EP1299876A1

EP1299876A1 - Method and device for controlling a matrix electron source, with regulation by the emitted charge

Info

Publication number: EP1299876A1
Application number: EP01954091A
Authority: EP
Inventors: Pierre Nicolas; Denis Sarrasin
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2000-07-13
Filing date: 2001-07-12
Publication date: 2003-04-09
Anticipated expiration: 2021-07-12
Also published as: FR2811799B1; JP4874500B2; US7280088B2; FR2811799A1; US20040021623A1; JP2004504639A; DE60137425D1; EP1299876B1; ATE421134T1; WO2002007139A1

Abstract

Process and device for control of an electron source with matrix structure, regulated by the emitted charge. The invention is applicable to an electron source comprising addressing rows and columns which intersect to define emission areas, the electrons being supplied by the columns. According to this invention, the emission of electrons is triggered by increasing the potential of the columns to a value that will enable preferential emission of unaddressed rows. Then, throughout the emission duration, the potential of columns will be kept equal to this value, while simultaneously making measurements in the columns of the quantity of charges emitted by the pixels in the said columns. Secondly, when the quantity of charges measured on a column reaches a required charge quantity, the potential of this column is switched to a value that will block the emission of electrons. The invention is particularly applicable to flat field emission displays.

Description

METHOD AND DEVICE FOR CONTROLLING A MATRIX-STRUCTURED ELECTRON SOURCE WITH REGULATION

BY THE LOAD ISSUED

DESCRIPTION

TECHNICAL AREA

The present invention relates to a method and a device for controlling an electron source with a matrix structure.

Various sources of electrons or electron emitting devices are known. These known devices are based on physical principles which can be very different from each other.

Hot cathodes, photoemissive cathodes and microdots with field effect microdots are known for example (see document [1] which, like the other documents cited below, is mentioned at the end of this description), nanofissures with field effect (see document [2]) flat electron sources of the graphite or carbon diamond type (see document [3]) and devices called LEDs.

Such electron sources find applications mainly in the field of visualization with flat screens but also in other fields, for example physical instrumentation, lasers and X-ray emission sources (see the document [4]). Examples of the invention which will be given in the following are limited to the field of visualization, which is the largest (it includes flat screens).

However, the present invention is not limited to this field and applies to any device using one or more sources of electrons (and including the case of a 1 row x 1 column matrix) this is the case for example of a monopixel screen in pulsed operation. FIG. 1 schematically illustrates the operating principle of a display screen which uses a field emission electron source 2.

The screen of FIG. 1 also includes an anode 4 comprising an anode conductor 6.

The cathode, which constitutes the electron source 2, is generally voltage-controlled. Under the influence of this voltage, it emits a flow of electrons 8. Let us consider the particular case of a microtip screen which is schematically and partially shown in perspective in FIG. 2. This screen comprises a cathode comprising a substrate 10, provided with cathode conductors 12 on which microtips 14 are formed, and grids 16 formed above the cathode conductors and provided with holes 18 opposite the microtips. The screen also includes an anode comprising a substrate 20 and an anode conductor 22 which is located opposite the grids 16. Let us return to FIG. 1. We see the voltage source 24 making it possible to apply the high voltage V _a to the anode conductor 6. We also see polarization means 26 intended to apply the voltage V _g to the gate of the electron source 2 and the voltage V _c to the cathode of this source. We denote V _gc the control voltage which is equal to V _g -V _c . Cathode characteristics are shown in Figure 3 (curves I and II). We denote V _tll the threshold voltage. For a control voltage Vo greater than Vth, the curve I corresponds to a cathode current lo while the curve II corresponds to a current Io-Δl. ^'

The electrons emitted by the electron source are accelerated and collected by the anode subjected to the high voltage V _a . If a layer of phosphor material (in English "phosphor") 28 is deposited on the anode conductor 6 then the kinetic energy of the electrons is converted into light.

