Abstract:
An exemplary liquid crystal display includes a power supply circuit, a scaler, and an alternating current off control circuit connected between the power supply circuit and the scaler. The alternating current off control circuit is configured to detect an operation state of the power supply circuit, and output a corresponding control signal to the scaler.

Description:
FIELD OF THE INVENTION 
     The present invention relates to liquid crystal displays (LCDs), and particularly to an LCD with an alternating current off control circuit. 
     GENERAL BACKGROUND 
     A typical LCD has the advantages of portability, low power consumption, and low radiation. Therefore, the LCD has been widely used in various portable information products, such as notebooks, personal digital assistants, video cameras, and the like. 
       FIG. 5  is a block diagram of a typical LCD. The LCD  100  includes a power supply circuit  11 , a scaler  15 , an LCD panel  16 , an inverter  17 , and a backlight module  18 . The power supply circuit  11  is used for transforming external alternating current (AC) voltages into direct current (DC) voltages. The DC voltages are transmitted to the LCD panel  16 , the scaler  15 , and the inverter  17 , respectively. The scaler  15  is used for receiving external video signals and generating control signals. The video signals are transmitted to the LCD panel  16 , and the control signals are transmitted to the LCD panel  16 , the power supply circuit  11 , and the inverter  17 , respectively. The inverter  17  is used for transforming the DC voltages into high-frequency AC voltages, and the high-frequency AC voltages are used for driving lamps (not shown) of the backlight module  18  to light up. 
     Shutting down the LCD  100  should be done by pressing a mechanical switch (not shown) located on a housing (not shown) of the LCD  100 . When the mechanical switch is pressed, the mechanical switch transmits a control signal to the scaler  15 . Firstly, the scaler  15  transmits a first shutting down signal to shut down the inverter  17 . Then, the scaler  15  stops transmitting the video signals to the LCD panel  16 . Finally, the scaler  15  transmits a second shutting down signal to shut down the power supply circuit  11 . The entire operation above is called “DC off.” 
     When the mechanical switch is not pressed and the external AC voltage suddenly drops to zero, the power supply circuit  11 , the inverter  17 , and the scaler  15  are shut down at the same time, as shown in  FIG. 6 . This is called “AC off.” If AC off occurs many times, electrical elements of the LCD  100  are liable to be damaged or even destroyed. 
     What is needed, therefore, is an LCD that can overcome the above-described deficiencies. 
     SUMMARY 
     In one aspect, a liquid crystal display includes a power supply circuit, a scaler, and an alternating current off control circuit connected between the power supply circuit and the scaler. The alternating current off control circuit is configured to detect an operation state of the power supply circuit, and output a corresponding control signals to the scaler. 
     In another aspect, a liquid crystal display includes a power supply circuit, a scaler, and an alternating current off control circuit. When an associated external alternating current voltage suddenly drops to zero, the alternating current off control circuit is configured to output a corresponding control signal to the scaler, and the scaler is configured to control the liquid crystal display to shut down according to a direct current off procedure. 
     Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an LCD according to an exemplary embodiment of the present invention, the LCD including an AC off control circuit. 
         FIG. 2  is a block diagram of the AC off control circuit of  FIG. 1 , the AC off control circuit including a sampling circuit and a switch circuit. 
         FIG. 3  is a diagram of details of the sampling circuit and the switch circuit of  FIG. 2 . 
         FIG. 4  is a waveform diagram illustrating an AC off procedure for the LCD of  FIG. 1 . 
         FIG. 5  is a block diagram of a conventional LCD. 
         FIG. 6  is a waveform diagram illustrating a DC off procedure for the LCD of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings to describe preferred and exemplary embodiments in detail. 
