Abstract:
Overheating while enabling a full flexibility in the display usage should be avoided. This object is solved by a method for avoiding the overheating of a driver circuit in a plasma display panel wherein the driver circuit receives serially display data in form of a sequence of sub-field data bits and forwards parallelly the display data in the form of data blocks each consisting of a predefined number of sub-field data bits, the method comprising the steps of counting sub-field data bits the value of which differs from that of a neighboring or preceding sub-field data bit and providing a respective counting signal indicative of heat contributions of sub-field data bits and, if said counting signal is above a pregiven threshold, taking countermeasures for reducing said temperature.

Description:
This application claims the benefit, under 35 U.S.C. §119 of EP Patent Application 06290298.6 filed 21 Feb. 2006. 
     FIELD OF THE INVENTION 
     The invention relates to a method for driving a plasma display panel including the steps of serially receiving display data in form of a sequence of subfield data bits and parallelly forwarding the display data in the form of data blocks each consisting of a pre-defined number of sub-field data bits. Furthermore, the present invention relates to a corresponding apparatus for driving a plasma display panel. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  shows the principal structure of the electronics of a known plasma display panel (PDP). 
     A video signal is sent to a Digital Board  1  that includes the heart of the PDP processing: the PDP IC controller. This IC takes care of all PDP relevant signal processing and converts video data to sub-field information as usual. Furthermore, the IC is responsible for sending all power signals to the hardware including:
         data drivers D 1  to D 6  of a PDP  2  for sending on the vertical electrodes the bits ( 1  or  0 ) for all cells  3  of the current selected lines,   line drivers L 1 , L 2 , L 3  for selecting lines to be written one after the other and   a common part  4  for generating global signals (in combination with the line drivers) like sustain, erase, priming.       

     As shown in  FIG. 1 , the PDP cell  3  is defined as the crossing point between a vertical electrode coming from a data driver output D 1 , a horizontal electrode coming from a line driver output L 1  and an horizontal electrode coming from the Common electronic  4 . The data drivers D 1  to D 6  are serial to parallel converters as described in connection with  FIG. 2 . Each data driver Dk (n outputs), receives n sub-field data bits (Cn,t) of line t serially from the PDP IC controller. The input occurs at a frequency defined by clk. 
     On each starting edge of the enable signal ENA, the n outputs of the driver Dk take the n values stored from the PDP IC. In fact when data C n,t  are send to the input of the driver Dk, the outputs take the values C n,t−1 . The enable signal ENA is included in the addressing signal used to activate the current line t−1. An important point is that the input signals are control logic signals (low voltage) whereas the output signals are power signals (high power ≅60V). 
     The activity of the driver Dk is defined by two important points:
         The activity at the input of the driver: how many changes are occurring during the loading of a driver?   The activity at the output of the driver: how many outputs are changing from one line to another? Furthermore, it is important to notice how these changes are appearing. Indeed if all outputs have the same value and are changing in one time, this is less energy consuming than if each output is different and is changing.       

     Based on all these assumptions, a critical test pattern can be defined per driver as illustrated in  FIG. 3 . 
     The pattern will introduce an overheating of the driver and above all when the addressing speed is fast (clk and ENA are high frequency signals) like for high-resolution displays. If the driver is overheated a long time (many frames) it can be definitely damaged. Moreover, today, the drivers are bonded on the PDP glass by using glue and it is almost impossible to remove them in order to perform an exchange. Therefore, if a driver has been damaged, the whole panel can be thrown away. 
     Today, in order to avoid such a problem, there are three possibilities:
         A technical one that tries to avoid such an overheating by limiting either the addressing speed (clk and ENA frequencies are low), or the number of sub-fields used per frame.   A coding one that tries to use a specific coding that should reduce the situation depicted in  FIG. 3  for a standard picture (reduce the toggling inside a codeword).   A signal-processing one that tries directly to detect critical patterns in order to reduce the number of sub-fields used during addressing.       

