Patent Publication Number: US-8120274-B2

Title: Controlling circuit for a light emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 98102576, filed on Jan. 22, 2009. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a control technology, and more particularly, to a control technology of light emitting diode channels (LED channels). 
     2. Description of Related Art 
     Along with the upgrowth of the semiconductor industry and the related electronic industry, many digital means, such as mobile phone, digital camera, digital video camera, notebook and desktop computer, have been getting continuous evolutions and improvements towards more usage convenience, multi functions and stylish design. In order to use the information products, a displaying screen is an indispensable man-machine interface, by which a user can more conveniently manipulate the above-mentioned apparatuses. Among various displaying screens, the liquid crystal display (LED) has played a major role in the market. However, an LED is not self-luminescent, so that a backlight module must be employed and placed under the LED, which serves as a light source so as to make display possible. 
       FIG. 1  is a diagram of a conventional backlight module and a controlling circuit thereof. Referring to  FIG. 1 , a conventional backlight module usually comprises a plurality of LED channels  21 - 2   x , wherein the LED channels  21 - 2   x  are respectively connected in series to a plurality of current sources  31 - 3 X and the connected current sources  31 - 3 X can respectively control the currents flowing the LED channels  21 - 2   x . The circuit for detecting short-circuit/open-circuit  10  in  FIG. 1  can detect a plurality of voltages Ch 1 -Chx so as to judge whether or not the LED channels  21 - 2   x  are short or open. 
       FIG. 2  is a diagram of the circuit for detecting short-circuit/open-circuit in  FIG. 1 . Referring to  FIGS. 1 and 2 , a comparator  41  compares the voltage Ch 1  with a voltage Vref_short. When the voltage Ch 1  is higher than the voltage Vref_short, the comparator  41  outputs a signal short to indicate the LED channel  21  is short already. Analogically for the comparators  42 - 4   x , the signal short output from an OR gate  61  is for indicating one of the LED channels  21 - 2   x  has short-circuit. 
     On the other hand, a comparator  51  compares the voltage Ch 1  with a voltage Vref_open. When the voltage Ch 1  is lower than the voltage Vref_open, the comparator  51  outputs a signal short to indicate the LED channel  21  is open already. Analogically for the comparators  52 - 5   x , the signal open output from an OR gate  62  is for indicating one of the LED channels  21 - 2   x  has open-circuit. 
     It should be noted that in the prior art, the quantity of the comparators is increased with the increasing quantity of the LED channels. However, since the comparators  42 - 4   x  and the comparators  52 - 5   x  are analog circuits, so that the circuit area thereof is considerably large with the increasing quantity thereof. In addition, the hardware cost would accordingly soars, which is disadvantageous for the circuit layout. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a controlling apparatus, which is capable of detecting whether or not any one of the employed LED channels works abnormally and reducing the demand on the comparators. 
     The present invention is also directed to a controlling method, which is capable of detecting whether or not any one of the employed LED channels works normally and reducing the demand on the comparators so as to lower down the hardware cost. 
     The present invention provides a controlling circuit, which includes a plurality of first switches and a comparator. The first terminals of the first switches are respectively coupled to one of a plurality of LED channels. The first switches are conducted according to a plurality of first switching signals respectively, wherein the first switching signals are asserted alternately. The first input terminal of the comparator is coupled to the second terminals of the first switches and the second input terminal of the comparator receives a reference voltage for comparing the voltage of the first input terminal with the voltage of the second input terminal so as to output a comparison result. 
     In an embodiment of the present invention, the above-mentioned, the controlling circuit further includes a signal generator, which is coupled to every of the first switches for generating the corresponding first switching signals. In more details, the signal generator includes an NOR gate and a flip-flop string. The NOR gate receives every of the first switching signals to generate a trigger signal. The flip-flop string receives the trigger signal to generates every of the first switching signals. 
     In an embodiment of the present invention, the controlling circuit includes a logic circuit. The logic circuit includes a flip-flop string, wherein the flip-flop string includes a plurality of flip-flops. The input terminal of each of the flip-flops receives the comparison result. The flip-flops respectively receive the first switching signals to generate a plurality of indication signals respectively, wherein the first switching signal serves as a clock signal of the corresponding flip-flop. In another embodiment, the logic circuit further includes an OR gate, and the OR gate receives the indication signals so as to generate an output signal. 
