Liquid crystal display device having an improved lighting device

• US 20040051692A1
• Filed: 08/08/2003
• Published: 03/18/2004
• Est. Priority Date: 10/12/2000
• Status: Active Grant

• Current Assignee: Panasonic Liquid Crystal Display Company Limited (Panasonic Holdings Corporation)
• Original Assignee: Hitachi, Ltd., Hitachi Device Engineering Co., Ltd. (Japan Display Incorporated)
• Inventors: Ono, Kikuo, Shingai, Akira, Hirakata, Junichi
• Granted Patent: US 7,138,974 B2
• Time in Patent Office: Days
• US Class Current: 345/102
• 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_off×V_off, where I_off and V_off 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 the at least one light source on the liquid crystal display panel; and

a circuit supplying a first current during a first period and a second current smaller than the first current during a second period to the at least one light source alternately,

wherein the circuit has an ammeter monitoring the first current and the second current and adjusting a ratio of the first period to the second period with reference to a result monitored by the ammeter.

2. A liquid crystal display device according to claim 1, wherein a sum of the first period and the second period is kept constant without reference to the result monitored by the ammeter.

3. A liquid crystal display device according to claim 1, wherein an integral of brightness produced by the at least one light source over the second period is equal to or lower than 90% of brightness of the at least one light source during the first period.

4. A liquid crystal display device according to claim 1, wherein the result monitored by the ammeter reflects a temperature variation of the at least one light source.

5. A liquid crystal display device according to claim 1, wherein the at least one light source is a fluorescent lamp.