Patent Document

BACKGROUND OF THE INVENTION 
     The present invention relates to plasma screens and more specifically to the control of cells of a plasma screen. 
     1. Field of the Invention 
     A plasma screen is an array type of screen, formed of cells arranged at the intersections of lines and columns, a cell includes a cavity filled with a rare gas, and at least two control electrodes. To create a light point on the screen by using a given cell, the cell is selected by applying a potential difference between its control electrodes, after which the cell gas is ionized, generally by means of a third control electrode. This ionization goes along with an emission of ultraviolet rays. The creation of the light point is obtained by excitation of a red, green, or blue luminescent material by the ultraviolet rays. 
     2. Discussion of the Related Art 
       FIG. 1  shows a conventional structure of a plasma screen formed of cells  2 . Each cell  2  has two control electrodes (not shown) respectively connected to a line  4  and to a column  6 . Each cell  2  is represented by its equivalent capacitor. A line control circuit  8  includes, for each line  4 , a line activation/deactivation block  10  having an output connected to the considered line. A column control circuit  12  includes, for each column  6 , a column control block  14  having an output terminal O connected to the considered column  6 . Each block  14  includes an input terminal E. Circuit  12  also includes a storage register  16  connected to receive column control signals (COL) from means not shown. Register  16  includes as many Q outputs as there are blocks  14 . Each Q output is coupled to input terminal E of a block  14  via a logic switch  18 . All logic switches  18  (here, AND gates) are controlled by the same enable signal VAL, provided by means not shown. Circuits  8  and  12  are conventionally integrated on the same semiconductor chip of a control circuit. 
     Conventionally, the cells of a plasma screen are activated line by line. The non-activated lines are submitted to a quiescent voltage (for example, 150 V). The activated line is brought to an activation voltage (for example, 0 V), the columns being at a deactivation voltage GND (0 V). Then, to activate selected cells in the activated line, the corresponding columns are brought from deactivation voltage GND to an activation voltage VPP (80 V) for a predetermined duration. Thus, the columns corresponding to the selected cells are each submitted to a voltage square pulse of the same amplitude and of same amplitude and the same duration. The columns corresponding to the unselected cells of the activated line are maintained at voltage OND. Thus, the cells to be activated are submitted, during the voltage square pulse, to a column-line voltage equal to VPP-GND (80 V). All non-activated lines are at the quiescent voltage (150 V). The column voltage being either 0 V or 80 V, the cells of the non-activated lines are reverse biased and are not submitted to a voltage capable of starting the gas ionization. 
       FIG. 2  shows a conventional column control block  14 . An N-type MOS transistor T 1  has its drain connected to voltage VPP and its source connected to output terminal O. An N-type MOS transistor T 2  has its drain connected to output terminal O and its source connected to voltage GND. A zener diode  20  is connected by its cathode to the gate of transistor T 1  and by its anode to the source of transistor T 1 . A P-type MOS transistor T 3  has its source connected to voltage VPP and its drain connected to the gate of transistor T 1 . An N-type MOS transistor T 4  has its drain connected to the gate of transistor T 1  and its source connected to ground (GND). P-type MOS transistors T 5 , T 6  have their sources connected to voltage VPP. The gate of transistor T 5  is connected to the drain of transistor T 6  and the gate of transistor T 6  is connected to the drain of transistor T 5 . An N-type MOS transistor T 7  has its source connected to ground and its drain connected to the drain of transistor T 5 . An N-type MOS transistor T 8  has its source connected to ground and its drain connected to the drain of transistor T 6 . The gate of transistor T 3  is connected to the drain of transistor T 6 . The gates of transistors T 2 , T 4 , and T 7  are connected to input terminal E via an inverter  22 . The gate of transistor T 8  is connected to the output of inverter  22  via an inverter  24 . Output terminal O is connected to a column  6 . In  FIG. 2 , a capacitor C 2  connects column  6  to ground. Capacitor C 2  is the equivalent capacitor of column  6 . It is mainly formed of a first component corresponding to the capacitance between the selected column and the screen lines, and of a second component corresponding to the capacitance between the selected column and its neighboring lines. Capacitance C 2  does not have a constant value, as will be seen hereafter. 
