Abstract:
The present invention discloses a digital dimming device and a digital dimming method, for controlling a plurality of light emitting device channels. The method comprises: generating a corresponding plurality of driving signals to control the plurality of light emitting device channels; receiving a PWM input signal having a duty ratio, and phase shifting the PWM input signal to generate multiple PWM output signals with about the same duty ratio as the PWM input signal, but with respectively shifted phases; and enabling or disabling corresponding driving signals by the multiple PWM output signals, respectively.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a digital dimming device and method, in particular to one that uniformly distributes the illumination timings of multiple strings of light emitting devices. 
     2. Description of Related Art 
     In a circuit for controlling light emitting devices (such as light emitting diodes, LEDs), a dimming function is often required. Two dimming methods have been proposed in prior art to deal with the case where there are multiple strings of light emitting devices. The first one is shown in  FIG. 1 , wherein the same dimming control signal is used to adjust the brightness of every string of light emitting devices. The second one is shown in  FIG. 2 , wherein different dimming control circuits are used to dim corresponding light emitting device strings. More specifically, in the first prior art shown in  FIG. 1 , a light emitting device control circuit comprises a power stage control circuit  21  for controlling a power transistor in a power stage  22  to convert an input voltage Vin to an output voltage, which is provided to multiple LED channels CH 1 -CHn. The power stage circuit  22  may be, for example but not limited to, asynchronously or asynchronously buck, boost, buck-boost, inverting or fly back voltage converter shown in  FIGS. 3A-3G , wherein if the power stage circuit  22  is the fly back voltage converter shown in  FIG. 3G , the power stage control circuit  21  and the dimming control circuit  23  are usually separated in different integrated circuits. In other cases, the power stage control circuit  21  and the dimming control circuit  23  can be integrated in the same integrated circuit. The dimming control circuit  23  provides the same dimming control signal to all LED channels CH 1 -CHn, synchronously controlling the transistors Q 1 -Qn on the channel paths. An example of such prior art is U.S. Pat. No. 7,259,687. 
     In the second prior art shown in  FIG. 2 , different dimming control circuits  23 A,  23 B, and  23 N are used to adjust the brightness of corresponding light emitting device strings. An example of such prior art is US Patent Publication No. 2009/0134817. 
     However, if the same dimming control signal is used to synchronously control all light emitting device strings, all light emitting devices would be synchronous in their ON/OFF cycles, which would cause a larger ripple in the output voltage and current, and also a more serious flicker effect. A better arrangement is to turn ON the light emitting device strings in sequential order and to uniformly distribute the illumination timings of the light emitting device strings. Although the illumination timing of each light emitting device string can be independent from another string by controlling the light emitting device strings respectively, this can not ensure that the illumination timings of the light emitting device strings are uniformly distributed. 
     Besides, the frequency of a digital dimming signal is in a range of about 60-500 Hz, but in some applications it is difficult to provide a signal of such low frequency. 
     In view of the above, the present invention proposes a digital dimming device and method which can solve the problem of non-uniform distribution of illumination timings, and furthermore it can receive digital dimming signals in any frequency range. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a digital dimming device. 
     Another objective of the present invention is to provide a digital dimming method. 
     To achieve the foregoing objectives, in one perspective of the present invention, it provides a digital dimming device for controlling a plurality of light emitting device channels, comprising: a driving signal generation circuit generating a driving signal; a plurality of driver circuits which control currents in the plurality of light emitting device channels according to the driving signal, respectively; and a phase shift circuit receiving a PWM (pulse width modulation) input signal having a duty ratio, and shifting the phase of the PWM input signal to generate multiple PWM output signals with about the same duty ratio as the PWM input signal, but with respectively shifted phases, wherein the multiple PWM output signals respectively enable or disable corresponding driver circuits, and the duty ratio of each PWM output signal determines an average current of a corresponding one of light emitting device channels. 
     The foregoing digital dimming device may further comprise: a frequency conversion circuit receiving a dimming input signal with a first frequency and generating the PWM input signal with a second frequency which is sent to the phase shift circuit. 
     In another perspective of the present invention, it provides a digital dimming method for controlling a plurality of light emitting device channels, comprising: generating a corresponding plurality of driving signals to control the plurality of light emitting device channels; receiving a PWM input signal having a duty ratio, and phase shifting the PWM input signal to generate multiple PWM output signals with about the same duty ratio as the PWM input signal, but with respectively shifted phases; and enabling or disabling corresponding driving signals by the multiple PWM output signals, respectively. 
     The foregoing digital dimming method may further comprise: receiving a dimming input signal with a first frequency and generating the PWM input signal with a second frequency. 
     The foregoing digital dimming method may generate multiple PWM output signals with respectively shifted phases by the following way: recording a pulse width of the PWM input signal; recording a cycle period of the PWM input signal; dividing the cycle period by the number of light emitting device channels to obtain a quotient; and generating multiple PWM output signals according to the recorded cycle period and the quotient, wherein each of the cycle periods of the multiple PWM output signals starts at a different timing which differs from one another by the quotient. 