It is possible to obtain a display screen by organizing the basic structure of FIG. 1 in the form of a matrix structure. The latter must allow the addressing of each pixel of the screen and therefore the control of the luminance of the pixel considered (see document [5]).

A screen with a matrix structure using an electron source with a matrix structure 30 is schematically represented in FIG. 4. Each pixel of the electron source 30 is defined by the intersection of a line electrode and a storage electrode. column from this source. We note L _x , L ₂ ... Lj _. ... L _n the line electrodes of this source and the column electrodes of the source are denoted C _x ,

C ₂ ... C _j ... C _m . The screen in Figure 4 includes a line scan generator 34. This generator is provided with a source 36 of voltage Vι _ns and a source 38 of voltage Vχ _s . We denote by V _xi the control voltage of the line Lj _. . The screen also includes means 40 for generating the column control voltages. V _Cj denotes the control voltage of column C _j .

More precisely, a control circuit is assigned to each line and to each column of the screen and addressing is carried out one line at a time for a time tι _ig . The rows are therefore brought sequentially to a potential V _ls called row selection potential while the columns are brought to a potential corresponding to the information to be displayed. During this time t _iig the non-selected lines are brought to a potential Vχ _ns such that the voltages present on the columns do not affect the display on these lines. To obtain gray levels, it is possible to act on the value of the control voltages Vij _. - _Cj or over their duration _co this duration must remain less than or equal to t _lig .

Other control methods are possible. We know for example the control method using electrical charges, more simply called "charge control method" (see document [6]). There is also known a control method using a current, more simply called "current control method" (see document [7]).

We are interested in what follows in the various control methods and, more particularly, in the load control method. STATE OF THE PRIOR ART

The three control methods mentioned above do not provide a completely satisfactory solution for controlling an electron source with a matrix structure. There is generally a need to obtain a uniform and quantified electronic emission which is at the same time achievable without major technical constraint.

The voltage control in these different gray level obtaining methods is widely used because it is easy to implement. This assumes, however, that the electrical response of the electron source is both stable and uniform. However, these conditions of stability and uniformity are difficult to achieve in electron sources with known matrix structure. Indeed, a high requirement of uniformity for a screen leads to rejection rates which can be high. Likewise, we are faced with problems of differential aging which, by destroying the uniformity of the sources as a function of the more or less repeated use of such or such zone of the source, harm their real lifespan.

A current command may seem to solve this problem because we are then required to inject a current and therefore a determined quantity of electrons. Such a principle is effectively valid under static conditions. On the other hand, as soon as one wants to quickly vary the current of the electron source, one is confronted with a problem of capacitive load. Indeed, a column electrode is similar to a capacitor with respect to the lines that this column crosses and the current necessary for the rapid charging of this capacitor proves to be greater, by several orders of magnitude, than the emission current.

By way of example, in a microtip screen having a definition of 1/4 VGA and an area equal to approximately 1 dm ² , the capacity of a column relative to the lines C _co ι is approximately 400 pF. If we want to "turn on", that is to say excite a pixel, we pass the current of this pixel from an almost zero value to a value of about 10 μA and, to do this, we increases the line-column voltage by approximately 40 V. If the switching must be done in 1 μs (time which is to be compared to a line time of 60 μs), the capacitive current amounts to:

I = C _co ι-dV / dt i.e. around 16 mA. The capacitive current is thus 1000 times greater than the emission current that we want to adjust. It is understood that such a method is not suitable for the rapid control of a source with a matrix structure.