     In an embodiment of the present invention, the controlling circuit further includes an alert device coupled to the comparator. The alert device decides whether or not to send out an alert according to the comparison result. In another embodiment, the controlling circuit further includes a reference voltage generator, an error amplifier, a power supply, a voltage-dividing circuit and a voltage-regulating capacitor. The reference voltage generator is coupled to the comparator and outputs a second reference voltage according to the comparison, result. The error amplifier is coupled to the reference voltage generator and receives a second reference voltage and a dividing voltage so as to output an adjusting signal. The power supply is coupled to the error amplifier and all the LED channels, adjusts a supply voltage according to the adjusting signal and outputs the supply voltage to all the LED channels. The voltage-dividing circuit is coupled to the power supply and the error amplifier and generates the dividing voltage according to the supply voltage. The voltage-regulating capacitor is coupled to the power supply to stabilize the voltage. 
     In an embodiment of the present invention, the controlling circuit further includes a plurality of second switches. The first terminal for each of the second switches are respectively coupled to one of a plurality of voltages, and the second terminals of the second switches are coupled to the second input terminal of the comparator. The second switches are conducted according to a plurality of second switching signals respectively so as to select one of the voltages as the first reference voltage, wherein the second switching signals are asserted alternately. 
     In an embodiment of the present invention, the controlling circuit further includes a plurality of current sources. The current sources respectively connect in series one of the LED channels and respectively control the current flow each of the LED channels according to one of a plurality of light-adjusting signals. 
     In an embodiment of the present invention, the controlling circuit further includes a signal generator, which includes a first NOR gate, a second NOR gate, a third NOR gate, a first flip-flop string, a second flip-flop string, a plurality of AND gates, an inverter, a flip-flop and an OR gate. The first NOR gate receives a plurality of filtering signals to generate a trigger signal. The first flip-flop string receives the trigger signal to generate the filtering signals. The second flip-flop string receives a reference light-adjusting signal to generate all the light-adjusting signals. The first terminals of the AND gates respectively receive one of the filtering signals, and the second terminals of the AND gates respectively receive a corresponding light-adjusting signal so as to generate each of the first switching signals. The inverter receives a first filtering signal in the filtering signals. The first input terminal of the second NOR gate receives a first light-adjusting signal in the light-adjusting signals and the second input terminal of the second NOR gate is coupled to the output terminal of the inverter. The input terminal of the flip-flop is coupled to the output terminal of the second NOR gate. The third NOR gate receives the light-adjusting signals. The first input terminal and the second input terminal of the OR gate are respectively coupled to the output terminal of the flip-flop and the output terminal of the third NOR gate so as to output a reset signal to the first flip-flop string. 
     The present invention further provides a controlling method, which includes generating a plurality of signals, wherein the signals are asserted alternately and using a timing of the signals so as to monitor a voltage at a terminal of each of a plurality of LED channels respectively. 
     In an embodiment of the present invention, the step of using a timing of the signalsso as to monitor a voltage at a terminal of each of a plurality of LED channels respectively includes receiving the voltage at a terminal of one of the LED channels as a stand-by voltage according to the timing of the signals and comparing the stand-by voltage with a reference voltage so as to obtain a comparison result. 
     In an embodiment of the present invention, the step of generating the signals includes progressively delaying a reference signal so as to generate the signals. 
     Based on the described above, the present invention takes a novel scheme that generating a plurality of signals and respectively monitoring the voltage at a terminal of one of a plurality of LED channels according to the timing of the signals, wherein the signals are asserted alternately, so that the present invention is advantageous not only in monitoring whether or not the LED channels work abnormally, but also in reducing the hardware cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a diagram of a conventional backlight module and a controlling circuit thereof. 
         FIG. 2  is a diagram of the circuit for detecting short-circuit/open-circuit in  FIG. 1 . 
         FIG. 3A  is a diagram of a controlling circuit according to the first embodiment of the present invention. 
