CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of the filing date of Chinese Application Ser. No. 200810046538.0, filed on Nov. 12, 2008, and incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a pulse dimming circuit for cold cathode fluorescent lamps (CCFL) and method thereof. 
       BACKGROUND 
       [0003]    Pulse dimming is usually adopted by CCFL Inverters. Generally there are two methods for pulse dimming. One method uses direct current (DC) input voltage, and the other method utilizes an external PWM input signal. For the first method, a comparison wave generator is required. By comparing a DC input signal with a reference comparison wave, a pulse dimming signal is generated. For the second method, the pulse dimming signal follows the PWM input signal directly, thus there is no need for a reference comparison wave generator. As a result, the two pulse dimming methods are not generally compatible. 
         [0004]    For either of the two dimming methods, a separate circuit board is needed. Thus, in the prior art, two circuit boards are required, which adds to cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description serve to explain the principles of the invention. 
           [0006]      FIG. 1  illustrates a schematic diagram of a pulse dimming circuit  100  in accordance with an embodiment of the present invention. 
           [0007]      FIG. 2  illustrates a graph of a pulse dimming waveform when both PWM input signal and DC input signal are input in accordance with an embodiment of the present invention. 
           [0008]      FIG. 3  illustrates a graph of a pulse dimming waveform in operation condition 2 in accordance with an embodiment of the present invention. 
           [0009]      FIG. 4  illustrates a graph of a pulse dimming waveform at the beginning of operation condition 2 in accordance with an embodiment of the present invention. 
           [0010]      FIG. 5  illustrates a graph of a pulse dimming waveform in operation condition 5 in accordance with an embodiment of the present invention. 
           [0011]      FIG. 6  illustrates a graph of a pulse dimming waveform in operation condition 5 in accordance with an embodiment of the present invention. 
           [0012]      FIG. 7  illustrates a schematic diagram of pulse dimming integrated circuit  200  in accordance with another embodiment of the present invention. 
           [0013]      FIG. 8  illustrates a flow chart  300  of a pulse dimming method in accordance with yet another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Reference will now be made in detail to different embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with different embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
         [0015]      FIG. 1  shows a schematic diagram of a pulse dimming circuit  100  in accordance with an embodiment of the present invention. The pulse dimming circuit  100  includes: a first input port  101 , which optionally receives a DC input signal; a second input port  102 , which optionally receives a PWM input signal; a comparison signal generator  103 , which is coupled to the second input port  102  for receiving the PWM input signal if provided, and provides a comparison signal and a logical signal accordingly; and a comparator  104 , which is coupled to the first input port  101  through a buffer  107  for receiving the DC input signal if provided, coupled to the comparison signal output of the comparison signal generator  103 . 
         [0016]    The comparator  104  provides a DC pulse signal according to the comparison signal and the DC input signal. The circuit  100  also includes a logic module  105 , which is coupled to the second input port  102  for receiving the PWM input signal if provided, and is further coupled to the logical signal output terminal of the comparison signal generator  103 . The logic module provides a selective signal accordingly. Finally, a switch network  106 , coupled to the output terminal of the comparator  104  and the output terminal of the logic module  105 , and coupled to the second input port  102  through buffer  108  for receiving the PWM input signal if the PWM input signal is provided, provides a pulse dimming signal accordingly. 
         [0017]    Note that, buffers  107  and  108  are optional. In addition, the first buffer module  107  could be used to regulate the voltage level of the DC input signal, the second buffer module  108  could be used to regulate the voltage level of the PWM input signal. In one embodiment, a PWM pulse signal is generated by processing the PWM input signal via the second buffer module  108 . Thereupon, in one embodiment, the switch network  106  receives the PWM pulse signal instead of the PWM input signal. 
         [0018]    In one embodiment, the PWM input signal has a higher priority than the DC input signal. When in operation, the comparison signal generator  103  identifies whether the PWM input signal is provided or not. If the PWM input signal is input, the comparison signal has a first status, which is delivered to the comparator  104 . Further, the logical signal has a first logic value, i.e., logic “0”, which is sent to the logic module  105 . As a result, the logic module  105  sends the selective signal with a first status to the switch network  106 , so that the switch network  106  operates to provide the pulse dimming signal according to the PWM pulse signal. 
         [0019]    In contrast, if the PWM input signal is not input, the comparison signal has a second status, which is compared with the DC input signal at the comparator  104 , the comparator  104  provides the DC pulse signal to the switch network  106  accordingly. At the same time, the logical signal has a second logic value, i.e., logic “1”, which is sent to the logic module  105 . As a result, the logic module  105  sends the selective signal with a second status to the switch network  106 , so that the switch network  106  operates to provide the pulse dimming signal according to the DC pulse signal. 