To solve the previous problem, a load command has already been proposed (see document [6]). FIG. 5 schematically illustrates a display screen comprising an electron source with a matrix structure using a charge command. The known screen of FIG. 5 differs from that of FIG. 4 only by the means for applying the control voltages to the columns of the source of the screen. In the case of Figure 5, the means 42 for applying a control voltage to a column, for example column C _j , comprise a logic block 44, which receives as input a line sync signal El, and a comparator 46, which receives as input a value setpoint A1 and which is connected to logic block 44 as seen in FIG. 5. The voltage application means 42 also include a three-state output stage 48 which is also connected to logic block 44 and receives voltages respectively denoted V _c _ _on and V _c _ _of f by unrepresented voltage sources. The three-state output stage and the comparator are connected to the corresponding column of the electron source (Cj in the example considered)

In the case of the command in charge, the column conductor considered is preloaded to ensure the emission of the sources (V _c _ _on ). Then the circuit is opened to let the column capacitor discharge on its internal impedance, until the floating potential V _C j reaches the set value Al corresponding to the quantity of electrons desired. The column is then brought back to the extinction potential (V _c -. _Off ). Such a way of doing things seems perfect but supposes the use of equally perfect components and the implementation of such a method proves difficult.

In fact, we saw above that a column electrode was similar to a capacitor with respect to the lines of the source with matrix structure, but there are also leakage currents which circulate between the column considered and the lines and these currents vary. with the potential difference between these electrodes. Therefore, when the circuit is opened, the voltage drop does not depend only on the emission current but also on leakage currents which themselves vary as a function of this drop. More precisely, this change in potential is required to measure the charge taken from the column's own capacity, but this variation poses a problem. Indeed, during the time t _ϋg each of the columns will leak with respect to the selected row but also with respect to all of the non-selected rows. For simplicity, it is assumed that this defect is similar to a leakage resistance R _ic identical for all the pixels. This value represents the row / column leakage impedance for any row and column. For a column and during the emission time, this leakage current I _{f is} expressed as follows:

I _f = If (ls) + 1f (lns) (Vι _s -V _cj (t)) / R _lc + (n-1). (V _lns -V _cj (t)) / R _lc

With:

I _f ≈ Leakage current of a column with respect to all rows I _{f (} i _s ) = Leakage current of a column with respect to the selected row

^I _f (i _ns ) = Leakage current of a column compared to the lines not selected Vι _s = Potential applied to the selected line Vι _ns = Potential applied to the lines not selected v _Cj (t) = Floating potential of column j during transmission time n = Number of lines.

To simplify, we can take V _lns equal to OV and, knowing that V _c j (t) is much less than Vι _s , we then have:

different from (V _lΞ / R _lc ) - (n-1). (V _cj (t) / R _lc )

This imposes severe constraints on the R _lc values of the different columns of the screen. Either the leakage currents are negligible (which corresponds to high Rj_ _c values) or they are not completely so and it is then necessary to ensure at least very good homogeneity of these resistors R _lc .

We also see that a single defective pixel from the point of view of Rχ _c imposes its leakage on the whole of the column considered, via the term (n- 1) of the formula given above. In the example considered, the drop in column voltage due to the emission is worth:

ΔV _cj = I • ti _g / C _co ι, so that, with 1 = 10 μA, tu _g = 50 μs and C _co ι = 400pF, we obtain ΔV _cj = 1.25V. It is recalled that this variation ΔV _Cj must be compared with the set value Al. This variation of the voltage ΔV _C j depends on the value of the capacity of the column, which brings back technological variables of the screen (related to the dimensions from this screen) in the control circuit design parameters. For its implementation, we also see that the comparator 46 is located at the output stage of the assembly forming the means for generating the column control voltages. This means that this comparator must either support the voltage dynamic range necessary for controlling the columns (approximately 40 V) or be able to isolate itself from this output by an additional stage.

STATEMENT OF THE INVENTION

The object of the present invention is to remedy the various preceding drawbacks. It relates to a method for controlling a source of electrons with a matrix structure, this source comprising at least one line and at least one addressing column, the intersection of which defines one or more emissive zones called pixels and where the electrons are supplied by the column, this process being a sequential process characterized in that:

- firstly, the emission of electrons is triggered by application of potentials on the selected line and the column (s) with a vapor capable of allowing this emission, then the potential of the column (s) is maintained at this value throughout the duration of the emission while, simultaneously, the measurement of the quantity of charges emitted by the pixel or pixels respectively of said column or columns is ensured in the column or columns, and

- secondly, when the quantity of charges measured on a column reaches a required quantity of charges, the potential of this column has a value which blocks the emission of electrons.