         FIG. 3B  is a diagram of the detecting circuit in  FIG. 3A . 
         FIG. 4  is a flowchart of a controlling method according to the first embodiment of the present invention. 
         FIG. 5  is a diagram showing the timing of a light-adjusting signal and a plurality of switching signals according to the first embodiment of the present invention. 
         FIG. 6  is a circuit diagram of a signal generator according to the first embodiment of the present invention. 
         FIG. 7A  is a diagram of another detecting circuit. 
         FIG. 7B  is a diagram of the logic circuit in  FIG. 7A . 
         FIG. 8A  is diagram of yet another detecting circuit and  FIG. 8B  is a diagram showing the timing of a light-adjusting signal and a plurality of switching signals. 
         FIG. 9  is a diagram of a controlling circuit according to the second embodiment of the present invention. 
         FIG. 10  is a diagram of the detecting circuit of  FIG. 9 . 
         FIG. 11  is a diagram showing the timing of a light-adjusting signal and a plurality of switching signals according to the second embodiment of the present invention. 
         FIG. 12  is a circuit diagram of a signal generator according to the second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A controlling circuit of a conventional backlight module requires a number of comparators, which occupies a great circuit area, but also cost more. To avoid the disadvantage in the prior art, the embodiments of the present invention make a plurality of switches coupled to the first input terminal of a comparator, wherein the switches are turned on alternately, so that the voltages at a terminal of every of a plurality of LED channels is alternately input to the first input terminal of the comparator. The second input terminal of the comparator receives a reference voltage, and then the comparator compares the voltage of the first input terminal with the voltage of the second input terminal so as to output a comparison result. In this way, the embodiments of the present invention may detect whether or not the LED channels work abnormally by means of the comparison result. The embodiments of the present invention are also advantageous in effectively reducing the quantity of the employed comparators. Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
       FIG. 3A  is a diagram of a controlling circuit according to the first embodiment of the present invention and  FIG. 3B  is a diagram of the detecting circuit in  FIG. 3A . Referring to  FIGS. 3A and 3B , the controlling circuit may include a detecting circuit  70 , or further include a signal generator  80  and a plurality of current sources  31 - 3   x . In the embodiment, the detecting circuit  70  includes a plurality of switches  301 - 30   x  and switches  311 - 31   x  and two comparators  91  and  92 . A plurality of LED channels  21 - 2   x  in the controlling circuit respectively include a plurality of LEDs. Although in  FIG. 3A , only two LEDs are shown, but the present invention is not limited to two LEDs. In other embodiments, the LED channels  21 - 2   x  may be respectively composed of LEDs of different numbers and the LEDs are connected in parallel or in series. 
     The anodes of the LED channels  21 - 2   x  receive a voltage Vin and the cathodes thereof are respectively coupled to the current sources  31 - 3 X. The currents flowing the current sources  31 - 3 X may be controlled according to a light-adjusting signal, so that the luminance of the LED channels  21 - 2   x  may be adjusted. The detecting circuit  70  is coupled to the cathodes of the LED channels  21 - 2   x  for detecting the voltages Ch 1 -Chx of the LED channels  21 - 2   x.    
     The first terminals of the switches  301 - 30   x  may respectively receive the voltages Ch 1 -Chx and the second terminals thereof are coupled to the first input terminal of the comparator  91 . The switches  301 - 30   x  are turned on according to the switching signals s 1 - sx  respectively. The second input terminal of the comparator  91  receives a voltage Vref_open. The comparator  91  compares the voltage of the first input terminal with the voltage of the second input terminal thereof so as to output a comparison result ouput 1  to indicate whether or not one of the LED channels  21 - 2   x  has open-circuit. In more details, when the voltage of the first input terminal of the comparator  91  is lower than the voltage Vref_open, it means one of LED channels  21 - 2   x  may be open. 
