Liquid crystal display having an improved lighting device

• US 6,636,190 B2
• Filed: 08/23/2001
• Issued: 10/21/2003
• Est. Priority Date: 10/12/2000
• Status: Expired due to Term

• Current Assignee: Panasonic Liquid Crystal Display Company Limited (Panasonic Holdings Corporation)
• Original Assignee: Hitachi Device Engineering Co., Ltd., Hitachi, Ltd.
• Inventors: Ono, Kikuo, Shingai, Akira, Hirakata, Junichi
• Primary Examiner(s): Shankar, Vijay
• Assistant Examiner(s): Patel, Nitin
• Application Number:
  US09/934,696
• Publication Number:
  US 20020036608A1
• Time in Patent Office: 789 Days
• Field of Search: 345/74.1, 345/87, 345/52, 315/86, 315/247, 315/169.3, 315/209, 349/65, 349/58
• US Class Current: 345/74.1
• CPC Class Codes: G02F 1/133604 with lamps
• CPC Class Codes: G02F 1/133612 Electrical details
• CPC Class Codes: G09G 2310/024 Scrolling of light from the illumination source over the display in combination with the scanning of the display screen
• CPC Class Codes: G09G 2320/0261 in the context of movement of objects on the screen or movement of the observer relative to the screen
• CPC Class Codes: G09G 2320/041 Temperature compensation
• CPC Class Codes: G09G 2320/0633 by amplitude modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2320/064 by time modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2320/066 for control of contrast
• CPC Class Codes: G09G 2330/045 Protection against panel overheating
• CPC Class Codes: G09G 3/342 using several illumination sources separately controlled corresponding to different display panel areas, e.g. along one dimension such as lines
• CPC Class Codes: H05B 41/2821 by means of a single-switch converter or a parallel push-pull converter in the final stage H05B41/285 takes precedence
• CPC Class Codes: H05B 41/2824 using control circuits for the switching element H05B41/2822 takes precedence
• CPC Class Codes: H05B 41/3927 by pulse width modulation H05B41/3922 takes precedence

Abstract

A liquid crystal display device includes a liquid crystal display panel and a lighting device. The lighting device is supplied with alternately a first current during a period t₁ and a second current during a second period t₂ such that electric power E1 is lower than electric power E2. E1 is defined as (t₁×i_p-p(1)×V_p-p(1))/2+(t₂×i_p-p(2)×V_p-p(2))/2, where i_p-p(1)=a peak-to-peak value of the first current, V_p-p(1)=a peak-to-peak value of a voltage across the light source during the period t1, i_p-p(2)=a peak-to-peak value of the second current, and V_p-p(2)=a peak-to-peak value of a voltage across the light source during the period t₂. E2 is defined as (t₁+t₂)×I_eff×V_eff, where I_eff and V_eff are effective values of the current and voltage of the light source, respectively.

Claims

1. A liquid crystal display device comprising:

a liquid crystal display panel having a plurality of pixels;

a lighting device having at least one light source and projecting light generated by said at least one light source on said liquid crystal display panel; and

a circuit for supplying alternately a first alternating current of a first amplitude during a first period t₁ and a second alternating current of a second amplitude during a second period t₂ to said at least one light source,

said first amplitude being greater than said second amplitude,

wherein said circuit controls said first alternating current and said second alternating current such that the following relationship is satisfied;

first electric power E1 is lower than second electric power E2,

where said first power E1 is defined as {(t₁×

i_p-p(1)×

V_p-p(1))/2}+{(t₂×

i_p-p(2)×

V_p-p(2))/2},

i_p-p(1)=a peak-to-peak value of said first alternating current flowing through a respective one of said at least one light source during said first period t₁,

V_p-p(1)=a peak-to-peak value of a voltage across said respective one of said at least one light source during said first period t₁,

i_p-p(2)=a peak-to-peak value of said second alternating current flowing through said respective one of said at least one light source during said second period t₂,

V_p-p(2)=a peak-to-peak value of a voltage across said respective one of said at least one light source during said second period t₂,

said second electric power E2 is defined as (t₁+t₂)×

(I_eff×

V_eff),

I_eff is an effective value of a current flowing through said respective one of said at least one light source during said first period t₁ plus and said second period t₂, and

V_eff is an effective value of a voltage across said respective one of said at least one light source during said first period t₁ plus and said second period t₂.

2. A liquid crystal display device according to claim 1, wherein said at least one light source comprises at least one lamp, a reflector for reflecting light from said at least one lamp, a light guide plate for directing said light reflected from said reflector toward a liquid crystal layer of said liquid crystal display panel, and said at least one lamp is disposed along at least one of sides of said light guide plate.