According to a preferred embodiment of the method which is the subject of the invention, the value capable of allowing transmission is equal to the potential of the unaddressed line or lines.

The invention also relates to a device for controlling a source of electrons with a matrix structure, this source comprising at least one line and at least one column of a-dressage, each intersection of which defines an area called a pixel and where the electrons are supplied by the column, this device being characterized in that it comprises:

means for controlling the address line or lines by applying a selection potential to the selected line, while outside the selection time the line or lines remain at a potential ensuring the blocking of the emission of the pixels corresponding, - means for controlling the column or columns, these control means comprising, for each column, means of application, during a line selection, either of a first voltage ensuring the emission of either a second tension ensuring blocking on said column,

means allowing both the measurement, in the column or columns, of the quantity of charges emitted during the emission and the constant maintenance of the voltage ensuring the emission on the said column during this measurement, and means for comparing the quantity of charges measured with a quantity of reference charges, with feedback on the control means of the columns. According to a particular embodiment, the quantity of charge already emitted is converted into a voltage level. The device which is the subject of the invention may further comprise means for compensating for residual leakage currents. This device can also include means for compensating intercolumn capacitive couplings.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limiting, with reference to the appended drawings in which: "Figure 1 schematically illustrates the operating principle of a display screen using a field emission device and has already been described, "FIG. 2 schematically illustrates the structure of a microtip screen and has already been described,

"FIG. 3 shows the characteristics I _cat = f (V _gC ) in the case of a microdot screen of the triode type and has already been described,

"FIG. 4 schematically illustrates a display screen using a device for transmitting field with a matrix structure and has already been described, • Figure 5 is a schematic view of a known device for controlling an electron source with a matrix structure and has already been described, »Figure 6 is a schematic view d '' a particular embodiment of the device object of

1 invention "Figure 7 illustrates schematically an example of a column control device in a device according to the invention," Figure 8 is a timing diagram of various voltages used in the device of Figure 7, ^has the FIG. 9 schematically illustrates a variant of FIG. 7, "FIG. 10 schematically illustrates an example of a device for controlling a column with compensation for the leakage current, in a device according to the invention,

"FIG. 11 schematically illustrates an exemplary embodiment of a device for controlling a column with filtering by diodes of the parasitic loads due to the inter-column capacities, in accordance with the invention,

"FIG. 12 diagrammatically illustrates an exemplary embodiment of a device for controlling a column with filtering by parasitic loads due to the inter-column capacities, in accordance with the invention, and "FIG. 13 schematically illustrates an exemplary embodiment of a device for controlling a column with analog compensation for parasitic loads due to the inter-column capacities, in accordance with the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The load control technique, which has been described above and which is also mentioned in document [6], therefore poses the main problem of the evolution of the potential of the columns ordered.

Consider in fact the expression of the leakage current I _fU i _te that we have seen previously:

I _f ui _te = 1 _f ui _t e _ls ^{+ T} - _f ui _te lns = (Vι _s -V _c - _OI1 (t)) / R _ic + (n-1) X (V _lns -

This expression clearly highlights the leakage current component with respect to the selected line and the leakage current component with respect to the (n-1) unselected lines. The first of these components is inevitable since it is linked to the very principle of screen scanning. The second of these components can be canceled provided that V _0j (t) and Vι _ns are both equal to the same constant.

The present invention provides a control circuit which operates under these conditions.

We have seen above the different functional blocks necessary for load control in the prior art (document [6]) in conjunction with the FIG. 5. The different functional blocks of a device according to the invention are schematically represented in FIG. 6. In this FIG. 6, the reference 50 represents the means for controlling a column of the screen (Cj). These control means 50 comprise, as can be seen, a control logic 52, a comparator 54, a current integrator with control of V _co χ and an output stage 58.

