Abstract:
An LCD driving circuit comprising an over-voltage protection circuit includes an input terminal to receive an input voltage, a voltage-dividing circuit, a voltage-stabilizing circuit including a voltage-stabilizing element, a control circuit, a switching element, and an output terminal. The voltage-dividing circuit provides a reference voltage according to the input voltage to the voltage-stabilizing circuit, the voltage-stabilizing circuit determines whether the voltage-stabilizing element conducts according to the reference voltage, and the control circuit controls the switching element to switch on or off according to a working stage of the voltage-stabilizing element to determine whether the output terminal outputs an output voltage.

Description:
BACKGROUND 
   1. Technical Field 
   The present disclosure relates to an over-voltage protection circuit and a driving circuit of a liquid crystal display (LCD) device using the same. 
   2. Description of Related Art 
   Because LCD devices have the advantages of portability, low power consumption, and low radiation, they are widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras, and others. An LCD device usually needs a power board circuit providing power thereto. 
   The power board circuit is usually connected to an external main power supply, and receives power output from the external main power supply. Therefore, when the external main power supply is unstable or the power board circuit is disturbed by circuit variation or other interference, the power board circuit is liable to output a voltage exceeding an acceptable voltage range of the LCD device, and the LCD device is at the risk of severe damage. Other elements connected to the power board circuit may also be affected. 
   What is needed, therefore, is an over-voltage protection circuit and an LCD driving circuit which can overcome the described limitations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an LCD driving circuit according to the present disclosure, the driving circuit including an over-voltage protection circuit. 
       FIG. 2  is a circuit diagram of the over-voltage protection circuit of  FIG. 1 . 
       FIG. 3  is a circuit diagram of an over-voltage protection circuit for an LCD driving circuit of the present disclosure. 
   

   DETAILED DESCRIPTION 
   Reference will now be made to the drawings to describe the disclosure in detail. 
     FIG. 1  is a block diagram of an LCD driving circuit  20  comprising an over-voltage protection circuit  23  according to the present disclosure. The driving circuit  20  further comprises a power board circuit  21 , a manual switch  22 , and a panel circuit  24 . The power board circuit  21  receives alternate current (AC) voltage provided by an external main power supply (not shown) and converts the AC voltage to direct current (DC) voltage of approximately 5V, in one example. Upon activation of the manual switch  22 , the 5V DC voltage is output from the power board circuit  21  to the panel circuit  24  via the manual switch  22  and the over-voltage protection circuit  23 . The panel circuit  24  converts the 5V DC voltage into various driving signals to drive a liquid crystal panel (not shown) for displaying images. When the manual switch  22  is deactivated, electrical connection between the power board circuit  21  and the panel circuit  24  is cut off and a liquid crystal panel powers down. 
   The over-voltage protection circuit  23  is connected between the manual switch  22  and the display circuit  24 . When a voltage output from the power board circuit  21  is less than a predetermined value such as 5.5V, the over-voltage protection circuit  23  electrically connects the power board circuit  21  and the panel circuit  24 . When voltage output from the power board circuit  21  exceeds or equals the predetermined value, the over-voltage protection circuit  23  automatically cuts off an electrical connection between the power board circuit  21  and the panel circuit  24 . 
   Referring to  FIG. 2 , the over-voltage protection circuit  23  includes an input terminal  300 , an output terminal  350 , a voltage-dividing circuit  310 , a voltage-stabilizing circuit  320 , a control circuit  330 , and a first transistor  340  as a switching element. 
   The voltage-dividing circuit  310  provides a reference voltage to the voltage-stabilizing circuit  320 , and includes two resistors  311 ,  312  connected in series between the input terminal  300  and ground. A proportion of resistances of the two resistors  311 ,  312  is 6:5, in one example. For example, when a voltage received by the input terminal  300  is 5.5V, a dividing voltage on the resistor  312  is 2.5V. 
   The voltage-stabilizing circuit  320  includes a voltage-stabilizing element  322  and two resistors  321 ,  323 . The voltage-stabilizing element  322  may be an adjustable shunting voltage regulator having a type of TLV431, and includes an anode (not labeled), a cathode (not labeled), and a reference terminal (not labeled). A conducting voltage of the voltage-stabilizing element  322  is about 2.5V, namely, the voltage-stabilizing element  322  conducts when the reference voltage provided by the voltage-dividing circuit  310  to the reference terminal exceeds or equals about 2.5V. Accordingly, the voltage-stabilizing element  322  does not conduct when the reference voltage provided by the voltage-dividing circuit  310  is less than about 2.5V. The reference terminal of the voltage-stabilizing element  322  is connected between the two resistors  311 ,  312 , the cathode is connected to the input terminal  300  via the resistor  321 , and the anode is grounded via the resistor  323 . A resistance of the resistor  321  is less than that of the resistor  323 , such that current through the voltage-stabilizing element  322  is limited. 
   The control circuit  330  includes a second transistor  331  and three resistors  332 ,  333 ,  334 . The second transistor  331  is a p-channel metal oxide semiconductor (PMOS) transistor, and includes a gate (not labeled), a drain (not labeled), and a source (not labeled). The gate of the second transistor  331  is connected to the anode of the voltage-stabilizing element  322  via the resistor  332 , the drain is connected to the input terminal  300  via the resistor  333 , and the source is grounded. The resistor  334  is connected between the drain and the source of the second transistor  331 . When no voltage is applied to the input terminal  300  of the over-voltage protection circuit  23 , a parasitic capacitor between the drain and the source of the second transistor  331  discharges through the resistor  334 . 
