Abstract:
An exemplary backlight system includes a power input circuit ( 21 ), a detection circuit ( 27 ), a control circuit ( 28 ), a light emitting diode driving circuit ( 25 ), and a light emitting diode array ( 26 ). The detection circuit is configured to detect a connection state of the power input circuit with respect to an external power source, and send a corresponding connection states signal to the control circuit. The control circuit is configured to output a control signal to the light emitting diode driving circuit according to the connection state signal. The light emitting diode driving circuit is configured to drive or shut down the light emitting diode array according to the control signal.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to backlight systems and liquid crystal display devices, and particularly to a backlight system with a detection circuit and a liquid crystal display device using the backlight system. 
       BACKGROUND 
       [0002]    A typical liquid crystal display (LCD) device 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]    One kind of conventional LCD device includes an LCD panel, a backlight source for illuminating the LCD panel, and a power supply circuit for driving the LCD panel and the light source. 
         [0004]    Referring to  FIG. 3 , this is a block diagram of a conventional LCD device. The LCD device  1  includes an alternating current (AC) filter circuit  11 , a rectifier and voltage step-down circuit  12 , an LCD panel  13 , a scaler  14 , an LED driving circuit  15 , and an LED array  16 . 
         [0005]    The AC filter circuit  11  generally includes capacitors with large capacitance and windings with large inductance, in order to filter surge voltages from an external electrical network, and then output steady AC current to the rectifier and voltage step-down circuit  12 . Then the AC current is rectified and reduced to variety of direct current (DC) voltages, such as 3 volts (V), 5V, 9V, and the like. The DC voltages are used for driving the LCD panel  13  and the scaler  14 . 
         [0006]    The scaler  14  receives video signals from an external device, processes the video signals, and then sends the video signals to the LCD panel  13  for display of corresponding images. At the same time, the scaler  14  outputs a pulse width modulation (PWM) signal to the LED driving circuit  15 . The LED driving circuit  15  receives the PWM signal, and correspondingly drives the LED array  16  to emit light beams for illuminating the LCD panel  13 . 
         [0007]    The AC filter circuit  11  and the rectifier and voltage step-down circuit  12  both include large capacitors and large windings. In operation, electrical power is stored in the capacitors and the windings. However, when the AC power is cut off, electrical power stored in the capacitors and the windings is released. The released electrical power drives the LCD panel  13  and the LED array  16  to continue working for a moment after the AC power is cut off. Because the released power is abnormal and uncontrolled, colors displayed on the LCD panel  13  are correspondingly abnormal and uncontrolled. For example, so-called red and blue noise colors are displayed. The noise colors reduce a display quality and aesthetic appeal of the LCD device  1 . 
         [0008]    Accordingly, what is needed is an LCD device that can overcome the above-described deficiencies. 
       SUMMARY 
       [0009]    In a first aspect, a backlight system includes a power input circuit a detection circuit, a control circuit, a light emitting diode driving circuit, and a light emitting diode array. The detection circuit is configured to detect a connection state of the power input circuit with respect to an external power source, and send a corresponding connection states signal to the control circuit. The control circuit is configured to output a control signal to the light emitting diode driving circuit according to the connection state signal. The light emitting diode driving circuit is configured to drive or shut down the light emitting diode array according to the control signal. 
         [0010]    In a second aspect, a liquid crystal display device includes a power input circuit, a detection circuit, a control circuit, a light emitting diode driving circuit, a light emitting diode array, a scaler, and a liquid crystal display panel. The power input circuit is capable of receiving electrical power from an external power source and providing electrical power to the liquid crystal display panel and the scaler. The scaler is configured to provide video signals to the liquid crystal display panel. The detection circuit is configured to detect a connection state of the power input circuit with respect to an external power source, and send a corresponding connection state signal to the control circuit, the control circuit is configured to output a control signal to the light emitting diode driving circuit according to the connection state signal. The light emitting diode driving circuit is configured to switch driving of the light emitting diode array according to the control signal. 
         [0011]    In a third aspect, a liquid crystal display device includes a power input circuit, a liquid crystal panel, a scaler configured for providing video signals to the liquid crystal display panel, and a backlight system configured for providing light beams for the liquid crystal display panel. the power input circuit is capable of receiving alternating current from an external power source and providing direct current to the liquid crystal display panel and the scaler, the backlight system is coupled to the power input circuit and is configured to detect a connection state of the power input circuit with respect to an external power source, and is further configured to provide or stop providing light beams to illuminate the liquid crystal display panel according to the connection state detected by the power input circuit. 
