Display device and method of driving the same

• US 6,828,950 B2
• Filed: 08/09/2001
• Issued: 12/07/2004
• Est. Priority Date: 08/10/2000
• Status: Expired due to Term

• Current Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Original Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Inventors: Koyama, Jun
• Primary Examiner(s): Liang, Regina
• Assistant Examiner(s): Dinh, Duc Q
• Application Number:
  US09/924,610
• Publication Number:
  US 20020017643A1
• Time in Patent Office: 1,216 Days
• Field of Search: 345/76-84, 315/169.3
• US Class Current: 345/76
• CPC Class Codes: G09G 2300/0408 Integration of the drivers onto the display substrate
• CPC Class Codes: G09G 2300/0426 Layout of electrodes and connections
• CPC Class Codes: G09G 2300/0814 used for selection purposes, e.g. logical AND for partial update
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2300/0866 by means of changes in the pixel supply voltage
• CPC Class Codes: G09G 2320/029 by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel
• CPC Class Codes: G09G 2320/041 Temperature compensation
• CPC Class Codes: G09G 3/2022 using sub-frames
• CPC Class Codes: G09G 3/2025 the sub-frames having all the same time duration
• CPC Class Codes: G09G 3/2081 with combination of amplitude modulation and time modulation space and time error diffusion G09G3/2066
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/3258 with pixel circuitry controlling the voltage across the light-emitting element
• CPC Class Codes: G09G 3/3291 in which the data driver supplies a variable data voltage for setting the current through, or the voltage across, the light-emitting elements
• CPC Class Codes: H01L 2029/7863 with an LDD consisting of more than one lightly doped zone or having a non-homogeneous dopant distribution, e.g. graded LDD
• CPC Class Codes: H01L 27/12 the substrate being other than a semiconductor body, e.g. an insulating body
• CPC Class Codes: H01L 27/1214 comprising a plurality of TFTs formed on a non-semiconducting substrate, e.g. driving circuits for AMLCDs
• CPC Class Codes: H01L 27/1259 Multistep manufacturing methods
• CPC Class Codes: H01L 27/127 with a particular formation, treatment or patterning of the active layer specially adapted to the circuit arrangement
• CPC Class Codes: H01L 29/66757 Lateral single gate single channel transistors with non-inverted structure, i.e. the channel layer is formed before the gate
• CPC Class Codes: H01L 29/78621 with LDD structure or an extension or an offset region or characterised by the doping profile
• CPC Class Codes: H10K 59/12 : Active-matrix OLED [AMOLED] displays

Abstract

A display is conducted by using a time gray-scale system, in which one frame period is divided into a plurality of sub-frame periods, and a voltage applied to an EL element of a pixel is varied on a sub-frame period basis. Because of this, a display device is provided in which the fluctuations in brightness caused by an environment temperature of an EL element are suppressed with a gray-scale display method that is unlikely to be influenced by variations in characteristics of TFTs in a pixel portion and that is unlikely to be influenced by variations in a display period.

Claims

1. A method of driving a display device that includes a buffer amplifier, a first EL element and a second EL element, with each EL element including a first electrode, a second electrode and an EL layer provided between the first electrode and the second electrode, the method comprising:

dividing a frame period into sub-frame periods;

placing the first EL element into one of a light-emitting state and a non-light-emitting state in each of the sub-frame periods; and

allowing a constant current to flow between the first electrode and the second electrode of the second EL element in each of the sub-frame periods,

wherein;

one of the electrodes of the first EL element is connected to a constant current source through at least the buffer amplifier,

one of the electrodes of the second EL element is connected to the constant current source and an input terminal of the buffer amplifier,

a voltage between the first electrode and the second electrode of the first EL element when the first EL element is in the light-emitting state is equal to a voltage between the first electrode and the second electrode of the second EL element when the constant current is flowing, and

respective values of the constant current are different during two sub-frame periods.

5. A method of driving a display device according to claim 1, wherein said display device is used for one of a video camera, an image play back device, a head mounted type display device, a personal computer, and a portable information terminal.

2. A method of driving a display device that includes a first EL element and a second EL element, with each EL element including a first electrode, a second electrode and an EL layer provided between the first electrode and the second electrode, the method comprising:

dividing a frame period into sub-frame periods;

placing the first EL element into one of a light-emitting state and a non-light-emitting state in each of the sub-frame periods; and

allowing a constant current to flow between the first electrode and the second electrode of the second EL element in each of the sub-frame periods,

wherein;

one of the electrodes of the first EL element is connected to a constant current source through at least the buffer amplifier,

one of the electrodes of the second EL element is connected to the constant current source,

a voltage between the first electrode and the second electrode of the first EL element when the first EL element is in the light-emitting state is equal to a voltage between the first electrode and the second electrode of the second EL element when the constant current is flowing, and

a value of the constant current is different during each of the sub-frame periods.

3. A method of driving a display device according to claim 2, wherein a length of each of the sub-frame periods is the same.

