1. Technical Field
The present invention relates to an organic light emitting diode display device, and more particularly to an organic light emitting diode driving circuit with minimized characteristic changes.
2. Related Art
Various flat panel display devices gradually replace a cathode ray tube (CRT) because they may be compact, light and thin. Flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), a light emitting diode (LED) display device and so on.
An LED display device uses an LED which emits light by recombining electrons and holes. The LED display device is divided into an inorganic LED display device which uses inorganic compounds and an organic light emitting diode (OLED) display device which uses organic compounds. OLED display devices are expected to be a next generation display device because they have many advantages such as low voltage driving, self-luminescence, thinness, wide viewing angle, rapid response speed and high contrast.
An OLED is generally made up of an electron injection layer, an electron transport layer, a light emitting layer, a hole transport layer and a hole injection layer which are deposited between a cathode and an anode. In an OLED, if a designated voltage is applied between the anode and the cathode, electrons generated from the cathode move to the light emitting layer through the electron injection layer and the electron transport layer, and holes generated from the anode move to the light emitting layer through the hole injection layer and the hole transport layer. Accordingly, electrons and holes supplied from the electron transport layer and the hole transport layer are recombined in the light emitting layer, thereby emitting light.
FIG. 1 illustrates an active matrix type of OLED display device 10 using an OLED. The OLED display device 10 includes an OLED panel 13 having n×m number of pixels P[i,j]. P[i,j] is a pixel located at the ith row and the jth column, where i is a positive integer which is equal to or smaller than n, and j is a positive integer which is equal to or smaller than m. The pixels are arranged in n×m matrix at an area which is defined by n numbers of gate lines G1 to Gn (n is a positive integer) and m numbers of data lines D1 to Dm (m is a positive integer). A gate drive circuit 12 drives the gate lines G1 to Gn of the OLED panel 13 and a data drive circuit 11 drives the data lines D1 to Dm of the OLED panel 13. The m number of power voltage supply lines S1 to Sm are arranged in parallel to the data lines D1 to Dm to supply the high potential power voltage Vdd to each pixel P[i,j].
The gate drive circuit 12 supplies scan pulses to the gate lines G1 to Gn to sequentially drive the gate lines G1 to Gn. The data drive circuit 11 converts a digital data voltage input from the outside into an analog data voltage. The data drive circuit 11 supplies the analog data voltage to the data lines D1 to Dm whenever the scan pulse is supplied. Each of the pixel P[i,j] receives the data voltage from the jth data line Dj to generate a light corresponding to the data voltage when the scan pulse is supplied to the ith gate line Gi.
Each pixel P[i,j] includes an OLED having an anode connected to the jth power voltage supply line Sj. An OLED drive circuit 15 is connected to the cathode of the OLED and the ith gate line Gi and the jth data line Dj to supply a low potential power voltage Vss. The OLED drive circuit 15 includes a first transistor T1 and a second transistor T2 and a storage capacitor Cs. The first transistor T1 supplies the data voltage from the jth data line Dj to a first node N1 in response to the scan pulse from the ith gate line Gi. The second transistor T2 controls a current flowing in the OLED in response to the voltage of the first node N1. The storage capacitor Cs is charged with the voltage on the first node N1.
FIG. 2 illustrates driving waveforms of the OLED drive circuit 15. In FIG. 2, ‘1F’ is one frame period, ‘1H’ is one horizontal period, ‘Vg_i’ is a gate voltage supplied from the ith gate line Gi′, ‘Psc’ is a scan pulse, ‘Vd_j’ is a data voltage supplied from the jth data line Dj, ‘VN1’ is a voltage on the first node N1, and ‘IOLED’ is a current flowing through the OLED. Referring to FIGS. 1 and 2, the first transistor T1 is turned on to supply the data voltage Vd supplied from the data line Dj to the first node N1 when the scan pulse is supplied through the gate line Gi. The data voltage Vd supplied to the first node N1 is charged to the storage capacitor Cs and supplied to a gate terminal of the second transistor T2. In this way, the second transistor T2 is turned on by the supplied data voltage Vd, and the current flows through the OLED. Because the current flowing through the OLED is generated by the high potential power voltage Vdd, the current is proportional to the magnitude of the data voltage Vd applied to the second transistor T2. When the first transistor T1 is turned off, the second transistor T2 remains turned on with the first node voltage VN1 from the storage capacitor Cs. As a result, the current which flows through the OLED may be controlled until the data voltage Vd of the next frame is supplied.
In FIG. 2, a positive data voltage Vd is applied for a long time to the gate electrode of the second transistor T2. An accumulated gate bias stress may be generated in the second transistor T2 with the positive data voltage Vd, as shown in FIG. 3. The accumulated gate-bias stress may cause deterioration, which in turn may cause characteristic changes, as shown in FIG. 4A. FIG. 4A represents a characteristic change of a transistor caused by a positive gate bias stress, and FIG. 4B represents a characteristic change of a transistor caused by a negative gate bias stress. The arrow marks in FIGS. 4A and 4B represent a threshold voltage change of the second transistor T2. The characteristic change of the OLED drive circuit, in particular, the second transistor T2 may deteriorate reliability of operations of the OLED drive circuit 15 by changing the current flowing in the OLED. Reliability of the entire OLED display device may be further affected.