   The first transistor  340  is a PMOS transistor, and includes a gate (not labeled), a drain (not labeled), and a source (not labeled). The gate of the first transistor  340  is connected to the drain of the second transistor  331 , the source of the first transistor  340  is connected to the input terminal  300 , and the drain of the first transistor  340  is connected to the output terminal  350 . 
   When the voltage applied to the input terminal  300  is less than about 5.5V, the dividing voltage on the resistor  312  is lower than about 2.5V, thus the reference voltage applied to the reference terminal of the voltage-stabilizing element  322  is lower than about 2.5V and the voltage-stabilizing element  322  is cut off, and no dividing voltage is generated on the resistor  323 . The gate of the second transistor  331  is grounded via the resistors  332 ,  323 , thus the second transistor  331  conducts due to the gate thereof at a low potential. The gate of the first transistor  340  is grounded via the second transistor  331 , thus the first transistor  340  also conducts due to the gate thereof at a low potential. Therefore, a voltage signal applied to the input terminal  300  is output from the output terminal  350  via the first transistor  340 . 
   When the voltage applied to the input terminal  300  exceeds or equals about 5.5V, the dividing voltage on the resistor  312  exceeds or equals about 2.5V, thus the reference voltage applied to the reference terminal of the voltage-stabilizing element  322  exceeds or equals about 2.5V and the voltage-stabilizing element  322  conducts, and a dividing voltage is generated on the resistor  323 . The second transistor  331  is cut off when the gate thereof reaches a high potential. Current flows through the resistors  333 ,  334 , and a dividing voltage is generated on the resistor  334 , thus the first transistor  340  is also cut off due to the gate thereof reaching a high potential. Accordingly, electrical connection between the input terminal  300  and the output terminal  350  is cut off, and voltage applied to the input terminal  300  cannot be output from the output terminal  350  via the first transistor  340 . 
   The over-voltage protection circuit  23  is connected between the power board circuit  21  and the panel circuit  24 , when the voltage applied by the power board circuit  21  is too large or larger than the predetermined value, the over-voltage protection circuit  23  cuts off the electrical connection between the power board circuit  21  and the panel circuit  24 . Therefore, the panel circuit  24  can be protected from over voltage. Furthermore, the over-voltage protection circuit  23  has simple structure, low cost, and is liable to be obtained. 
     FIG. 3  is a circuit diagram of an over-voltage protection circuit  43  for an LCD driving circuit of the present disclosure. The driving circuit is not shown her for brevity. The disclosed over-voltage protection circuit  43  of the driving circuit includes a voltage-stabilizing circuit  420 . 
   The voltage-stabilizing circuit  420  includes a voltage-stabilizing element  422  and a resistor  423 . The voltage-stabilizing element  422  is a voltage-stabilizing diode, and includes an anode (not labeled) and a cathode (not labeled). The cathode of the voltage-stabilizing element  422  is connected between two resistors  411 ,  412  of a voltage-dividing circuit  410  of the over-voltage protection circuit  43 , and the anode of the voltage-stabilizing element  422  is grounded via the resistor  423 . A gate of a transistor  431  of a control circuit  430  of the over-voltage protection circuit  43  is connected to the anode of the voltage-stabilizing element  422 . A conducting voltage of the voltage-stabilizing element  422  is about 2.5V, namely, the voltage-stabilizing element  422  conducts when a voltage provided to the cathode is about 2.5V or more than that to the anode. Accordingly, the voltage-stabilizing element  422  does not conduct when the voltage provided to the cathode is less than about 2.5V. 
   When a voltage applied to an input terminal  400  of the over-voltage protection circuit  43  is less than about 5.5V, a dividing voltage on the resistor  312  is lower than about 2.5V and the voltage-stabilizing element  422  is cut off, with no dividing voltage generated on the resistor  423 . The gate of the transistor  431  is grounded via the resistors  432 ,  423 , thus the transistor  431  conducts due to the gate thereof having a low potential. A transistor  440  of the over-voltage protection circuit  43  also conducts due to a gate thereof having a low potential. Accordingly, a voltage signal applied to the input terminal  400  is output from an output terminal  450  of the over-voltage protection circuit  43  via the transistor  440 . 
   When voltage applied to the input terminal  400  exceeds or equals about 5.5V, the dividing voltage on the resistor  412  exceeds or equals about 2.5V, thus the voltage-stabilizing element  422  conducts, and a dividing voltage is generated on the resistor  423 . The transistor  431  is cut off due to the gate thereof having a high potential. The transistor  440  is also cut off due to the gate thereof having a high potential. Therefore, an electrical connection between the input terminal  400  and the output terminal  450  is cut off. Over-voltage protection is thus achieved. 
   Alternatively, the predetermined value of the over-voltage protection circuit of this present disclosure needs not be limited to 5.5V, and can be changed by adjusting the proportion of resistances of the two resistors of the voltage-dividing circuit while the reference voltage does not need to be changed. That is, the over-voltage protection circuit as disclosed can protect a variety of electronic devices having different input voltages. Furthermore, the transistors of the over-voltage protection circuit of this present disclosure can be PNP bipolar junction transistors. 
   It is to be understood, however, that even though numerous characteristics and advantages of preferred and exemplary 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 that changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.