Abstract:
A power supply for an LCD and a voltage sequence control method in which the sequence for voltages applied to a gate driver IC for outputting a driving voltage of an LCD panel is controlled by arranging a switching element between a DC-to-DC converter and the gate driver IC so as to switch a turn-on voltage to a turn-off voltage to be applied to the gate driver IC, and a latch up is prevented by arranging diodes in reverse and forward directions to the lines for applying the turn-on and turn-off voltages respectively so that the applied voltage is not deviated from the latch up preventing scope. Voltages are applied or removed from the gate driver IC in accordance with a predetermined sequence, and an abnormal voltage is prevented from being applied in an early stage of driving, to thereby stabilize the LCD panel operation.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a power supply of an LCD and voltage sequence control methods that enhance the LCD panel performance and prevent a latch up by applying stabilized voltages to the gate driver integrated circuits (ICs). The voltage sequence is determined by the turn-off voltage level among voltage levels applied to a gate driver IC that outputs a driving voltage for the panel. 
     (b) Description of the Related Art 
     Generally, LCD devices include an LCD panel where a liquid is injected between the two glass substrates on which pixels and electrodes are formed, a printed circuit board (PCB) where various integrated circuits for driving the LCD panel are mounted and interfacing with the electrode of the LCD panel, a back light unit providing the display with required light, a power supply providing various driving voltages, and an assembly of mold frames or chassis. 
     A predetermined voltage level applied to the LCD panel, operates a thin film transistor (TFT) that constitutes each pixel to display a certain image. 
     Controlling the voltage applied to display the desired image is considerably important in LCD display technologies. U.S. Pat. No. 5,777,611 discloses an apparatus for controlling power sequence of an LCD module. 
     Specifically, a plurality of voltages are applied in a predetermined sequence to a plurality of gate driver ICs mounted on the PCB, and each gate driver IC outputs a voltage to drive the LCD panel. 
     In a conventional method, as shown in FIG. 1, a turn-on voltage V on  with approximately 20V and a turn-off voltage V off  with approximately −7V are applied to a driver IC  3 . Such voltages are applied or removed in accordance with a predetermined sequence. An incorrectly controlled sequence causes a latch up, which may result in a failure in driving the LCD panel. Here the driver IC  3  is a gate driver IC. 
     Voltages V on  and V off  are generated by applying a constant voltage V DD  to a DC-to-DC converter. 
     Generally, the sequence for applying a voltage to a driver IC is set in such a manner that V off  voltage is applied first and V on  voltage later when the device is turned on, and V on  voltage is applied first and V off  voltage later when the device is turned off. 
     If necessary, an LCD device includes a power sequence controlling circuit. V on  voltage and V off  voltage are generated independently from each other in a conventional power sequence controlling circuit. A sequence of such voltages is controlled by time constants of a plurality of DC-to-DC converters  1  and  2  for outputting V on  voltage and V off  voltage, or only by a time constant of V on  voltage. 
     However, in the above-described sequence control method, V on  voltage and V off  voltage are applied to a driver IC independently from each other. Thus, a relative time control for keeping the sequence is difficult to achieve. Specifically, the above-described conventional method allows a sequence control only when a power is turned on. 
     Accordingly, as shown in FIG. 2 the voltage applying sequence of the driver IC  3  is not followed correctly, which causes a latch up. This may result in a failure in driving an LCD panel  4 . 
     In the meantime, a latch up may occur while controlling the voltage applying sequence, by failing to keep the voltage level applied to the driver ICs. 
     In more detail, after voltage V DD  is applied in accordance with the normal sequence, voltage V off  of approximately −7V and voltage V on  of approximately 20V are applied to the driver IC as a reference voltage for controlling a TFT. However, a current may flow to the path for applying voltage V on  or V off  before the voltages of V on  and V off  are stabilized to the level of −7V and 20V respectively. Thus, due to such a current, a voltage exceeding the scope of −0.5V of V off  (and 0.5V for V on ), a requisite for preventing latch up of the driver IC, is applied to the driver IC. As a result, an excessive current is generated to the CMOS (complementary metal-oxide semiconductor) circuit which constitutes the driver IC. Thus, the DC-to-DC converter is shut down due to an excessive current applied thereto, which impedes driving of the LCD module. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to apply a plurality of voltages to a driver IC for outputting a driving voltage of an LCD panel in accordance with a predetermined sequence by allowing the plurality of voltages to be dependent upon each other. 
     It is another object of the present invention to stabilize driving of the LCD panel by applying the voltages to the driver IC in accordance with a predetermined sequence. 
