Patent Publication Number: US-8120280-B2

Title: Circuits and methods for controlling a light source

Description:
RELATED APPLICATION 
     This application claims priority to Patent Application No. 201010225108.2, titled “Driving Circuits, Methods and Controllers for Driving a Light Source,” filed on Jul. 2, 2010, with the State Intellectual Property Office of the People&#39;s Republic of China. 
     BACKGROUND 
     Currently, light sources such as light emitting diodes (LEDs) or cold cathode fluorescent lamps (CCFLs) are widely used in the lighting industry, e.g., for backlighting liquid crystal displays (LCDs), street lighting, and home appliances. A light driving circuit can be used to adjust power delivered to the light source according to a dimming signal, e.g., a pulse width modulation (PWM) signal. 
       FIG. 1  shows a block diagram of a conventional light driving circuit  100 . The light driving circuit  100  includes an alternating current (AC) to direct current (DC) converter  104 , a power converter  106 , and a dimming module  112 . The AC to DC converter  104  converts an input AC voltage provided by an AC power source  102  to a first DC voltage. The power converter  106  transforms the first DC voltage to a second DC voltage having a voltage level suitable for powering an LED string  108 . The dimming module  112  can operate in a burst-dimming control mode, in which the dimming module  112  generates a pulse width modulation (PWM) signal  120  to adjust the power delivered to the LED string  108  so as to regulate the brightness of the LED string  108 . More specifically, the light driving circuit  100  further includes a switch  110  coupled to the LED string  108  and operable for controlling a current I LIGHT  flowing through the LED string  108  according to the PWM signal  120 , which further determines the brightness of the LED string  108 . 
       FIG. 2  shows a timing diagram  200  of signals generated by the light driving circuit  100 .  FIG. 2  is described in combination with  FIG. 1 . In the example of  FIG. 2 , the timing diagram  200  shows the PWM signal  120  and the current I LIGHT  flowing through the LED string  108 . When the PWM signal  120  is high, e.g., during a time duration T ON  from t 1  to t 2 , the switch  110  is turned on. The current I LIGHT  having a predetermined level I 1  flows through the LED string  108 . When the PWM signal  120  is low, e.g., during a time duration T OFF  from t 2  to t 3 , the switch  110  is turned off. The current I LIGHT  drops to substantially zero ampere. Thus, by adjusting the duty cycle of the PWM signal  120 , an average level of the current I LIGHT  is varied to regulate the brightness of the LED string  108 . 
     However, due to the characteristics of semiconductor devices such as the power converter  106 , the current I LIGHT  needs a delay time T DELAY  to reach the predetermined level I 1  after the switch  110  is turned on, e.g., at t 1  or t 3 . As such, the dimming control of the LED string  108  may be affected by frequency noise of the light driving circuit  100 . For example, if the frequency of the PWM signal  120  is greater than a predetermined threshold F MAX  when the duty cycle is relatively low (e.g., the duty cycle is in a range of 0˜5%), the time duration T ON  is close to or less than the delay time T DELAY . Thus, the average level of the current I LIGHT  does not vary in accordance with the duty cycle of the PWM signal  120 , which results in a failure in dimming control of the light driving circuit  100 . 
     SUMMARY 
     In one embodiment, a driving circuit for controlling a light source includes a frequency controller and a switch module. The frequency controller is operable for receiving a first dimming signal for controlling the light source to achieve a predetermined brightness, and for generating a second dimming signal having a frequency out of one or more predetermined ranges according to the first dimming signal when the frequency of the first dimming signal is within the predetermined ranges. The switch module coupled to the frequency controller is operable for switching on and off alternately to achieve the predetermined brightness of the light source according to the second dimming signal when the frequency of the first dimming signal is within the predetermined ranges and according to the first dimming signal when the frequency of the first dimming signal is out of the predetermined ranges. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of embodiments of the claimed subject matter will become apparent as the following detailed description proceeds, and upon reference to the drawings, wherein like numerals depict like parts, and in which: 
         FIG. 1  shows a block diagram of a conventional light driving circuit. 
         FIG. 2  shows a timing diagram of signals generated by the light driving circuit in  FIG. 1 . 