     A typical real pattern introducing the problem of  FIG. 3  is shown in  FIG. 4 . 
     The problem is that, even if this pattern is a seldom one and could mainly appear only in case of PC applications, the display should be made robust enough in order not to be destroyed. This needs solutions as those described just before. The problem is that such solutions do not cover all possibilities or all risks. Moreover, some solutions (e.g. coding ones) are limiting the flexibility of the display that can have an impact on the picture quality (e.g. less sub-fields or not optimized coding). 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to avoid overheating while enabling a full flexibility in the display usage. 
     According to the present invention this object is solved by a method for avoiding the overheating of a driver circuit in a plasma display panel wherein the driver circuit receives serially display data in form of a sequence of sub-field data bits and forwards parallelly the display data in the form of data blocks each consisting of a predefined number of sub-field data bits, the method comprising the following steps
         counting sub-field data bits the value of which differs from that of a neighbouring or preceding sub-field data bit and providing a respective counting signal representative of the temperature of said driver circuit and,   if said counting signal is above a pregiven threshold, taking countermeasures for reducing said temperature.       

     Furthermore, there is provided a method for avoiding the overheating of a driver circuit in a plasma display panel wherein the driver circuit receives serially display data in form of a sequence of sub-field data bits and forwards parallelly the display data in the form of data blocks each consisting of a predefined number of sub-field data bits, the method comprising the following steps
         determining transition information for each sub-field data bit, the transition information representing a relation between the value of the sub-field data bit and the value of a neighbouring sub-field data bit,   counting sub-field data bits the value of which differs from that of a neighbouring or preceding sub-field data bit and/or the transition information the value of which differs from that of a preceding sub-field data bit and providing a respective counting signal representative of the temperature of said driver circuit and,   if said counting signal is above a pregiven threshold, taking countermeasures for reducing said temperature.       

     Moreover, the above object is solved by an apparatus for avoiding the over-heating of a driver circuit in a plasma display panel wherein the driver circuit receives serially display data in form of a sequence of sub-field data bits and forwards parallelly the display data in the form of data blocks each consisting of a predefined number of sub-field data bits, the apparatus including
         counting means for counting sub-field data bits the value of which differs from that of a neighbouring or preceding sub-field data bit and for providing a respective counting signal representative of the temperature of said driver circuit and,   controlling means for taking countermeasures for reducing said temperature if said counting signal is above a pregiven threshold.       

     Finally, according to the present invention there is provided an apparatus for avoiding the overheating of a driver circuit in a plasma display panel wherein the driver circuit receives serially display data in form of a sequence of sub-field data bits and forwards parallelly the display data in the form of data blocks each consisting of a predefined number of sub-field data bits, the apparatus including
         data processing means for determining transition information for each sub-field data bit, the transition information representing a relation between the value of the sub-field data bit and the value of a neighbouring sub-field data bit,   counting means for counting sub-field data bits the value of which differs from that of a neighbouring or preceding sub-field data bit and/or the transition information the value of which differs from that of a preceding sub-field data bit and for providing a respective counting signal representative of the temperature of said driver circuit,   controlling means for taking countermeasures for reducing said temperature if said counting signal is above a pregiven threshold.       

     Thus, there is provided a solution that is quiet robust in order to avoid any data driver overheating while enabling a full flexibility in the display usage (as many sub-fields as needed, fastest possible addressing, fully optimized coding etc.). Preferably, an input counter is incremented, if the value of a received sub-field data bit is different from the neighbouring sub-field data bit received previously. Thus, the number of changes occurring during the loading of a driver can be regarded. 
     Furthermore, an output counter may be incremented, if the value of a sub-field data bit of a data block is different from the corresponding sub-field data bit of the preceding data block. Alternatively or additionally, a stage counter may be incremented, if the transition information of a sub-field data bit of a data block is different from the corresponding sub-field data bit of the preceding data block. With that, the activity of the output of the driver, i.e. how many outputs are changing from a one line to another, can be regarded. 
     Advantageously, taking countermeasure includes generating an overheat signal for optionally reducing the gain of the plasma display panel or the number of sub-fields used per frame on the basis of the counter values of at least two counters of the input counter, the output counter and the stage counter. So, a helpful value as to the level of overheating can be produced. If a plasma display device includes plural driving apparatuses as described above, each associated to a driver circuit of the display panel, an overheat signal should be generatable for each apparatus and the gain or the number of sub-fields should be reducible, if the overheat signal of one single apparatus exceeds a pre-given threshold, each overheat signal of more than a pre-given number of apparatuses exceeds the pre-given threshold or each overheat signal of more than a pre-given number of neighbouring apparatuses exceeds the pre-given threshold. This leads to a reliable decision on the status of overheating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. The drawings showing in 
         FIG. 1  an overall PDP electronic structure, 
         FIG. 2  a data driver principal, 
         FIG. 3  a critical test pattern, 
         FIG. 4  a critical video pattern, 
         FIG. 5  an emulator block, and 
         FIG. 6  the concept of an implementation of a plasma display panel according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to provide a robust system for avoiding any data driver overheating, each driver of a driver system is emulated inside the PDP IC controller by a block called EMU_DR k  where k represents the number of the driver. Such a block is described in  FIG. 5 . 
     Each information C x,t  used for evaluating the heating contribution contains two types of information:
         Its value—0 or 1 and   its horizontal transition to previous C x−1,t  having three possible stages: &lt;, =, &gt;.       

