Current load device and method for driving the same

• US 20040041750A1
• Filed: 08/27/2002
• Published: 03/04/2004
• Est. Priority Date: 08/29/2001
• Status: Active Grant

• Current Assignee: HannStar Display Corporation
• Original Assignee: NEC Corporation
• Inventors: Abe, Katsumi
• Granted Patent: US 7,209,101 B2
• Time in Patent Office: Days
• US Class Current: 345/76
• 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 2310/0251 Precharge or discharge of pixel before applying new pixel voltage
• CPC Class Codes: G09G 2310/0262 The addressing of the pixel, in a display other than an active matrix LCD, involving the control of two or more scan electrodes or two or more data electrodes, e.g. pixel voltage dependent on signals of two data electrodes
• CPC Class Codes: G09G 2320/0252 Improving the response speed
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: H03K 17/04106 in field-effect transistor switches H03K17/0412, H03K17/0416 take precedence

Abstract

This invention provides a precise current load device. A cell includes a power supply line, a ground line, first and second voltage supply lines, a signal line, first, third and fourth control lines, first to fourth switches, a p-type TFT, a capacitance element, and a current load element. A source of the p-type TFT is connected to the power supply line, one terminal of the current load element is connected to the ground line, the first switch is connected between the signal line and a drain of the p-type TFT, the second switch is connected between the drain and the gate of the p-type TFT, the third switch is connected between the drain of the p-type TFT and the current load element, and the fourth switch is connected between the voltage supply line and the current load element.

Claims

1. A current load device, comprising:

a driving transistor having a source connected to a power supply line or ground line;

a first switch connected between a signal line supplied with a current or voltage and a drain of said driving transistor;

a second switch connected between said signal line or the drain of said driving transistor and a gate of said driving transistor;

a capacitance element having one terminal connected to a first voltage supply line and the other terminal connected to the gate of said driving transistor; and

a serially-connected assembly of a current load element and a third switch, the serially-connected assembly being connected between the ground line or power supply line and the drain of said driving transistor.

3. The current load device according to claim 1, wherein in a first operation state, said first and second switches are turned on and said third switch is turned off to store a gate voltage in accordance with current/voltage characteristics of said driving transistor in said capacitance element so that the current flowing through said signal line flows between the drain and source of said driving transistor without passing a current through said current load element, and then

in a second operation state, said first and second switches are turned off and the third switch is turned on to let said driving transistor supply the current whose value is the same that of current having flowed through the signal line in said first operation state to said current load element through said third switch.

4. The current load device according to claim 1, wherein in a first operation state, said first and second switches are turned on and said third switch is turned off to apply the voltage applied to said signal line to the gate of said driving transistor and said capacitance element without passing a current through said current load element, and then a gate voltage in accordance with current/voltage characteristics of said driving transistor is stored in said capacitance element so that the current flowing through said signal line flows between the drain and source of said driving transistor, and

in a second operation state, said first and second switches are turned off and the third switch is turned on to let said driving transistor supply the current whose value is the same that of current having flowed through the signal line in said first operation state to said current load element through said third switch.

5. The current load device according to claim 1, wherein in a first operation state, said second switch between said signal line and said driving transistor is turned on and said first and third switches are turned off to apply the voltage applied to said signal line to the gate of said driving transistor and said capacitance element without passing a current through said current load element, and then, said first and second switches are turned on and said third switch is turned off to store a gate voltage in accordance with current/voltage characteristics of said driving transistor in said capacitance element so that the current flowing through said signal line flows between the drain and source of said driving transistor, and

in a second operation state, said first and second switches are turned off and the third switch is turned on to let said driving transistor supply the current whose value is the same that of current having flowed through the signal line in said first operation state to said current load element through said third switch.

6. The current load device according to claim 3, wherein said third switch is turned off before said first switch is turned on, and is turned on after said first switch is turned off.

7. The current load device according to claim 3, wherein said second switch is turned off before said first switch is turned off.

8. The current load device according to claim 1, wherein said driving transistor is constituted by a thin film transistor (TFT).

9. The current load device according to claim 1, wherein the first, second and third switches are each constituted by a TFT.

