Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-03-30
2001-01-16
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C315S169200, C345S076000, C345S079000, C345S204000, C345S208000, C345S209000
Reexamination Certificate
active
06175193
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon and claims benefit of priority of Japanese Patent Application No. Hei-11-92130 filed on Mar. 31, 1999, the content of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a display device having a display panel in which capacitive luminescent elements such as electroluminescent elements are used.
2. Description of Related Art
An example of a circuit for driving an electroluminescent display panel is disclosed in U.S. Pat. No. 5,847,516. In this example, an electroluminescent panel having scanning and data electrodes and pixels arranged in a matrix is driven by scanning and data electrode driving circuits. The display panel is sequentially scanned with a positive voltage in a positive field and with a negative voltage in a negative field. Specifically, in the positive field, a scanning voltage Vr is sequentially supplied to the scanning electrodes and an offset voltage that is the same as a modulation voltage Vm is used as a base voltage. A ground voltage Vg is supplied to pixels to be activated and a modulation voltage Vm is supplied to the pixels not to be activated from the data electrodes. In the negative field, a scanning voltage −(Vr−Vm) is sequentially supplied to the scanning electrodes and the ground voltage Vg is used as a base voltage. The modulation voltage Vm is supplied to pixels to be activated and the ground voltage Vg is supplied to the pixels not to be activated from the data electrodes.
The pixels on which voltage Vr is imposed emit light, while the pixels on which voltage (Vr−Vm) is imposed do not emit light. Thus, the pixels arranged in a matrix are selectively activated thereby to display images on the panel. After scanning on the scanning electrodes is completed, electric charges stored in the pixels connected to the scanned electrodes are discharged.
However, there is a following problem in the conventional device disclosed. Since the charges stored in the scanned pixels are discharged when the fields are switched and after the scanning of a selected scanning electrode is completed, a turnaround current is supplied to the scanned pixels when other scanning electrodes are scanned thereafter. That is, since the base voltage of each scanning electrode is set to the voltage Vm that is the same as the modulation voltage in the positive field, the discharged pixels are charged with Vm when the data electrode voltage becomes the ground level voltage Vg (Vg is zero volt). The base voltage of each scanning electrode is set to the ground voltage Vg in the negative field. Therefore, the discharged pixels are charged with the modulation voltage Vm when the data electrode voltage becomes Vm. The turnaround current flowing in this manner does not contribute to luminescence of the pixels, and accordingly this turnaround current is useless and only increases power consumption.
Further, since the turnaround current flows when the data voltage becomes a level to activate the pixels, driving voltage waveforms are deformed or distorted especially when output current for driving the pixels is low, thereby causing uneven brightness among pixels. Such a waveform deformation or distortion is caused by changes of driving voltages and data voltages. For example, when the scanning voltages (Vr and Vm in the positive field, and −(Vr−Vm) and Vg in the negative field) are supplied from a common power source circuit in both fields, the offset voltage Vm is decreased by the turnaround current due to resistance between the scanning electrode driving circuit and the power source circuit in the positive filed, and the ground voltage Vg is increased in the negative field. In accordance with those voltage changes, the voltages Vr and −(Vr−Vm) also change, making the driving voltage insufficient to activate the pixels. Especially, when a scanning electrode having many pixels to be activated is scanned after a series of scanning of electrodes having no pixels to be activated, the amount of the turnaround current becomes large, and thereby the driving voltages Vr and −(Vr−Vm) change in a high degree. In addition to the driving voltage changes, the data voltage waveforms are also changed by the turnaround current, thereby causing the uneven brightness mentioned above. The degree of the uneven brightness becomes especially large when it is required to make a pulse width narrow for a stepwise control of brightness, because a sufficient voltage cannot be imposed on the pixels on a selected scanning electrode.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved display device, in which unnecessary turnaround current is suppressed in scanning of a display panel and thereby to prevent uneven brightness among pixels.
A display device includes a flat display panel having a luminescent layer such as an electroluminescent layer, an array of scanning electrodes disposed on one surface of the luminescent layer, and another array of data electrodes disposed on the other surface of the luminescent layer. Both electrode arrays are arranged to perpendicularly cross each other. Pixels arranged in a matrix are formed at each intersection of the scanning and data electrodes together with the luminescent layer. A scanning electrode driving circuit is connected to the scanning electrodes to supply scanning voltages thereto, and a data electrode driving circuit is connected to the data electrodes to supply data voltages thereto.
The scanning electrodes are sequentially scanned, e.g., from the top of the display panel toward the bottom thereof, and composite voltages consisting of the scanning and data voltages are supplied to the pixels thereby to selectively activate the pixels to emit light therefrom. The scanning and data voltages may be supplied from both ends of the respective electrodes to quickly activate the pixels. The scanning is performed by supplying positive scanning voltages in a positive field and negative scanning voltages in a negative field, both fields being consecutively alternated.
After the scanning of one scanning electrode is completed, electric charges stored in the pixels on the scanned electrode are discharged. The discharged pixels are charged again to a level of a data voltage modulation voltage by supplying a turnaround current thereto before the scanning moves to the next scanning electrode. When other scanning electrodes are scanned, a harmful and useless turnaround current is prevented from flowing into the pixels on the electrode already scanned, because the already scanned pixels are charged to the level of the modulation voltage. Since the turnaround current is eliminated in this manner, driving voltages imposed on all the pixels are stabilized, and thereby uneven brightness among the scanning electrodes is suppressed. Moreover, less power is consumed for driving the display panel because the useless turnaround current is eliminated.
The pixels on other scanning electrodes that are not yet scanned may be charged with the modulation voltage at the same time the scanned pixels are charged. Further, all the pixels may be preliminarily charged to the level of the modulation voltage before the first scanning electrode in each field is scanned. In this manner, the effect of suppressing the uneven brightness is further enhanced.
Alternatively, electric charges stored in the pixels on the scanned electrode are transferred to the pixels on other scanning electrodes that are not yet scanned. All the pixels including the scanned pixels are equally charged to the level of the modulation voltage, thereby eliminating the turnaround current. This can be done by bringing all the data electrodes to a high impedance state after scanning of one scanning electrode is completed and before the scanning moves to the next scanning electrode.
REFERENCES:
patent: 4864182 (1989-09-01), Fujioka e
Hanaki Takashi
Kishita Hiroyuki
Matsumoto Naoki
Ogusu Koji
Osada Masahiko
Denso Corporation
Law Office of David G. Posz
Philogene Haissa
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