     It is still another object of the present invention to provide a normal operation of the LCD panel by stabilizing the driving voltage applied to the driver IC and preventing a latch up of the driver IC. 
     To achieve the above objects and other advantages, there is provided a power supply of an LCD including a first and a second DC-to-DC converters for converting a constant voltage and outputting a first and a second voltages which are different from each other, a switching device for switching the first voltage based on a level of the second voltage and outputting a converted third voltage and a gate driver integrated circuit (IC) for determining the second voltage as a turn-off voltage and the third voltage as a turn-on voltage and outputting a signal for driving an LCD panel. 
     The switching device consists of a switching element and voltage dividing resistances connected thereto. A pnp-type bipolar transistor or a p-type MOS transistor can be used as the switching element. 
     For the pnp-type bipolar transistor, a potential difference caused by the voltage dividing resistance is required to be set higher than those between an emitter and a base. For the p-type MOS transistor, the potential difference caused by the voltage dividing resistance is required to be set higher than a threshold voltage. 
     The voltage sequence according to the above-described constitution is controlled by applying voltages for turning on and off the LCD panel to a driver IC for outputting on and off signals for driving the LCD panel. In addition, the level of the turn-off voltage switches to control the level and time for applying the voltage for turning on the LCD panel. 
     Accordingly, when an external power is applied to the LCD panel, the voltage for turning on the LCD panel is applied after the voltage for turning off the LCD panel is applied to the driver IC. When the external power is turned off, the turn-on voltage level is removed prior to the removal of the turn-off voltage level and these sequences are controlled automatically. 
     The present invention may use as a latch up preventive device a first diode connected in forward direction to a portion of the driver IC to which the first voltage is applied, and a second diode connected in reverse direction to a portion of the driver IC to which the second voltage is applied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and other advantages of the present invention will become more apparent by describing in detail the preferred embodiments thereof with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram showing a conventional power supply of an LCD; 
     FIG. 2 illustrates a voltage sequence error of the conventional power supply of an LCD; 
     FIG. 3 is a block diagram showing a power supply apparatus of an LCD according to a first embodiment of the present invention; 
     FIG. 4 illustrates a voltage sequence of an LCD according to the present invention; 
     FIG. 5 is a block diagram showing a power supply of an LCD according to a second embodiment of the present invention; and 
     FIG. 6 is a block diagram showing a power supply of an LCD according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. 
     Referring to FIG. 3, a power supply apparatus of a first embodiment of the present invention includes DC-to-DC converters  10  and  12  to which the constant voltage V DD , i.e., an external input power having a predetermined level, is applied. The DC-to-DC converter  10  outputs a turn-on voltage V on 1  while the DC-to-DC converter  12  outputs a turn-off voltage V off . 
     An output terminal of the DC-to-DC converter  10  is connected to an emitter of a transistor Q 1 , a resistance R 1  is connected between the emitter and a base of the transistor Q 1 , and a collector of the transistor Q 1  is connected to a first input terminal of a driver IC  20 . Here, a turn-on voltage V on2  that appears at the collector when the transistor Q 1  is turned on, is applied to the first input terminal of the driver IC  20 . 
     An output terminal of the DC-to-DC converter  12  is connected to a second input terminal of the driver IC  20  so that the turn-off voltage V off  can be applied. In addition, a node connected to the base of the transistor Q 1  via a resistance R 2  is formed between the second input terminal of the driver IC  20  and the output terminal of the DC-to-DC converter  12 . A node connected to the resistance R 1  is formed between the resistance R 2  and the base of the transistor Q 1 . Here, the voltage applied to the base of the transistor Q 1  is called V b . 
     Preferably, the transistor Q 1  used as a switching device is a pnp-type bipolar transistor. Resistances R 1  and R 2  are for dividing the potential difference between the turn-on voltage V on1  and the turn-off voltage V off . The ratio between the two resistances is determined with reference to the following equation which shows a voltage applied to            R11     R11   +   R22       ×     (       V   on1     -     V   off       )       &gt;     V   eb                            
     the resistance R1 between the emitter and the base. 
     Wherein, R 11  and R 22  indicate values of the resistances R 1 , R 2 , and V eb  is a constant representing a voltage drop between the emitter and the base of the transistor Q 1 . 
     That is, resistance values R 11  and R 22  can be set within the scope that satisfies the above-described equation. The potential difference applied to the emitter and the base of the transistor Q 1 , i.e., the difference (‘T’ as shown in FIG. 4) between the turn-on voltage V on  and the base applying voltage V b , is required to be set higher than the voltage drop value V eb  between the collector and the base of the transistor Q 1 . 
