Abstract:
The control of a plasma display panel, successively comprises, at least for all the cells of a current line having to switch state for the next line: a connection of a terminal of application of an intermediary supply voltage to output terminals of column control stages corresponding to the junction points of first and second switches between two terminals of application of a supply voltage, to perform a precharge or a predischarge of the screen cells; a disconnection of said output terminals from this intermediary voltage; and a connection of each output terminal to a first or to a second power supply voltage by the turning-on of the first or second switch of the corresponding stage, according to a luminance reference value, delayed with respect to the disconnection of the corresponding output terminal from the terminal of application of the intermediary voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to plasma display panels and, more specifically, to the control of a plasma display panel power stage. 
     2. Description of the Related Art 
     A plasma display panel is formed of an array of cells arranged at the intersection of lines and columns. Each cell of the display panel comprises a cavity filled with a gas and at least two control electrodes. To create a light spot on the display panel by using a given cell, a potential difference is applied between the control electrodes thereof, the gas contained in the cell being then ionized, generally by means of a third electrode. This ionization comes along with an ultraviolet ray emission, the light spot creation being obtained by excitation of a red, green, or blue light-emitting material by these rays. 
       FIG. 1  very schematically shows in the form of blocks a conventional example of a plasma display panel formed of a network of cells represented in  FIG. 1  by their equivalent capacitances  2 . Each cell comprises two electrodes respectively connected to a line  4  and to a column  6 . A line control circuit  8  (SCAN) comprises, for each line  4 , an activation/deactivation circuit having an output connected to the considered line. A column control circuit  12  comprises an element  16  (DATA) (generally of shift register type) for parallelizing address data received in series (signal COL) and, for each column  6 , a control circuit or stage  14  having an output O connected to the considered column  6  and receiving, on input data E, reference signals generated from the luminance data. Elements  14  and  16  are generally integrated in a same circuit  12 . A general circuit  10  (CTRL) for controlling the display panel synchronizes the operation of circuits  8  and  12 . 
     The display panel cells are activated in a line scanning by means of circuit  8 . The non-activated lines are submitted to a quiescent voltage (generally greater than 100 volts), while the activated line is brought to an activation voltage (generally, 0 volt). The quiescent voltage of a column corresponds to ground. To activate cells based on the data provided by circuit  16  on the active line, the corresponding columns are brought to an activation voltage Vpp generally on the order of 70 volts for a given period. 
     The voltage difference between an activated line and a column (about 70 volts) provides lighting of the selected cell. The third electrode (not shown in  FIG. 1 ), called support electrode, provides for adjusting the luminance of the selected cells (memory effect). 
       FIG. 2  illustrates, in a very simplified partial representation of three control stages  14   i−1 ,  14   i , and  14   i+1  of columns C i−1 , C i , and C i+1 , a conventional example of precharge or predischarge of cells of a plasma display panel of the type shown in  FIG. 1 . The function is to limit the screen consumption to bring the respective column electrodes to the activation voltage. For example, an external capacitor with a capacitance greater than the total equivalent capacitance of the panel is used, to store power on discharge of a line which has just been addressed and prepare the charge of the next line. Each output terminal O of a circuit  14  is connected to the junction point of two switches P 1  and N 1  in series between two terminals of application of activation voltage V PP . Switches K connect terminals O to a terminal  24  which is at a voltage V PP /2 (for example, the first electrode of the capacitor, which has its second electrode at ground). The control of switches P 1 , N 1 , and K of each stage is organized to, between each line L j , enable recovering charges of the columns to be discharged (cells to be turned off) for the benefit of columns to be charged (cells to be turned on). It is then spoken of charge sharing. Voltage V PP /2 of terminal  24  may also be obtained by an internal or external voltage source or by any other means. In  FIG. 2 , the cumulated equivalent capacitances of the cells of columns C i−1 , C i , and C i+1  have been represented by capacitances { 2 } i−1 , { 2 } i , and { 2 } i+1  in dotted lines. 