     Block  14  is provided to submit column  6  to a voltage square pulse when its input E receives a logic “1” (for example, a voltage VDD equal to 5 V), then a logic “0” (0 V). When input E receives a logic “1”, block  14  charges capacitor C 2  to a voltage substantially equal to VPP (which will be called VPP for simplicity). When input E receives a logic “0”, block  14  discharges capacitor C 2  and the voltage of column  6  switches from VPP to GND. The value of the capacitor C 2  of a column  6  depends on the voltages to which the neighboring columns located on either side of this column  6  are submitted. Thus, when a column  6  is submitted to the voltage square pulse, the capacitor C 2  of this column has a maximum value if none of the two neighboring columns is submitted to a voltage square pulse. Capacitor C 2  has a minimum value if the two neighboring columns are submitted to a voltage square pulse, and a value substantially equal to half of the sum of the maximum and minimum values, which will be called hereafter the median value, if only one of the neighboring columns is also submitted to a voltage square pulse. 
     It is important for the proper operation of a plasma screen that the rise and fall times of the voltage square pulse provided to each selected column be smaller than a predetermined maximum duration. The maximum rise time of the voltage square pulse may be different from the maximum fall time of the voltage square pulse. For simplicity, they will be assumed to be equal. The maximum admissible rise/fall duration of the voltage square pulse and the different values of capacitance C 2  are features of each type of plasma screen. For a given type of screen, blocks  14  are provided, to each provide (and receive) a predetermined current enabling charging (and discharging) the capacitor C 2  with the maximum capacitance of he considered screen type in a time shorter than the maximum admissible rise/fall duration of the voltage square pulse for this type of screen. Especially, transistors T 1  and T 2  are sized to conduct this predetermined current when on. 
     However, when capacitance C 2  has its median value or its minimum value, the rise/fall durations of the voltage square pulse are shorter than the rise/fall durations observed for the maximum capacitance C 2 . Accordingly, block  14  provides or absorbs the preceding predetermined current for a variable duration depending on the selection of the neighboring columns. As a result, each block  14  introduces, when capacitance C 2  has its minimum value, intense variations in the current consumption for very short durations, which may create electromagnetic disturbances on the power supply and the ground of the control circuit, which is not desirable. 
     Further, a control circuit having its blocks  14  sized to control a screen of a specific type may not be usable to control another type of screen. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a circuit for controlling cells of a plasma screen having an operation which is rather unlikely to create electromagnetic disturbances. 
     Another object of the present invention is to provide such a control circuit which can easily be adapted to various types of plasma screens. 
     To achieve these and other objects, the present invention provides a circuit for controlling a plasma screen formed of cells arranged at the intersections of lines and columns, including, for each screen column, a column control block enabling selection of the column associated therewith by applying to said column a voltage square pulse during which said column is brought to a first voltage substantially equal to a first predetermined voltage, then to a second voltage substantially equal to a second predetermined voltage, said column having a different capacitance according to whether the neighboring columns are selected or not, each column control block including a first means adapted to charging the capacitor of said column in a first predetermined duration when said column is brought to said first voltage, and a second means for discharging the capacitor of said column in a second predetermined duration when said column is brought to said second voltage, the second means is controlled by a control means as a function of an estimation of the capacitance of said column obtained from data indicating the selection of the non-selection of the columns adjacent to said columns. 
     The foregoing and other objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments, in conjunction with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , previously described, schematically shows a plasma screen provided with a control circuit; 
         FIG. 2 , previously described, schematically shows a conventional column control block of a control circuit; 
         FIG. 3  schematically shows a first embodiment of a column control block according to the present invention; 
         FIG. 4  schematically shows an element of the control block of  FIG. 3 ; 
         FIG. 5  schematically illustrates the operation of the control means of  FIG. 3 ; 
         FIG. 6  shows in more detail an example of forming of the control block of  FIG. 3 ; 
         FIG. 7  schematically shows a second embodiment of a column control block according to the present invention; and 
         FIG. 8  schematically shows the variable current source of FIG.  7 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides a control circuit in which each column control block includes means for having the rise and/or fall time of the voltage square pulse provided to each column take a same predetermined value whatever the value of the capacitor of said column. 