     The emulator block  5  illustrated in  FIG. 5  is a complex counter that will evaluate for each driver:
         The activity of the input by counting the number of differences between two consecutive, i.e. horizontally neighbouring inputs C x,t+1 , and C x+1,t+1 . Each time that a transition is detected (1→0 or 0→1), the input counter Cnt_IN k  is increased by a value HEAT_IN representing the impact of heating due to such a transition on driver Dk.   The activity of the output by storing in a memory MEM_BLK k  the data of a complete driver output data block (e.g.  96  values in case of  96  outputs). Each time a new data C x,t+1  is coming, this will replace the former C x,t  in the memory and a counter called Cnt_OUT k  is increased by value HEAT_OUT if C x,t+1 , and C x,t  are different in value. Respectively, a counter Cnt_OUT_DIFF is increased by value HEAT_DIFF if the stage of C x,t+1  and C x,t  are different (e.g. changing from &lt; to = . . . etc.). The value HEAT_OUT and HEAT_OUT_DIFF represent the heat contribution of the output toggling.       

     A general heating counter HEAT k =Cnt_IN k +Cnt_OUT k +Cnt_OUT_DIFF k  represents the heat of the driver Dk. This driver is reset on each new output frame based on the vertical synchronism signal V. This value is compared with a threshold OVERHEAT. 
     Now it is possible to react when:
     (1) One single driver Dk is overheated having HEAT k &gt;OVERHEAT   (2) More than p different drivers have HEAT k &gt;OVERHEAT   (3) More than p neighbouring drivers have HEAT m &gt;OVERHEAT with m ∈ [k-p;k].   

     It is possible to use all  3  conditions by using different thresholds OVERHEAT  1 , OVERHEAT  2  and OVERHEAT  3 , wherein OVERHEAT  1 &gt;OVERHEAT  2 &gt;OVERHEAT  3 . 
     The final decision if an overheating occurs or not is based on the three possibilities listed above. This decision is programmable depending on electronic behaviour. 
     As soon as the overheating has been detected some modification of the addressing concept should be applied to reduce the overheating. However, the overheating problem is not a “punctual” problem appearing on only one frame and able to destroy the panel during this frame. This means that only when the overheating exists during a long time such a problem may appear. 
     Therefore, the number of frames having an overheating shall be counted. The detection will be done as following:
         When one of the three overheat criteria has been detected (1), (2) or (3), OVERHEAT_FRAME is incremented. (Here also one can use all three conditions by using OVERHEAT FRAME  1 , OVERHEAT FRAME  2 , OVERHEAT FRAME  3 .   As soon as the overheat criteria is no more valid, the OVERHEAT_FRAME is decremented.       

