difference with a narrow-width-pulse shape or a potential difference with a ramp waveform is applied between scan electrodes SC 1  through SCn and sustain electrodes SU 1  through SUn. The application of voltage erases wall voltage on scan electrode SCi and sustain electrode SUi, with the positive wall voltage on data electrode Dk maintained. 
     Next will be described the workings of the sub-field (the selective initializing subfield) that carries out a selective initializing operation. In the selective initializing operation of the initializing period, voltage Ve 1  is applied to sustain electrodes SU 1  through SUn, and 0 (V) is applied to data electrodes D 1  through Dm. A ramp voltage gradually decreasing toward voltage Vi 4  is applied to scan electrodes SC 1  through SCn. In the discharge cell having undergone a sustain discharge in the sustain period in the previous subfield, a weak initializing discharge occurs. The discharge weakens wall voltage on scan electrode SCi and sustain electrode SUi. A sufficient amount of positive wall voltage has been accumulated on electrode Dk by an immediately preceding sustain discharge. The surplus amount of the wall voltage is discharged, so that a proper amount of wall voltage is left for the address operation. 
     On the other hand, a discharge cell without a sustain discharge in the previous subfield has no initializing discharge and therefore maintains the wall voltage the same as that at the end of the initializing period of the previous subfield. As described above, the selective initializing operation is selectively carried out in a discharge cell having undergone the sustain operation in the sustain period of the immediately preceding subfield. 
     The operations of address period of the selective-cell initializing subfield are similar to those of the all-cell initializing subfield and descriptions thereof will be omitted. The operations of the subsequent sustain period are also similar to those of the all-cell initializing subfield except for the number of sustain pulses. 
     Next will be described the subfield structure employed for the driving method of the embodiment. The distinctive feature of the driving method of the embodiment is that the subfields are temporally disposed in a manner that magnitude of luminance weight has monotonous decrease from an all-cell initializing subfield and to a subfield disposed immediately before the next all-cell initializing subfield. That is, a selective initializing subfield that follows an all-cell initializing subfield has a luminance weight equal to or smaller than that of the immediately preceding subfield, and similarly, a selective initializing subfield that follows the aforementioned selective initializing subfield has a luminance weight equal to or smaller than that of the immediately preceding subfield. Hereinafter, the aforementioned subfield structure where magnitude of luminance weight has monotonous decrease between an all-cell initializing subfield and a subfield disposed immediately before the next all-cell initializing subfield will be simply referred to “descending coding”. 
       FIG. 7  shows the subfield structure in accordance with the exemplary embodiment of the present invention. In the structure of the embodiment, one field is divided into 10 subfields (first SF, second SF, . . . , 10th subfield) and the subfields have luminance weights of 80, 60, 44, 30, 18, 11, 6, 3, 2, and 1. The first SF is an all-cell initializing subfield, and the second SF through 10th subfields are selective initializing subfields.  FIG. 7  schematically shows the waveform of the driving voltage to be applied to scan electrodes  22  for one field. The detail of the driving voltage waveform in each period of each subfield is shown in  FIG. 6 . 
     In the embodiment, panel  10  is driven with the subfield structure of descending coding. Driving a panel with descending coding enhances the speed and stability of an address operation, getting the best performance from panel  10  capable of high-speed driving. This provides the plasma display panel with high quality of image display. Besides, the driving with descending coding further reduces address pulse voltage, and accordingly, reducing the power consumption of the plasma display device. 
     The reason will be described hereinafter. The inventors have measured the discharge delay time of panel  10  of the present invention. The measured panel is the panel of the present invention, which has protective layer  26  formed of base protective layer  26   a  and particle layer  26   b . Particle layer  26   b  is formed in a manner that single-crystal particles  27  are stuck on base protective layer  26   a  so as to have almost uniform distribution over the entire surface of base protective layer  26   a . Single-crystal particle  27  has the peak in emission spectrum at 200-300 nm being twice or grater than the peak at 300-550 nm. The panel is a 42-inch panel of high luminance and high definition and employs 100% xenon gas as discharge gas. For comparison, the discharge delay time has been measured on a conventional panel having base protective layer  26   a  only, that is, without particle layer  26   b.    
     The measurement on the discharge delay time of an address discharge is carried out in a discharge cell controlled so that the discharge cell has no influence of a discharge generated in the surrounding discharge cells and an address discharge is not generated in the adjacent discharge cells. The discharge delay time is affected by a phosphor material. In the measurement, the discharge delay time is measured in a discharge cell coated with green phosphor that has a tendency to increase the discharge delay time. 
     To find the relation between the discharge delay time and a lapse of time since the all-cell initializing operation, the inventors has measured the discharge delay time when the address operation is carried out in only one subfield of the first SF through the 10th SF. The number of sustain pulses in the measurement is set at two for all the subfields. To obtain the relation between the discharge delay time and the number of sustain pulses, the address operation is carried out only in the fifth SF, and thereafter, the discharge delay time is measured while the number of sustain pulses in the subsequent sustain period is being varied from 2 to 256. 
