1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly to a driving method and apparatus for a plasma display panel that can be driven stably under a high temperature environment.
2. Description of the Related Art
A plasma display panel PDP displays a picture by having an ultraviolet ray make light emitted from a phosphorus material, the ultraviolet ray is generated when inert mixture gas is discharged. The PDP has its picture quality improved in debt to recent technology development as well as being easy to be made thin in thickness and big in size.
Referring to FIG. 1, a discharge cell of a three electrode AC surface discharge PDP includes a pair of sustain electrodes having a scan electrode 30Y and a common sustain electrode 30Z formed on an upper substrate 10, and an address electrode 20X formed on a lower substrate 18 to cross the pair of sustain electrodes perpendicularly. The sustain electrode 30Y and the sustain electrode 30Y each has a structure where transparent electrodes 12Y and 12Z and metal bus electrodes 13Y and 13Z are deposited. There are an upper dielectric layer 14 and an MgO passivation film 16 deposited on the upper substrate 10 where the scan electrode 30Y and the sustain electrode 30Z.
There are lower dielectric layer 22 and barrier ribs 24 formed on the lower substrate 18 where the address electrode 20X is formed. There is a fluorescent layer 26 spread on the lower dielectric layer 22 and surface of the barrier ribs 24.
There is inert gas such as He+Xe, Ne+Xe and He+Xe+Ne etc. interposed in a discharge space provided between the upper/lower substrates 10 and 18 and the barrier ribs 24.
In order to realize the gray level of a picture, the PDP is time-division driven by dividing one frame into several sub-fields that have their light emission frequencies different. Each sub field can be divided into an initialization period (or a reset period) to initialize a full screen, an address period to select scan lines and select cells among the selected scan lines, and a sustain period to realize gray levels in accordance with a discharge frequency. The initialization period is again divided into a setup period for which a rising ramp waveform is applied and a set-down period for which a falling ramp waveform is applied. For example, in the event of displaying a picture with 256 gray levels, the frame period (16.67 ms) corresponding to 1/60 second as in FIG. 2 is divided into 8 sub-fields (SF1 to SF8).
Each of the 8 sub-fields (SF1 to SF8), as described above, is divided into the initialization period, the address period and the sustain period. The initialization period and the address period of each sub-field are the same for each sub-field, while the sustain period increases at the rate of 2n (n=0, 1, 2, 3, 4, 5, 6, 7) in each sub-field.
FIG. 3 illustrates a driving waveform of a PDP which is applied to two sub-fields.
In FIG. 3, Y represents a scan electrode, Z does a sustain electrode and X does an address electrode.
Referring to FIG. 3, the PDP is driven by being divided into an initialization period to initialize a full screen, an address period to select cells and a sustain period to sustain discharges of the selected cells.
In the initialization period, a rising ramp waveform Ramp-up is simultaneously applied to all scan electrodes Y for a setup period SU. The rising ramp waveform Ramp-up causes a discharge to occur within the cells of the full screen. The setup discharge causes positive wall charges to be accumulated in the address electrode X and the sustain electrode Z, and negative wall charges to be accumulated in the scan electrode Y. A falling ramp waveform Ramp-down is simultaneously applied to the scan electrodes Y for the set-down period after the rising ramp waveform Ramp-up being applied. Herein, the falling ramp waveform begins to fall at the positive voltage lower than the peak voltage of the rising ramp waveform Ramp-up.
The falling ramp waveform Ramp-down causes a weak erasure discharge within the cells so as to eliminate the wall charges formed excessively. The wall charges are uniformly sustained within the cells so that an address discharge can be stably caused by the set-down discharge.
In the address period, negative scan pulses SCAN are sequentially applied to the scan electrodes Y and at the same time positive data pulses DATA synchronized with the scan pulses SCAN are applied to the address electrodes X. When the voltage difference between the scan pulse SCAN and the data pulse DATA is added to the wall voltages generated in the initialization period, the address discharge is generated within the cell to which the data pulse DATA is applied. When sustain voltages are applied, wall charges are formed within the cells selected by the address discharge so that the discharge can be caused.
Positive DC voltage Zdc is applied to the sustain electrode Z for the set-down period and the address period. The DC voltage Zdc sets the voltage difference between the sustain electrode Z and the scan electrode Y or the sustain electrode Z and the address electrode X so as to cause the set-down discharge to occur between the sustain electrode Z and the scan electrode Y for the set-down period, and at the same time so as not to cause a discharge to be generated on a large scale between the scan electrode Y and the sustain electrode Z for the address period.
In the sustain period, sustain pulses SUS are alternately applied to the scan electrodes Y and the sustain electrodes Z. In the cells selected by the address discharge, there occurs a sustain discharge, i.e., display discharge, between the scan electrode Y and the sustain electrode Z whenever each sustain pulse SUS is applied as the wall voltage within the cell is added to the sustain pulse SUS.
Lastly, after completion of the sustain discharge, a ramp waveform ERASE with narrow pulse width and low voltage level is applied to the sustain electrode Z, thereby to erase the wall charges remaining behind within the cells of the full screen.
However, the prior art PDP has a problem that the driving is not stable, i.e., there is no discharge generated in the event that it is made to run in a high temperature environment. For instance, in a high temperature environment of 50° C. or more, when the PDP, as in FIG. 4, is divided into an upper part and a lower part so that the upper part is scanned from top downward and the lower part is scanned from bottom upward, there occurs no address discharge in a middle part 41 where it is scanned late in order. If no address discharge is generated with respect to the selected cell, because the sustain discharge is not generated in the selected cell though the sustain voltage is applied, thus it is not possible to display a picture. In the same way, when the PDP is sequentially scanned from the first line till the last line as in FIG. 5 in the high temperature of 50° C. or more, there occurs no address discharge in a lower part 51 of the screen, which is scanned late in order.
Upon the high temperature environment experiment and the analysis result thereof, the principal cause for the occurrence of mis-discharge under the high temperature environment is the scanning order, as it gets later, the amount of loss of the wall charges generated in the initialization period is increased. To describe this cause on the basis of the discharge characteristic within the cell, firstly, as the internal/external temperature of the cell increases, the insulation characteristic of a dielectric material and a passivation material within the cell is deteriorated to generate leakage current, thereby leaking the wall charges. More specifically, in the event that the wall charges of the scan electrode Y and the sustain electrode Z is made to leak, it is easy for the address discharge to be mis-discharged. Secondly, as the movement of the space charge within the cell generated by the discharge in the high temperature environment gets active, the space discharge is easily recombined with the atom that has lost electrons so that the wall charges and space charges contributing to the discharge are lost as time passes by.