     The foregoing digital dimming method may convert the first frequency to the second frequency by the following way: recording a high-level pulse width of the dimming input signal; dividing the high-level pulse width by m which is the ratio of the second frequency to the first frequency; recording a low-level pulse width of the dimming input signal; dividing the low-level pulse width by m; and generating the PWM input signal with the second frequency according to the high-level and low-level pulse widths divided by m. 
     The foregoing digital dimming method may convert the first frequency to the second frequency also by the following way: generating an operating frequency substantially equal to or near to the first frequency; generating the second frequency; recording a high-level pulse width of the dimming input signal by the operating frequency; recording a low-level pulse width of the dimming input signal by the operating frequency; and generating the PWM input signal according to the high-level and low-level pulse widths, by the second frequency. 
     The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below, with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a conventional light emitting device control circuit whose drawback is that the illumination timings of the light emitting device strings are not uniformly distributed. 
         FIG. 2  illustrates another conventional circuit which has the same drawback as well. 
         FIGS. 3A-3G  show several embodiments of the power stage circuit  22 . 
         FIG. 4  shows an embodiment of the present invention. 
         FIG. 5  shows another embodiment of the present invention. 
         FIG. 6  shows an embodiment of a phase shift circuit. 
         FIG. 7  shows, by way of example, the output waveforms of the phase shift circuit. 
         FIG. 8  shows an embodiment of a frequency conversion circuit. 
         FIG. 9  shows, by way of example, the input and output waveforms of the frequency conversion circuit in  FIG. 8 . 
         FIG. 10  shows another embodiment of the frequency conversion circuit. 
         FIG. 11  shows, by way of example, the input and output waveforms of the frequency conversion circuit in  FIG. 10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In present invention, the digital dimming device or the digital dimming method generates multiple PWM output signals with respectively shifted phases, to turn ON the light emitting devices of different channels in sequential order, so that the illumination timings are uniformly distributed. Please refer to  FIG. 4 , which shows the first embodiment of the present invention, wherein a digital dimming device  30  comprises a phase shift circuit  35 , a light emitting diode (LED) driving signal generation circuit  37 , a plurality of driver circuits  39  (only one is shown), and optionally, a frequency conversion circuit  31 . When the frequency of a dimming input signal is not in a proper range (60-500 Hz, for example), no matter it is too low or too high, the frequency conversion circuit  31  can receive and convert the dimming input signal into a signal with about the same duty ratio but with a proper frequency. The details about the frequency conversion circuit  31  will be described later. If the dimming input signal is already in the proper range, then the frequency conversion circuit  31  is not required. 
     The LED driving signal generation circuit  37  generates n driving signals QC 1 -QCn through the driver circuits  39  to control gates of transistors Q 1 -Qn in corresponding LED channels CH 1 -CHn; the driving signals QC 1 -QCn determine the current amounts on the corresponding LED channels CH 1 -CHn when the transistors Q 1 -Qn are conducted. The phase shift circuit  35  generates n dimming control signals  1 - n  with shifted phases according to the dimming input signal or the output of the frequency conversion circuit  31 , the number of the signals corresponds to the number of LED channels. The dimming control signals  1 - n  are digital square wave signals which enable the driver circuits  39  at high level while disable the outputs of the driver circuits  39  at low level. In other words, the duty ratio of the dimming control signals  1 - n  determines the average currents on the corresponding LED channels CH 1 -CHn, that is, the average brightness of the LEDs on each LED channel. The details about the phase shift circuit  35  will be described later. 
       FIG. 5  shows another embodiment of the present invention, wherein the LED driving signal generation circuit  37  generates n driving signals IC 1 -ICn through the driver circuits  39  to control current sources CS 1 -CSn in corresponding LED channels CH 1 -CHn, instead of the gates of transistors Q 1 -Qn. This also is an alternative to achieve the same function as the foregoing embodiment. 
       FIG. 6  shows an example of the phase shift circuit  35 . In this embodiment, the phase shift circuit  35  comprises a pulse width recording circuit  351 , a cycle period recording circuit  353 , a divider circuit  355 , and a dimming control signal generator  357 . When the phase shift circuit  35  receives a dimming signal (directly, or after frequency conversion by the frequency conversion circuit  31 ), the pulse width recording circuit  351  records a high-level pulse width of the dimming input signal and sends a corresponding digital data to the dimming control signal generator  357 . On the other hand, the cycle period recording circuit  353  records a cycle period T of the dimming input signal, and the divider circuit  355  divides the recorded cycle period T by n and sends the digital data (T/n) to the dimming control signal generator  357 , wherein n corresponds to the number of the light emitting device channels. The dimming control signal generator  357  generates multiple phase-shifted dimming control signals  1 - n  according to the high-level pulse width and the digital data (T/n). 
     What is described above can be better understood with reference to the example shown in  FIG. 7 . In this example, n (the number of channels) is 4, so each of the dimming control signals  1 - n  generated by the phase shift circuit  35  starts at a different timing which differs from one another by T/4, but has the same high-level pulse width. 