10. The current load device according to claim 1, wherein said first and second switches are TFTs having a same polarity, and said third switch is a TFT having a polarity opposite to that of said first and second switches.

11. The current load device according to claim 1, wherein said first, second and third switches are controlled by one control line.

12. The current load device according to claim 1, wherein said first and second switches are controlled by one control line.

13. The current load device according to claim 1, wherein said first and third switches are controlled by one control line.

14. The current load device according to claim 1, wherein a TFT which has the drain and source short-circuited and operates oppositely to said second switch TFT is connected between said second switch TFT and said driving transistor TFT as a dummy switch.

15. The current load device according to claim 14, wherein a ratio (W/L) between a length (L) and a width (W) of the gate of said dummy switch TFT is a half the ratio between the length and the width of the gate of said second switch TFT.

16. The current load device according to claim 1, further comprising a fourth switch having one terminal connected between said third switch and said current load element and another terminal connected to a third voltage supply line.

17. The current load device according to claim 16, wherein a value of a voltage applied to said third voltage supply line is lower than a voltage value for said current load element to start to operate.

18. The current load device according to claim 17, wherein said fourth switch is turned on when said third switch is turned off, whereby charges stored in said current load element are forcedly removed and the current flowing through said current load element is quickly stopped.

19. The current load device according to claim 16, wherein said fourth switch is constituted by a TFT.

20. The current load device according to claim 19, wherein said third and fourth switches are TFTs having opposite polarities, whereby said third and fourth switches are controlled by one control line.

21. The current load device according to claim 1, wherein said first voltage supply line is the power supply line or ground line.

22. The current load device according to claim 1, wherein the voltage applied through said first voltage supply line differs between said first operation state and said second operation state.

30. The current load device according to claim 1, wherein said current load element is a light emitting device.

31. The current load device according to claim 1, wherein said current load element is an organic EL device.

2. A current load device, comprising:

a first transistor having a source connected to a power supply line or ground line and a gate connected to a second voltage supply line;

a driving transistor serially connected to said first transistor;

a first switch connected between a signal line supplied with a current or voltage and a drain of said driving transistor;

a second switch connected between said signal line or the drain of said driving transistor and a gate of said driving transistor;

a capacitance element having one terminal connected to a first voltage supply line and the other terminal connected to the gate of said driving transistor; and

a serially-connected assembly of a current load element and a third switch, the serially-connected assembly being connected between the ground line or power supply line and the drain of said driving transistor.

23. A method for driving a current load device, the current load device being active-matrix driven and comprising a plurality of cells each comprising a current load element, a driving transistor for driving the current load element and a retention capacitance element for retaining a voltage to be applied to said driving transistor, wherein said current load element has not been driven not only for the period in which said retention capacitance element is setting an appropriate voltage level to be retained therein, but also for a part of the period in which said retention capacitance element does not perform the setting operation.

24. The method for driving a current load device according to claim 23, wherein the supply of the current to said current load element is stopped before the operation for setting the voltage level for said retention capacitance element.

25. The method for driving a current load device according to claim 23, wherein when the supply of the current to said current load element is stopped, charges stored in said current load element is forcedly removed.

26. The method for driving a current load device according to claim 23, wherein when setting the voltage level for said retention capacitance element, said driving transistor operates in a saturation region.

27. The method for driving a current load device according to claim 23, wherein when setting the voltage level for said retention capacitance element, the performance of passing the current through said driving transistor is set after the performance of supplying the voltage to said retention capacitance element and said driving transistor.

28. The method for driving a current load device according to claim 23, wherein the operation for setting the voltage level for said retention capacitance element includes a period in which the current flowing through the signal line is passed across a drain and source of said driving transistor.

29. The method for driving a current load device according to claim 23, wherein a performance of said current load element is set based on two factors, a first factor being the performance of said current load element in the case of being driven by said driving transistor, and a second factor being a ratio between a period in which said current load element operates and a period in which said current load element does not operate.

32. The method for driving a current load device according to claim 23, wherein said current load element is a light emitting device.

33. The method for driving a current load device according to claim 23, wherein said current load element is an organic EL device.