       FIG. 3  shows the electric diagram of a circuit  14  for controlling a column (represented by its equivalent capacitance { 2 } in dotted lines). Switches P 1  and N 1  formed of MOS transistors, respectively with a P and N channel, in series between two terminals  20  and  22  of application of voltage V PP , are each in parallel with a diode D 16  or D 18  (for example, their respective parasitic diodes). The anode of diode D 16  is connected to the drain of transistor P 1  (output terminal O of the stage), the source of transistor P 1  being connected to terminal  20 . The anode of diode D 18  is connected to ground  22 , the source of transistor N 1  being also connected to ground  22 , and its drain being connected to terminal O. Bidirectional switch K is formed of two N-channel MOS transistors N 2  and N 3  in series and with a common source of terminal  24  at voltage V PP /2 and terminal O. Two diodes D 26  and D 28 , for example corresponding to the parasitic diodes of transistors N 2  and N 3 , have their respective anodes connected to midpoint  30  of switch K. The gates of transistors N 2  and N 3  are connected together to the drain of a P-channel MOS transistor P 2 , mirror-assembled on a P-channel MOS transistor P 3 . Transistor P 3  is in series with a control transistor N 4  and a current source  34  between terminal  20  and ground  22 . 
     The control of circuit  14  is performed by means of three signals V H , V L , and V M . A level-shifting circuit  36  (LS), controlled by signal V H  referenced to ground, is interposed between terminal  20  and the gate of transistor P 1 . Signal V L  is directly applied to the gate of transistor N 1  while signal V M  is applied to that of transistor N 4 . The function of signals V L , V H , and V M  is to control circuit  14  to organize the precharge and predischarge of the addressed cells between the actual display periods. 
       FIG. 4  very schematically shows in the form of blocks an amplifier  14  and partially shows column control circuit  16 , to illustrate the different signals received by these circuits. Circuit  16  receives, from circuit  10 , a signal CSE (Charge Sharing Enable) for controlling the precharge or predischarge and a synchronization signal Str. Signal CSE is active at state  1  while signal Str indicates, by ground pulses, the times of switching of the column data of the shift register of circuit  16  to circuits  14  for generation of signals Out. 
       FIGS. 5A ,  5 B,  5 C,  5 D,  5 E, and  5 F illustrate in timing diagrams the operation of amplifier  14  of  FIGS. 3 and 4  for the lighting (signal DATA at  1 ) of a cell at the intersection of a line L j  and of the considered column C i . In  FIG. 5 , preceding and next lines L j−1  and L j+1  are assumed not to have to be lit for the current column (signal DATA at  0 ). 
     Signals V L  ( FIG. 5C ), V M  ( FIG. 5D ), and V H  ( FIG. 5E ) are generated by circuit  16  based on signals Str ( FIG. 5A ) and CSE ( FIG. 5B ) by taking into account the data to be displayed of the preceding columns. An example of a circuit for generating signals V L , V M , and V H  is described in U.S. Pat. No. 7,122,968. 
     The function of signals V L , V M , and V H  is to control amplifier  14  to obtain a precharge to level V PP /2 of the concerned column (voltage Vout,  FIG. 5F ) before completing this charge through transistor P 1 . Conversely, at the end of the column addressing, these signals are used to organize the cell discharge towards terminal  24  before ending this discharge through transistor N 1 . 
     Assuming that the datum of the preceding line L i−1  is 0, signals V M  and V H  are low until time t 1  of the pulse of signal Str, so that transistors P 1  and N 4  are blocked while transistor N 1  is on. At a time t 0 , preceding time t 1  towards the end of the addressing of line L i−1 , signal CSE is switched to state  1  to activate the charge transfer system. At time t 1  when signal Str switches to the low state to transfer the data from the shift register to circuits  14 , signal V L  switches to the low state to block transistor N 1  while signal V M  switches to the high state to turn on transistor N 4 . Since terminal O is in the low state, this results in a turning-on of transistor N 2  and a precharge ( FIG. 5F ) of node O approximately up to level V PP /2 via transistor N 2  and diode D 28 , which is then forward biased. With a capacitor providing level V PP /2, the increase in voltage Vout actually lasts until the charges are balanced between this capacitor and the equivalent capacitances of the addressed display panel cells. At a time t 2 , signal CSE returns to the low state, which causes a low switching of the transistor of signal V M  and a high switching of signal V H . This results in a turning-off of transistor N 4 , which in turn results in a turning-off of transistor N 2  and of switch K, and a turning-on of transistor P 1  to complete the charge of the cells of the addressed column up to level V PP . A little before the end of the addressing of current line L i  (time t 0 ′), signal CSE switches back to the high state, indicating an activation of the precharge or predischarge circuit. At a following time t 1 ′, the pulse on signal Str causes the high switching of signal V M  as at time t 1  and due to the data level  0  desired for the next line L i+1 , signal V H  switches to the low state while signal V L  remains therein. This results in a discharge of the cells charged to level V PP  during the previous period to reach level V PP /2. As for the previous period, when signal CSE switches back to the low state (time t 2 ′), this causes the carrying on of the discharge to 0 by the switching to the high state of signal V L  and the turning-off of transistor N 4  (switching to the low state of signal V M ). 
