1.  Field of Invention
The present invention relates to a backlight unit.  More particularly, the present invention relates to a backlight unit capable of reducing tube current variation between light sources, and a liquid crystal display device having the same. 
2.  Description of Related Art
Recently, with the development of semiconductor technologies, flat display devices having compact size and light weight with superior performance have been extensively used. 
Among the flat display devices, a liquid crystal display (LCD) has advantages of compact size, light weight and low power consumption, so the LCD has been spotlighted as a substitute for a conventional cathode ray tube (CRT).  Such an LCD is installed in various information processing appliances that require the display device. 
In general, the LCD applies voltage to liquid crystal having specific molecular alignment to change the molecular alignment of the liquid crystal, and generates visual signals based on variation in optical characteristics of liquid crystal cells, such as double refraction, optical rotary power, dichroism, and light scattering characteristics.  That is, the LCD displays information by performing light modulation using the liquid crystal cells that emit light according to the molecular alignment of the liquid crystal. 
A liquid crystal display panel of the LCD is a non-emissive device, so a backlight unit is installed at a rear of the liquid crystal display panel to provide light to the liquid crystal display panel. 
The backlight unit is classified into an edge-illumination type backlight unit and a direct-illumination type backlight unit according to a position of a light source thereof. 
The edge-illumination type backlight unit includes a light source unit provided at a lateral side of a light guiding plate used for guiding light.  The light source unit includes a lamp for generating light, a lamp holder provided at both sides of the lamp to protect the lamp, and a lamp housing that surrounds an outer peripheral surface of the lamp to reflect the light, which is generated from the lamp, toward a light guiding plate.  The edge-illumination type backlight unit is employed in the LCD having a relatively small size, such as a monitor of a lap-top computer or a desk-top computer.  The edge-illumination type backlight unit represents advantages in terms of light uniformity, endurance, and slimness of the LCD. 
The direct-illumination type backlight unit is developed as the size of the LCD has been enlarged up to 20 inches.  According to the direct-illumination type backlight unit, a plurality of lamps are provided below the liquid crystal display panel to directly illuminate light to the liquid crystal display panel.  The direct-illumination type backlight unit has high light efficiency, so the direct-illumination type backlight unit is mainly employed in a large-size LCD requiring high brightness. 
A light source unit provided in the direct-illumination type backlight unit includes a plurality of lamps, lamp sockets provided at both ends of the lamps to support the lamps while being electrically connected with electrodes provided at both ends of the lamps, a printed circuit board (PCB) electrically connected to the lamp sockets to receive a lamp driving signal from an exterior, and a plurality of condensers installed at both ends of the lamps on the PCB. 
The condensers uniformly maintain a swing width of an AC (alternate current) signal input into both ends of the lamp. 
The condensers may have tolerance and temperature coefficient, which vary according to the condensers.  If the condensers have tolerance and temperature coefficient, which are different from each other, the swing width of the AC signal input into both ends of the lamp may not be uniformly maintained. 
The condensers having various tolerance and temperature coefficient may cause tube current variation of the lamps, resulting in non-uniform brightness. 
In addition, since the large-size LCD having the direct-illumination type backlight unit displays an image in a state in which the large-size LCD is uprightly positioned, the temperature may increase at the upper portion of the backlight unit due to convection phenomenon, so that brightness difference may occur between the upper and lower portions of the backlight unit.  That is, the lamps provided at the upper portion of the backlight unit represent higher tube current as compared with that of the lamps provided at the lower portion of the backlight unit. 
In detail, if the temperature of a conductor subject to the same voltage rises, resistance of the conductor decreases so that high current is applied to the conductor.  Thus, the ambient temperature of the lamp becomes high, so that non-uniform brightness may occur. 
In order to solve this problem, capacitance values of the condensers are finely adjusted depending on the lamps.  However, since capacitance values of high-voltage condensers in use are set with few pF intervals, not only is it difficult to develop new high-voltage condensers having capacitance values with 0.1 pF interval, but also the new high-voltage condensers may cause the great manufacturing cost.