In this example of the invention, the following functions are performed chronologically. The pixel of this column C _j is initialized in transmission

(V _Cj = V _c _ _on ) using the output stage 58. The current supplied by the emitters is integrated while keeping the column potential stable at the value V _c - _on . To do this, we use a functional block which will be described below. A voltage is thus obtained in A2 (see FIG. 6) which is proportional to the charge emitted. The emission of the pixel is cut (V _Cj = V _c _ _{0 f} ) by the output stage 58 when the required charge, chosen by an external reference value Al, has been supplied. In this operating mode and during the emission of the pixel considered equation (1) becomes equation (2):

I _f ui _t e = I _f ui _{te l} s + Ifuite lns ⁼ (Vι _s -V _c j _ _on ) / Rχ _c + (n- 1) X (V _lns -V _c _{j_} _on ) / Rj _.c

The column tension has become fixed and

V _cj - _on (t) and V _c _ _orι are both equal to the same constant value. We can then cancel the leakage term with respect to (n-1) lines by choosing V _c _ _on equal to Vι _ns . For the sake of simplification, we define these two potentials as being equal to the mass reference of the entire device. We then obtain:

I _f i _te = I _fu i _{te ls} ⁼ Vι _s / Rι _c (3)

We can immediately see the advantage of the control method used in this example of the invention since, even if there is a leakage current, the latter only depends on the addressed pixel and no longer on the (n-1) other pixels. not addressed in the same column. In other words, ^'the method of addressing used in this example of the invention allows, for the same screen (in terms of resistance R _e), better image quality.

Under these conditions, a compensation of this residual current is possible because, the voltage V _iΞ being fixed, this current is constant. We can therefore inject, on each column and during each row time, the current of opposite sign.

The invention therefore relates to a sequential control process for an electron source which ^allows':

- maintaining, throughout the duration of the transmission, the potential of the columns equal to that of the unaddressed lines as well as the simultaneous measurement of the quantity of charges emitted by the pixels of these columns,

the return of this column potential to a level ensuring the blocking of the emission when the quantity of charge measured reaches a quantity of charge required. We now consider an example of a column control device, in accordance with the invention, which is shown in FIG. 7.

This control device 60 comprises an output stage 62 of the push-pull type, a current integrator assembly 64 and a comparator 66.

The output stage 62 makes it possible to switch, on the column electrode (C _j ), either the supply voltage V _c _ ₀ f _f corresponding to the pixel extinction level or the input- of the integrator circuit 64 which imposes by its virtual mass the level V _c _ _0n (putting it at the potential of the lines not selected). The output stage 62 comprises in known manner means 68 of logic level translation and two MOSFET transistors 70 and 72, the transistor 70 being of type P and the transistor 72 of type N, these means 68 and these transistors being arranged as seen in Figure 7.

The integrator assembly 64 comprises an amplifier 74 which is looped over a capacitor 76 of capacity Ci _nt which is itself mounted in parallel with a controlled switch SW1, the output A2 of this amplifier being connected to the input (-) of the comparator 66. The controlled switch enables the potential A2 to be brought to zero at the start of each line.

The comparator input (+) is connected to a reference voltage Al corresponding to the quantity of charges to be emitted. In the present invention, this voltage setpoint can be provided by various means which depend on the desired application of the invention. In the example shown in FIG. 7, a digital analog converter CDA is used which receives as input a digital datum DN of setpoint voltage and whose output provides the setpoint potential Al.

The output S2 of the comparator assembly constitutes the control of the push-pull output stage thus allowing the device to be looped.