     Analogically for the rest, the first terminals of the switches  311 - 31   x  respectively receive the voltages Ch 1 -Chx, and the second terminals of the switches  311 - 31   x  are coupled to the first input terminal of the comparator  92 . The switches  311 - 31   x  are turned on according to the switching signals s 1 - sx  respectively. The second input terminal of the comparator  92  receives a voltage Vref_short. The comparator  92  compares the voltage of the first input terminal with the voltage of the second input terminal thereof so as to output a comparison result ouput 2  to indicate whether or not one of the LED channels  21 - 2   x  has short-circuit. In more details, when the voltage of the first input terminal of the comparator  92  is higher than the voltage Vref_short, it means one of LED channels  21 - 2   x  may be short. 
       FIG. 4  is a flowchart of a controlling method according to the first embodiment of the present invention and  FIG. 5  is a diagram showing the timing of a light-adjusting signal and a plurality of switching signals according to the first embodiment of the present invention. First in step S 401 , a plurality of signals are generated, wherein the signal are asserted alternately. For example, the signal generator  80  may generate the switching signals s 1 - sx , which are asserted alternately. It should be noted that when the light-adjusting signal is asserted, the misjudgement risk by the detecting circuit  70  may be reduced by alternately enabling the switching signals s 1 - sx . In the following, an implementation of the signal generator is described for anyone skilled in the art as a reference. 
       FIG. 6  is a circuit diagram of a signal generator according to the first embodiment of the present invention. A signal generator  80  in the embodiment includes an NOR gate  100  and a plurality of flip-flops  101 - 10   x . The NOR gate  100  receives a plurality of switching signals s 1 - sx  so as to output a signal start. The flip-flop  101  outputs the switching signal s 1  according to a clock signal clk and the signal start. The flip-flop  102  outputs the switching signal s 2  according to the clock signal clk and the switching signal s 1 . Analogically for the flip-flops  103 - 10   x , which are omitted to describe. In addition, the flip-flops  101 - 10   x  may be reset according to the light-adjusting signal. By the above-mentioned scheme, the embodiments of the present invention may ensure to alternately enable the switching signals s 1 - sx  under the condition of enabling the light-adjusting signal, which is advantageous in reducing a misjudgement risk by the detecting circuit  70 . 
     Next in step S 402 , a timing of the signals is used so as to monitor a voltage at a terminal of each of the LED channels respectively. For example, when the signal s 1  is asserted and the switching signals s 2 - sx  are deasserted, the switch  301  is turned on and the switches  302 - 30   x  are turned off, so that the first input terminal of the comparator  91  receives the voltage Ch 1  at the cathode terminal of the LED channel  21 . Meanwhile, the comparator  91  compares the voltage Ch 1  with the reference voltage Vref_open so as to output the comparison result ouput 1 . 
     It should be noted that when the LED channel  21  is open, the equivalent resistance of the LED channel  21  is near to infinity, so that the voltage Ch 1  approaches a ground voltage. Based on the above [mentioned consideration, anyone skilled in the art should appropriately define the reference voltage Vref_open, so that when the voltage Ch 1  is lower than the reference voltage Vref_open, the comparison result ouput 1  is able to indicate the LED channel  21  may be open. In addition, anyone skilled in the art may also dispose an alert device (not shown), for example, a light alert device or a sound alert device, and the alert device is coupled to the comparator  91  to send out an alert according to the comparison result ouput 1 . 
     When the signal s 2  is asserted and the switching signals s 1  and s 3 - sx  are deasserted, the switch  302  is turned on and the switches  301  and  303 - 30   x  are turned off, so that the first input terminal of the comparator  91  receives the voltage Ch 2  at the cathode terminal of the LED channel  22 . Meanwhile, the comparator  91  compares the voltage Ch 2  with the reference voltage Vref_open so as to output the comparison result ouput 1 . At the time, the comparison result ouput 1  is able to indicate whether or not the LED channel  22  may be open. Analogically for indicating whether or not the LED channels  23 - 2   x  are open, which is omitted to describe. 
     On the other hand, when the signal s 1  is asserted and the switching signals s 2 - sx  are deasserted, the switch  311  is turned on and the switches  312 - 31   x  are turned off, so that the first input terminal of the comparator  92  receives the voltage Ch 1  at the cathode terminal of the LED channel  21 . Meanwhile, the comparator  92  compares the voltage Ch 1  with the reference voltage Vref_short so as to output the comparison result ouput 2 . 