       FIG. 8A  shows the relation between the discharge delay time and a lapse of time since the all-cell initializing operation in panel  10  in accordance with the exemplary embodiment of the present invention.  FIG. 8B  shows the relation between the discharge delay time and the number of sustain pulses in accordance with the exemplary embodiment of the present invention. For comparison,  FIGS. 8A and 8B  show the characteristic of the conventional panel with a broken line. 
     As is apparent from the drawings, compared to the conventional panel, panel  10  of the embodiment has an extremely shortened discharge delay time. High electron emission performance of panel  10  of the embodiment contributes to the shortened discharge delay time. As is shown in  FIG. 8A , panel  10  of the embodiment has a tendency that the discharge delay time becomes long with increase in a lapse of time since the all-cell initializing operation. This is true also in the conventional panel. It is considered that the priming generated in the all-cell initializing operation decreases with time, which suppresses the generation of discharge. 
     On the other hand, in terms of the relation between the discharge delay time and the number of sustain pulses, as is shown in  FIG. 8 , the conventional panel has a tendency that the discharge delay time decreases as the number of sustain pulses increases. In contrast, panel  10  of the embodiment has a tendency that the discharge delay time increases as the number of sustain pulses increases. The general view is that increase in the number of sustain pulses promotes priming following a sustain discharge, decreasing the discharge delay time. However, panel  10  of the embodiment shows the opposite tendency. Although the reason why such a tendency appears in panel  10  of the embodiment is not fully clarified, the following can be one possibility. That is, of a formative delay time and a statistical delay time that determine the discharge delay time, the statistical delay time that is significantly affected by the priming is sufficiently short, so that the priming following the sustain discharge does not largely contribute to the discharge delay time. Panel  10  of the embodiment has charge retention performance higher than that of the conventional panel, but the wall charge reduces slightly. Therefore, the wall charge reduces in response to the sustain discharge, the voltage substantially applied between the electrodes decreases, increasing the formative delay time, and as a result, the discharge delay time increases. 
     In a panel of low electron emission performance, the influence of the priming on the statistical delay time sometimes extends to a range from 100 to 1000 ns, whereas the influence of the decrease in wall voltage on the formative delay time is relatively small, for example, around 100 ns. That is, in the panel of low electron emission performance, the influence of the priming on the statistical delay time is stronger, and the discharge delay time decreases with increase in the number of sustain pulses. In contrast, in panel  10  of high electron emission performance, the influence of the priming on the discharge delay is small, and the influence of the decrease in wall voltage on the statistical delay time is strong even when the charge retention performance is high. As a result, the discharge delay time increases with increase in the number of sustain pulses. 
     Thus, panel  10  of the embodiment has tendencies that increase in the number of sustain pulses increases the discharge delay time, and increase in the lapse of time since the all-cell initializing operation increases the discharge delay time. Therefore, employing the subfield structure of descending coding—where an increased number of the sustain pulses is used when the lapse of time since the all-cell initializing operation is small and a decreased number of the sustain pulses is used when the lapse of time since the all-cell initializing operation is large—allows the conditions of extending/shortening the discharge delay time to be cancelled out with each other. As a result, the plasma display device achieves high-speed driving, taking full advantage of panel  10  of the embodiment. 
     In addition, the subfield structure of descending coding reduces the voltage applied to data electrodes D 1  through Dm.  FIG. 9  is a diagram showing the lowest voltages applied to data electrodes D 1  through Dm when panel  10  of the embodiment is driven with a subfield structure of descending coding where subfields are disposed so that the luminance weight monotonically decreases and when panel  10  is driven with a subfield structure of ascending coding where subfields are disposed so that the luminance weight monotonically increases. According to  FIG. 9 , the required address pulse voltage increases in response to increase in light-emitting rate, but employing the subfield structure of descending coding decreases address pulse voltage Vd by about 5(V). This reduces the electric power of the data electrode driving circuit. 
     Next will be described an example of the panel driving circuits for generating the aforementioned driving voltage. 