         [0020]    In general, the pulse dimming circuit  100  includes the following five operational conditions according to the input signal provided: 1. only PWM input signal is provided; 2. only DC input signal is provided; 3. both PWM input signal and DC input signal are provided; 4. PWM input signal is inserted when DC input is used as the dimming input signal during operation; 5. PWM input signal disappears which is used as the dimming input signal originally, then DC input signal is used as the dimming input signal instead thereafter. The detailed operation mode and principles of the pulse dimming circuit  100  under each condition will be described hereinafter. 
         [0021]    During operational conditions 1 and 3, only PWM input signal is provided or both PWM input signal and DC input signal are provided. These two conditions are virtually the same, because the PWM input signal is preset to have a higher priority in one embodiment, and the logic module  105  is preset to output a logic “1” selective signal. As a result, the switch network  106  outputs the pulse dimming signal based on the PWM pulse signal, i.e. the system follows the PWM input signal automatically and produces the pulse dimming signal. For example, when both PWM input signal and DC input signal are provided, the PWM input signal is buffed by the second buffer module  108 , then a PWM pulse signal is provided to the switch network  106 . Because the PWM input signal is preset to have a higher priority, the logic module  105  will output a logic “1” selective signal to the switch network  106 , so that the switch network  106  outputs the pulse dimming signal based on the PWM pulse signal. 
         [0022]      FIG. 2  is a graph of a pulse dimming waveform when both PWM input signal and DC input signal are input in accordance with an embodiment of the present invention. When the selective signal is logic “1”, the system utilizes the PWM input signal. The comparison signal generated by the comparison signal generator  103  has a first status. As shown in  FIG. 2 , when the PWM input signal is high, the comparison signal is reset. When the PWM input signal is low, the comparison signal generator  103  will be slowly charged up by a relatively small current and outputs the comparison signal rising up at a small rate. When a next rising edge of the PWM input signal comes, the comparison signal is reset. As can be seen from  FIG. 2 , the comparison signal is comprised by the rising part and reset part. Thus, the comparison signal generator  103  also functions as a detection timer, which detects whether the PWM input signal is constantly provided. The comparison signal generator  103  also provides a logical signal to the logic module  105 , wherein the logical signal is a high or low level signal that is produced by comparing the comparison signal with the threshold of the comparison signal generator  103 . If the comparison signal is lower than the threshold all the while, the logical signal is logic “0”, which is low level. If the comparison signal rises up to the threshold, the logical signal is a narrow pulse. If the PWM input signal is provided all the time, the comparison signal rises up at a relatively small rate and then it is reset. Thus it cannot reach the threshold, and the logical signal is low all the time. The selective signal provided by the logic module  105  will keep logic “1”. Thus the switch network  106  will output the pulse dimming signal based on the PWM pulse signal. 
         [0023]    Turning to operational condition 2, only DC input signal is provided.  FIG. 3  is a graph of pulse dimming waveform when only DC input signal is provided in accordance with an embodiment of the present invention. As shown in  FIG. 3 , the DC input signal is delivered to one input terminal of the comparator  104  via the first buffer module  107 . The comparison signal which is generally a triangular or a sawtooth wave generated from the comparison generator  103 , is input to the other input terminal of the comparator  104 . After comparison, a DC pulse signal is provided by the comparator  104  and is delivered to the switch network  106 . During this operation condition 2, the comparison signal rises up at a normal (relatively greater) rate. The logical signal produced by the comparison signal generator  103  is a narrow pulse (not shown) when the comparison signal rises to be higher than the predetermined threshold. As a result, the logic module  105  outputs a logic “0” selective signal, and the switch network  106  switches to the output terminal of the comparator  104  and output a pulse dimming signal according to the DC pulse signal. 