The signal SI (corresponding to the start of the time which is allocated to a line), - according to a chronology which will be described below, controls the switch SW1. It can be seen that the control logic 52, which supplies SI, also controls a line control circuit PL, not shown. FIG. 8 represents the time diagram of the different voltages within the device, during a line addressing cycle. The cycle (see figure 8 part A) starts at time t ₀ , by the signal start signal SI (see figure 8 part B) triggering the rise of S2 (see figure 8 part C) which, by output stage, changes column V _c j to V _c - _on (virtual mass). After a time allowing V _c j to settle at the voltage V _c _ _on (time t _on ), the signal SI goes low to open the switch SWl, which begins the integration of current in Cι _nt • To trigger the emission, V _Li goes from its potential Vι _nΞ (defined as the mass of the assembly) to the selection potential Vι _s , the set Ul and C _int then loads into A2 (see Figure 8 part D ) , according to the law :

A2 = -I _x t / C _int . When the potential A2 reaches the set potential Al, the comparator U2 switches to its output S2 (descent of S2), which, through the output stage, requires the return of V _C j to V _c _ _ofÊ (see the Figure 8 part E), at time t = t _off such that:

Q = I. (t _0ff -t _on ) = C _int xAl.

It can therefore be seen that the device described makes it possible to deliver to the pixel considered a charge controlled by the setpoint supplied A1, ^" and this without variation of the voltage applied to the column, during the transmission time.

It will be noted that the line potential V _L i is switched over to the selection potential Vι _s , after the column potential has been established (V _Cj ), so as to reduce the capacity to charge only to that of the pixel considered. . The capacitive current in the column will therefore be minimized.

If the potential V _L ι increases before t _on , the emission current is established before the start of integration (and the corresponding charges are therefore not measured). If V _L i rises during or after the start of integration (t _on ), the charges corresponding to the capacitive pixel current are measured and result in an initial voltage offset on A2. A slight difference in phase, between the rise of V _Li and the falling edge of Si, can therefore be adjusted to adopt the best compromise according to the application.

To avoid unnecessarily switching to V _c _ _on a column to display black, it should be noted that this level can be managed directly by the control unit while keeping the signal SI in the corresponding column low.

Another embodiment of a column control device according to the invention is shown diagrammatically in FIG. 9. It is a variant of FIG. 7.

In summary, the previous system (FIG. 7) converts the quantity of charge already emitted into a voltage level, which makes it possible to switch the piloting of the column control stage at time _{0 f} when the quantity of charge ( Q _re f) the setpoint is reached.

A similar result can be obtained by using a DC current-voltage converter type assembly. The current (I _j ) being stable during the line time, the instantaneous measurement of this current, associated with a CCN circuit of digital or analogical calculation, makes it possible to calculate, from the start of the line time, the time t _off of switching of the column, time such that t ₀ £ = Qref / lj-

This solution is shown in FIG. 9. In this figure, the switch SW2 makes it possible to directly discharge the currents outside the measurement instant. Indeed, during row / column switching, high capacitive currents could disturb the CCT voltage current converter.

It can be seen in FIG. 9 that the CCT converter comprises the amplifier 74 already used in the example of FIG. 7 but associated, in the case of FIG. 9, at a resistor R mounted between the input (-) and the output of the amplifier 74.

It can also be seen that the CCN circuit receives digital or analog data from appropriate DNA means.

We now consider the compensation for residual leakage currents.

To inject, on each column and during each row time, a compensation current of sign opposite to I _fU i _te i _s it suffices to connect a current source to the measurement input of the integrator (see figures 6 and 7). This may for example consist of a transistor mounted as a current generator or of a resistance R _Comp controlled by an adjustable voltage generator GT, as seen in FIG. 10.