       FIG. 11  is a circuit block diagram of plasma display device  100  in accordance with the exemplary embodiment of the present invention. Plasma display device  100  has panel  10  and a panel driving circuit. Protective layer  26  of panel  10  is formed of base protective layer  26   a  and particle layer  26   b . Base protective layer  26   a  is formed of a thin film of metal oxide containing at least one of magnesium oxide, strontium oxide, calcium oxide, and barium oxide. Particle layer  26   b  is formed in a manner that magnesium oxide single-crystal particle  27 , which has the peak of emission intensity at 200-300 nm being twice or greater than the peak of emission intensity at 300-550 nm, are stuck onto base protective layer  26   b . The panel driving circuit drives the plasma display panel in a manner that an initializing period has one of the all-cell initializing operation for generating an initializing discharge in all discharge cells and the selective initializing operation for generating an initializing discharge in a discharge cell having undergone a sustain discharge before the all-cell initializing operation, and the subfields are temporally disposed so that the luminance weight monotonically decreases from a subfield where the all-cell initializing operation is performed to a subfield where the next all-cell initializing operation is performed. The panel driving circuit has image signal processing circuit  41 , data electrode driving circuit  42 , scan electrode driving circuit  43 , sustain electrode driving circuit  44 , timing generating circuit  45 , and a power supply circuit (not shown) for supplying power to each circuit block. Receiving an image signal, image signal processing circuit  41  converts it into image data for light-emitting or non-light-emitting on a subfield basis. Data electrode driving circuit  42  converts the image data of each subfield into a signal for data electrodes D 1  through Dm to drive them. Timing generating circuit  45  generates timing signals that control each circuit block according to a horizontal synchronizing signal and a vertical synchronizing signal. Such generated timing signals are fed to each circuit block. According to the timing signals, scan electrode driving circuit  43  drives scan electrodes SC 1  through SCn. According to the timing signals, sustain electrode driving circuit  44  drives sustain electrodes SU 1  through SUn. 
       FIG. 11  is a circuit diagram showing scan electrode driving circuit  43  and sustain electrode driving circuit  44  of plasma display device  100  of the embodiment of the present invention. 
     Scan electrode driving circuit  43  has sustain pulse generating circuit  50 , initializing waveform generating circuit  60 , and scan pulse generating circuit  70 . Sustain pulse generating circuit  50  has switching element Q 55  for applying voltage Vs to scan electrodes SC 1  through SCn, switching element Q 56  for applying 0 (V) to scan electrodes SC 1  through SCn, and power recovering section  59  for recovering power for the application of sustain pulses to scan electrodes SC 1  through SCn. Initializing waveform generating circuit  60  has Miller integrating circuit  61  and Miller integrating circuit  62 . Miller integrating circuit  61  applies voltage having up-ramp waveform to scan electrodes SC 1  through SCn, whereas Miller integrating circuit  62  applies voltage having down-ramp waveform to scan electrodes SC 1  through SCn. Switching elements Q 63 , Q 64  prevent backflow of electric current via a parasitic diode of other switching elements. Scan pulse generating circuit  70  has floating power supply E 71 , switching elements Q 72 H 1  through Q 72 Hn and Q 72 L 1  through Q 72 Ln, and switching element Q 73 . Switching elements Q 72 H 1  through Q 72 Hn apply voltage on the high-voltage side of floating power supply E 71  to scan electrodes SC 1  through SCn, whereas switching elements Q 72 L 1  through Q 72 Ln apply voltage on the low-voltage side of floating power supply E 71  to scan electrodes SC 1  through SCn. Switching element Q 73  fixes voltage on the low-voltage side of floating power supply E 71  to voltage Va. 
     Sustain electrode driving circuit  44  has sustain pulse generating circuit  80  and initializing/address voltage generating circuit  90 . Sustain pulse generating circuit  80  has switching element Q 85  for applying voltage Vs to sustain electrodes SU 1  through SUn, switching element Q 86  for applying 0 (V) to sustain electrodes SU 1  through SUn, and power recovering section  89  for recovering power for the application of sustain pulses to sustain electrodes SU 1  through SUn. Initializing/address voltage generating circuit  90  has switching element Q 92  and diode D 92  for applying voltage Ve 1  to sustain electrodes SU 1  through SUn, switching element Q 94  and diode D 94  for applying voltage Ve 2  to sustain electrodes SU 1  through SUn. 
     The switching elements above are formed of generally well-known devices, such as a metal oxide semiconductor field-effect transistor (MOSFET) and an insulated gate bipolar transistor (IGBT). The switching elements are controlled by each of timing signals generated in timing generating circuit  45 . 
     The driving circuit of  FIG. 11  is introduced as an example for generating the driving voltage waveforms shown in  FIG. 6 . The plasma display device of the present invention does not necessarily have the circuit structure. 
     In the embodiment, one field is divided into 10 subfields, and only the first SF is an all-cell initializing subfield. The present invention is not limited to this.  FIG. 12  is a diagram showing a subfield structure in accordance with another exemplary embodiment of the present invention. The subfield structure of  FIG. 12  has the following conditions: 
     one field is divided into 14 subfields; and 
     the first SF and the seventh SF are all-cell initializing subfields; 
     the luminance weight monotonically decreases between the first SF and the sixth SF, and between the seventh SF and the 14th SF. 
     It is important that the luminance weight monotonically decreases from an all-cell initializing subfield to the subfield immediately before the next all-cell initializing subfield. The number of subfields forming one period, the subfields for all-cell initializing operation and the number of the subfields may be arbitrarily determined as required. 
     Besides, specific values seen throughout the description of the embodiment are cited merely by way of example and without limitation. They should be optimally determined according to characteristics of a panel and specifications of a plasma display device. 
     Industrial Applicability 
     The plasma display device of the present invention offers stable address operation at high speed and image display with high quality. 
     Therefore, the plasma display device is useful for a display device.