Patent Application
20060176249
1. Field of the Invention
The present disclosure relates to plasma displays, and more particularly, to a technique of generating voltages for electrodes of a pixel in a plasma display in a manner that maintains stability of a discharge of gas in a region of the pixel.
2. Description of the Related Art
A plasma display includes a front plate and a rear plate sealed together and having a space therebetween filled with a dischargeable gas. The front plate includes horizontal rows of electrodes, each row being configured with a sustain electrode in parallel with a scan electrode. The scan electrodes and the sustain electrodes are covered by a dielectric layer and a magnesium oxide (MgO) layer. The rear plate supports vertical barrier ribs and plural vertical column conductors. In a color display, individual column electrodes are covered with red, green, or blue (RGB) phosphors. A pixel is defined as a region proximate to an intersection of (i) a scan electrode and a sustain electrode, and (ii) three column conductors, one for each color. In a monochrome display, a single column conductor is used for each pixel, and a phosphor combination is used to achieve the monochromatic color. Visible light is emitted by the phosphors following UV excitation, produced when a voltage of a sufficient magnitude is applied across a volume of the gas to cause the gas to discharge. When the gas discharges, the atoms of the gas are excited, when the atoms relax, the atoms emit UV photons, which, in turn, excite the phosphor.
In operation, the plasma display partitions a frame of time into sub-fields, each of which produces a portion of the light required to achieve a proper intensity of each pixel. Each sub-field is partitioned into a setup period, an addressing period and a sustain period. The sustain period is further partitioned into a plurality of sustain cycles.
The setup period resets any ON pixels to an OFF state, and provides priming to the gas and to the MgO surface to allow for subsequent addressing. In the setup period, each interior surface of the pixel's electrodes is placed at a voltage very close to a firing voltage of the gas.
During the addressing period, the sustain electrodes are driven with a common potential, while scan electrodes are driven such that a row of pixels is selected so that pixels in that row can be addressed via an addressing discharge triggered by an application of a data voltage on a vertical column electrode. Thus, during the addressing period, each row is sequentially addressed to place desired pixels in the ON state.
During the sustain period, a common sustain pulse is applied to all scan electrodes to repetitively generate plasma discharges at each pixel addressed during the addressing period. That is, if a pixel is turned ON during the address period, the pixel is repetitively discharged in the sustain period to produce a desired brightness.
The dimension of space between adjacent electrodes, and the overall width of the electrodes, influence the pixel's discharge capacitance, which in turn influences discharge power and therefore brightness. Each discharge yields a certain level of brightness, and therefore a number of discharges in a predetermined period of time is chosen to meet an overall brightness requirement for an image being displayed.
For the setup operation to precisely control the interior surface voltage of the electrodes within the pixel, ramping waveforms are employed. The slope of the ramp allows the gas breakdown to be exceeded causing weak positive resistance discharges to reduce the voltage to just below the gas breakdown voltage. In the three-electrode topology of the pixel, it is desirable to setup of all three electrodes uniformly.
There is provided a method that includes generating a first waveform for a first electrode of a pixel of a plasma display, and generating a second waveform for a second electrode of the pixel. The first waveform includes a first setup waveform and a first sustain waveform, and the second waveform includes a second setup waveform and a second sustain waveform. The first and second setup waveforms are generated during a setup period for setting a wall charge of the pixel, and the first and second sustain waveforms are generated during a sustain period for sustaining a discharge of a gas in a region of the pixel. The first setup waveform attains a voltage that is more positive than a maximum positive voltage of the first sustain waveform, and the second setup waveform attains a voltage that is more negative than a maximum negative voltage of the second sustain waveform. There is also provided a plasma display and a controller that employ the method.
A pixel in a plasma display has inherently symmetrical properties as its front electrodes typically have equal widths and so, are symmetrical about a sustain gap separating the electrodes. A gap between the front electrodes and a back electrode is constant, therefore, a voltage relationship of each front electrode to the back electrode is inherently symmetrical. A technique for driving the pixel, as described herein, takes advantage of the symmetrical nature of the electrodes to provide stable gas discharges during setup operations for the pixel.
The MgO layer covering dielectric layer 105 aids in a discharge of the gas by providing secondary electron emission, necessary for the gas discharge to avalanche. Additionally, MgO will emit electrons at a slow rate for a long period of time following discharge activity. This emission is an aid to addressing a plasma display.