         [0012]    Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. In the drawings, all the views are schematic. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a block diagram of an LCD device according to a preferred embodiment of the present invention. 
           [0014]      FIG. 2  is a circuit diagram of a portion of the LCD device marked by dashed lines in  FIG. 1 . 
           [0015]      FIG. 3  is a block diagram of a conventional LCD device. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0016]      FIG. 1  is a block diagram of an LCD device according to a preferred embodiment of the present invention. The LCD device  2  includes an AC filter circuit  21 , a rectifier and voltage step-down circuit  22 , an LCD panel  23 , a scaler  24 , an AC detection circuit  27 , an LED driving control circuit  28 , an LED driving circuit  25 , and an LED array  26 . 
         [0017]    The AC filter circuit  21  generally includes capacitors with large capacitance and windings with large inductance. The AC filter circuit  21  receives AC voltage from an external electrical network, and filters surge voltages and electromagnetic interference (EMI) of the external electrical network. The LCD device  2  thereby complies with relevant Federal Communication Commission (FCC) standards, European Community (CE) standards, and the like. Then the AC filter circuit  21  outputs steady AC voltage to the rectifier and voltage step-down circuit  22 . 
         [0018]    The rectifier and voltage step-down circuit  22  generally includes a rectifier circuit (not shown) and a DC-DC voltage step-down circuit (not shown). The rectifier circuit transforms the AC voltage to a high DC voltage. The DC-DC voltage step-down circuit reduces the high DC voltage to a variety of low DC voltages, such as 3V (volts), 5V, 9V, 12V, and the like. Then the low DC voltages are supplied to the LCD panel  23  and the scaler  24 . 
         [0019]    The scaler  24  receives video signals such as low voltage differential signals (LVDS), processes the video signals, and sends the video signals to the LCD panel  23 . At the same time, the scaler  24  outputs a PWM signal to the LED driving control circuit  28 . 
         [0020]    The AC detection circuit  27  is coupled to an output of the AC filter circuit  21 . The AC detection circuit  27  is capable of sampling the AC voltage of the AC filter circuit  21 , in order to detect whether the AC filter circuit  21  is connected to an external electrical network. Then the AC detection circuit  27  outputs a corresponding connection state signal to the LED driving control circuit  28 . Typically, the connection state signal indicates that the AC filter circuit  21  is connected to an external electrical network, or that the AC filter circuit  21  is not connected to an external electrical network. 
         [0021]    The LED driving control circuit  28  receives signals from both the AC detection circuit  27  and the scaler  24 , analyzes the signals, and outputs appropriate control signals to the LED driving circuit  25 . Then the LED driving circuit  25  outputs voltage to turn on or turn off the LED array  26  according to the control signals from the LED driving control circuit  28 . 
         [0022]    Upon a condition that the AC filter circuit  21  is connected to an external electrical network, the AC detection circuit  27  detects the output of the AC filter circuit  21 , and sends a first signal to the LED driving control circuit  28 . Under the control of the first signal from the AC detection circuit  27  and the PWM signal from the scaler  24 , the LED driving control circuit  28  sends a switch-on signal to the LED driving circuit  25 . Then the LED driving circuit  25  outputs a high voltage to switch on the LED array  26  for illuminating the LCD panel  23 . 
         [0023]    Upon a condition that the AC filter circuit  21  is disconnected or cut off from an external electrical network, the AC detection circuit  27  detects that no AC voltage is sent out from the AC filter circuit  21 . Then the AC detection circuit  27  sends a second signal to the LED driving control circuit  28 . Under the control of the second signal from the AC detection circuit  27  and the PWM signal from the scaler  24 , the LED driving control circuit  28  sends a switch-off signal to the LED driving circuit  25 . Accordingly, the LED driving circuit  25  outputs a low voltage to switch off the LED array  26 . Thus, once the LCD device  2  is cut off from the external electrical network, the backlight provided by the LED array  26  is switched off immediately without any time delay. 
         [0024]    Referring to  FIG. 2 , this is a circuit diagram of a portion of the LCD device  2  marked by dashed lines in  FIG. 1 . The marked portion includes the scaler  24 , the LED driving circuit  25 , the LED array  26 , the AC detection circuit  27 , and the LED driving control circuit  28 . 
