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  Driving method of light emitting deviceUSPTO Application #: 20060076567Title: Driving method of light emitting device Abstract: If a potential of a gate electrode of a driving transistor varies after a gray scale signal is inputted into each pixel, a current value of a current supplied to a light emitting element varies so that accurate gray scale display cannot be obtained. In particular, in the case of performing black display, current may flow, which makes clear black display difficult. Accordingly, the invention provides a light emitting device capable of performing accurate gray scale display, and a driving method thereof. According to the invention, a signal for display is inputted plural times within a predetermined timing period, or a writing operation period is lengthened. Consequently, the gate voltage of the transistor is determined after the anode potential of the light emitting element is stabilized, and therefore accurate gray scale display can be performed. (end of abstract)
Agent: Fish & Richardson P.C. - Minneapolis, MN, US Inventors: Keisuke Miyagawa, Ryota Fukumoto USPTO Applicaton #: 20060076567 - Class: 257094000 (USPTO) Related Patent Categories: Active Solid-state Devices (e.g., Transistors, Solid-state Diodes), Incoherent Light Emitter Structure, With Heterojunction The Patent Description & Claims data below is from USPTO Patent Application 20060076567. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a configuration for performing accurate gray scale display in a display device having a light emitting element (a light emitting device), and a driving method thereof. [0003] 2. Description of the Related Art [0004] In a conventional light emitting device, a pixel configuration as shown in FIG. 9 has been proposed in which a switching element 810 whose on/off is controlled by a video signal inputted from a signal line 814, a transistor 811 for driving a light emitting element 813, and a capacitor 812 provided between a power source line 815 and a gate electrode of the transistor 811 to hold a gate-source voltage of the transistor 811 (see Patent Document 1) are provided. [Patent Document 1] Japanese Patent Laid-Open No. 2001-343933 [0005] It is considered that an equivalent circuit of a light emitting element described in Patent Document 1 can be shown by a parallel circuit including a diode 816 and a capacitor (C.sub.EL) in FIG. 9. Operation in the case where a current value of a current supplied to the light emitting element 813 varies is described below with reference to FIG. 9. [0006] First, it is assumed that a current at a current value I.sub.0 flows constantly into the light emitting element 813. Then, in the case where the current value of the current flowing into the light emitting element 813 increases from I.sub.0 to I.sub.1, a current value of a current flowing into the diode 816 does not become I.sub.1 immediately. This is because an increased amount of the current value of the light emitting element 813 is equal to a sum of an increased amount of the current value of the current flowing into the diode 816 and a current value of a current flowing into the capacitor (C.sub.EL). Therefore, the current value of the current flowing into the diode 816 becomes equal to I.sub.1 when the charging of the capacitor (C.sub.EL) is completed. [0007] Meanwhile, assuming that the current at the current value I.sub.0 flows constantly into the light emitting element 813 and then the current value decreases from I.sub.0 to I.sub.2, a sum of the current value of the current flowing into the diode 816 and a current value of a current discharged from the capacitor (C.sub.EL) becomes I.sub.2. The current value of the current flowing into the diode 816 becomes equal to I.sub.2 when the discharging of the capacitor (C.sub.EL) is completed. In the above-described cases, the time until which the current value of the constant current flowing into the diode 816 changes is equal to the time until which changing of a potential between an anode and a cathode of the light emitting element 813 is completed, which becomes longer as the size of the capacitor (C.sub.EL) is larger and as the changed amount of the current value of the light emitting element 813 is larger. [0008] The pixel circuit shown in FIG. 9 further includes overlap capacitance (C.sub.gd) between a gate electrode and a drain electrode of the driving transistor 811 and parasitic capacitance (C.sub.P) caused by overlap between the gate electrode and the anode and the like depending on the layout in addition to the capacitor (C.sub.EL) between both the electrodes of the light emitting element 813. [0009] At this time, the switching element 810 is turned on, and a current corresponding to a gray scale signal inputted into the gate of the transistor 811 is supplied to the light emitting element 813 and an anode potential thereof changes. However, when the capacitor (C.sub.EL) of the light emitting element 813 is large and a changed amount of the current value of the current supplied to the light emitting element 813 is large, it takes a long time to complete the charging/discharging of the capacitor (C.sub.EL) and complete the changing of the anode potential. Therefore, there is a case where the changing of the anode potential does not complete in the on-period of the switching element 810. [0010] Then, in the case where the anode potential of the light emitting element 813 changes (a value of change is .DELTA.V.sub.A) after the switching element 810 is turned off in FIG. 9, the potential of the gate electrode of the transistor 811 changes due to capacitive coupling of the parasitic capacitance (C.sub.P), the overlap capacitance (C.sub.gd), and the capacitor (C.sub.S) 812. A value of change at this time, .DELTA.V.sub.B is expressed by .DELTA.V.sub.B=(C.sub.P+C.sub.gd)/(C.sub.P+C.sub.gd+C.sub.S).times..DELTA- .V.sub.A. [0011] As set forth above, in the case where the potential of the gate electrode of the transistor 811 changes after a gray scale signal is inputted into each pixel, there is a problem in that the current value of the current supplied to the light emitting element 813 changes so that accurate gray scale display cannot be obtained. In particular, in the case of performing black display, a current may flow into the light emitting element so that clear black display cannot be easily performed. SUMMARY OF THE INVENTION [0012] It is an object of the invention to provide a light emitting device capable of performing accurate gray scale display and a driving method thereof. [0013] In view of the foregoing problem, according to the invention, a signal for display is inputted plural times within a predetermined timing period, or a writing operation period thereof is lengthened. As a result, the gate voltage is determined after the anode potential of the light emitting element is stabilized, so that accurate gray scale display can be performed. [0014] A specific mode of the invention is a driving method of a light emitting device, in which one frame period is divided into a plurality of subframe periods SF1, SF2, . . . , and SFn (n is a positive integer), each subframe period SFn has a writing operation period Ta, and the period Te for inputting an erasing signal is provided plural times in at least one subframe period. [0015] Another mode of the invention is a driving method of a light emitting device, in which one frame period is divided into a plurality of subframe periods SF1, SF2, . . . , and SFn (n is a positive integer), each subframe period SFn has a writing operation period Ta, and the writing operation period Ta is provided plural times in at least one subframe period. [0016] Another mode of the invention is a driving method of a light emitting device for performing gray scale display by inputting a video signal and an erasing signal formed of a digital signal, in which a period for inputting the erasing signal is provided longer than a period for inputting the video signal. [0017] A pixel configuration of such a light emitting device comprises a switching transistor having a source electrode or a drain electrode connected to a signal line and a gate electrode connected to a scan line, a driving transistor having a gate electrode connected to the switching transistor, and a light emitting element which is connected to a source electrode or a drain electrode of the driving transistor. [0018] In addition, the pixel configuration may additionally include an erasing transistor for discharging a charge corresponding to a gate-source voltage of the driving transistor. [0019] In addition, the pixel configuration may further additionally include a transistor which is connected to the driving transistor in series and the gate potential of which is fixed. [0020] According to the driving method of the invention, a light emitting device capable of performing accurate gray scale display can be provided. BRIEF DESCRIPTION OF THE DRAWINGS Continue reading... Full patent description for Driving method of light emitting device Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Driving method of light emitting device patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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