Pixel 102 includes several capacitances represented in
Capacitances C1 and C2 represent capacitances between the surface of dielectric layer 105 and phosphor surface 110. Capacitance C4 represents a capacitance between electrode A and the surface of dielectric layer 105. Capacitance C5 represents a capacitance between electrode B and the surface of dielectric layer 105. Capacitances C4 and C5 are of much larger values than capacitances C1 and C2, respectively, and so, for purposes of the present discussion, voltage drops across capacitances C4 and C5 are considered as being negligible. Thus, for the present discussion, a voltage on electrode A is regarded as being the same as a voltage at the top of C1, and a voltage on electrode B is regarded as being the same as a voltage at the top of capacitance C2. Capacitance C3 represents a capacitance within the gas between electrode A and electrode B.
The gas has a breakdown voltage. If a voltage across a volume of the gas exceeds the breakdown voltage, the gas discharges. Depending on the magnitude of the voltage across the gas, the discharge will be either a weak positive resistive discharge or a strong negative resistance discharge.
The weak positive resistance discharge occurs when the voltage applied across the volume of gas is only slightly greater than the breakdown voltage. During the weak positive resistance discharge, the gas retains a resistive characteristic, and the discharge persists only until the voltage is reduced to marginally less than the breakdown voltage. Thus, when the discharge ends, the voltage across the gas is marginally less than the breakdown voltage.
The strong negative resistance discharge occurs when the voltage applied across the volume of gas is more than slightly greater than the gas discharge voltage. During the strong negative resistance discharge, the resistance of the gas becomes very low, and all voltage across the gas is dissipated. Thus, a strong negative resistance discharge is an avalanching discharge.
Because the gas is dischargeable, the maximum voltage obtainable across capacitances C1, C2 and C3 is the breakdown voltage of the gas. For example, if the voltage across capacitance C1 exceeds the breakdown voltage by a small amount, a positive resistance discharge occurs, reducing the voltage across capacitance C1 to, or marginally below, the breakdown voltage. Similarly, exceeding the breakdown voltage by a small amount across either of capacitances C2 or C3 results in a positive resistance discharge that reduces the voltage across capacitance C2 or C3, respectively, to, or marginally below, the breakdown voltage. However, if the voltage across any of capacitances C1, C2 or C3 is exceeded by a sufficient amount, a strong negative resistance discharge occurs that reduces the voltage to zero across all three capacitances.
Electrode A and electrode B may be driven to create a weak positive resistance discharge across capacitance C3 while creating very little discharge activity across capacitances C1 and C2. This is accomplished by applying voltage waveforms to electrodes A and B that are approximately equal in peak-to-peak voltage but opposite in polarity. Dielectric layer 105 has a dielectric area DA adjacent to electrode A, and a dielectric area DB adjacent to electrode B. The weak positive resistance discharge across capacitance C3 sets a charge on dielectric area DA and dielectric area DB such that the voltage across capacitance C3 is slightly below the breakdown voltage of the gas. Similarly, a slight discharge activity is required across capacitances C1 and C2 to reference the dielectric areas DA and DB to phosphor surface 110. That is, with data electrode 115 at a fixed voltage, electrode A may be driven to create a weak positive resistance discharge across capacitances C1 and C3, and electrode B may be driven to create a weak positive resistance discharge across capacitances C2 and C3. With voltages across each of capacitances C1, C2 and C3 being close to the gas breakdown voltage, an addressing voltage applied to data electrode 115 during an addressing operation can be minimized. The minimum peak-to-peak voltage to create a weak positive resistance discharge across capacitance C3 is typically slightly greater than twice the breakdown voltage across capacitances C1 and C2. This is of particular importance for a case in which electrode A is used for row selection, since the strong address discharge is triggered by a weak discharge between the phosphor surface 110 and a selected row. Once the address discharge is triggered, a strong negative resistance discharge depletes the voltage across all three capacitances C1, C2 and C3.
Controller 104 includes a module 106 and a module 108. Module 106 generates voltage waveforms for electrode A, and module 108 generates voltage waveforms for electrode B. The operation of controller 104 is described below in greater detail in association with
The term “module” is used herein to denote a functional operation that may be embodied either as a stand-alone component or as an integrated configuration of a plurality of sub-ordinate components. Controller 104, and modules 106 and 108, can be implemented in any of hardware, firmware, software, or a combination thereof.