     The same references represent the same elements in the different drawings. Only those elements necessary to the understanding of the present invention have been shown in the following drawings. 
       FIG. 3  shows a column control block  14 ′ according to a first embodiment of the present invention. Block  14 ′ has an output terminal O connected to a column  6 . Column  6  is grounded via a capacitor C 2 . Block  14 ′ includes transistors T 1 , T 2 , T 3 , T 4 , T 5 , T 6 , T 7 , and T 8  and inverters  22  and  24  substantially connected as in FIG.  2 . Further, according to the present invention, a capacitor C is connected between the gate of transistor T 1  and the ground. A constant current source CS 1  has a first terminal connected to voltage VPP and a second terminal connected to the source of transistor T 3 . The gate of transistor T 2  is connected to an output terminal O 28  of a control means  28 . Control means  28  has an input terminal E 28  connected to the output of inverter  22 . 
     When input terminal E receives a logic “1”, transistors T 7 , T 6 , and T 4  turn off, transistors T 8 , T 5 , and T 3  turn on and the current I 1  provided by constant current source CS 1  charges capacitor C. It is assumed that at the beginning, capacitor C is discharged. The charge of capacitor C occurs at constant current and the gate voltage of transistor T 1  changes from 0 to a maximum value (substantially VPP) in a constant duration. Transistor T 1  is connected as a voltage follower. The voltage of output terminal O increases with the gate voltage of transistor T 1 , in a constant duration, whatever the value of capacitor C 2  of column  6 . The rise time of the voltage square pulse thus is constant. 
       FIG. 4  schematically shows an embodiment of current source CS 1  of FIG.  3 . Current source CS 1  includes a P-type MOS transistor T 9 , having its source connected to voltage VPP and its drain connected to the source of transistor T 3 . A P-type MOS transistor T 10  has its source connected to voltage VPP and its drain connected to its gate. The gate of transistor T 9  is connected to the gate of transistor T 10  so that the current flowing through transistor T 9  is proportional (to simplify, it is considered to be equal) to the current flowing through transistor T 10 . A constant current source CS 2  has a first terminal connected to the drain of transistor T 10  and a second terminal connected to ground. Constant current I 2  flowing through current source VS 2  is reproduced in transistor T 9 , and determines the value of current I 1  generated by current source CS 1 . Current I 2  determines the rise time of the voltage square pulse to which column  6  is submitted. Current source CS 2  may be adjustable to provide different constant currents I 2  and adjust the rise time of the voltage square pulse to the features of different types of plasma screens. Transistor T 10  and current source CS 2  may be common to all the current sources CS 1  of all the column control blocks  14 ′ of a control circuit. In this case, each block  14 ′ will only include a transistor T 9  having its gate connected to the gate of common transistor T 10 . Further, it is possible to arrange a switch, for example an N-type MOS transistor, between current source CS 2  and transistor T 10 . Such a switch would enable deactivating of current source CS 1  when block  14 ′ is not desired to be used, for example, in a screen ionization hold phase, and thus to limit the consumption of the control circuit. 
     When input terminal E of the column control block receives a logic “0”, transistors T 8 , T 5 , T 3 , and T 1  turn off and transistors T&amp;, T 6 , and T 4  turn on. Control means  28  is activated and it submits the gate of transistor T 2  to an activation voltage selected from among three predetermined activation voltages. According to the present invention, the activation voltage provided by means  28  is different according to whether the value of capacitor C 2  is maximum, median, or minimum, so that transistor T 2  is respectively conducts a maximum, median, or minimum current and that the discharge duration of capacitor C 2  is constant. Control means  28  includes three control terminals Q i , Q i−1 , Q i+1 . Terminal Q i  is connected to the Q output of register  16 , which is coupled to input E of control block  14 ′ of the considered column  6 , said to be of rank i. Terminal Q i−1  is connected to the Q output of register  16 , which is coupled to control block  14 ′ of the preceding column, of rank i−1. Terminal Q i+1  is connected to output Q of register  16 , which is coupled to the control block  14 ′ of the next column, of rank i+1. 