     When the LED channel  21  has short-circuit, the equivalent resistance of the LED channel  21  drops down; therefore, anyone skilled in the art should appropriately define the reference voltage Vref_short, so that when the voltage Ch 1  is higher than the reference voltage Vref_short, the comparison result ouput 2  is able to indicate the LED channel  21  may be open. In addition, anyone skilled in the art may also dispose an alert device (not shown), for example, a light alert device or a sound alert device, and the alert device is coupled to the comparator  92  to send out an alert according to the comparison result ouput 2 . 
     When the signal s 2  is asserted and the switching signals s 1  and s 3 - sx  are deasserted, the switch  312  is turned on and the switches  311  and  313 - 31   x  are turned off, so that the first input terminal of the comparator  92  receives the voltage Ch 2  at the cathode terminal of the LED channel  22 . Meanwhile, the comparator  92  compares the voltage Ch 2  with the reference voltage Vref_short so as to output the comparison result ouput 2 . At the time, the comparison result ouput 2  is able to indicate whether or not the LED channel  22  may be short. Analogically for indicating whether or not the LED channels  23 - 2   x  are short, which is omitted to describe. 
     Based on the described above, the controlling circuit of the embodiment may detect whether or not the LED channels  21 - 2   x  are short or open. In addition, the embodiment employs two comparators ( 91  and  92 ) only. In particular, the quantity of the comparators is not increased with an increasing quantity of the LED channels, which may effectively reduce the circuit area and save the hardware cost. 
     Although the above-mentioned embodiment provides an implementation of the controlling circuit and the controlling method, but anyone skilled in the art should understand the relevant manufactures have their own designs of the controlling circuit and the controlling method. Therefore, the present invention is not limited to the above-mentioned implementation. In fact, whenever the voltage at a terminal of each of a plurality of LED channels is respectively monitored according to the timing of a plurality of signals which are asserted alternately, the scheme is counted to fall in the scope of the present invention. More embodiments of the present invention are described hereinafter for anyone skilled in the art to further understand the spirit of the present invention and to realize the present invention. 
       FIG. 3B  in the above-mentioned embodiment is one of the implementations only, which the present invention is not limited to. The implementation or the architecture of the detecting circuit may vary depending on the requirement. For example,  FIG. 7A  is a diagram of another detecting circuit.  FIG. 7A  is similar to  FIG. 3B , except that two additional logic circuits  111  and  112  are disposed in  FIG. 7A . The logic circuits  111  and  112  respectively analyze the signals c 1  and c 2  output from the comparators  91  and  92 , so as to judge whether or not each of the LED channels  21 - 2   x ) works abnormally in association with the timing of the switching signals s 1 - sx . The logic circuits  111  and  112  are similar to each other. In the following, the logic circuit  111  is depicted, and anyone skilled in the art may easily and similarly implement the logic circuit  112 . 
       FIG. 7B  is a diagram of the logic circuit in  FIG. 7A . The logic circuit  111  includes a plurality of flip-flops D 11 -D 1   x  and an OR gate  115 . The flip-flops D 11 -D 1   x  may be reset according to a reset signal. The flip-flops D 11 -D 1   x  respectively receive the switching signals s 1 - sx  serving as the clock signals. The flip-flops D 11 -D 1   x  also receive the signal c 1 , so as to respectively output signals s 1 ′- sx ′ in association with the clock signals s 1 - sx . The OR gate  115  receives the signals s 1 ′- sx′  so as to output the signal ouput 1 . The signals s 1 ′- sx′  are for respectively indicating whether or not the LED channels  21 - 2   x  are open; the signal ouput 1  is for indicating whether or not one of the LED channels  21 - 2   x  is open. In this way, the function of the above-mentioned embodiment is also achieved. 
