Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to plasma screens and more specifically to the control of cells of a plasma screen. 
     2. Discussion of the Related Art 
     A plasma screen is an array type 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 emitted rays. 
       FIG. 1  shows a conventional structure of a plasma screen formed of cells  4 . Each cell  4  has two control electrodes respectively connected to a line  6  and to a column  8 . 
     The selection of the cells, to create images, is performed, conventionally, by logic circuits generating control signals. The logic states of these signals determine the cells that are controlled to generate a light point and those that are controlled not to generate one. The ionization of a gas of a cell requires that potentials on the order of some hundred volts be applied between the two control electrodes for a predetermined duration, on the order of 2 microseconds. Each cell has an equivalent capacitance on the order of several tens of picofarads. 
       FIG. 2  shows a plasma screen, the cells  4  of which are represented by an equivalent capacitor. A line control circuit  10  includes, for each line  6 , a line control block  14 , an output of which is connected to line  6 . A column control circuit  12  includes, for each column  8 , a column control block  18 , an output  20  of which is connected to column  8 . Circuits  10  and  12  are generally integrated on a same semiconductor chip. 
     Conventionally, the cells of a plasma screen are activated line by line. The non-activated lines are set to a quiescent voltage VDD1 (for example, 150 V). The activated line is brought to an activation voltage GND (0 V). To light chosen points of the activated line, the corresponding columns are brought to a voltage VDD2 (80 V). The columns corresponding to the other points of the activated line are brought to voltage GND (0 V). Thus, the lit cells of the activated line see a column-line voltage equal to VDD2−GND (80 V) and the unlit cells of the activated line see a column-line voltage equal to GND−GND (0 V). For all non-activated lines, the line voltage is VDD1 (150 V) and the column voltage is 0 or 80 V. In both cases, the cells of the non-activated lines are reverse biased. 
     Each line control block  14  includes a pair of complementary power transistors  22  and  24 . Transistor  24  receives voltage VDD1 on its source. Its drain is connected to a line  6  and its gate receives a line deactivation control signal LSN. The source of transistor  22  is connected to voltage GND. Its drain is connected to line  6  and its gate receives a control signal LS complementary to signal LSN. Signals LS and LSN are generated, for example, by a microprocessor, not shown. 
     Each column control block  18  includes an output stage  26  including a couple of power transistors (not shown) enabling bringing output  20  to voltages VDD2 or GND according to a logic column selection signal LCS provided to stage  26 . Each control block  18  also includes a memory element  28  connected, for example, to a microprocessor, not shown, for receiving and storing the value of logic signal LCS intended for output stage  26 . Each control block  18  further includes a logic switch  30  controlled by an enable signal VAL, connected between memory element  28  and output stage  26 . Logic switch  30  is provided to provide an inactive signal to output stage  26  as long as enable signal VAL is inactive, for example at a low logic level. Switch  30  is also provided for, when signal VAL is active, providing output stage  26  with signal LCS stored in memory element  28 . Signal VAL is conventionally activated for a predetermined duration after each activation of a screen line. 
       FIG. 3  is a timing diagram illustrating voltage V6 of a line  6 , enable signal VAL, voltage V8 of a column  8 , and current I 22  in transistor  22  of line control circuit  14 . At a time t0, the line is selected and voltage V6 switches from voltage VDD1 to voltage GND. Voltage V8 then is at GND. At a time t1, signal VAL is activated and column  8  is connected to potential VDD2, for a point to be lit. The selected cell charges between time t1 and a time t2 and voltage V8 switches from GND to VDD2. During this charge, transistor  22  conducts a first current peak P 1 . For physical reasons associated with the cell structure, a short time after this first current peak, a second current peak P 2 , more intense than the first one, occurs between times t3 and t4. As an example, time t1 may occur from 10 to 20 ns after time t0, time t2 may occur from 50 to 100 ns after time t1, and times t3 and t4 may occur from 150 to 200 ns after times t1 and t2, respectively. The charge of a cell can correspond to current peaks P 1  and P 2  respectively of 0.1 and 0.3 mA. A control circuit is conventionally used to control more than 3000 columns. Thus, if all the columns  8  of a selected line must be lit, the second current peak crossing transistor  22  can reach 1 A. Transistors  22  must have a large size to be able to conduct such a current. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a control circuit of the cells of a plasma screen, which is of reduced size and low cost. 
     To achieve this object, the present invention provides delaying the selection of the different columns so that the charge of the equivalent capacitors of the cells in a same screen line is not simultaneous. 
     More specifically, the present invention provides a method for controlling cells of a plasma screen of array type, formed of cells arranged at the intersections of lines and columns, including the step of sequentially applying to each line an activation potential and, during the activation of a line, applying an activation potential to selected columns, in which, while a line is activated, the selected columns are non-simultaneously activated. 