     For the case where a next line in the scan order has to keep the same level, the predischarge (times t 1 ′ to t 2 ′) does not occur. 
     As compared with still prior solutions based on the use of a PMOS transistor to form switch K, the use of two DMOS transistors N 2  and N 3  space, a switch K having to be provided for each column. 
     However, a disadvantage of the circuit of  FIG. 3  is a static consumption on turning-on of switch K. 
     Another disadvantage is a risk of simultaneous conduction of transistors N 2  and N 3  and of transistor P 1  at time t 2 , causing a short-circuit between supply line  20  at level Vpp and terminal  24  at level V PP /2. The same problem occurs at time t 2 ′ with the ground. 
     The risk of simultaneous conduction is partly linked to the stray capacitances of the gates of transistors N 2  and N 3  which, when added to the stray drain capacitance of transistor P 1 , generate a switching delay. The risk of simultaneous conduction also originates from the recovery time of diodes D 26  or D 28  according to the initial cell biasing. 
     An additional constraint in display panels of the type to which the present invention applies is that it is not desirable to multiply the number of input signals of the column control circuits, which are in practice made in an integrated circuit. This is among others justified by a need for a compatibility of the column control circuit with the rest of the circuits. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment of the present invention overcomes all or part of the disadvantages of known circuits for controlling power stages of circuits of plasma display panel columns. 
     One embodiment of the present invention more specifically addresses the problems of simultaneous conduction of precharge transistors of the cells of such a display panel with one of the transistors for providing the bias voltage to the concerned cell. 
     One embodiment of the present invention provides a solution that does not require an additional terminal for the column control circuit. 
     One embodiment of the present invention provides a method for controlling a plasma display panel, successively comprising, at least for all the cells of a current line having to switch state for the next line: 
     a connection of a terminal of application of an intermediary supply voltage to output terminals of column control stages corresponding to the junction points of first and second switches between two terminals of application of a supply voltage, to perform a precharge or a predischarge of the screen cells; 
     a disconnection of said output terminals from this intermediary voltage; and 
     a connection of each output terminal to a first or to a second power supply voltage by the turning-on of the first or second switch of the corresponding stage, according to an addressing reference value, delayed with respect to the disconnection of the corresponding output terminal from the terminal of application of the intermediary voltage. 
     According to an embodiment of the present invention, the delay is obtained by a resistive and capacitive cell for shifting an edge of deactivation of a signal of activation of the precharge or predischarge. 
     According to an embodiment of the present invention, said delay is selected according to the recovery time of parasitic diodes of N-channel MOS transistors forming a switch of connection of said intermediary voltage to the output terminals. 
     According to an embodiment of the present invention, an internal signal is generated from the precharge or predischarge activation signal. 
     According to an embodiment of the present invention, said internal signal is used to generate signals of activation and reset of flip-flops placed at the output of a circuit for generating control signals of said column control stage switches. 
     One embodiment of the present invention provides a circuit for controlling a column of a plasma display panel. 
     One embodiment of the present invention provides a plasma display panel. 