6. A method of driving a display device according to claim 2, wherein said display device is used for one of a video camera, an image play back device, a head mounted type display device, a personal computer, and a portable information terminal.

4. A method of driving a display device that includes a first EL element and a second EL element, with each EL element including a first electrode, a second electrode and an EL layer provided between the first electrode and the second electrode, the method comprising:

dividing a frame period into n (n is a natural number of 2 or more) sub-frame periods;

placing the first EL element into one of a light-emitting state and a non-light-emitting state in each of the sub-frame periods; and

allowing a constant current to flow between the first electrode and the second electrode of the second EL element in each of the sub-frame periods,

wherein;

one of the electrodes of the first EL element is connected to a constant current source through at least the buffer amplifier,

one of the electrodes of the second EL element is connected to a constant current source,

a voltage between the first electrode and the second electrode of the first EL element when the first EL element is in the light-emitting state is equal to a voltage between the first electrode and the second electrode of the second EL element when the constant current is flowing, and

a ratio of a value of the constant current during each of the n sub-frame periods is 2⁰;

2<sup>−

1</sup>;

2<sup>−

2</sup>;

. . . ;

2<sup>−

(n−

2)</sup>;

2<sup>−

(n−

1)</sup>.

7. A method of driving a display device according to claim 4, wherein said display device is used for one of a video camera, an image play back device, a head mounted type display device, a personal computer, and a portable information terminal.

8. A display device comprising:

a plurality of pixels, each of which includes a TFT and a first EL element;

a power supply line; and

a circuit that includes a buffer amplifier, a second EL element, a switch, and first and second constant current sources for outputting constant currents with different values,

wherein;

each of the first EL element and the second EL element respectively includes a first electrode, a second electrode, and an EL layer provided between the first electrode and the second electrode,

the switch functions for selecting whether an output terminal of the first constant current source is connected to the first electrode of the second EL element, or an output terminal of the second constant current source is connected to the first electrode of the second EL element,

the first electrode of the second EL element is connected to a non-inversion input terminal of the buffer amplifier,

an output terminal of the buffer amplifier is connected to the power supply line, and

an electric potential of the power supply line is applied to the first electrode of the first EL element of a pixel through the TFT of the pixel.

10. A display device according to claim 8, wherein the first electrodes of the first EL element and the second EL element are anodes, and the second electrodes thereof are cathodes.

12. A display device according to claim 8, wherein the first electrodes of the first EL element and the second EL element are cathodes, and the second electrodes thereof are anodes.

14. A display device according to claim 8, wherein said display device is used for one of a video camera, an image play back device, a head mounted type display device, a personal computer, and a portable information terminal.

9. A display device comprising:

a plurality of pixels, each of which includes a TFT and a first EL element;

a power supply line; and

a circuit that includes a buffer amplifier, a second EL element, a switch, and n (n is a natural number of 2 or more) constant current sources for outputting constant current with the same value,

wherein;

each of the first EL element and the second EL element respectively includes a first electrode, a second electrode, and an EL layer provided between the first electrode and the second electrode,

the switch functions for selecting whether m (m is a natural number of n or less) output terminals of the n constant current sources are connected to the first electrode of the second EL element, or k (k is a natural number of n or less, different from m) output terminals of the n constant current sources are connected to the first electrode of the second EL element,

the first electrode of the second EL element is connected to a non-inversion input terminal of the buffer amplifier,

an output terminal of the buffer amplifier is connected to the power supply line, and

an electric potential of the power supply line is applied to the first electrode of the first EL element of a pixel through the TFT of the pixel.

11. A display device according to claim 9, wherein the first electrodes of the first EL element and the second EL element are anodes, and the second electrodes thereof are cathodes.

13. A display device according to claim 9, wherein the first electrodes of the first EL element and the second EL element are cathodes, and the second electrodes thereof are anodes.

15. A display device according to claim 9, wherein said display device is used for one of a video camera, an image play back device, a head mounted type display device, a personal computer, and a portable information terminal.

16. A display device comprising:

a plurality of pixels, each of the plurality of pixels including;

a first EL element, and

a power supply line;

a buffer amplifier;

a switching portion;

a plurality of constant current sources; and

a second EL element,

wherein;

an output terminal of the buffer amplifier is electrically connected to the first EL element through the power supply line; and

an input terminal of the buffer amplifier is electrically connected to the second EL element, and to one of the plurality of constant current sources through the switching portion.

17. A display device comprising:

a plurality of pixels each of the plurality of pixels including;

a first EL element, and

a power supply line;

a buffer amplifier;

a switching portion;

a plurality of constant current sources for outputting constant current with different values; and

a second EL element,

wherein;

the switching portion selects one of the plurality of constant current sources;

an output terminal of the buffer amplifier is electrically connected to the first EL element through the power supply line; and

a non-inversion input terminal of the buffer amplifier is electrically connected to the second EL element, and to the selected one of the plurality of constant current sources through the switching portion.