         FIG. 3  illustrates a block diagram of a driving circuit for controlling a light source, in accordance with one embodiment of the present invention. 
         FIG. 4  illustrates a diagram of a driving circuit for controlling a light source, in accordance with one embodiment of the present invention. 
         FIG. 5  illustrates an example of a timing diagram of signals received and generated by a frequency converter, in accordance with one embodiment of the present invention. 
         FIG. 6  illustrates an example of a frequency controller, in accordance with one embodiment of the present invention. 
         FIG. 7  illustrates another block diagram of a driving circuit for controlling a light source, in accordance with one embodiment of the present invention. 
         FIG. 8  illustrates a flowchart of operations performed by a driving circuit, in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiments of the present invention. While the invention will be described in conjunction with these 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, it will be recognized by one of ordinary skill in the art that 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 as not to unnecessarily obscure aspects of the present invention. 
     Embodiments in accordance with the present disclosure provide a driving circuit for controlling a light source, e.g., a light emitting diode (LED) string. The driving circuit includes a frequency controller and a switch module. The frequency controller receives a first dimming signal, e.g., a pulse width modulation signal, for controlling the light source to achieve a predetermined brightness. Advantageously, when the frequency of the first dimming signal is within one or more predetermined ranges, the frequency controller can generate a second dimming signal having a frequency outside the predetermined ranges according to the first dimming signal. For example, a predetermined range can be greater than a maximum frequency threshold. In addition, duty cycles of the first dimming signal and the second dimming signal are the same. 
     Therefore, the switch module can switch on and off alternately to achieve the predetermined brightness of the light source according to the second dimming signal when the frequency of the first dimming signal is within the predetermined ranges and according to the first dimming signal when the frequency of the first dimming signal is outside the predetermined ranges. Thus, the dimming control of the light source will not be affected by the frequency noise, which improves the accuracy of the driving circuit. 
       FIG. 3  illustrates a block diagram of a driving circuit  300  for controlling a light source, in accordance with one embodiment of the present invention. In one embodiment, the driving circuit  300  includes an alternating current (AC) power source  302 , an AC to direct current (DC) converter  304 , a power converter  306 , a light source  308 , a switch module  310 , a dimming module  312 , and a frequency controller  320 . The light source  308  can include one or more light source strings such as a light emitting diode (LED) string having multiple series-connected LEDs. Although one light source string is shown in the example of  FIG. 1 , other number of light source strings can be included in the light source  308 . The AC power source  302  provides an input AC voltage, e.g., a 120 volt commercial voltage supply. The AC to DC converter  304  coupled to the AC power source  302  converts the input AC voltage to a first DC voltage. The power converter  306  transforms the first DC voltage into a second DC voltage having a voltage level suitable for powering the light source  308 . The operations of the AC to DC converter  304  and the power converter  306  are further described in relation to  FIG. 4 . 
     In one embodiment, the switch module  310  includes a switch coupled to the LED string  308 , and is operable for controlling power delivered to the LED string  308  according to a dimming signal, such that the LED string  308  can achieve a predetermined brightness. More specifically, in one embodiment, the dimming signal can be a pulse signal such as a pulse width modulation (PWM) signal. When the dimming signal has a logic high level, the switch  310  is turned on. Thus, a current I LIGHT  flows through the LED string  308 , and the LED string  308  is lit up to emit light, which is referred to as an ON state of the LED string  308 . When the dimming signal has a logic low level, the switch  310  is turned off. Thus, the current I LIGHT  drops to substantially zero ampere, and the LED string  308  is cut off, which is referred to as an OFF state of the LED string  308 . When a switching frequency of the switch  310  is greater than a predetermined minimum threshold F MIN , the flicker of the LED string  308  (e.g., caused by the switching between ON and OFF states of the LED string  308 ) is imperceptible, e.g., by human eyes. In this circumstance, an average level of the current I LIGHT  can be adjusted by adjusting the duty cycle of the dimming signal, which can further determine the brightness of the LED string  308 . 
     In one embodiment, the dimming module  312  can be a signal generator operable for generating a dimming signal DIM 1 , e.g., a PWM signal, to control the power delivered to the LED string  308  to achieve the predetermined brightness. For example, a user can set the duty cycle of DIM 1  to set the predetermined brightness. 