We now consider another aspect of the invention, relating to the compensation of inter-column capacitive couplings. When switching the potential of any column j from V _c - _on to V _c _ _0ff , a parasitic charge Qar = C _par is induced on the neighboring columns (j-1) and (j + 1). (C _by (Vc-on-o-of); C _by being the intercolumn coupling capacity. If the columns (j-1) or j + 1) are at this instant, still be in transmission, this charge Q _by will then be measured by the integrators located on these columns, thus distorting the measurement of the charge emitted by the pixels of said columns. This charge Q _by being constant for a given screen format, several solutions are possible to solve this problem. These solutions are can be combined with one another to achieve the specifications for the number of gray levels desired. Apart from technological improvements aimed at reducing capacity C _by , two main classes are possible:

I) Avoid the charge Q _by being measured by the charge integrator, which requires analog filtering solutions, solutions to be implemented upstream of this same integrator: Figure 11 presents an example of diode filtering parasitic loads due to the inter-column capacities. This solution is an asynchronous solution, which is based on fast switching diodes, responding much faster than the integrator. In other words, we play on the fact that the variations of the capacitive currents are rapid compared to those of the emission current, practically zero variations in steady state during the line time. In the same state of mind, we could try to implement analog or logic filters, filters used to discriminate the emission currents from the parasitic capacitive currents.

In the example of FIG. 11, two filter diodes DF1 and DF2 are used to filter the parasitic loads due to the inter-column capacities.

FIG. 12 provides another exemplary embodiment of a device for controlling a column with filtering, this time by transistors, parasitic loads due to the inter-column capacities. This solution is of the synchronous type. The comparator output is this time revalidated by logic to supply S2 at specific times. The switching times of the columns from V _c _ _on to V _c _ _ofÊ are now fixed. It is therefore possible, synchronously, to prevent the capacitive currents associated with this consumption from being integrated into the measurement of loads. In the example of FIG. 12, it suffices to close SW2 and to open SW3 in synchronism with S2, and this for all the columns. After a minimum time necessary for the "evacuation" of the capacitive currents (if the neighboring column or columns have switched), one returns to the standard measurement mode by switching to SW2 open and SW3 closed. The operating frequency (F _sw ) of switches SW2, SW3, must be fast enough to be compatible with the number of gray levels desired (N _gr i _s ). We must respect the following condition: F _sw > N _grαs . Fι _ιgne, _gne Fi being the frequency of addressing lines of the screen.

II) Compensate this charge Q _by (since it is a fixed charge), which makes it possible to implement solutions of analog or digital type downstream of the integrator:

FIG. 13 shows an exemplary embodiment of a device for controlling a column with analog compensation for parasitic loads due to the inter-column capacities of the neighboring columns. It can be seen that, for any column j, the switching signals, at V _c _ _0ff / of columns j-1 and j + 1 make it possible to readjust, using an adder, the setpoint level Al d ' a quantity V _par = (Q _par ) / C _ιn t • Obviously, this addition can be carried out digital upstream of the digital to analog converter CDA.

In the example in Figure 13, the adder has the reference ADD. The switching signals of columns j-1 and j + 1 have the references S2 _j _ι and S2j + ι respectively.

It can be seen that these signals control respective switches SWj and SWj ₊₁ which are connected to the adder ADD as shown in FIG. 13.

Various advantages provided by the invention are given below.

The control method proposed in this invention consists in summary of a command at constant column voltage, of the Pulse Width Modulation (PWM) type, width controlled by the load emitted. Such a column control circuit, an embodiment of which has just been given provides various advantages: - a limitation of the leakage currents of the columns, to those of the single row addressed, which, for the same screen, makes it possible to obtain a better image quality in terms of uniformity,

a stabilization over the line time of this residual leakage current, a current which becomes independent of the quantity of charges which it is desired to have emitted by the pixel considered,

- always for this leakage current, its effect in the current integrator circuit becomes linear over time, which could simplify any compensation for this leakage current, - unlike the command in charge of the prior art, this command mode maintains a constant column voltage during transmission, which makes it possible to stay at the maximum of the emission of the pixel considered, and therefore for a given line time , maximum shine,

- the proposed control circuit is "independent" of the technological and dimensional characteristics of the screen - the control circuit fully decouples, with regard to the voltages, the functions for measuring the charge emitted (integrator plus comparator), from those of the output stage. One can for example imagine measurement functions operating at 5 volts, while the potentials of the columns switched by the output stage are worth several tens of volts.