3. A liquid crystal display device according to claim 2, wherein said at least one lamp includes two lamps disposed along two respective sides of said light guide plate.

4. A liquid crystal display device according to claim 2, wherein said at least one lamp includes a lamp disposed along one side of said light guide plate.

5. A liquid crystal display device according to claim 1, wherein a sum of said first period t₁ and said second period t₂ is made different from a picture frame period of said liquid crystal display panel.

6. A liquid crystal display device according to claim 1, wherein a sum of said first period t₁ and said second period t₂ is made equal to a picture frame period of said liquid crystal display panel, and a time of starting said at least one light source is synchronized with a time of starting of one of scanning lines from (0.3×

n)th to (0.7×

n)th, where n is a number of scanning lines formed in said liquid crystal display panel.

7. A liquid crystal display device according to claim 1, wherein a sum of said first period t₁ and said second period t₂ is made equal to a picture frame period of said liquid crystal display panel, and a time of starting a first scanning line forming said liquid crystal display panel is delayed by a fixed time from a time of starting said at least one light source.

8. A liquid crystal display device according to claim 1, wherein said second period t₂ is equal to or more than one twentieth of said first period t₁.

9. A liquid crystal display device comprising:

a liquid crystal display panel having a plurality of pixels;

a lighting device having at least one light source and projecting light generated by said at least one light source on said liquid crystal display panel; and

a circuit for alternately supplying a lamp current to said at least one light source during a first period t₁ and ceasing to supply said lamp current to said at least one light source during a second period t₂,

wherein the following relationship is satisfied;

first electric power E1 is lower than second power E2, where said first power E1 is defined as (t₁×

i_p-p×

V_p-p)/2,

i_p-p=a peak-to-peak value of said lamp current flowing through a respective one of said at least one light source during said first period t₁,

V_p-p=a peak-to-peak value of a voltage across said respective one of said at least one light source during said first period t₁,

said second power E2 is defined as (t₁+t₂)×

(I_eff×

V_eff),

I_eff is an effective value of said lamp current flowing through said respective one of said at least one light source during said first period t₁ plus and said second period t₂, and

V_eff is an effective value of a voltage across said respective one of said at least one light source during said first period t₁ plus and said second period t₂.

10. A liquid crystal display device according to claim 9, wherein a sum of said first period t₁ and said second period t₂ is made different from a picture frame period of said liquid crystal display panel.

11. A liquid crystal display device according to claim 9, wherein a sum of said first period t₁ and said second period t₂ is made equal to a picture frame period of said liquid crystal display panel, and a time of starting said at least one light source is synchronized with a time of starting of one of scanning lines from (0.3×

n)th to (0.7×

n)th, where n is a number of scanning lines formed in said liquid crystal display panel.

12. A liquid crystal display device according to claim 9, wherein a sum of said first period t₁ and said second period t₂ is made equal to a picture frame period of said liquid crystal display panel, and a time of starting a first scanning line forming said liquid crystal display panel is delayed by a fixed time from a time of starting said at least one light source.

13. A liquid crystal display device comprising:

a liquid crystal display panel having a plurality of pixels;

a lighting device having at least one light source and projecting light generated by said at least one light source on said liquid crystal display panel; and

a circuit for supplying alternately a first voltage having a first effective value V₁ during a first period t₁ and a second voltage having a second effective value V₂ during a second period t₂ to said at least one light source,

said first voltage producing a first current having a first effective value i₁ flowing through said respective one of said at least one light source during said first period t₁,

said second voltage producing a second current having a second effective value i₂ flowing through said respective one of said at least one light source during said second period t₂,

said second effective value i₂ being smaller than said first effective value i₁,

wherein a first ratio of a first brightness to a first electric power is greater than a second ratio of a second brightness to a second electric power,

where said first brightness is a brightness produced by said respective one of said at least one light source during said first period t₁ plus said second period t₂,

said first electric power is defined as {(t₁×

V₁×

i₁)+(t₂×

V₂×

i₂)}/(t₁+t₂),

said second electric power is defined as (V_eff×

i_eff)

V_eff is an effective value produced by a combination of said first voltage supplied during said first period t₁ and said second voltage supplied during said second period t₂,

i_eff is an effective value produced by a combination of said first current flowing during said first period t₁ and said second current flowing during said second period t₂, and

said second brightness is a brightness produced by said respective one of said at least one light source supplied with said second electric power.