     The frequency controller  320  coupled between the dimming module  312  and the switch  310  receives the dimming signal DIM 1  and determines whether the frequency F DIM1  of the dimming signal DIM 1  is within one or more predetermined ranges. By way of example, a predetermined range can be greater than a predetermined maximum threshold F MAX . In some circumstances, the accuracy of the dimming control may be affected by the frequency noise if the frequency F DIM1  of the dimming signal DIM 1  is within the predetermined range, e.g., greater than F MAX . The present disclosure is described in relation to the predetermined range of greater than F MAX  for illustrative purposes; however, this invention is not so limited, the one or more predetermined ranges can include other ranges such as a range of less than F 1  and/or a range of greater than F 2  but less than F 3 , where F 1 &lt;F 2 &lt;F 3 , in an alternative embodiment. 
     In one embodiment, if the frequency of the dimming signal DIM 1  is within a predetermined range, e.g., greater than F MAX , the frequency controller  320  generates a dimming signal DIM 2 , e.g., a second PWM signal, according to the dimming signal DIM 1 . The frequency F DIM2  of the dimming signal DIM 2  is different from the frequency F DIM1  of the dimming signal DIM 1 . For example, F DIM2  is less than the maximum threshold F MAX  such that F DIM2  is outside the predetermined range. Moreover, the frequency controller  320  maintains duty cycles of the dimming signal DIM 1  and the dimming signal DIM 2  to be the same. As such, the predetermined brightness of the LED string  308  can be achieved by controlling the power delivered to the LED string  308  according to the dimming signal DIM 2 . In this condition, the frequency controller  320  transfers the dimming signal DIM 2  to the switch  310 . The switch  310  controls the power delivered to the LED string  308 , e.g., by controlling the current I LIGHT , according to the dimming signal DIM 2 . 
     If the frequency of the dimming signal DIM 1  is outside the predetermined range, e.g., less than F MAX , the frequency controller  320  transfers the dimming signal DIM 1  to the switch  310 . In this condition, the switch  310  controls the power delivered to the LED string  308 , e.g., by controlling the current I LIGHT , according to the dimming signal DIM 1 . 
     Therefore, based upon the frequency F DIM1  of the dimming signal DIM 1 , the switch  310  controls the power delivered to the LED string  308  according to a dimming signal selected from at least the first dimming signal DIM 1  and the second dimming signal DIM 2 . As a result, the frequency of the dimming signal that is used to control the LED string  308  remains below the maximum threshold F MAX . As such, the current I LIGHT  flowing through the LED string  308  is not be affected by the frequency noise. For example, although the current I LIGHT  may need a delay time T DELAY  to reach a predetermined level I 1  after the switch  310  is turned on and although the duty cycle of the dimming signal may have a relatively small value, e.g., 0-5%, the time duration T ON  of the ON state of the LED string  308  can be enforced to be greater than the delay time T DELAY . Thus, the accuracy of the driving circuit  300  is improved. 
       FIG. 4  illustrates a diagram of a driving circuit  400  for controlling a light source, e.g., the LED string  308 , in accordance with one embodiment of the present invention. Elements labeled the same as in  FIG. 3  have similar functions.  FIG. 4  is described in combination with  FIG. 3 . 
     In one embodiment, the AC to DC converter  304  includes a rectifier circuit and a filter. The rectifier circuit can include, but is not limited to, a half-wave rectifier, a full-wave rectifier, or a bridge rectifier. The rectifier circuit commutates the input AC voltage to provide a first DC voltage. For example, the rectifier circuit can exclude negative waves of the input AC voltage, or converts the negative waves to corresponding positive waves. Therefore, the first DC voltage having positive voltage waves is obtained at the output of the rectifier circuit. The filter can be a low pass filter operable for filtering the first DC voltage, such that ripples of the first DC voltage can be reduced or eliminated. Alternatively, the AC power source  302  and the AC to DC converter  304  can be substituted by a DC power source. For example, the first DC voltage can be provided by a battery pack coupled to the power converter  306 . 