       FIG. 8A  is diagram of yet another detecting circuit and  FIG. 8B  is a diagram showing the timing of a light-adjusting signal and a plurality of switching signals.  FIG. 8A  is similar to  FIG. 3B , except that two switches  321  and  322  rather than the switches  311 - 31   x  and the comparator  92  in  FIG. 3B  are employed in  FIG. 8A . The switch  321  is coupled between the reference voltage Vref_short and the second input terminal of the comparator  91 ; the switch  322  is coupled between the reference voltage Vref_open and the second input terminal of the comparator  91 . The switches  321  and  322  are conducted according to two switching signals s_short and s_open respectively. When the switching signal s_short is asserted and the switching signal s_open is deasserted, the second input terminal of the comparator  91  receives the reference voltage Vref_short; when the switching signal s_open is asserted and the switching signal s_short is deasserted, the second input terminal of the comparator  91  receives the reference voltage Vref_open. 
     In more details, when the switching signals s 1  and s_open are asserted and the switching signals s 2 - sx  and s_short are deasserted, the signal c 3  output from the comparator  91  may indicate whether or not the LED channel  21  is open; when the switching signals s 1  and s_short are asserted and the switching signals s 2 - sx  and s_open are deasserted, the signal c 3  output from the comparator  91  may indicate whether or not the LED channel  21  is short. Analogically for the rest, when the switching signals s 2  and s_open are asserted and the switching signals s 1 , s 3 - sx  and s_short are deasserted, the signal c 3  output from the comparator  91  may indicate whether or not the LED channel  22  is open; when the switching signals s 2  and s_short are asserted and the switching signals s 1 , s 3 - sx  and s_open are deasserted, the signal c 3  output from the comparator  91  may indicate whether or not the LED channel  22  is short. In this way, not only the function of the first embodiment may be achieved, but also the embodiment may further reduce the circuit area and save the hardware cost. 
     The controlling circuit of the first embodiment is one of the implementations only, and anyone skilled in the art may modify the above-mentioned implementation of the controlling circuit and appropriately adjust the circuit architecture depending on the requirement. For example,  FIG. 9  is a diagram of a controlling circuit according to the second embodiment of the present invention and  FIG. 10  is a diagram of the detecting circuit of  FIG. 9 . Referring to  FIGS. 9 and 10 , the second embodiment is similar to the first embodiment except that the controlling circuit of the second embodiment further includes an error amplifier  121 , a power supply  131 , a voltage-dividing circuit  141  and a voltage-regulating capacitor  151 . 
     The power supply  131  provides a voltage to the LED channels  21 - 2   x  according to the voltage Vin. The voltage-regulating capacitor  151  is for stabling the voltage. The voltage-dividing circuit  141  is composed of, for example, a plurality of resistors in series connection, so that a dividing voltage Vin′ is generated according to the voltage provided by the power supply  131  and outputs the voltage Vin to the error amplifier  121 . The error amplifier  121  adjusts the voltage provided by the power supply  131  according to the dividing voltage Vin′ and the voltage Vref provided by the detecting circuit. In more details, the error amplifier  121  adjusts the dividing voltage Vin′ to approach the voltage Vref. It should be noted that the detecting circuit  71  in the second embodiment is for detecting and deciding whether or not the voltages Ch 1 -CHx at the cathodes of the LED channels  21 - 2   x  are normal. Once an abnormal voltage is revealed by the detecting circuit  71 , the voltage provided by the power supply  131  may be changed by altering the voltage Vref. 
     Differently from the first embodiment, the detecting circuit  71  of the embodiment further includes a reference voltage generator  161 . The comparator  91  respectively compares the voltages Ch 1 -CHx with the voltage Vref_low_limit one by one, so as to output a comparison result c 4  to the reference voltage generator  161 . In more details, when one of or a plurality of the voltages Ch 1 -CHx is lower than the voltage Vref_low_limit, it indicates the voltage or the voltages is abnormal, and the reference voltage generator  161  would accordingly increase the value of the voltage Vref according to the comparison result c 4 , for example, the voltage Vref may be increased by selecting a set of voltages with higher values as the voltage Vref from a plurality of sets of voltages. 
     On the other hand, the comparator  92  respectively compares the voltages Ch 1 -CHx with the voltage Vref_high_limit one by one, so as to output a comparison result c 5  to the reference voltage generator  161 . In more details, when one of or a plurality of the voltages Ch 1 -CHx is higher than the voltage Vref_high_limit, it indicates the voltage or the voltages is abnormal, and the reference voltage generator  161  would accordingly reduce the value of the voltage Vref according to the comparison result c 5 , for example, the voltage Vref may be reduced by selecting a set of voltages with lower values as the voltage Vref from a plurality of sets of voltages. 