     According to an embodiment of the present invention, the activation of the selected columns is controlled by a single signal activating several control blocks, each of which controls with a specific delay the application of the activation potential to the column. 
     The present invention also aims at a circuit for controlling the cells of a plasma screen of array type, formed of cells arranged at the intersections of lines and columns, including line control blocks for sequentially applying, to each line, an activation potential, and including column control blocks for, as each line is activated, applying an activation potential to selected columns, each column control block including a means with a predetermined delay for delaying the application of the activation potential to the selected columns. 
     According to an embodiment of the present invention, the predetermined delay means of each column control block is connected to be activated by a same enable signal. 
     According to an embodiment of the present invention, each predetermined delay means delays the application of the activation potential to a selected column with a predetermined delay from its activation. 
     According to an embodiment of the present invention, each column control block includes:
         an output stage coupled to the column activated by the control block, and receiving an input signal,   a memory element for receiving and storing a column selection signal, and   a predetermined delay means including a NAND gate having a first input connected at the output of the memory element, a second input which receives said enable signal and an output connected to the input of the output stage via an inverter including a P-type MOS transistor, the dimensions of which are such that said inverter switches at a predetermined speed.       

     According to an embodiment of the present invention, the column control blocks form several groups, the column control blocks of a same group each activating a column with a same predetermined delay and each column control block including:
         an output stage coupled to the column activated by the control block, and receiving an input signal,   a memory element for receiving and storing a column selection signal, and   a predetermined delay means including a NAND gate having a first input connected at the output of the memory element, a second input which receives said enable signal and an output connected to the input of the output stage via an inverter supplied between a ground and a supply node, the supply nodes of the column control blocks of a same group being interconnected and separated from the supply nodes of the other column control blocks by a resistor, the supply nodes of a first group of column control blocks being connected to a supply voltage.       

    
    
     
       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 conventional plasma screen structure; 
         FIG. 2 , previously described, schematically shows a plasma screen connected to a conventional control circuit; 
         FIG. 3 , previously described, illustrates the charge of a cell of a line of the screen of  FIG. 2 ; 
         FIG. 4  schematically shows column control blocks according to the present invention; 
         FIG. 5  illustrates the charge of cells of a line of a plasma screen controlled by the control circuit according to the present invention; 
         FIG. 6  schematically shows an embodiment of a logic switch of a column control block according to the present invention; and 
         FIG. 7  schematically shows another embodiment of the logic switch of a column control block according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings, only those elements necessary to the understanding of the present invention have been shown. The same references represent the same elements in the difference drawings. 
       FIG. 4  schematically shows a circuit  12 ′ for controlling the columns of a plasma screen (not shown) according to the present invention. Circuit  12 ′ includes, for each column  8  of the plasma screen, a column control block  18 ′, an output  20  of which is connected to column  8 . Each control block  18 ′ includes an output stage  26  controlled by a logic column activation signal LCS, and a memory element  28  connected to receive and store the value of the logic signal to be provided to stage  26 . Each control block  18 ′ further includes a logic switch  30 ′ controlled by an enable signal VAL and connected between memory element  28  and output stage  26 . According to the present invention, the logic switch  30 ′ of each control block  18 ′ is provided for, when signal VAL is activated, providing the signal LCS stored in memory element  28  to output stage  26  with a predetermined delay. The logic switches  30 ′ of the different blocks  18 ′ may each introduce a different delay with respect to signal VAL, or they may be distributed into several groups of switches introducing the same delay. As the number of blocks  18 ′ introducing a different delay is increased, the number of cells is reduced, and therefore the number of equivalent capacitors of the cells which need to be simultaneously charged is reduced, and therefore the maximum current conducted by transistor  22  is reduced. 
       FIG. 5  shows various voltages and currents appearing upon operation of the circuit of  FIG. 4. V 8a, V8b, V8c represent the voltages of three columns connected to three blocks  18 ′ according to the present invention, the logic switches of which respectively introduce delays Da, Db, Dc. At a time t0, a line  6  is selected and its voltage V6 switches from potential VDD1 to potential GND. Voltages V8a, V8b, V8c then are at voltage GND. Signal VAL is activated at a time t1. The logic switches  30 ′ of the three blocks  18 ′ respectively generate activation signals LCSa, LCSb, LCSc at times t1a, t1b, t1c delayed by Da, Db, Dc with respect to time t1. Columns  8   a ,  8   b , and  8   c  are connected to potential VDD2 substantially at times t1a, t1b, and t1c. The capacitors of the cells connected to columns  8   a ,  8   b , and  8   c  respectively charge between times t1a and t2a, t1b and t2b, t1c and t2c. Transistor  22  conducts first current peaks P 1   a , P 1   b , P 1   c  on the order of 0.1 mA, each during the charge of each of the three capacitors. As seen previously, each charge is followed by a second current peak. Transistor  22  conducts three second current peaks P 2   a , P 2   b , P 2   c  on the order of 0.3 mA, each between times t3a and t4a, t3b and t4b, t3c and t4c. When all the columns  8  of a line must be lit by a by a column control circuit according to the present invention, the maximum current conducted by transistor  22  is only equal to the sum of the current peaks generated by blocks  18 ′ introducing the same delay. If, for example, blocks  18 ′ are distributed in three groups a, b, c respectively introducing a delay Da, Db, Dc, the present invention reduces by a factor of three the maximum current in transistor  22 . 