     The above description is for easier understanding of the basic concept of the present invention. In fact, because the dimming input signal itself is a PWM signal having a correct duty ratio, the phase shift circuit  35  can merely generate (n−1) dimming control signals  2 - n , and the dimming input signal can be used as the first dimming control signal  1  without being processed by the phase shift circuit  35 . Under the teaching of the present invention, those skilled in this art can readily conceive other variations and modifications. 
     Hereafter we will illustrate two examples to embody the frequency conversion circuit  31 . As described above, the dimming input signal might not be in the proper range, and the function of the frequency conversion circuit  31  is to divide the frequency of the dimming input signal (if its frequency is too high) or to multiply the frequency of the dimming input signal (if its frequency is too low), so as to generate a frequency-converted dimming input signal which is in a proper frequency range, with the same duty ratio. 
     First referring to  FIG. 8 , the frequency conversion circuit  31  in this embodiment comprises a high-level recording circuit  311 , a multiplier or divider circuit  312 , a low-level recording circuit  313 , a multiplier or divider circuit  314 , and a signal generator  316 . When the frequency conversion circuit  31  receives a dimming input signal with a frequency F 1 , the high-level recording circuit  311  records the high-level pulse width of the dimming input signal and the low-level recording circuit  313  records the low-level pulse width of the dimming input signal. Depending on whether multiplication or division is required, the multiplier or divider circuits  312  and  314  multiply or divide the recorded high-level and low-level pulse widths by m, which represents a ratio of the frequency F 1  of the dimming input signal to a frequency F 2  of the dimming signal to be generated, that is, m=F 2 /F 1  (when the frequency of the dimming input signal is below the proper range) or F 1 /F 2  (when the frequency of the dimming input signal is above the proper range). The signal generator  316  generates the dimming signal having the frequency F 2  according to the high-level and low-level pulse widths from the multiplier or divider circuits  312  and  314 . 
     What is described above can be better understood with reference to the example shown in  FIG. 9 . In this example, F 2 =(1/2)F 1 , that is, the circuits  312  and  314  are divider circuits and m=2. The signal generator  316  combines the high-level and low-level signals with double pulse widths to generate the dimming signal shown in the figure, which can be used as the dimming input signal in  FIG. 6 . 
       FIG. 10  shows another embodiment of the frequency conversion circuit  31 , wherein the frequency conversion circuit  31  comprises a high-level recording circuit  311 , a low-level recording circuit  313 , an oscillator (OSC)  315 , and a signal generator  316 . In this embodiment, the frequency of the dimming input signal is F 0  while the frequency of the dimming signal to be outputted is F 3 , and the conversion ratio to be achieved is, for example, 1/2 (F 3 /F 0 =1/2). The oscillator (OSC)  315  generates two sample frequencies F 1  and F 2  in high-frequency, which are both much higher than the frequency F 0  of the dimming input signal, and the ratio of F 1  to F 2  is the same as the ratio of F 0  to F 3 , that is, F 2 /F 1 =1/2. The high-level and low-level recording circuits  311  and  313  operate under the frequency F 1  generated by the oscillator (OSC)  315 , and the signal generator  316  operates under the frequency F 2  generated by the oscillator (OSC)  315 . Similar to the previous embodiment, the high-level and low-level recording circuits  311  and  313  record the high-level and low-level pulse widths, and send corresponding digital data to the signal generator  316  which combines the high-level and low-level pulse widths to generate an output. But, because the signal generator  316  operates under a frequency F 2  which is half of F 1 , the frequency F 3  of the dimming signal outputted from the signal generator  316  is also half of F 0 , as shown in  FIG. 11 . 
     As described above, the frequency conversion circuit  31  converts the dimming input signal to a signal with about the same duty ratio but with a proper frequency, such that the dimming control can be based on the proper frequency. Note that, such frequency conversion can be applied to a single-channel LED controller circuit, not limited to multi-channel LED controller circuit. In the case of single-channel LED control, referring to  FIGS. 4 and 5 , it is not required for the digital dimming device  30  to include the phase shift circuit  35 ; the digital dimming device  30  only includes the frequency conversion circuit  31 , one LED driving signal generation circuit  37  and one driver circuit  39 . 
     The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. For example, it is described that the high-level pulse width of the dimming signal is used to determine the light emitting time of the light emitting devices, but the light emitting time can alternatively be determined by the low-level pulse width. As yet another example, the light emitting device is not necessarily a light emitting diode, but can be any light emitting device whose brightness can be controlled by current. Further, in the present invention, the power stage control circuit  21  and the dimming control circuit  23  can be integrated in the same integrated circuit or separated into two integrated circuits, and in the latter case the current sources CS 1 -CSn can be, for example, integrated with the digital dimming device  30  in one integrated circuit. Thus, the present invention should cover all such and other modifications and variations, which should be interpreted to fall within the scope of the following claims and their equivalents.