The documents which are mentioned in the present description are the following:

[1] Fluorescent microtip screens, R. Baptist,

L'onde électrique, November-December 1991, vol.71, n ° 6, pp 36-42 [2] Fiat panel displays based on surface-conduction electron emitters, K. Sa ai et al., Proceedings of the 16th. international display research conference, ref.18.3L., pp 569-572 [3] Carbon nanotube FED elements, S. Uemura et al.,

SID 1998 Digest, pp 1052-1055 [4] Recent progress in field emitter array development for high performance applications, Dorota Temple, Materials science & engineering, vol.R24, n ° 5, January 1999, pp 185-239 [5] Microtips displays addressinging, T. Leroux et al., SID 91 Digest, pp 437-439 [6] FR 2632436 A, Process for addressing a fluorescent microtip matrix screen, Invention of JF Clerc and A. Ghis, corresponding to EP 0345148 A and also to US 5138308 A [7] US 5359256 A, Regulatable field emitter device and method of production thereof, HF Gray.

Let us return to the present invention. Load control circuits for operating electron sources are known from documents WO 96 05589 and US 6020804. These circuits make it possible to apply voltages to the rows and columns of a matrix source in order to allow the 'emission of electrons and measure the quantity of charge emitted to compare it with a set value.

The fundamental difference between these known techniques and the present invention lies in the fact that, in these known techniques, the measurement of the charge takes place "on the anode side" where the high voltage (a few kV) is established, whereas in the invention it is carried out "cathode side", that is to say on the low voltage side (a few tens of volts) and, moreover, simultaneously with the emission.

However, the measurement of the load is generally made on a resistor and causes a voltage variation of the order of IV taking into account the other quantities involved. This measurement voltage comes disturb the supply circuit: in the prior art, the variation of 1 volt with respect to the applied kV causes a negligible error. In the invention, the error would become very large (one volt relative to a few tens of volts) and absolutely unacceptable.

The measurement techniques disclosed in the above-mentioned documents are therefore unusable "cathode side". The problem thus posed is resolved by the present invention.

Claims

1. Method for controlling an electron source with a matrix structure, this source comprising at least one line and at least one addressing column, the intersection of which defines one or more emissive zones called pixels and where the electrons are supplied by the column, this process being a sequential process, characterized in that: - firstly, -the emission of electrons is triggered by application of potentials on the selected line and the column (s) at a value capable of enabling this emission then the potential of the column or columns is maintained at this value throughout the duration of the emission while, simultaneously, the measurement of the quantity of charges emitted by the pixel or pixels respectively of said column or columns is ensured in the or the columns, and - in a second step, when the quantity of charges measured on a column reaches a required quantity of charges, the potential of this collar is switched onne a value which ensures the blocking of the emission of electrons.

2. Method according to claim 1, in which said value is equal to the potential of the unaddressed line or lines.

3. Device for controlling an electron source with matrix structure, this source comprising at least one row and at least one addressing column, each intersection of which defines an area called pixel and where the electrons are supplied by the column, this device being characterized in that it comprises:

means for controlling the address line or lines by applying a selection potential to the selected line, while outside the selection time the line or lines remain at a potential ensuring blocking of the emission of the pixels corresponding, - means for controlling the column or columns, these control means comprising, for each column, means of application, during a line selection, either of a first voltage ensuring the emission of either a second tension ensuring blocking on said column,

means allowing both the measurement, in the column or columns, of the quantity of charges emitted during the emission and the constant maintenance of the voltage ensuring the emission on the said column during this measurement, and

means for comparing the quantity of charges measured with a quantity of reference charges, with feedback on the control means of the columns.

4. Device according to claim 3, in which the quantity of charge already emitted is converted into a voltage level.

5. Device according to claim 3, further comprising means for compensating for residual leakage currents.

6. Device according to claim 3, further comprising means for compensating for inter-column capacitive couplings.