         [0024]    It should be noted that at the beginning of operation condition 2, there might be a transition from PWM input signal to DC input signal, since the selective signal is predetermined to be logic “1” since the PWM input signal has a higher priority.  FIG. 4  illustrates pulse dimming waveform when started utilizing the DC input signal, in accordance with an embodiment of the present invention. As shown in  FIG. 4 , there is no PWM input signal at the second input terminal  102 . However, due to the preset higher priority of the PWM input signal, it is considered that the PWM input signal is provided at the beginning of the system, and a logic “1” selective signal is provided by the logic module  105 . Meanwhile the comparison signal generator  103  is charged up slowly by a relatively small current, and it outputs a comparison signal rising up at a small rate during the first charging cycle. Because the PWM input signal is not input virtually, when the comparison signal is charged up to the threshold, the selective signal output by the logic module  105  will turn to logic “0” according to the high level logical signal and the low level PWM input signal. The DC input signal is then selected as the dimming input signal and delivered to the comparator  104  via the first buffer module  107 . Meanwhile the comparison signal generator  104  is charged up by a relatively great current and provides a comparison signal rising up at a normal rate which is generally a triangular wave or a sawtooth wave. The DC pulse signal is output by the comparator  104  to the switch network  106 . The switch network  106 , which will switch to couple to the output terminal of the comparator  104  according to the logic “0” selective signal, will output a pulse dimming signal according to the DC pulse signal. 
         [0025]    Under operational condition 4, the PWM input signal is inserted when DC input is used as the dimming input signal during operation.  FIG. 5  is a graph of pulse dimming waveform in operation condition 4 in accordance with an embodiment of the present invention. As shown in  FIG. 5 , when no PWM input signal is provided, the DC input signal is selected as the dimming input signal, the logic module  105  outputs a logic “0” selective signal. The comparison signal generator  103  outputs a comparison signal that is a triangular wave or sawtooth wave to the comparator  104 . The comparator  104  compares the comparison signal with the DC input signal, and outputs a DC pulse signal according to which the switch network  106  provides the pulse dimming signal. 
         [0026]    Once the PWM input signal is input, the comparison signal is reset so long as the PWM input signal is high. When PWM input signal is low, the comparison signal generator  103  is charged up slowly by a relatively small current, and the comparison signal is rising up slowly at a relatively small rate until coming of a next rising edge of the PWM input signal. Thus the comparison signal generator  103  is also functions as a detection timer, which detects whether the PWM input signal is provided all the while or not. If the PWM input signal is provided, the selective signal keeps logic “1”. The PWM input signal is delivered to the second buffer module  108  where PWM pulse signal is produced. The switch network  106  receives the PWM pulse signal and outputs pulse dimming signal based on the PWM pulse signal. As shown in  FIG. 5 , during the operation that the DC input signal is selected as dimming input signal, once the PWM input signal is inserted, the pulse dimming signal will immediately follow the PWM input signal. Thus, the pulse dimming circuit  100  achieves a self-adapting switching from DC input signal to PWM input signal. 
         [0027]    Operation condition 5: the PWM input signal disappears which is used as the dimming input signal originally, then the DC input signal is used as the dimming input signal instead.  FIG. 6  shows the pulse dimming waveform in operation condition 5 in accordance with an embodiment of the present invention. As shown in  FIG. 6 , when PWM input signal is provided as dimming input signal, the logic module  105  outputs a logic “1” selective signal and the comparison signal generator functions as a detection timer. When the PWM input signal is high, the comparison signal is reset. When the PWM input signal turns to low, the comparison signal generator  103  is charged up slowly by a relatively small current, and the comparison signal rises up slowly at a relatively small rate. The comparison signal generator  103  is not reset until coming of a next rising edge of the PWM input signal. 
         [0028]    As illustrated hereinbefore, the pulse dimming signal follows the PWM input signal. However, if the PWM input signal disappears, i.e. the PWM input signal keeps low for a long interval, the comparison signal generator  103  will be charged up by a relatively great current to the threshold. And a comparison signal rising up at a normal rate is output. The comparison signal is generally a triangular wave or sawtooth wave, which is sent to the comparator  104 . At one hand, the DC input signal is selected as the dimming input signal and is delivered to the comparator  104  via the first buffer module  107 . The DC pulse signal is then provided by the comparator  104 , and is sent to the switch network  106 . At the other hand, the logical signal is high and the PWM input signal is low, thus a logic “0” selective signal is output by the logic module  105 . As a result, the switch network  106  will choose to switch to the output terminal of the comparator  104  according to the logic “0” selective signal and a pulse dimming signal based on the DC pulse signal is produced. Therefore, the dimming circuit  100  achieves self-adapting switch to DC input signal when the PWM input signal disappears. 
         [0029]      FIG. 7  is a schematic diagram of a pulse dimming integrated circuit  200  in accordance with another embodiment of the present invention. 