     It should be noted that in  FIG. 5 , the illustrated charge durations, that is, the width of the current peaks, and delays Da, Db, Dc, are such that the current peaks corresponding to the different delays are distinct. In practice however, the charge durations and the delays may be such that the different peaks overlap. 
       FIG. 6  schematically shows an embodiment of a logic switch  30 ′. Switch  30 ′ includes a conventional NAND gate  34 . The two input terminals of gate  34  are the two input terminals of logic switch  30 ′. The output of gate  34  is connected to output S of switch  30 ′ via an inverter  36 . Inverter  36  includes an N-type MOS transistor connected between the ground and output S and a P-type MOS transistor connected between output S and a supply line VDD, for example 3 or 5 V. According to the present invention, the width-to-length ratio (W/L) specific to the P-type MOS transistor of inverter  36  is used to obtain a specific delay. The W/L ratio of the P-type transistor especially determines the current that can be conducted by this transistor, and thereby, the speed at which switch  30 ′ can bring a load (stage  26 ) connected to its output S to a voltage corresponding to a high logic state. Thus, the W/L ratio of the P-type MOS transistor of inverter  36  enables adjusting the delay introduced by logic switch  30 ′. 
       FIG. 7  shows logic switches  30 ″ of a control circuit according to another embodiment of the present invention. Each logic switch  30 ″ includes a NAND gate  34 , the inputs of which form the inputs of the logic switch, and the output of which is connected to output S of logic switch  30 ″ via an inverter  38 . Each inverter  38  is supplied between a supply node A and the ground. According to the present invention, the logic switches  30 ″ are distributed into n groups G 1 , G 2 , . . . Gn (where n is an integer), introducing different delays.  FIG. 7  shows groups of two switches  30 ″. Nodes A of switches  30 ″ belonging to a same group are interconnected. Nodes A of the switches of group G 1  are connected to a supply voltage VDD. Nodes A of the switches of group G 2  are connected to nodes A of the switches of group G 1  via a resistor  40 . Similarly, nodes A of the switches of a group Gi (where i ranges between 2 and n) are connected to nodes A of the switches of group Gi- 1  via a resistor  40 . 
     In this embodiment, inverters  38  of switches  30 ″ of a same group have the same supply voltage and the inverters of two different groups have different supply voltages. The speed at which each inverter can bring a load (stage  26 ) connected to its output S to a voltage corresponding to a high logic state depends on the supply voltage of this inverter. Thus, the delays introduced by switches  30 ″ of groups G 1 , G 2 , . . . Gn depend on the supply voltage of the respective inverters  38  of these switches. The supply voltage of inverters  38  depends on the voltage drops in resistors  40  and these voltage drops depend on the number of inverters  38  with a state that switches. When the number of activated cells is large, which, in prior art, would cause high current peaks in transistor  22 , the number of inverters  38  having a state that switches is large and the voltage drops in resistors  40  are significant. As a result, the delays introduced by switches  30 ″ of groups G 1 , G 2 , . . . Gn are long, which reduces the current peaks in transistor  22 . When the number of activated cells is small, the number of inverters  38  having a state that switches is small and the voltage drops in resistors  40  are small. The delays introduced by switches  30 ″ of groups G 1 , G 2 , . . . Gn are then short and the line selection time is thus short. Such a control circuit thus operates at an optimal speed while having transistors  22  of reduced size. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, embodiments of the present invention in which the column activation signal is delayed from a single enable signal VAL have been described, but those skilled in the art will easily adapt the present invention to an embodiment in which several delayed enable signals VAL generated based on an initial signal VAL are used. 
     The present invention has been described in relation with logic switches ( 30 ′,  30 ″) provided for receiving and providing logic signals that are active at a high state, but those skilled in the art will easily adapt the present invention to logic switches provided for receiving and providing logic signals that are active at a low state. 
     Further, the present invention has been described in relation with a logic switch ( 30 ′,  30 ″), the output of which is provided by an inverter ( 36 ,  38 ) provided for introducing a predetermined delay, but those skilled in the art will easily adapt the present invention to a logic switch also including other elements (such as a logic NAND gate) provided for introducing a predetermined delay. 
     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.

Technology Category: 3