       FIG. 5  illustrates the operation of control means  28  of FIG.  3 . When input terminal E 28  receives a logic “0”, block  14 ′ controls the rising of the voltage square pulse and output terminal O 28  is grounded to turn transistor T 2  off. When input terminal E 28  receives a logic “1” and when terminal Q i  receives a logic “0”, the column  6  coupled to control block  14 ′ is not selected. Output terminal O 28  then takes a logic value “1”, transistor T 2  is turned on and connects capacitor C 2  to ground. When input terminal E 28  receives a logic “1” and terminal Q i  receives a logic “1”, and terminal Q i−1  and Q i+1  receives a logic “0” (none of the columns neighboring column  6  is selected), output O 28  is brought to a voltage V max . When input terminal E 28  receives a logic “1”, terminal Q i−1 receives a logic “1” and only one of terminals Q i−1  and Q i+1  receives a logic “0” (only one of the columns next to column  6  is selected), output O 28  is brought to a voltage V med . When input terminal E 28  receives a logic “1”, and terminals Q i−1  and Q i+1  receive a logic “1” (the two columns next to column  6  are also selected), output O 28  is brought to a voltage V min . Voltages V max , V med , and V min , smaller than voltage VDD, are chosen to control transistor T 2  so that it is respectively run through by currents I max , I med , and I min  adapted to discharging capacitor C 2  from voltage VPP to ground in a constant time, when capacitance C 2  respectively has its maximum, median, and minimum value. 
     It should be noted that voltages V max , V med , and V min  can be generated by adjustable voltage sources, to adapt the control circuit to different types of plasma screens. 
       FIG. 6  shows in further detail an example of a structure of control block  14 ′. In  FIG. 6 , means  28  is formed by means of inverters, of NAND, X-OR gates, and of transistors assembled as switches, but those skilled in the art will easily form a means  28  having the same functions by means of other elements. Further, in  FIG. 6 , the gate of transistor T 4  is connected at the output of inverter  22  via two series-connected inverters  23 ,  25 . 
       FIG. 7  schematically shows a column control block  14 ″ according to a second embodiment of the present invention. Block  14 ″ includes an input terminal E and an output terminal O. Block  14 ″ includes a P-type MOS transistor T 11 , having its source connected to voltage VPP and its drain connected to terminal O. An N-type MOS transistor T 2  has its source connected to ground and its drain connected to the drain of transistor T 11 . The gate of transistor T 2  is connected to output O 28  of a control means  28  having three control terminals Qi, Qi −1 , and Q i+1 . Terminals Qi, Qi −1 , Q i+1  are connected to register  16  as described in relation with FIG.  3 . Means  28  has an input terminal E 28  connected to terminal E via an inverter  22 . A P-type MOS transistor T 12  has its source connected to voltage VPP and its drain connected to the gate of transistor T 11 . Transistor T 12  forms a current mirror with a P-type MOS transistor T 13  having its source connected to voltage VPP and having an interconnected drain and source. The drain of transistor T 13  is connected to the drain of an N-type transistor T 7  having its source connected to ground and its gate connected to the output of inverter  22 . A P-type MOS transistor T 14  has its source connected to the drain of an N-type MOS transistor T 15 , having its gate connected via an inverter  24  to the output of inverter  22 . A variable current source CS 3  has a first terminal connected to the source of transistor T 15  and a second terminal connected to ground. Current source CS 3  has a first terminal connected to the source of transistor T 15  and a second terminal connected to ground. Current source CS 3  includes three control terminals connected to the terminals Qi, Qi −1 , and Q i+1  Current source CS 3  is provided to provide a current I 3  capable of having three different values I 3   max , I 3   med , and I 3   min  according to the values of the signals received on terminals Qi, Qi −1 , and Q i+1 . The current flowing through transistor T 11 , proportional to current I 3  running through current source CS 3 , determines the rise time of the voltage square pulse provided to column  6 . 
     When input terminal E of the column control block is at a logic “0”, transistors T 7 , T 13 , and T 12  are on, transistors T 15 , T 14 , and T 11  are off and means  28  is activated. As in the preceding block  14 ′, control means  28  is controlled according to the Q outputs of register  16  and it submits the gate of transistor T 2  to an activation voltage selected from among three predetermined voltages, so that the discharge duration of capacitor C 2  is constant. 