     The power converter  306  converts the first DC voltage to a second DC voltage suitable for powering the LED string  308 . In the example of  FIG. 4 , the power converter  306  can be a boost converter including an inductor L 1 , a diode D 1 , a capacitor C 1 , and a switch S 1 . By adjusting an on time and an off time of the switch S 1 , e.g., according to a PWM signal CP, the power converter  306  can adjust energy stored in the inductor L 1  and the capacitor C 1 . In this way, the power converter  306  generates a second DC voltage greater than the first DC voltage, in one embodiment. The second DC voltage is capable of forward biasing the LED string  308 , e.g., when the switch  310  is turned on. The power converter  306  can have other configurations, e.g., the power converter  306  can include a buck converter, a buck-boost converter, or a flyback converter, and is not limited to the example of  FIG. 4 . 
     The dimming module  312  generates the dimming signal DIM 1 . For example, the dimming signal DIM 1  can be a pulse signal such as a PWM signal, and the duty cycle of the dimming signal DIM 1  represents the predetermined brightness of the LED string  308 . The duty cycle can be set by users. The dimming signal DIM 1  is received by the frequency controller  320 . In one embodiment, the frequency controller  320  includes a frequency detector  402 , a frequency converter  404 , and a logic circuit  406 . 
     The frequency detector  402  can detect the frequency of the dimming signal DIM 1  to determine whether the frequency of the dimming signal DIM 1  is within a predetermined range, e.g., the range is F MAX  to the positive infinity (+∞). In one embodiment, the frequency detector  402  includes a counter  420  operable for measuring the frequency of the dimming signal DIM 1 . More specifically, the dimming signal DIM 1  can be clocked by (synchronized with) a predetermined sample clock signal. The predetermined sample clock signal can be a periodical square-wave signal having a fixed cycle period T CLOCK , in one embodiment. In operation, the counter  420  can count the number M of the cycles of the sample clock signal clocked during a cycle period of the dimming signal DIM 1 . The frequency F DIM1  of the dimming signal DIM 1  is obtained according to the number M and the cycle period T CLOCK  of the sample clock signal, which can be given by:
 
 F   DIM1 =1/( M*T   CLOCK ).  (1)
 
     Furthermore, the frequency detector  402  can include a comparator  422  operable for comparing the detected frequency F DIM1  to one or more predetermined thresholds so as to determine whether the frequency F DIM1  is within the predetermined range. In one embodiment, the comparator  422  compares the frequency F DIM1  to the predetermined maximum threshold F MAX . If the frequency F DIM1  is greater than F MAX , it indicates that the frequency F DIM1  is within the predetermined range. Thus, the comparator  422  transfers the dimming signal DIM 1  to the frequency converter  404 . If the frequency F DIM1  is less than F MAX , it indicates that the frequency F DIM1  is outside the predetermined range. Thus, the comparator  422  transfers the dimming signal DIM 1  to the logic circuit  406 . The logic circuit  406  further transfers the dimming signal DIM 1  to the switch  310 . The switch  310  can adjust the current I LIGHT  through the LED string  308  accordingly. The frequency detector  402  can include other components and is not limited to the configuration in the example of  FIG. 4 . 
     The frequency converter  404  is operable for generating the dimming signal DIM 2  according to the dimming signal DIM 1 . In one embodiment, the frequency converter  404  varies the frequency F DIM1  and maintains the duty cycle D DIM1  to generate the dimming signal DIM 2 . The dimming signal DIM 2  has a frequency F DIM2  and a duty cycle D DIM2 . The frequency F DIM2  is less than F MAX  and outside the predetermined range. The duty cycle D DIM2  is the same as the duty cycle D DIM1  of the dimming signal DIM 1 . As such, the predetermined brightness indicated by the dimming signal DIM 1  is also indicated by the dimming signal DIM 2 . 
     More specifically, the frequency converter  404  can employ a first sample clock signal and a second sample clock signal to generate the dimming signal DIM 2  whose frequency is a fraction of that of the dimming signal DIM 1 . In one embodiment, both the first sample clock signal and the second sample clock signal can be periodical square-wave signals with fixed frequencies. A frequency of the second sample clock signal, e.g., F CLOCK2 , is a fraction of a frequency of the first sample clock signal e.g., F CLOCK1 , which can be given by:
 
 F   CLOCK2 =(1 /N )* F   CLOCK1 .  (2)
 
The frequency converter  404  counts the first sample clock signal to obtain result data indicating the cycle period and the duty cycle of DIM 1 , and then uses the result data and the second sample clock signal to generate the dimming signal DIM 2 .