         [0025]    The AC detection circuit  27  includes an AC detection terminal  270 , a first resistor  271 , a second resistor  272 , a diode  273 , a capacitor  275 , and a transistor  276 . The AC detection terminal  270  is connected to the output of the AC filter circuit  21  for detecting the AC voltage. The AC detection terminal  270 , the first resistor  271 , and the second resistor  273  are connected in series to ground. A positive electrode of the diode  273  is connected to a node between the first resistor  271  and the second resistor  272 . The capacitor  275  is connected between a negative electrode of the diode  273  and ground. The transistor  276  is a positive-negative-positive (PNP) type bipolar junction transistor, which includes a base “b”, an emitter “e”, and a collector “c”. The base “b” is connected to the negative electrode of the diode  273 . The emitter “e” is connected to a constant DC power supply, such as a 3.3 V DC power supply. 
         [0026]    The scaler  24  includes an output pin  241  for outputting the PWM signal. The LED driving control circuit  28  includes a NOT gate  281 , and an AND gate  283  with two input terminals. An input terminal of the NOT gate  281  is connected to the collector “c” of the transistor  276 . An output terminal of the NOT gate  281  is connected to a first input terminal of the AND gate  283 . A second input terminal of the AND gate  283  is connected to the output pin  241  of the scaler  24 . 
         [0027]    The LED driving circuit  25  includes an LED driver IC  250 . The LED driver IC  250  can for example be a G5950 model, which includes a control signal input pin DIM and eight output pins CH 1 ˜CH 8 . The control signal input pin DIM is connected to an output of the AND gate  28 . The eight output pins CH 1 ˜CH 8  are connected to the LED array  26  for supplying driving voltages thereto. Upon a condition that the control signal input pin DIM receives a logic high voltage, the output pins CH 1 ˜CH 8  output high voltage to switch on the LED array  26 . Upon a condition that the control signal input pin DIM receives a logic low voltage, the output pins CH 1 ˜CH 8  output low voltage to switch off the LED array  26 . 
         [0028]    Typical detailed operation of the LCD device  2  is as follows: 
         [0029]    AC voltage output from the AC filter circuit  21  is sampled by the second resistor  272 . The diode  273  and the capacitor  275  function as a rectifier and filter circuit, which transforms the sampling AC voltage to a sampling DC voltage. The sampling DC voltage is supplied to the base “b” of the transistor  276 . Upon a condition that the AC filter circuit  21  is connected to an external electrical network, the sampling DC voltage that is applied to the base “e” of the transistor  276  is a high voltage. Thus, the transistor  276  is switched off and the collector “c” outputs a low voltage. The low voltage is converted into a high voltage by the NOT gate  281  of the LED driving control circuit  28 . In this situation, the output of the AND gate  283  is determined by the PWM signal from the output pin  241 . If the PWM signal is a high voltage, the output of the AND gate  283  is a high voltage. Thus, the LED driver IC  250  outputs high voltage at the output pins CH 1 ˜CH 8 . The LED array  26  is switched on and emits light beams. If the PWM signal is a low voltage, the output of the AND gate  283  is a low voltage. The LED driver IC  250  outputs low voltages at the output pins CH 1 ˜CH 8 . The LED array  26  is switched off. 
         [0030]    Upon the other condition that the AC filter circuit  21  is cut off from an external electrical network, the sampling DC voltage that is applied to the base “e” of the transistor  276  is a low voltage. Thus, the transistor  276  is switched on and the collector “c” outputs a high voltage. The high voltage is converted into a low voltage by the NOT gate  281  of the LED driving control circuit  28 . Thus no matter what kind of voltages the output pin  241  outputs, the AND gate  283  outputs a low voltage to the control signal input pin DIM. The eight output pins CH 1 ˜CH 8  then output low voltages to the LED array  26 . The LED array  26  is switch off. 
         [0031]    In summary, the LCD device  2  includes the AC detection circuit  27  and the LED driving control circuit  28 . The AC detection circuit  27  samples an AC signal, and detects whether the AC filter circuit  21  is connected to an external electrical network. Upon a condition that the AC filter circuit  21  is cut off or disconnected from an external electrical network, the AC detection circuit  27  sends a cut-off signal to the LED driving control circuit  28 . With the cut-off signal, the LED driving control circuit  28  outputs a control signal to lower the output voltage of the LED driving circuit  25 . Therefore the LED array  26  is switched off immediately. Accordingly, once the LCD device  2  is cut off from the external electrical network, the LED array  26  stops emitting light beams. This reduces or even eliminates color noise when the AC filter circuit  21  is cut off from the external electrical network, and thereby improves a display quality and aesthetic appeal of the LCD device  2 . 
         [0032]    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.