Abstract:
An exemplary liquid crystal display (LCD) ( 20 ) includes: a gate driving integrated circuit (IC) ( 22 ) for scanning an LCD panel ( 24 ) of the LCD; a data driving IC ( 23 ) for providing a plurality of gradation voltages to the LCD panel; a primary control circuit board ( 21 ) configured for providing the operation voltage to the data driving IC; and a flexible printed circuit board ( 25 ) connected between the LCD panel and the primary control circuit board. The data driving IC includes a voltage detecting circuit ( 230 ), which detects an operation voltage applied to the gate driving IC and is configured to provide an all-scanning signal to the gate driving IC.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates a driving circuit having a voltage detecting circuit for eliminating residual image and a liquid crystal display (LCD) using the same.  
       GENERAL BACKGROUND  
       [0002]     An LCD has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the LCD is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.  
         [0003]     Usually, an LCD needs an external power supply for providing operating power. When the LCD operates, much electric charge is stored therein. When the LCD is powered off, electric charge stored therein is not discharged quickly. This makes the voltage at the external power supply connection drop slowly. As a result, a gate driving circuit and a data driving circuit that drive the LCD operate incorrectly, thereby producing a residual image on the LCD.  
         [0004]      FIG. 4  is a schematic circuit diagram of a typical LCD. The LCD  10  includes an LCD panel  14  and a driving circuit (not labeled). The driving circuit includes a gate driving integrated circuit (IC)  12 , a data driving IC  13 , a primary control circuit board  11 , and a flexible printed circuit board (FPCB)  15 . The gate driving IC  12  and the data driving IC  13  are respectively formed on two adjacent sides of the LCD panel  14  by chip on glass (COG) technology. The FPCB  15  is connected between the LCD panel  14  and the primary control circuit board  11 . The gate driving IC  13  scans the LCD panel  14 . The data driving IC  13  provides a plurality of gradation voltages to the LCD panel  14  when the LCD panel  14  is scanned.  
         [0005]     The primary control circuit board  11  includes a driving control circuit  111 , a power supply circuit  112  and a voltage detecting circuit  113 . The power supply circuit  112  directly provides an operation voltage (not labeled) to the driving control circuit  111 , and respectively provides two operation voltages V 2 , V 1  to the gate driving IC  12  and the data driving IC  13  via the FPCB  15 .  
         [0006]     The voltage detecting circuit  113  generates an all-scanning signal “Xon” when the operation voltage V 2  falls below a predetermined threshold voltage. Then, the voltage detecting circuit  113  transmits the all-scanning signal “Xon” to the gate driving IC  12  through a conducting lead (not labelled) configured on the FPCB  15 . As the gate driving IC  12  receives the all-scanning signal “Xon”, the gate driving IC turns on all of TFTs (thin film transistors) of the LCD panel  14 . Thus the electric charge stored in the TFTs can be discharged quickly. As a result, the residual image on the LCD  10  does not appear when the LCD  10  is power off.  
         [0007]     However, because the primary control circuit board  11  includes the voltage detecting circuit  113 , the volume occupied by the primary control circuit board  11  is large. Furthermore, because the voltage detecting circuit  113  is configured on the primary control circuit board  11 , the FPCB  15  needs a special conducting lead formed thereon for transmitting the all-scanning signal “Xon” from the voltage detecting circuit  113  to the gate driving IC  12 .  
         [0008]     What is needed, therefore, is a driving circuit of an LCD that can overcome the above-described deficiencies.  
       SUMMARY  
       [0009]     An exemplary driving circuit of an LCD includes a gate driving IC for scanning an LCD panel of the LCD; a data driving IC for providing a plurality of gradation voltages to the LCD panel; a primary control circuit board configured for providing the operation voltage to the data driving IC; and a flexible printed circuit board connected between the LCD panel and the primary control circuit board. The data driving IC includes a voltage detecting circuit. The voltage detecting circuit is configured for detecting an operation voltage applied to the gate driving IC and providing an all-scanning signal to the gate driving IC.  
         [0010]     Other advantages and novel features will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic circuit diagram of an LCD according to a preferred embodiment of the present invention.  
         [0012]      FIG. 2  is a schematic circuit diagram of a voltage detecting circuit of the circuit of  FIG. 1 .  
         [0013]      FIG. 3  is an abbreviated timing chart illustrating operation of the voltage detecting circuit of  FIG. 2  and  
         [0014]      FIG. 4  is a schematic circuit diagram of a conventional LCD. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0015]      FIG. 1  is a schematic circuit diagram of an LCD according to a preferred embodiment of the present invention. The LCD  20  includes an LCD panel  24  and a driving circuit (not labeled). The driving circuit includes a gate driving IC  22 , a data driving IC  23 , a primary control circuit board  21 , and an FPCB  25 . The gate driving IC  22  and the data driving IC  23  are respectively formed on two adjacent sides of the LCD panel  24  by chip on glass (COG) technology. The FPCB  25  is connected between the LCD panel  24  and the primary control circuit board  21 . The gate driving IC  23  scans the LCD panel  24 . The data driving IC  23  provides a plurality of gradation voltages to the LCD panel  24  when the LCD panel  24  is scanned.  
         [0016]     The primary control circuit board  21  includes a driving control circuit  211  and a power supply circuit  212 . The power supply circuit  212  directly provides an operation voltage to the driving control circuit  211 , and respectively provides operation voltages V 2 , V 1  to the gate driving IC  22  and the data driving IC  23  via the FPCB  25 .  
         [0017]     The LCD panel  24  includes a conducting lead (not labeled) thereon formed by semiconductor technology. The conducting lead is configured between the gate driving circuit  22  and the data driving circuit  23  for electrically connecting the gate driving circuit  22  and the data driving circuit  23 .  