 
     In the example of  FIG. 4 , the frequency converter  404  includes a multiplexer  414 , and one or more count modules such as a count module  410  and a count module  412 . In one embodiment, when one count module is used to detect the duty cycle and cycle period of the dimming signal DIM 1 , the other count module is used to determine the duty cycle and cycle period of the dimming signal DIM 2 . In one embodiment, each of the count modules  410  and  412  includes a period counter and a duty counter. When a corresponding count module, e.g., the count modules  410 , is working to detect the dimming signal DIM 1 , the period counter in the count modules  410  can count the number N 1 A of the cycles of the first sample clock signal clocked during a cycle period of the dimming signal DIM 1 . In this way, the period counter obtains period data indicative of the cycle period of the dimming signal DIM 1 . Moreover, the duty counter can count the number N 1 B of the cycles of the first sample clock signal clocked during a time period T STATE1  when the dimming signal DIM 1  has a predetermined state (e.g., a logic high level or a logic low level) in one cycle period of the dimming signal DIM 1 . In this way, the duty counter obtains duty data indicative of the duty cycle of the dimming signal DIM 1 . For example, when the time period T STATE1  represents the logic high level of the dimming signal DIM 1 , the duty data indicative of the duty cycle D DIM1  of DIM 1  can be obtained according to a combination of N 1 A and N 1 B, e.g., D DIM1 =N 1 B/N 1 A. When the time period T STATE1  represents the logic low level of the dimming signal DIM 1 , the duty data indicative of the duty cycle D DIM1  of DIM 1  can be obtained according to a combination of N 1 A and N 1 B, e.g., D DIM1 =1−(N 1 B/N 1 A). As such, the result data including the period data and the duty data is obtained. The operation of the count module for detecting the dimming signal DIM 1  is further described in relation to  FIG. 5 . 
     When a corresponding count module, e.g., the count module  412 , is working to generate the dimming signal DIM 2 , the period counter in the count modules  412  can determine the cycle period T DIM2  of the dimming signal DIM 2  by counting the number of the cycles of the second sample clock signal according to the period data, e.g., the number N 1 A. For example, T DIM2  is equal to N 1 A times the cycle period of the second sample clock signal. Moreover, the duty counter in the count modules  412  can determine the duty cycle of the dimming signal DIM 2  by counting the number of the cycles of the second sample clock signal according to the duty data. For example, the time duration T STATE2  of a corresponding predetermined state (e.g., a logic high level or a logic low level) of DIM 2  is equal to N 1 B times the cycle period of the second sample clock signal. The duty cycle of the dimming signal DIM 2  is given by, e.g., D DIM2 =T STATE2 /T DIM2  (when the time period T STATE2  represents the logic high level of the dimming signal DIM 2 ) or D DIM2 =1−(T STATE2 /T DIM2 ) (when the time period T STATE2  represents the logic low level of the dimming signal DIM 2 ). The operation of the count module for generating the dimming signal DIM 2  is further described in relation to  FIG. 5 . 
     As a result, both T DIM1  and T STATE1  of the dimming signal DIM 1  are multiplied by the same number N to obtain T DIM2  and T STATE2  of the dimming signal DIM 2 , where N is determined according to equation (2). Thus, the frequency F DIM2  is a fraction of the frequency F DIM1 , which can be given by:
 
 F   DIM2 =(1 /N )* F   DIM1 .  (3)
 
As shown in equation (3), the fraction 1/N is also determined by a ratio of the frequency of the second sample clock signal to the frequency of the first sample clock signal obtained from equation (2). In addition, the duty cycle D DIM2  can be the same as the duty cycle D DIM1  according to equation (4).
