Display device and driving method thereof

• US 20030090447A1
• Filed: 09/18/2002
• Published: 05/15/2003
• Est. Priority Date: 09/21/2001
• Status: Active Grant

• Current Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Original Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Inventors: Kimura, Hajime
• Granted Patent: US 7,138,967 B2
• Time in Patent Office: Days
• US Class Current: 345/82
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2300/0861 with additional control of the display period without amending the charge stored in a pixel memory, e.g. by means of additional select electrodes
• CPC Class Codes: G09G 2360/18 Use of a frame buffer in a display terminal, inclusive of the display panel
• CPC Class Codes: G09G 3/2022 using sub-frames
• CPC Class Codes: G09G 3/3241 the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror
• CPC Class Codes: G09G 3/3266 Details of drivers for scan electrodes
• CPC Class Codes: G09G 3/3283 in which the data driver supplies a variable data current for setting the current through, or the voltage across, the light-emitting elements
• CPC Class Codes: G09G 5/008 Clock recovery
• CPC Class Codes: G09G 5/399 using two or more bit-mapped memories, the operations of which are switched in time, e.g. ping-pong buffers

Abstract

Each pixel of a display device has a current supply circuit, a switch portion, and a light emitting element. The light emitting element, the current supply circuit, and the switch portion are connected in series between a power supply reference line and a power supply line. The switch portion is switched between ON and OFF using a digital video signal. The amount of constant current flowing in the current supply circuit is determined by a control signal inputted from the outside of the pixel. When the switch portion is ON, a constant current determined by the current supply circuit flows in the light emitting element and light is emitted. As a result, a low-cost display device can be provided in which the light emitting element can emit light at a constant luminance even when the current characteristic is changed by degradation or the like, which is fast in writing signals in pixels, which can display in gray scales accurately, and which can be reduced in size with a low cost, as well as a driving method of the display device.

Claims

1. A display device characterized by including a pixel that has means for converting a first current into a voltage, means for holding the converted voltage, means for converting the voltage held into a second current, and means for causing the second current to flow into a light emitting element using a digital video signal.

4. A display device according to any one of claims 1 to 3, characterized by having means for preventing the second current from flowing into the light emitting element using a signal different from the digital video signal.

14. A display device according to any one of claims 1 to 4 and 7 to 13, characterized in that the first current is not changed by the digital video signal.

15. A display device according to any one of claims 1 to 14, characterized in that the pixel has means for holding the digital video signal.

16. A display device according to any one of claims 1 through 14, characterized in that the pixel has means for selecting input of the digital video signal to the pixel and means for holding the digital video signal.

17. A display device according to any one of claims 1 to 4 and 7 to 16, characterized in that the pixel is more than one and at least some of the plural pixels have the same current value for the first current.

18. A display device according to any one of claims 1 through 17, characterized by having a driving circuit for inputting a constant current to the pixel.

2. A display device characterized by including a pixel that has means for converting a first current into a voltage, means for holding the voltage converted, means for converting the voltage held into a second current having the same current value as the first current, and means for causing the second current to flow into a light emitting element using a digital video signal.

3. A display device characterized by including a pixel that has means for converting a first current into a voltage, for holding the voltage converted, and for converting the voltage held into a second current whose current value is in proportion to the first current, and means for causing the second current to flow into a light emitting element using a digital video signal.

5. A display device including a pixel having a current supply circuit (102) that causes a constant current to flow and a switch portion (101) that uses a digital video signal to switch between ON and OFF, which controls light emission of a light emitting element (106), characterized in that the switch portion (101), the current supply circuit (102), and the light emitting element (106) are connected in series.

6. A display device including a pixel having a current supply circuit (102), a switch portion (101), a power supply line, and a power supply reference line, with the current supply circuit (102) having a first terminal (A) and a second terminal (B) and fixing a current flowing between the first terminal (A) and the second terminal (B) to a constant value, and the switch portion (101) having a third terminal (C) and a fourth terminal (D) and using a digital video signal to switch a path between the third terminal (C) and the fourth terminal (D) to a conductive or nonconductive state, the display device being characterized in that the current supply circuit (102), the switch portion (101), and a light emitting element (106) are connected between the power supply line and the power supply reference line such that a current flowing between the first terminal (A) and the second terminal (B) flows between an anode and a cathode of the light emitting element (106) when the path between the third terminal (C) and the fourth terminal (D) is made conductive.

7. A display device characterized by including a pixel having means for setting a first current as a drain current of a first transistor, means for holding a gate voltage of the first transistor, means for setting the gate voltage as a gate voltage of a second transistor (112) that has the same polarity as the first transistor, and means for causing a drain current of the second transistor (112) to flow into a light emitting element (106) using a digital video signal.

8. A display device according to claim 7, characterized in that a ratio of a gate length to a gate width of the first transistor is different from the ratio of the gate length to the gate width of the second transistor (112).

9. A display device according to claim 7 or 8, characterized by having means for electrically connecting a gate electrode of the first transistor to its drain terminal.

10. A display device according to any one of claims 7 through 9, characterized by having means for preventing a drain current of the second transistor (112) from flowing into the light emitting element (106) using a signal different from the digital video signal.

11. A display device characterized by including a pixel that has means for inputting a first current into a transistor (112) to set it as a drain current of the transistor (112), means for holding a gate voltage of the transistor (112), and means for causing the drain current of the transistor (112) to flow into a light emitting element (106) using a digital video signal to apply a voltage between source-drain terminals of the transistor (112), the drain current being determined by the gate voltage held.

12. A display device according to claim 11, characterized by having means for electrically connecting a gate electrode and a drain terminal of the transistor (112).

13. A display device according to claim 11 or 12, characterized by having means for preventing the drain current of the transistor (112) from flowing into the light emitting element (106) using a signal different from the digital video signal.

19. A method of driving a display device, characterized by performing a first operation in which a first current inputted to a pixel is converted into a voltage to hold the converted voltage and a second operation in which the voltage held is converted into a second current to cause the second current to flow into a light emitting element using a digital video signal inputted.

20. A method of driving a display device according to claim 19, characterized in that the second operation includes an operation of choosing input of the digital video signal to the pixel and holding the digital video signal inputted.

21. A method of driving a display device according to claim 19 or 20, characterized in that the first operation and the second operation are conducted independently.

22. A method of driving a display device according to any one of claims 19 to 21, characterized in that a ratio of a period in which the second current flows into the light emitting element in one frame period is changed to display gray scales.

23. A method of driving a display device according to any one of claims 19 to 21, characterized in that one frame period is divided into plural sub-frame periods, and the second operation is conducted in each of the plural sub-frame periods to display gray scales.

24. A method of driving a display device according to claim 23, characterized in that a non-display period in which a signal different from the digital video signal is used to prevent the second current from flowing into the light emitting element is provided in at least one of the plural sub-frame periods.

25. A method of driving a display device according to claim 24, characterized in that the first operation is conducted in the non-display period.