     The foregoing and other features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1 , previously described, very schematically shows in the form of blocks an example of architecture of a plasma display panel of the type to which the present invention applies; 
         FIG. 2 , previously described, shows an example of conventional architecture of precharge and predischarge circuits of the type to which the present invention applies; 
         FIG. 3 , previously described, shows the electric diagram of a conventional plasma display panel column control circuit; 
         FIG. 4 , previously described, illustrates the signals received by a conventional control circuit; 
         FIGS. 5A ,  5 B,  5 C,  5 D,  5 E, and  5 F illustrate in timing diagrams an example of operation of the circuit of  FIGS. 3 and 4 ; 
         FIGS. 6A ,  6 B,  6 C,  6 D,  6 E,  6 F, and  6 G illustrate in timing diagrams an embodiment of the control method according to the present invention; 
         FIG. 7  shows an example of a circuit for obtaining an internal signal exploited by the method of one embodiment of the present invention; 
         FIG. 8  very schematically shows in the form of blocks an embodiment of a circuit for generating signals exploited by the method of one embodiment of the present invention; 
         FIGS. 9A ,  9 B,  9 C,  9 D, and  9 E illustrate an example of the shape of signals internal to the circuit of  FIG. 8 ; and 
         FIG. 10  shows an embodiment of a detail of the circuit of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Same elements have been designated with same reference numerals in the different drawings which have been drawn out of scale. For clarity, only those steps and elements which are useful to the present invention have been shown and will be described. In particular, the generation of the luminance reference values and the generation of the scan control signals have not been shown, the present invention being compatible with any conventional circuit generating such signals. 
     A feature of an embodiment of the present invention is to shift the switching of the transistors bringing a complement to the charge or discharge of the display panel cells with respect to the turning-off of the precharge or discharge control switch. 
     Another feature of an embodiment of the present invention is to provide a generation of control signals internal to the column control circuit, that is, exclusively based on the signals for making data available and activating the precharge and predischarge stage. 
     One embodiment of the present invention exploits the conventional architecture of column control circuits such as previously described in relation with  FIGS. 1 ,  2 , and  3 . For simplification, embodiments of the present invention will be described hereafter in relation with the elements and reference numerals of these drawings and will not be described again. 
       FIGS. 6A ,  6 B,  6 C,  6 D,  6 E,  6 F, and  6 G illustrate in timing diagrams to be compared with those of  FIGS. 5A-5F  an embodiment of the present invention. The same situation as in  FIGS. 5A-5F  of a precharge to display a line L j  with respect to a previous line L j−1 , then for a predischarge to turn off the next line L j+1 , is assumed. 
     As previously, signals Str ( FIG. 6A ) of control of the shift register of circuit  16  ( FIG. 1 ) and CSE ( FIG. 6B ) of activation of the precharge or predischarge, originating from general control circuit  10 , switch at times t 1 , t 0  and t 2 , t 0 ′, t 1 ′, and t 2 .  FIGS. 6A and 6B  are identical to  FIGS. 5A and 5B . 
     As previously still, control signal V M′  of transistor N 4  ( FIG. 3 ) is switched to the high state at times t 1  and t 1 ′, then to the low state at times t 2  and t 2 ′, and signals V L′  and V H′  are switched to their respective low states according to the content of the columns to be addressed (in this example at times t 1  and t 1 ′). 
     According to this embodiment of the present invention, time t 3 , respectively t 3 ′, of switching to the high state of signals V H′  and V L′  to turn on switch P 1  or N 1  and bring the charge or discharge complement, is delayed by a delay τ with respect to times t 2  and t 2 ′ of switching of signal V M′  to the low state, and thus with respect to the control signal for turning on switch K. 
     Delay τ may be obtained by internal generation of a signal CSEINT common to all circuits  14 . Signal CSEINT exhibits a rising edge triggered by the rising edge of signal CSE (time t 0 ) and a falling edge (time t 3 ) delayed with respect to the falling edge of signal CSE. Signal CSEINT is obtained, for example, by delaying the falling edge of signal CSE by a time period τ by means of a resistive and capacitive cell based on signal CSE. 
       FIG. 7  shows an example of a circuit for generating signal CSEINT from signal CSE. Other embodiments are of course possible. 
     In this example, an OR-type logic gate  411  combines signal CSE with a signal DELCSE obtained by delaying signal CSE by means of a resistive and capacitive cell formed of a resistor R between a terminal  412  receiving signal CSE and an input terminal of gate  411 , and of a capacitor C connecting this input terminal to ground. The other terminal of gate  411  is directly connected to terminal  412  and the output of gate  411  provides signal CSEINT. 
     Delay τ (corresponding to the time constant of the RC cell) is selected to enable the diodes (D 26  and D 28 ,  FIG. 3 ) to recover before turning-on of transistor P 1  by signal V H . The interval between times t 1  and t 2  is selected for level V PP /2 to be reached at time t 2  even on a maximum charge (dotted lines in  FIG. 6F ). 