The waveform for electrode A attains voltages Va, Vb and Vc. The waveform for electrode B attains voltages Vd, Ve and Vf. Voltages Va, Vb and Vc are referenced to a point on phosphor surface 110 at the bottom of capacitance C1, and voltages Vd, Ve and Vf are referenced to a point on phosphor surface 110 at the bottom of capacitance C2. Voltages Va, Vb and Vc are approximately equal to voltages Vd, Ve and Vf, respectively. Consequently, the peak-to-peak voltages applied to electrode A and to electrode B are about equal to each other. That is Va-Vc is about equal to Vd-Vf. Furthermore, each of the peak-to-peak voltages, i.e., Va-Vc and Vd-Vf, are about equal to twice the breakdown voltage of the gas.
During setup1, controller 104 generates a ramp having a negative slope for electrode B while generating a higher level for electrode A. The ramp traverses from voltage Ve to a maximum negative voltage of Vf. The level generated for electrode A is at voltage Va. This configuration of waveforms produces a weak positive resistance discharge between electrode A and electrode B such that at the end of the ramp, the voltage across capacitance C3 within the gas volume is at, or near, the breakdown voltage of the gas. Additionally, the maximum positive voltage on electrode A, Va, is chosen so that the voltage across capacitance C1 will be close to or slightly over the breakdown voltage of the gas across capacitance C1. Similarly, the maximum negative voltage applied to electrode B, Vf, is chosen so that the voltage across capacitance C2 will be close to or slightly over the breakdown voltage of the gas across capacitance C2. Thus at the end of setup1, if voltage on electrode A is increased positively form Va, a discharge activity will occur across capacitances C1 and C3 with a current flow sourced from electrode A. Similarly, if voltage on electrode B is increased negatively from Vf, then a discharge activity will occur across capacitances C2 and C3 with a current flowing into electrode B. Thus Va is the upper bound for electrode A to prevent discharge activity, and Vf is the lower bound for electrode B to prevent discharge activity.
During address1, controller 104 maintains voltage Va for electrode A, and generates a row select pulse 205 for electrode B. If a data voltage (not shown) is applied to data electrode 115 coincident with row select pulse 205, that is, if pixel 102 is addressed, then a strong negative resistance discharge forms across capacitance C2, which extends across capacitance C3, resulting in a depletion of the voltages across capacitances C1, C2 and C3, and a charging of capacitances C4 and C5. With capacitances C1, C2 and C3 short-circuited by the gas discharge, capacitances C4 and C5 form a capacitive divider between voltages Va and Vf, achieving a level close to voltages Vb and Ve respectively. Thus, during address1, controller 104 generates an addressing waveform for electrode B to enable an initial gas discharge of pixel 102 during sub-field1.
During sustain1, controller 104 generates a plurality of sustain pulses for each of electrodes A and B. The sustain pulses for electrode A achieve a maximum positive voltage of Va and a maximum negative voltage of Vb. The sustain pulses for electrode B achieve a maximum positive voltage of Ve and a maximum negative voltage of Vf. If pixel 102 was addressed during address1, the sustain pulses on electrodes A and B during sustain1 will cause the gas in the region of pixel 102 to be repetitively discharged.
Because an application, across the gas, of a voltage of greater than the breakdown voltage causes a gas discharge, the magnitudes of the sustain pulses are effectively limited by the breakdown voltage. Since setup1 established a positive boundary on electrode A, the sustain pulses applied to electrode A during sustain1 are limited to a voltage nearly equal to, or below, voltage level Va to prevent an erroneous discharging of capacitance C1. During the sustain period, OFF pixels, which were not addressed, need to remain off. This is guaranteed if the bounds established during the setup operation are not exceeded. Similarly, electrode B's low level during sustain1 is limited to level Vf to prevent an erroneous discharging of capacitance C2.