         [0030]    In one embodiment, the logic module is implemented by a RS Flip-Flop  205 . The comparison signal generator  203  comprises a second comparator, a charge current source Icharge, a discharge current source Idischarge, a capacitor C 0 , a first switch SW 1 , a second switch SW 2 , and a third switch SW 3 . The charge current source Icharge, the discharge current source Idischarge, and the first switch SW 1  are coupled in series, the common coupled terminal of the charge current source Icharge and the discharge current source Idischarge is coupled to the non-inverting input terminal of the second comparator. The second switch SW 2 , the third switch SW 3 , and the capacitor C 0  are coupled in parallel between the non-inverting input terminal of the second comparator and ground. The control terminal of the first switch SW 1  is coupled to the output terminal Q of the RS Flip-Flop  205 , the control terminal of the second switch SW 2  is coupled to the second input port  202 , the control terminal of the third switch SW 3  is coupled to the output terminal of the second comparator, respectively. The second comparator receives a threshold signal Vth at its inverting terminal. 
         [0031]    At the beginning, the PWM input signal has a higher priority and a logic “1” selective signal S is output by the RS Flip-Flop  205 . The selective signal S turns on the first switch SW 1 . 
         [0032]    As can be seen from  FIG. 7 , when the selective signal S is logic “1”, the first switch SW 1  is turned on; when the selective signal S is logic “0”, the first switch SW 1  is turned off. When the PWM input signal B is high, the second switch SW 2  is turned on, the capacitor C 0  is reset accordingly; when the PWM input signal B is low, the second switch SW 2  is turned off. In the case when the first switch SW 1  is turned on, the discharege current source Idischarge is shunted from the charge current source Icharge to ground, only the rest relatively small part of the charge current source Icharge is used to charge the capacitor C 0 , thus the comparison signal rising up at a relatively small rate until the PWM input signal B turns to high again. And the capacitor C 0  is reset. According to the selective signal S, the switch network  206  selects the PWM input signal B as the dimming input signal of the dimming circuit  200 . Thus a pulse dimming signal is output based on the PWM input signal B. 
         [0033]    When the PWM input signal keeps low for a predetermined time, the second switch SW 2  is turned off, the capacitor C 0  is charged up till the voltage across it exceed the threshold Vth. Then the second comparator, i.e., the comparison signal generator  203  outputs a high signal R to the reset terminal of RS Flip-Flop. The RS Flip-Flop outputs a logic “0” selective signal S accordingly to turn off the first switch SW 1 . As a result, the capacitor C 0  will be charged up by the current Icharge and the comparison signal generator  203  outputs a comparison signal rising up at a normal rate. Generally the comparison signal is a triangular wave or sawtooth wave and is sent to the inverting terminal of the comparator  204 . The non-inverting terminal of the comparator  204  receives a DC input signal or the DC input signal level shifted by a level shift module  207 . The comparator  204  provides a DC pulse signal to the switch network  206 . The switch network  206  switches to the output terminal of the comparator  204  according to the logic “0” selective signal S and outputs a pulse dimming signal O based on the DC pulse signal C. 
         [0034]    In one embodiment, the logic module in the pulse dimming circuit  200  may also be implemented by a D Flip-Flop. 
         [0035]      FIG. 8  illustrates a flow chart  300  of a pulse dimming method in accordance with yet another embodiment of the present invention. Referring to  FIG. 8 , the method comprises: process  301 , judging whether a PWM input signal is provided, and if provided, jumping to process  302 , and if not provided, jumping to process  304 . At process  302 , a selective signal is output with a first status. At process  303 , a switch network is controlled to provide a pulse dimming signal according to the PWM input signal based on the selective signal with a first status. 
         [0036]    In one embodiment, at process  302 , a comparison signal is provided with a first status, and a logical signal is provided with a first logic value. When the PWM input signal is high, the comparison signal is reset; when the PWM input signal is low, the comparison signal with a first status rises up at a relative low rate. The logical signal with a first logic value is a logic “0” signal; the selective signal with a first status is a logic “1” signal. 
         [0037]    If input signal contains DC input voltage signal only, that is, the PWM input signal is not provided, then control goes to process  304  where the DC input voltage signal is converted to a DC pulse signal. At process  305 , a selective signal is output with a second status. At process  306 , the switch network is controlled to output a pulse dimming signal according to the DC pulse signal based on the selective signal with a second status. 
         [0038]    In one embodiment, at process  304 , a comparison signal is provided with a second status, and a logical signal is provided with a second logic value. Further, the comparison signal is compared with the DC input signal to get the DC pulse signal. The comparison signal with a second status is a sawtooth or triangular wave, rising up at a relative high rate; the logical signal with a second logic value is a logic “1” signal; the selective signal with a second status is a logic “0” signal. 
         [0039]    While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
The present invention provides a pulse dimming circuit and a method thereof. The pulse dimming circuit is compatible for both DC input signal and PWM input signal using only one circuit board.