 
 D   DIM2   =T   STATE2   /T   DIM2 =( N*T   STATE1 )/( N*T   DIM1 )= T   STATE1   /T   DIM1   =D   DIM1 .  (4)
 
       FIG. 5  illustrates an example of a timing diagram  500  of signals received and generated by the frequency converter  404  in  FIG. 4 , in accordance with one embodiment of the present invention. In the example of  FIG. 5 , the timing diagram  500  shows the dimming signal DIM 1 , the first sample clock signal SIGNAL 1 , the dimming signal DIM 2 , and the second sample clock signal SIGNAL 2 . In addition, the frequency F CLOCK2  of SIGNAL 2  is a fraction 1/N of the frequency F CLOCK1  of SIGNAL 1 . For example, in  FIG. 5 , F CLOCK2  is ½ of F CLOCK1 . 
     During the time interval from t 1  to t 7 , one or more corresponding count modules perform counting operation to obtain the result data. At time t 1 , the corresponding count module counts the number of cycles of the first sample clock signal SIGNAL 1 . As shown in the example of  FIG. 5 , 5 cycles of the first sample clock signal SIGNAL 1  is clocked during a cycle period of the dimming signal DIM 1 , e.g., from t 1  to t 3  or from t 3  to t 5 . As such, the period counter obtains the period data  5 . Furthermore, 2 cycles of the first sample clock signal SIGNAL 1  is clocked during a time duration when the dimming signal DIM 1  has a logic high level in one cycle period of the dimming signal DIM 1 , e.g., from t 1  to t 2 , from t 3  to t 4 , or from t 5  to t 6 . Accordingly, the duty data indicative of the duty cycle of the dimming signal DIM 1  is 40%. 
     During the time interval from t 1 ′ to t 6 ′, one or more count modules use the result data (including the period data and the duty data) and the second sample clock signal SIGNAL 2  to generate the dimming signal DIM 2 . As shown in the example of  FIG. 5 , the cycle period of the dimming signal DIM 2  is equal to 5 times the cycle period of the second sample clock signal SIGNAL 2 , e.g., from t 1 ′ to t 3 ′ or from t 3 ′ to t 5 ′. Moreover, a time duration of the logic high level of the dimming signal DIM 2  is equal to 2 times the cycle period of the second sample clock signal SIGNAL 2 , e.g., from t 1 ′ to t 2 ′, from t 3 ′ to t 4 ′, or from t 5 ′ to t 6 ′. As such, the duty cycle of the dimming signal DIM 2  is also 40%. 
     As such, to generate the dimming signal DIM 2 , both the cycle period of the dimming signal DIM 1  and the time duration of the high electrical level of DIM 1  are multiplied by the same predetermined number N (e.g., N=2 in  FIG. 5 ). The predetermined number N is determined by the signals SIGNAL 1  and SIGNAL 2  according to equation (2). As a result, the frequency of the dimming signal DIM 2  is a fraction (1/N) of the frequency of the dimming signal DIM 1 . 
     In one embodiment, the signals SIGNAL 1  and SIGNAL 2  can have fixed frequencies that are predetermined or programmed by a user. For example, the user can set the ratio N to a substantially constant value. Alternatively, the signals SIGNAL 1  and SIGNAL 2  can be generated by a signal generator, in which the ratio N or the fraction 1/N is determined according to the frequency F DIM1  of the dimming signal DIM 1 . In other words, the ratio N can vary in accordance with the frequency F DIM1 . For example, if the frequency F DIM1  of the dimming signal DIM 1  is greater than F MAX  and is less than F 1 , e.g., F MAX &lt;F DIM1 &lt;F 1 , the ratio N is equal to N 1 . If the frequency F DIM1  of the dimming signal DIM 1  is greater than F 1 , the ratio N is equal to N 2 , where N 2  is greater than N 1 . 