     The driver IC  20  is structured in a way that an on/off signal for driving an LCD panel  30  is generated by the turn-on voltage V on2  and turn-off voltage V off  which are applied to the first and second input terminals thereof, and applied to the LCD panel  30 . 
     In the first embodiment of the present invention, a sequence for a normal operation of the driver IC  20  is as follows. When the constant voltage V DD , i.e., a main power, is applied, the turn-off voltage V off  applied to the driver IC  20  is generated prior to the generation of the turn-on voltage V on2 . When the constant voltage V DD  is dropped down to a ground level, the turn-on voltage V on2  applied to the driver IC  20  is removed prior to the removal of the turn-off voltage V off . 
     The first embodiment of the present invention considering such a sequence is shown in FIG. 3, and its voltage applying sequence is shown in FIG.  4 . 
     An operation of the first embodiment of the present invention can be explained with reference to FIGS. 3 and 4. 
     The driver IC  20  converts the voltage in accordance with the turn-on voltage V on2  and the turn-off voltage V off  inputted from the first and second input terminals and applies the converted voltage to the LCD panel  30 . 
     When V DD  of a high level is applied to each input terminal of DC-to-DC converters  10  and  12  of a low level, i.e., a ground level (“GND” as shown in FIG.  4 ), the DC-to-DC converter  10  outputs the voltage V on1  while the DC-to-DC converter  12  outputs the turn-off voltage V off . 
     The DC-to-DC converter  10  has a time constant smaller than that of the DC-to-DC converter  12 . Therefore, the voltage V on1  is output prior to the output of turn-off voltage V off . 
     The voltage V b  applied to the base of the transistor Q 1  after its voltage is divided by resistances R 1  and R 2 , is raised to a high level, while the voltage V on1  is being raised to a predetermined high level (approximately 20V). 
     In the meantime, the turn-off voltage V off  is output from the DC-to-DC converter  12 , and in parallel applied to the driver IC  20  and to the resistance R 2  connected to the transistor Q 1 . Here, the turn-off voltage V off  is lowered to a predetermined level (approximately, −7V), and the voltage V b  goes down to a predetermined level. The transistor Q 1  is turned on for switching when the turn-off voltage V off  is lowered to a predetermined level (approximately, −7V). 
     When the transistor Q 1  is turned on, the turn-on voltage V on2  with a predetermined level is applied from the collector of the transistor Q 1  to the driver IC  20 . 
     In the first embodiment of the present invention, the turn-on voltage V off  is applied to the driver IC  20  first when the constant voltage V DD , a power source, is turned on. Then, if the turn-off voltage V off  reaches a predetermined level, the turn-on voltage V on 2  generated by switching of the transistor Q 1  is applied to the driver IC  20 . 
     When the constant voltage V DD  drops down to the ground level GND, level of each voltage also drops to the ground level GND at the same time. When the turn-off voltage V off  goes out of the switching level, the transistor Q 1  is immediately turned off. Therefore, the turn-on voltage V on2  applied to the driver IC  20  first drops down to the ground level GND, and is removed. Then, after a predetermined time period, the turn-off voltage V off  rises up to the ground level GND, and is removed. Then, the turn-on voltage and the base applying voltage V b  of the transistor Q 1  having a potential difference relatively higher than that of the turn-off voltage V off , drop down to the ground level GND, and are removed. 
     Accordingly, in the first embodiment of the present invention, when the constant voltage V DD  is turned off, the turn-on voltage V on2  applied to the driver IC  20  is removed prior to the removal of the turn-off voltage V off . 
     Voltages are applied to the gate driver IC  20  when the power is turned on, and voltages are removed when the power is turned off, in accordance with a prearranged sequence, which is caused by a switching operation of the transistor Q 1 . 
     The transistor Q 1  is switched in accordance with the level of the turn-off voltage V off  applied to the base. As a result, sequences of the voltages applied to the driver IC  20  are determined by the turn-off voltage as a reference voltage. 
     In the first embodiment of the present invention, the switching device consists of a pnp-type bipolar transistor and voltage dividing resistances. However, a p-type MOS transistor and resistances may constitute the switching device. 
     For the p-type MOS transistor, the potential difference between the emitter and the base is required to be higher than the absolute value of the threshold voltage V th . 