     The second embodiment further provides another implementation of the signal generator.  FIG. 11  is a diagram showing the timing of a light-adjusting signal and a plurality of switching signals according to the second embodiment of the present invention. In the embodiment, the signal generator  81  generates the switching signals s 1 - sx  and a plurality of signals phase 1 -phasex, wherein the signals phase 1 -phasex respectively control the luminance of the LED channels  21 - 2   x  and the signals phase 1 -phasex are respectively asserted when the switching signals s 1 - sx  are asserted, so as to reduce a misjudgement risk by the detecting circuit  71 . 
       FIG. 12  is a circuit diagram of a signal generator according to the second embodiment of the present invention. In the embodiment, the signal generator  81  includes a plurality of flip-flops  171 - 17   x  and  181 - 18   x , a plurality of NOR gates  180 ,  192  and  194 , an inverter  191 , an OR gate  195  and a plurality of AND gates  201 - 20   x . The flip-flops  171 - 17   x  receive a signal CLK as the clock signal and a signal Reset as the reset signal. The flip-flop  171  receives a light-adjusting signal so as to output the signal phase 1 . The flip-flop  172  receives the signal phase 1  so as to output the signal phase 2 . Analogically for the flip-flops  173 - 17   x , which are omitted to describe. 
     The flip-flops  181 - 18   x  receive the signal CLK as the clock signal and the signal clear as the reset signal. The NOR gate  180  receives a plurality of signals Cs 1 -Csx so as to output a signal Cstart. The flip-flop  182  receives the signal Cstart so as to output the signal Cs 1 . The flip-flop  182  receives the signal Cs 1  so as to output the signal Cs 2 . Analogically for the flip-flops  183 - 18   x , which are omitted to describe. 
     The flip-flop  193  receives the signal CLK as the clock signal and the signal Reset as the reset signal. The inverter  191  receives the signal Cs 1  and outputs a signal L 1 . The NOR gate  192  receives the signal phase 1  and the signal L 1  so as to output a signal L 2 . The flip-flop  193  receives the signal L 2  so as to output a signal L 3 . The NOR gate  194  receives the signals phase 1 -phasex so as to output a signal L 4 . The OR gate  195  receives the signals L 3  and L 4  so as to output the signal clear. 
     The AND gate  201  receives the signals phase 1  and Cs 1  so as to output the switching signal s 1 . The AND gate  202  receives the signals phase 2  and Cs 2  so as to output the switching signal s 2 . Analogically for the AND gates  203 - 20   x , which are omitted to describe. In this way, the embodiment not only generates the switching signals s 1 - sx , but also ensures to respectively enable the signals phase 1 -phasex under the condition of respectively enabling the switching signals s 1 - sx , which is advantageous in reducing a misjudgement risk by the detecting circuit  70 . 
     In summary, the present invention takes a scheme, which respectively and alternately monitors the voltage at a terminal of one of a plurality of LED channels. In this way, the present invention may monitor whether or not the LED channels are abnormal and reduce the hardware cost (for example, reducing the quantity of the comparators). In addition, the embodiments of the present invention has following advantages:
         1. The embodiments may detect out whether or not the LED channels have short-circuit, open-circuit, excessive voltage or too low voltage by changing the reference voltage of the comparator (i.e., by providing an appropriate reference voltage to the comparator).   2. The embodiments may further analyze and identify which one of the LED channels is abnormal by disposing a logic circuit at the output terminal of the comparator in association with the timing of the switching signals.   3. The hardware cost may be further reduced by using the switches to select one of the reference voltages and inputting the selected reference voltage to the input terminal of the comparator.   4. When the voltage of an LED channel gets abnormal, a feedback mechanism may be used to adjust the voltage provided to the defective LED channel.   5. When the light-adjusting signal is asserted, a misjudgement risk may be reduced by monitoring the voltage at a terminal of each of the LED channels.       

     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.