     Referring to  FIG. 4  and  FIG. 5 , the count modules  410  and  412  can alternately count the number of cycles of the first sample clock signal SIGNAL 1  to obtain the result data and count the number of cycles of the second sample clock signal SIGNAL 2  according to the result data to generate the dimming signal DIM 2 , in one embodiment. By way of example, the count module  410  detects the dimming signal DIM 1  by counting the cycles of the first sample clock signal SIGNAL 1  from time t 1  to t 3 . At time t 3 , the count module  410  obtains the period data and the duty data. Then, the count module  410  generates the dimming signal DIM 2  by counting the number of cycles of the second sample clock signal SIGNAL 2  from time t 1 ′ to t 3 ′. In this instance, the time t 1 ′ corresponds to the time t 3 , and the time t 3 ′ corresponds to the time t 7 . Thus, at time t 3  or t 1 ′, the count module  412  starts to detect the dimming signal DIM 1  by counting the number of cycles of the first sample clock signal SIGNAL 1 . Similarly, the count module  412  obtains the period data and the duty data at time t 5 . After the count module  410  completes generating the dimming signal DIM 2  at time t 3 ′ or t 7 , the count module  410  goes back to detect the dimming signal DIM 1 , and the count module  412  starts to generate the dimming signal DIM 2 . In this way, the dimming signal DIM 2  can be a continuous PWM signal. 
     The multiplexer  414  transfers the dimming signal DIM 2  generated by the count module  410  or the count module  412  to the logic circuit  406 . The logic circuit  406  further transfers the dimming signal DIM 2  whose frequency is outside the predetermined range to the switch  310 . 
       FIG. 6  illustrates another example of the frequency controller  320 , in accordance with one embodiment of the present invention. Elements labeled the same as in  FIG. 4  have similar functions.  FIG. 6  is described in combination with  FIG. 3-FIG .  5 . 
     In the example of  FIG. 6 , the frequency converter  404  includes a count module  510 , a register  514 , and a count module  512 . The count module  510  is operable for detecting the dimming signal DIM 1  by counting the cycles of the first sample clock signal SIGNAL 1 , e.g., from time t 1  to t 7  in  FIG. 5 , and can store the result data including the period data and the duty data in the register  514  coupled to the count module  510 . The count module  512  coupled to the register  514  is operable for reading the result data, and for generating the dimming signal DIM 2  by counting the cycles of the second sample clock signal SIGNAL 2  accordingly, e.g., from t 1 ′ to t 6 ′ in  FIG. 5 . As such, in this instance, the time t 1 ′ corresponds to the time t 1 , and the time t 3 ′ corresponds to the time t 5 . 
     The frequency controller  320  can have other configurations, and is not limited to the example in  FIG. 4  and  FIG. 6 . In another embodiment, the count module  510  can be removed from the frequency controller  320  and the frequency detector  402  can be designed with the functional features of the count module  510 . For example, the frequency detector  402  can detect the frequency and the duty cycle of the dimming signal DIM 1  by counting the first sample clock signal SIGNAL 1 . If the detected frequency of the dimming signal DIM 1  is greater than F MAX , the frequency detector  402  can store the period data and the duty data in the register  514 . The count module  512  uses the second sample clock signal SIGNAL 2  and the result data to generate the dimming signal DIM 2 , which is further forwarded to the logic circuit  406 . If the frequency of the dimming signal DIM 1  is less than F MAX , the frequency detector  402  transfers the dimming signal DIM 1  to the logic circuit  406 . 
       FIG. 7  illustrates another block diagram of a driving circuit  700  for controlling a light source, in accordance with one embodiment of the present invention. Elements labeled the same as in  FIG. 3  and  FIG. 4  have similar functions.  FIG. 7  is described in combination with  FIG. 3 ,  FIG. 4  and  FIG. 6 . In the example of  FIG. 7 , the driving circuit  700  includes an AC power source  302 , an AC to DC converter  304 , a power converter  306 , a light source  308 , a switch module  310 , a dimming module  312 , and a controller  702 . The controller  702  coupled to the switch module  310  and the power converter  306  can be integrated in an integrated circuit (IC) chip and is used to control the dimming of the light source  308  by controlling the switch module  310  and the power converter  306 . 
     In one embodiment, the controller  702  includes a frequency controller  320 , a converter controller  704 , and a logic module  706 . The frequency controller  320  employs similar configurations as disclosed in relation to  FIG. 4  and  FIG. 6 . Thus, the controller  702  is capable of turning on and off the switch module  310  according to a selected dimming signal DIM 1 /DIM 2  to control the current flowing through the light source  308 , thereby achieving the predetermined brightness of the light source  308 . The selected dimming signal is DIM 1  when the frequency F DIM1  of DIM 1  is outside the predetermined range, e.g., less than F MAX , and is DIM 2  when the frequency F DIM1  is within the predetermined range, e.g., greater than F MAX . 