         [0018]     The data driving circuit  23  includes a voltage detecting circuit  230  integrated therein. The voltage detecting circuit  230  generates an all-scanning signal “Xon” when the operation voltage V 1  falls below a predetermined threshold voltage. Then, the voltage detecting circuit  230  transmits the all-scanning signal “Xon” to the gate driving IC  22  through the conducting lead. The gate driving IC  22  turns on all of TFTs of the LCD panel  24  when the gate driving IC  22  receives the all-scanning signal from the voltage detecting circuit  230 . Thus the electric charge stored in the TFTs can be discharged quickly. As a result, a residual image on the LCD  20  does not appear when the LCD  20  is power off.  
         [0019]     Referring to the  FIG. 2 , the voltage detecting circuit  230  includes an input terminal “IN” for receiving the operation voltage V 1 , an output terminal “OUT” connected to the conducting lead for providing the all-scanning signal “Xon” to the gate driving IC  22 , a first comparator  241 , a second comparator  251 , a first negative-positive-negative (NPN) bipolar transistor  271 , a second NPN bipolar transistor  281 , a first constant current power circuit  242 , a second constant current power circuit  252 , a constant voltage diode  261 , and a plurality of resistors  231 ,  232 ,  233 ,  234 ,  235 .  
         [0020]     The resistors  231 ,  232  are connected in series between the input terminal “IN” and ground. The first constant current circuit  242  and the constant voltage diode  261  are also connected in series between the input terminal “IN” and ground. A joint node between the resistors  231 ,  232  is connected to an inverting input of the first comparator  241 . A joint node between the first constant current circuit  242  and the constant voltage diode  261  provides a constant reference voltage to a noninverting input of the first comparator  241 . An output of the first comparator  241  is connected to a base electrode “b” of the first NPN bipolar transistor  271  via the resistor  233 . An emitter electrode “e” of the first NPN bipolar transistor  271  is connected to ground. A collector electrode “c” of the first NPN bipolar transistor  271  is connected to the input terminal “IN” via the resistor  234  and the second constant current power circuit  252  in series. The collector electrode “c” of the first NPN bipolar transistor  271  is connected to an inverting input of the second comparator  251  via the resistor  234 . A noninverting input of the second comparator  251  is connected to the output terminal “OUT”. An output of the second comparator  251  is connected to a base electrode “b” of the second NPN bipolar transistor  281  via the resistor  235 . An emitter electrode “e” of the second NPN bipolar transistor  281  is connected to ground. A collector electrode “c” of the second NPN bipolar transistor  281  is connected to the output terminal “OUT”.  
         [0021]      FIG. 3  is an abbreviated timing chart illustrating operation of the voltage detecting circuit  230 . Vin represents the voltage wave provided to the input terminal “IN”. Vth represents a predetermined threshold voltage lower than a maximum voltage provided to the input terminal “IN”. Vout represents an output voltage wave of the output terminal “OUT”. Va represents a voltage wave output from the output of the first comparator  241 .  
         [0022]     The operation of the voltage detecting circuit  230  is as follows. Normally, a constant operation voltage, such as the voltage V 1 , is applied to the input terminal “IN” from the power supply circuit  212 . Because the constant reference voltage is set to be lower than a division voltage provided from the joint node between the resistors  231 ,  232 , a voltage of the noninverting input of the first comparator  241  is lower than that of the inverting input of the first comparator  241 . The first comparator  241  outputs a lower voltage to the base electrode “b” of the first NPN bipolar transistor  271  via the resistor  233 . Thus, the first NPN bipolar transistor  271  turns off. The constant operation voltage, such as the voltage V 1 , is provided to the inverting input of the second comparator  251  via the second constant current circuit  252 . Then the second comparator  251  outputs a low voltage to the base electrode “b” of the second NPN bipolar transistor  281  via the resistor  235 . The second bipolar transistor  281  turns off. Thus the output terminal “OUT” of the voltage detecting circuit  230  outputs a high voltage to the gate driving IC  22  through the conducting lead.  
         [0023]     When the LCD is turned off, the voltage V 1  falls below a threshold voltage Vth. Thus the voltage of the noninverting input of the first comparator  241  is higher than that of the inverting input of the first comparator  241 . Then, the first comparator  241  outputs a high voltage to the base electrode “b” of the first NPN bipolar transistor  271 . The first NPN bipolar transistor  271  turns on. The inverting input of the second comparator  251  is connected to ground via the activated first NPN bipolar transistor  271 . Then, the second comparator  251  outputs a high voltage to the base electrode “b” of the second NPN bipolar transistor  281 . The second NPN bipolar transistor  281  turns on. Thus the output terminal “OUT” is connected to ground via the activated second NPN bipolar transistor  281 . Therefore, the output terminal “OUT” outputs a zero volt voltage as an all-scanning signal “Xon” to the gate driving circuit  22  through the conducting lead.  
         [0024]     In summary, because the voltage detecting circuit  230  is integrated in the data driving IC  23 , the primary control circuit board  21  need not generate an all-scanning signal “Xon” and provide the all-scanning signal “Xon” to the gate driving IC  22  through the FPCB  25 . Thus the configuration of the FPCB  25  and the primary control circuit board  21  is simple.  
         [0025]     In an alternative embodiment of the present invention, the voltage detecting circuit  230  can be integrated in the gate driving IC  22  for detecting an operation voltage applied thereon. When the operation voltage applied to the gate driving IC  22  falls below a predetermined threshold voltage, the gate driving IC  22  performs a function of turning on all the TFTs of the LCD.  
         [0026]     It is to be understood, however, that even though numerous characteristics and advantages of the present embodiments have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.