     When OVERHEAT_FRAME has been decremented down to 0, it won&#39;t be decremented anymore (0 is the minimum value for this counter). 
     When OVERHEAT_FRAME reaches OVERHEAT_DANGER then the real countermeasures will be applied. OVERHEAT_FRAME can for instance be incremented up to 2× OVERHEAT_DANGER+MARGIN (this is the maximum value reached by OVERHEAT_FRAME counter). MARGIN is a parameter that can be either positive or negative. 
     As soon as the danger has been detected, a counter measure is applied. The countermeasure should avoid a high activity in the data driver per frame. A possibility is to reduce the number of sub-fields used per frame in case of danger. 
     In order to do that, it is important to notice that the highest video level in a frame defines the maximal number of sub-fields used for this frame. Indeed, to encode the level 255 all sub-fields must be switched on. On the opposite, to encode the level 64, only a reduced amount of sub-fields is used. 
     The concept to reduce the driver overheating when a danger has been detected is based on a reduction of the signal amplitude of the incoming video. This is done by using a multiplier (like for contrast) with a gain lower than 1. In that case, the maximal video level is reduced leading to a need of fewer sub-fields. 
     The reduction will be done very slowly to avoid any visible picture change. This reduction will continue as long as the OVERHEAT_FRAME&gt;OVERHEAT_DANGER. As soon as this situation has gone, the video gain will be modify slowly back to 1. The aim is to adjust the gain automatically to have OVERHEAT_FRAME just below OVERHEAT_DANGER. 
     Furthermore, a hysteresis function should be added on the gain change to avoid any oscillations even if those are quite invisible. 
       FIG. 6  illustrates a possible implementation of the above described solution. 
     A digital board  1  controls the PDP  2  roughly in the same principal as illustrated in  FIG. 1 . Therefore, as to the data drivers D 1  to Dn, the line drivers L 1  to Lf at the common part  4  it is referred to the description of  FIG. 1 . However, according to  FIG. 6 , the line drivers L 1  to Lf and the common part  4  are specifically driven by a wave form generator  6  being included in the digital board  1 . The video input signal  10  is forwarded to a gamma transformation block  11  where the following operation is applied: I out =(I in ) y  usually with γ=2.2. The output of this block  11  goes through the new gain multiplier  12  required to adjust the signal amplitude to the driver heating. If not multiplier is used another solution to reduce the amount of sub-fields is also possible but less efficient. 
     Then its output is forwarded to the standard PDP functions  13  including video functions, dithering and sub-field encoding. The encoded information is stored sub-fields wise and pixel wise inside a frame memory  14 . 
     The output of this frame memory  14  is read sub-field wise and line wise and sent to the data drivers D 1  to Dn and at the same time to the driver heating emulation blocks EMU_DR k , wherein 0≦k≦n. Each of this block evaluating the value HEAT k =Cnt_IN k +Cnt_OUT k . Optionally, the counter CNT_OUT_DIFF k  can also be added to the value HEAT k . This value is then provided to controlling means for taking countermeasures for reducing the temperature of the data driver Dk if the value HEAT k  is above a pregiven threshold. 
     All the outputs of these emulators are collected and analyzed to determine if the counter OVERHEAT_FRAME  15  must be incremented or decremented according methods (1), (2) or (3). This value is filtered by means of a hysteresis functions  16  to reduce jumps and oscillations. 
     Finally, depending on a comparison  17 , if the value OVERHEAT_FRAME is bigger or lower than OVERHEAT_DANGER, the gain of multiplier  12  located directly after gamma block  11  is correspondingly decreased or increased. 
     The advantage of this solution is to avoid any loss of video information compared to a simple sub-field suppression (and also to avoid loss of gray-scale quality). Alternatively, the video gain may be before the gamma block  11  and therefore also before an APL measurement (not shown). Then, by reducing the gain, the APL is reduced and the number of sustains is increased by the standard PDP power management resulting in a quite stable light output. Only the grayscale dynamic is reduced here. 
     In order to improve the concept a low-pass filtering in the time domain could be applied on the gain to avoid oscillation following the encoding approach used. In that case the real gain will be defined as following: 
     
       
         
           
             
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     By increasing the value T, the influence of specific coding methods is reduced without introducing additional risks for the driver heat problem as long as T is shorter than the maximal heating time (time after that the driver temperature has reached a critical point in case of a critical test pattern shown in  FIG. 4 ). 
     A further improvement against critical sequences can be realized optionally. When a danger has been detected a specific spatial filtering can be implemented on the picture before the gain function as described below: 
     
       
         
           
             
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     This exemplary function will reduce the critical differences as shown in  FIG. 4  but introduces a minimal reduction of sharpness. It is an optional concept that can be activated depending on the system integrator or automatically if the OVERHEAT_FRAME reaches a very high value OVERHEAT_STRONG_DANGER.