       FIG. 8  very schematically shows in the form of blocks an embodiment of a circuit  40  for generating signals V H′ , V M′ , and V L′  based on signals V H , V M , and V L  provided by a decoding circuit  41  (DECOD) generating these signals based on signal CSE and on signal Str. An example of a circuit for obtaining signals V H , V M , and V L  will be described subsequently in relation with  FIG. 10 . In the representation of  FIG. 8 , the generation of signal CSEINT (for example, by means of the circuit of  FIG. 7 ) is assumed to be integrated to circuit  41 . As illustrated in  FIG. 8 , for each output  16   i  of the shift register receiving series data COL (addressing reference values), two flip-flops  43  and  44  are used to store two data from this columns for two successive lines to be able to take into account, for a current line L i , states of the previous line L i−1  in the generation of signals V H  and V L . 
     According to this embodiment of the present invention, two D-type flip-flops  44  and  45  respectively receive signals V H  and V L  generated by decoder  41  as the signals of  FIGS. 5E and 5C  and provide signals V H′  and V L′ . Flip-flops  44  and  45  are controlled by a signal Valid causing the transfer of the state present at the input (signal V H  or V L ) to the output of the concerned flip-flop. A third RS-type flip-flop  46  receives signal V M  and is controlled by signal Valid. Flip-flop  46  provides signal V M′  and receives a reset signal Reset. Signals Valid and Reset are generated from signals Str, CSE, and CSEINT and may be common to all circuits  14 . 
       FIGS. 9A ,  9 B,  9 C,  9 D, and  9 E illustrate an example of generation of signals Valid and Reset ( FIGS. 9D and 9E ) according to the shapes of signals Str ( FIG. 9A ), CSE ( FIG. 9B ), and CSEINT ( FIG. 9C ). 
     Signal Valid is, for example, obtained by logic recombination of signals Str, CSE, and CSEINT. Signal Reset exhibits a pulse between times t 2  and t 3 . This signal is, for example, obtained by a logic XOR-type combination of signals CSE and CSEINT. On the side of signal Valid, a first pulse (between times t 1  and t 4 ) corresponds to the pulse inverse to that of signal Str and a second pulse occurs between time t 3  and a slightly later time t 5 . This second pulse of signal Valid is, for example, obtained by means of a resistive and capacitive cell. The first pulse of signal Valid is obtained, for example, by AND-type combination of signal CSEINT with the result of an XOR-type combination of signals Str and CSE. 
     As a variation, the durations of all the pulses of signals Valid and Reset are set by resistive and capacitive cells. 
     The generation of signals Valid and Reset to control flip-flops  44  to  46  of  FIG. 8  enables taking into account the real operating conditions of the display panel and especially the extreme conditions of a need for precharge or predischarge of the display panel cells. 
       FIG. 10  shows an example of a circuit for generating signals V H , V L , and V M . Other circuits are of course possible. In the shown example, a logic AND-gate  413  combines signal L j  and the inverse of signal CSEINT (inverter  414 ), and provides signal V H . A logic AND-type gate  415  receives the output of inverter  414  (inverse of signal CSEINT) and the inverse of signal L j  (inverter  416 ), and provides signal V L . Signal V M  is provided by a logic AND-type gate  417  which combines signal CSEINT with the result provided by a logic OR-type gate  418  combining the respective results of two AND-type gates  419  and  420  respectively receiving signal L j−1  and the inverse of signal L j , and the inverse of signal L j−i  (inverter  421 ) and signal L j . 
     An advantage of the embodiments described above is that they enable in simple fashion and without using additional external signals, avoiding the problems of simultaneous conduction in a screen of plasma display panel type. 
     Another advantage is that they do not adversely affect the advantages brought by control circuits based on DMOS transistors over the use of PMOS transistors. 
     Another advantage is that they are compatible with any conventional structure of a plasma display panel column and line addressing circuit. 
     Of course, the present invention is likely to have various alterations, improvements, and modifications which will readily occur to those skilled in the art. In particular, the practical generation of the signals useful for the implementation of the present invention is within the abilities of those skilled in the art based on the functional indications given hereabove. For example, the active and inactive levels may be adapted according to the control circuits. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto. 
     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.