During setup2, controller 104 generates a ramp having a negative slope for electrode A while generating a higher level for electrode B. The ramp traverses from voltage Vb to a maximum negative voltage of Vc. The level generated for electrode B is at voltage Vd. This configuration of waveforms produces a weak positive resistance discharge between electrode A and electrode B such that at the end of the ramp, the voltage across capacitance C3 within the gas volume is at, or near, the breakdown voltage of the gas. Thus, electrode A is set to a negative boundary level, while electrode B is set to a positive boundary level. Operation of setup2 is similar to setup1, except that electrode A, rather than being setup with an upper bound, is being setup with a lower bound of voltage Vc, and electrode B, rather than being setup with a lower bound, is being setting up with a positive bound of voltage Vd. Thus, after execution of setup1 and setup2, the operating range of both electrode A and electrode B are defined, and phosphor surface 110 covering data electrode 115 is referenced to both electrode A and electrode B. The configuration of waveforms in setup2 also prepares pixel 102 for address2. More specifically, if pixel 102 was ON during sustain1, the waveforms in setup2 return pixels 102 to the OFF state, and if pixel 102 was not addressed during address1, and therefore not ON during sustain1, then pixel 102 will be setup by the falling ramp on electrode A to the opposite boundary compared to setup1.
During address2, controller 104 generates a row select pulse 210 for electrode A. If a data voltage (not shown) is applied to data electrode 115 coincident with row select pulse 210, that is, if pixel 102 is addressed, then a strong negative resistance discharge forms across C1 which extends across capacitance C3 resulting in a depletion of the voltages across capacitances C1, C2 and C3, and a charging of capacitances C4 and C5. Thus, during address2, controller 104 generates an addressing waveform for electrode B to enable an initial gas discharge of pixel 102 during sub-field2.
During sustain2, controller 104 generates a plurality of sustain pulses for each of electrodes A and B. The sustain pulses for electrode A achieve a maximum positive voltage of Vb and a maximum negative voltage of Vc. The sustain pulses for electrode B achieve a maximum positive voltage of Vd and a maximum negative voltage of Ve. If pixel 102 was addressed during address1, the sustain pulses on electrodes A and B during sustain1 will cause the gas in the region of pixel 102 to be repetitively discharged.
Note that the number of sustain pulses in sustain2 is not necessarily equal to the number of sustain pulses in sustain1. This is because different levels of illumination may be desired for different sub-fields, and the number of gas discharges is directly proportional to the desired level of illumination. For example, depending on the dynamics of an image being displayed, some sub-fields will be required to provide a relatively high level of illumination, and thus, a relatively high number of gas discharges, while other sub-field will be required to provide a relatively low level of illumination, and thus, a relatively low number of gas discharges.
Although not shown in
For electrode A, the setup waveform of setup1 attains a voltage that is more positive than a maximum positive voltage of the sustain waveform of sustain2. That is, Va is more positive than Vb. For electrode B, the setup waveform of setup1 attains a voltage that is more negative than a maximum negative voltage of the sustain waveform of sustain2. That is, Vf is more negative than Ve. Furthermore, since the waveforms of setup1 alternate with the waveforms of setup2, for electrode A, the setup waveform of setup3 (not shown) attains a voltage that is more positive than a maximum positive voltage of the sustain waveform of sustain2, and for electrode B, the setup waveform of setup3 attains a voltage that is more negative than a maximum negative voltage of the sustain waveform of sustain2. Note that for this comparison of waveforms of sustain2 and setup3, sustain2 occurs prior to setup3.
The setting up of electrodes A and B, as described above, has several advantages over prior art setup techniques. Firstly, The act of switching between a positive boundary and a negative boundary on both electrodes allows for better defined internal capacitance voltages, allowing for the application of lower peak voltages with reduced background brightness produced by the setup discharges. Lower peak voltages also reduce the voltage requirements of the panel's dielectric materials. Secondly, each falling ramp setup provides the MgO layer with a weak discharge during the falling ramp proceeding each addressing period, which improves address ability. Thirdly, the stability of the positive resistance ramp discharges is enhanced by encouraging sustain gap discharges, across C3, and discouraging plate gap discharges, across C1 and C2, as sustain gap discharges are more stable as both surfaces are covered my the MgO layer. The use of falling ramps for setup operations is advantageous as the phosphor surface covering the data electrodes acts as an anode during the application of the falling ramp. Common phosphor materials have a poor to negligible secondary electron emission characteristic, thus making the phosphor a poor cathode for a positive resistance discharge. However when the phosphor serves as an anode, there is less likelihood that a positive resistance discharge can over develop, go unstable, and avalanche. An avalanching setup discharge is undesirable because it results in a negative resistance discharge that, in turn, reduces the voltage across the gas to zero and effectively turns the pixel into the ON state regardless of addressing operations. Lastly, identical circuits and voltages are applied to each side, allowing for power supply simplification.