     When input terminal E receives a logic “1”, transistors T 7 , T 12 , T 13 , and T 2  are off and transistors T 15 , T 14  and T 11  are on. The current flowing through transistor T 11  charges capacitor C 2 . The three currents I 3 max, I 3 med, and I 3 min are adapted to ensuring a predetermined constant rise duration of the voltage square pulse when capacitance C 2  respectively has its maximum, median and minimum value. 
       FIG. 8  very schematically shows an embodiment of current source CS 3  of FIG.  7 . Current source CS 3  includes a first terminal E 3  connected to the source of transistor T 15 . An N-type MOS transistor T 16  has its drain connected to terminal E 3 . Transistor T 16  is assembled as a switch. The gate of transistor T 16  is connected to the output of a buffer circuit  56 . An N-type MOS transistor T 18  has its drain connected to the source of transistor T 16  and its source connected to ground. An N-type MOS transistor T 20  has its drain connected to terminal E 3 . Transistor T 20  is assembled as a switch. The gate of transistor T 20  is connected to the output of a buffer circuit  58 . An N-type MOS transistor T 22  has its drain connected to the source of transistor T 20  and its source connected to ground. An N-type MOS transistor T 24  has its drain connected to terminal E 3 . Transistor T 24  is assembled as a switch. The gate of transistor T 24  is connected to the output of a buffer circuit  60 . An N-type MOS transistor T 26  has its drain connected to the source of transistor T 24  and its source connected to ground. An N-type MOS transistor T 28  has its source connected to ground and its drain connected to supply voltage VDD via a constant current source CS 4 . The gate and drain of transistor T 28  are interconnected. The gates of transistors T 26 , T 22 , and T 18  are connected to the gate of transistor T 28 . Transistors T 26 , T 22 , and T 18  each behave as a constant current source. A decoder  64  has three outputs D 1 , D 2 , and D 3  respectively connected to control buffer circuits  56 ,  58 , and  60 . Decoder  64  has three input terminals corresponding to control terminals Q i−1 , Q i , and Q i+1  of constant current source CS 3 . 
     The operation of decoder  64  is the following. When only terminal Q i  is at “1”, output D 3  is at “1” and outputs D 2 , D 1  are at “0”. When terminal Q i  and only one of terminals Q i−1  and Q i+1  are at “1”, output D 2  is at “1” and outputs D 3 , D 1  are at “0”. When terminals Q i , Q i−1 , and Q i+1  are at “1”, output D 1  is at “1” and outputs D 3 , D 2  are at “0”. 
     Transistor T 24  is on and transistors T 20  and T 16  are off when capacitance C 2  has a maximum value. Transistor T 20  is on and transistors T 24  and T 16  are off when capacitor C 2  has a median value. Transistor T 16  is on and transistors T 24  and T 20  are off when capacitance C 2  has a minimum value. The channel width and length of transistors T 26 , T 22 , and T 18  are provided in such a way that these transistors are respectively run through by currents I 3 max, I 3 med, and I 3 min. Current source CS 4  may be fixed, or may be adjustable to adjust the rise time of the voltage square pulse to different types of plasma screens. 
     The present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the elements used to form column control blocks  14 ′ and  14 ″ are given as an example only, and those skilled in the art will easily adapt the present invention to other embodiments using other elements having equivalent functions. For example, the MOS transistors may be replaced with bipolar transistors. 
     Further, in the described embodiments, column control blocks  14 ′ and  14 ″ provide voltage square pulses having constant rise and fall times. However, these two aspects may be dissociated from each other and it is possible to provide a column control block providing voltage square pulses in which only the rise time is constant or only the fall time is constant, without departing from the field of the present invention. 
     Moreover, the described embodiments apply to plasma screens in which the capacitor C 2  of each column  6  can take three values, only the influence of the columns adjacent to the selected column having been considered. Of course, the influence of other columns neighboring the selected column may be taken into account, those skilled in the art easily adapting the present invention to the case where capacitance C 2  can take more than three values. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.

Technology Category: g