     The present invention concerns the control of the sequence of the voltage supplied to the driver IC that applies a driving voltage to the LCD panel, by means of a switching method. Either the turn-on voltage or turn-off voltage applied to the driver IC can be used as a reference signal to control the switching. A control method using the turn-off voltage as a reference signal is proposed here. As shown in the first embodiment of FIGS. 3 and 4, the turn-on voltage and the turn-off voltage are applied to or removed from the driver IC in accordance with a prearranged sequence. 
     Now referring to FIG. 5, a second embodiment of the present invention includes diodes for preventing a latch up of the gate driver IC. 
     In detail, the second embodiment includes DC-to-DC converters  50  and  52  to which the constant voltage V DD  is applied. DC-to-DC converter  50  is connected to a driver IC  54  to apply the voltage Von. DC-to-DC converter  52  is also connected to the driver IC  54  to apply the voltage V off . Driver IC  54  applies a driving voltage to an LCD panel  56 . A diode D 1  is connected in a reverse direction and in parallel to an output terminal of the DC-to-DC converter  50 , and a diode D 2  is connected in forward direction and in parallel to an output terminal of the DC-to-DC converter  52 . The two diodes D 1  and D 2  are grounded in common. 
     Thus, when voltage V DD  is applied to DC-to-DC converters  50  and  52 , the DC-to-DC converter  52  applies voltage V off  of −7V the DC-to-DC converter  52  to the driver IC  54 , and the DC-to-DC converter  50  applies voltage V on  of 20V to the driver IC  54 . 
     In the meantime, an unstable current may flow to the driver IC  54  via the line through which the turn-off voltage V off  is applied before the turn-off voltage V off  is stabilized to −7V, which may result in a supply of an abnormal voltage. If the abnormal voltage reaches the latch up preventive level, the diode D 2  is turned on so as to drop the voltage level down. Thus, the abnormal voltage which may generate a latch up is prevented from being applied to the driver IC  54 . 
     The turn-on voltage V on  is applied to the driver IC  54  after the turn-off voltage V off  is applied. At this time, the unstable current may flow to the driver IC  54  via the line through which the turn-on voltage V on  is applied, before the turn-on voltage V on  is stabilized to 20V, which may result in a supply of the abnormal voltage to the driver IC  54 . If the abnormal voltage reaches the latch up preventive level, the diode D 1  is turned on, which raises the voltage level. Thus, the abnormal voltage which may generate a latch up is prevented from being applied to the driver IC  54 . 
     In the embodiment shown in FIG. 5, the diode D 1  can satisfy the requisite of V on &gt;−0.5V and the diode D 2  can satisfy the requisite of V off &lt;0.5V. Thus, a latch up of the driver IC  54  can be prevented. 
     Diodes for preventing a latch up of the gate driver IC can also be constituted as shown in FIG.  6 . 
     Referring to FIG. 6, the constant voltage V DD  is applied to DC-to-DC converters  60  and  62  which respectively output voltages V on1  and V off . A transistor Q 61  converts voltage V on1  to V on2  when turned on by the turn-off voltage V off . Then, the turn-off voltage V off  of −7V is applied to the driver IC  64  before the turn-on voltage V on2  is applied. The driver IC  64  generates a driving voltage by voltages V on2  and V off , and applies it to an LCD panel  66 . 
     Diodes D 61  and D 62  are grounded in reverse and forward directions respectively to the portions of the driver IC  64  to which voltages V on2  and V off  are applied. 
     Thus, similarly to those shown in FIG. 5, diodes D 61  and D 62  remove the voltage that deviates from the latch up preventive condition. That is, diodes D 61  and D 62  prevent the abnormal voltage from being applied before voltages V on2  and V off  are stabilized to a normal level. 
     In the present invention, the diodes as shown in FIGS. 5 and 6 are employed as a latch up preventive device so as to prevent the voltage that violates the latch up preventive condition from being applied to the gate driver IC. Thus, an operation of the gate driver IC can be stabilized. 
     Schottky diodes can be used in the embodiments shown in FIGS. 5 and 6. 
     According to the present invention, voltages are applied to or removed from the gate driver in accordance with a prearranged sequence, which eliminates a latch up and stabilizes a driving of the LCD panel. Thus, a production yield for an LCD device can be enhanced while the product reliability is improved. 
     This invention has been described above with reference to the aforementioned embodiments. It is evident, however, that many alternative modifications and variations will be apparent to those having skills in the art in light of the foregoing description. Accordingly, the present invention embraces all such alternative modifications and variations falling within the spirit and scope of the appended claims.