During setup1A, controller 104 generates voltage Va for electrode A, and generates a ramp having a negative slope for electrode B. The ramp traverses from voltage Ve to a maximum negative voltage Vf. Setup1A occurs only once per frame, at the beginning of the frame, in subfield1, to setup boundary levels for electrodes A and B. Here, electrode A is set to a positive boundary and electrode B is set to a negative reference boundary. By setting up positive and negative boundary levels, operation of pixel 102 is well defined since the negative slope on electrode B defines the minimum operating point for capacitance C2, ensuring that when operating at the positive boundary level, capacitance C2 is operated close to but below its breakdown voltage.
During setup1B, controller 104 generates a ramp having a negative slope for electrode A, and generates voltage Vd for electrode B. The ramp traverses from voltage Vb to a maximum negative voltage Vc. This configuration of waveforms produces a weak positive resistance discharge between electrode A and electrode B such that at the end of the ramp, the voltage across capacitance C3 within the gas volume is at, or near, the breakdown voltage of the gas. Thus, pixel 102 is setup for address2.
During address1, controller 104 generates a row select pulse 305 for electrode A, and maintains voltage Vd for electrode B. If a data voltage (not shown) is applied to data electrode 115 coincident with row select pulse 305, that is, if pixel 102 is addressed, then a strong negative resistance discharge forms across C1, which extends across capacitance C3 resulting in a depletion of the voltages across capacitances C1, C2 and C3. As discharge current flows from electrode B to electrode A, capacitances C4 and C5 charge via a capacitor divider between electrode A and electrode B as capacitance C3 becomes a very low resistance during the discharge. The charge on capacitances C4 and C5 define the ON state for pixel 102. Note that in the ON state after the discharge, the interior voltage on each surface will be close to voltage Vb on electrode A and Ve on electrode B, thus Vb is close to or equal to Ve.
During sustain1, controller 104 generates a plurality of sustain pulses for each of electrodes A and B. The sustain pulses for electrode A achieve a maximum positive voltage of Vb and a maximum negative voltage of Vc. The sustain pulses for electrode B achieve a maximum positive voltage of Vd and a maximum negative voltage of Ve. If pixel 102 was addressed during address1, the charge on capacitances C4 and C5 will add to the voltages supplied by sustain pulses on electrodes A and B during sustain1 thus causing the gas in the region of pixel 102 to be repetitively discharged.
In setup2, address2 and sustain2, controller 104 generates the same waveforms as in setup1B, address1 and sustain1, respectively. Setup1B removes the charge from capacitances C4 and C5 of any ON pixels and restores the charge on capacitances C1, C2, and C3 in preparation for addressing. Furthermore, although not shown in
For electrode A, the setup waveform of setup1A attains a voltage that is more positive than a maximum positive voltage of the sustain waveforms of sustain1 and sustain2. That is, Va is more positive than Vb. For electrode B, the setup waveform of setup1A attains a voltage that is more negative than a maximum negative voltage of the sustain waveforms of sustain1 and sustain2. That is, Vf is more negative than Ve.
In
The waveforms of
The ramps for electrodes A and B in
For electrode A, the setup waveform of setup1A attains a voltage that is more positive than a maximum positive voltage of the sustain waveforms of sustain1 and sustain2. That is, Va is more positive than Vb. For electrode B, the setup waveform of setup1A attains a voltage that is more negative than a maximum negative voltage of the sustain waveforms of sustain1 and sustain2. That is, Vf is more negative than Ve.
For electrode A, the setup waveform of setup1A attains a voltage that is more positive than a maximum positive voltage of the sustain waveforms of sustain1 and sustain2. That is, Va is more positive than Vg. For electrode B, the setup waveform of setup1A attains a voltage that is more negative than a maximum negative voltage of the sustain waveforms of sustain1 and sustain2. That is, Vf is more negative than Ve.
As in the previous figures, the bounds of electrode A are Va and Vc, and the bounds of electrode B are Vd and Vf. However, electrode B reaches its upper bound during a sustain period. Electrode A's sustain pulses are operated within the bounds of Va and Vc, but are shifted above the negative bound Vc to a voltage Vh. This technique uses the established bounds to increase the applied voltage for the first sustain discharge, which can be slow or weak following a weak address discharge. Thus operating margins are improved.
In
The techniques described herein are exemplary, and should not be construed as implying any particular limitation on the present invention. It should be understood that various alternatives, combinations and modifications could be devised by those skilled in the art. The present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.