       FIG. 1  is a block diagram of an LCD according to an exemplary embodiment of the present invention. The LCD  200  includes a power supply circuit  21 , a scaler  25 , an LCD panel  26 , an inverter  27 , a backlight module  28 , and an AC off control circuit  30 . The power supply circuit  21  is used for transforming external AC voltages into DC voltages. The DC voltages are transmitted to the LCD panel  26 , the scaler  25 , and the inverter  27 , respectively. The scaler  25  is used for receiving external video signals and generating control signals. The video signals are transmitted to the LCD panel  26 , and the control signals are transmitted to the LCD panel  26 , the power supply circuit  21 , and the inverter  27 , respectively. The inverter  27  is used for transforming the DC voltages into high-frequency AC voltages, and the high-frequency AC voltages are used for driving lamps (not shown) of the backlight module  28  to light up. The AC off control circuit  30  is connected between the power supply circuit  21  and the scaler  25 . 
       FIG. 2  is a block diagram of the AC off control circuit  30 . The AC off control circuit  30  includes an input terminal  31 , a sampling circuit  32 , a switch circuit  37 , a control terminal  38 , and an output terminal  39 . The input terminal  31  is connected to the power supply circuit  21 . The control terminal  38  is connected to a DC voltage source (not shown). The output terminal  39  is connected to the scaler  25 . The sampling circuit  32  transforms input signals of the input terminal  31  into control signals, to turn on or turn off the switch circuit  37 . When the switch circuit  37  is turned on, the control terminal  38  is connected to the output terminal  39 . 
       FIG. 3  is a diagram of details of the sampling circuit  32  and the switch circuit  37 . The sampling circuit  32  includes a voltage division circuit (not labeled) and a commutating and filter circuit (not labeled). The voltage division circuit includes a first resistor  33  and a second resistor  34 . The commutating and filter circuit includes a capacitor  35  and a diode  36 . The switch circuit  37  includes a transistor  40 , and the transistor  40  is typically a positive-negative-positive (PNP) bipolar transistor. An anode of the diode  36  is connected to the input terminal  31  via the first resistor  33 , and is also connected to ground via the second resistor  34 . A cathode of the diode  36  is connected to a base electrode of the transistor  40 , and is also connected to ground via the capacitor  35 . An emitter electrode of the transistor  40  is connected to the control terminal  38 , and a collector electrode of the transistor  40  is connected to the output terminal  39 . 
     When the LCD  200  works normally, the input terminal  31  receives an AC voltage from the power supply circuit  21 . The sampling circuit  32  transforms the AC voltage into a DC voltage. A value of the DC voltage is higher than a value of the DC voltage source, thus the transistor  40  is turned off. 
     When the external AC voltage suddenly drops to zero, the AC voltage received by the input terminal  31  decreases rapidly. The DC voltage decreases correspondingly. When the value of the DC voltage is lower than the value of the DC voltage source, the transistor  40  is turned on. The DC voltage source outputs a DC voltage to the scaler  25  via the control terminal  38 , the actived transistor  40 , and the output terminal  39 . The scaler  25  firstly transmits a first shutting down signal to the inverter  27  in order to shut down the inverter  27 . After a short time T 1 , as shown in  FIG. 4 , the scaler  25  stops transmitting the video signals to the LCD panel  26 . T 1  can for example be 50 ms. After another short time T 2 , as shown in  FIG. 4 , the scaler  25  transmits a second shutting down signal to shut down the power supply circuit  21 . T 2  can for example be 30 ms. 
     That is, the AC off control circuit  30  switches what would otherwise be an AC off procedure to a DC off procedure. Thus, a risk of electrical elements of the LCD  200  being damaged or even destroyed due to repeated AC off occurrences is effectively eliminated. 
     In alternative embodiments, the transistor  40  can be a P-channel metal-oxide-semiconductor field effect transistor (P-MOSFET). In such case, a gate electrode of the P-MOSFET is connected to the cathode of the diode  36 , a source electrode of the P-MOSFET is connected to the control terminal  38 , and a drain electrode of the P-MOSFET is connected to the output terminal  39 . The AC off control circuit  30  can be integrally packaged in the power supply circuit  21  or in the scaler  25 . 
     It is to be further understood 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.