14. A liquid crystal display device according to claim 13, wherein said second period t₂ is equal to or more than one twentieth of said first period t₁.

15. A liquid crystal display device according to claim 13, wherein an integral of brightness produced by said at least one light source over said second period t₂ is equal to or lower than 90% of brightness of said at least one light source during said first period t₁.

16. A liquid crystal display device comprising:

a liquid crystal display panel having a plurality of pixels;

a lighting device having a cold-cathode tube having an end-to-end length L (cm) and projecting light generated by said cold-cathode tube on said liquid crystal display panel; and

a circuit for supplying alternately a first electric power W₁ (W) during a first period t₁ and a second electric power W₂ (W) during a second period t₂ to said cold-cathode tube,

said second electric power W₂ being lower than said first power W₁,

wherein a ratio of said first electric power W₁ to said end-to-end length L is greater than 0.2 W/cm, and a ratio of said second electric power W₂ to said end-to-end length L is equal to or less than 0.1 W/cm.

17. A liquid crystal display device according to claim 16, wherein said at least one light source is a tubular lamp having a diameter equal to or more than 2.5 mm.

18. A liquid crystal display device according to claim 16, wherein said at least one light source is a tubular lamp having a glass radial wall thickness equal to or more than 1 mm.

19. A liquid crystal display device comprising:

a liquid crystal display panel having a plurality of pixels;

a lighting device having at least one light source and projecting light generated by said at least one light source on said liquid crystal display panel; and

a circuit for supplying alternately a first current having a first effective value i₁ during a first period t₁ and a second current having a second effective value i₂ during a second period t₂ to a respective one of said at least one light source,

said first effective value i₁ being greater than a rated value of a lamp current flowing through said respective one of said at least one light source,

said second effective value i₂ being smaller than said rated value of said lamp current,

wherein said first effective value i₁, said second effective value i₂, said first period t₁ and said second period t₂ are selected such that an integral of brightness produced by said respective one of said at least one light source over said first period t₁ plus said second period t₂ is greater than an integral of brightness produced by said respective one of said at least one light source supplied with said lamp current of said rated value over said first period t₁ plus said second period t₂.

20. A liquid crystal display device according to claim 19, wherein an integral of brightness produced by said at least one light source over said second period t₂ is equal to or lower than 90% of brightness of said at least one light source during said first period t₁.

21. A liquid crystal display device according to claim 19, wherein said at least one light source is bent at at least one position of a length thereof.

22. A liquid crystal display device comprising:

a liquid crystal display panel having a plurality of pixels;

a lighting device having at least one light source and projecting light generated by said at least one light source on said liquid crystal display panel;

a control circuit configured so as to switch between a first operation and a second operation,

said first operation supplying a first current having a first effective value i₁ during a first period t₁ to said at least one light source, and said second operation supplying a second current having a second effective value i₂ during a second period t₂ to said at least one light source, said second effective value i₂ being smaller than said first effective value i₁; and

a temperature detector circuit for detecting a temperature of said at least one light source,

wherein said temperature detector circuit transmits a signal to said control circuit when a temperature of an outside wall of said at least one light source exceeds 65°

C., and said control circuit switches from said first operation to said second operation in response to said signal.

23. A liquid crystal display device according to claim 22, wherein an integral of brightness produced by said at least one light source over said second period t₂ is equal to or lower than 90% of brightness of said at least one light source during said first period t₁.

24. A liquid crystal display device according to claim 22, wherein said at least one light source is a lamp containing xenon therein.

25. A liquid crystal display device comprising:

a liquid crystal display panel having a plurality of pixels;

a lighting device having at least one light source and projecting light generated by said at least one light source on said liquid crystal display panel;

a control circuit configured so as to switch between a first operation and a second operation,

said first operation supplying a first current having a first effective value i₁ during a first period t₁ to said at least one light source, and said second operation supplying a second current having a second effective value i₂ during a second period t₂ to said at least one light source, said second effective value i₂ being smaller than said first effective value i₁; and

a brightness detector circuit for detecting brightness of said at least one light source,

wherein said brightness detector circuit transmits a signal to said control circuit when the brightness begins to reduce in said first period t₁, and said control circuit switches from said first operation to said second operation in response to said signal.

26. A liquid crystal display device according to claim 25, wherein an integral of brightness produced by said at least one light source over said second period t₂ is equal to or lower than 90% of brightness of said at least one light source during said first period t₁.

27. A liquid crystal display device according to claim 25, wherein said at least one light source is a lamp containing xenon therein.