     The converter controller  704  is operable for generating the PWM signal CP to drive the power converter  306 . The logic module  706  coupled to the converter controller  704  and the frequency controller  320  is operable for detecting the selected dimming signal, e.g., DIM 1 /DIM 2 , to obtain the switching condition of the switch module  310  and for controlling the power converter  306  accordingly. More specifically, in one embodiment, when the selected dimming signal indicates that the switch module  310  is turned on, the logic module  706  transfers the PWM signal CP to the power converter  306 . Then, the power converter  306  adjusts energy stored in the inductor L 1  and the capacitor C 1  by adjusting an on time and an off time of the switch S 1  according to the PWM signal CP, as mentioned in relation to  FIG. 4 . Thus, the first DC voltage is converted to the second DC voltage to forward bias the LED string  308 . 
     When the selected dimming signal indicates the switch module  310  is turned off, the current I LIGHT  drops to the substantially zero ampere. Then, the logic module  706  transfers a termination signal (e.g., a logic one signal instead of the PWM signal CP) to the switch S 1 , in order to terminate the operation of the power converter  306 . For example, the switch S 1  maintains on according to the logic one signal, such that the energy stored in the inductor L 1  and the capacitor C 1  is dissipated. In this way, the power converter  306  stops converting the first DC voltage to the second DC voltage. Moreover, the power converter  306  no longer consumes energy from the AC power source  302 , which reduces the power consumption of the driving circuit  700 . 
     In conclusion, the power converter  306  operates to provide the second DC voltage to drive the light source  308  when the switch module  310  is turned on, and stops operating when the switch module  310  is turned off. As such, the power efficiency of the driving circuit  700  is improved. 
       FIG. 8  illustrates a flowchart  800  of operations performed by a driving circuit, e.g., the driving circuit  300 ,  400  or  700 , in accordance with one embodiment of the present invention.  FIG. 8  is described in combination with  FIG. 3-FIG .  7 . Although specific steps are disclosed in  FIG. 8 , such steps are examples. That is, the present invention is well suited to performing various other steps or variations of the steps recited in  FIG. 8 . 
     In block  802 , a first dimming signal, e.g., the dimming signal DIM 1 , for controlling a light source to achieve a predetermined brightness is received. 
     In block  804 , the first dimming signal is detected to determine whether the frequency of the first dimming signal, e.g., the frequency F DIM1 , is within one or more predetermined ranges, e.g., greater than F MAX . If the frequency of the first dimming signal is out of the predetermined ranges, the flowchart  800  goes to block  806 . In block  806 , the light source is controlled to achieve the predetermined brightness according to the first dimming signal. If the frequency of the first dimming signal is within the predetermined ranges, the flowchart  800  goes to block  808 . 
     In block  808 , a second dimming signal, e.g., the dimming signal DIM 2 , having a frequency out of the predetermined ranges is generated according to the first dimming signal. In one embodiment, both the first dimming signal and the second dimming signal include PWM signals. Duty cycles of the first dimming signal and the second dimming signal are maintained to be the same. In one embodiment, to generate the second dimming signal, both a cycle period of the first dimming signal and a TON period of the first dimming signal are multiplied by the same number. In one embodiment, the number is adjustable according to the frequency of the first dimming signal. In one embodiment, the number of cycles of a first sample clock signal, e.g., the first sample clock signal SIGNAL 1 , is counted to obtain the result data indicative of the cycle period and the duty cycle of the first dimming signal. The number of cycles of a second sample clock signal, e.g., the second sample clock signal SIGNAL 2 , is counted according to the result data to generate the second dimming signal. The frequency of the first dimming signal is a fraction of the frequency of the second dimming signal. The fraction is determined by a ratio of the frequency of the first sample clock signal to the frequency of the second sample clock signal. 
     In block  810 , the light source is controlled to achieve the predetermined brightness according to the second dimming signal. 
     While the foregoing description and drawings represent embodiments of the present invention, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the principles of the present invention as defined in the accompanying claims. One skilled in the art will appreciate that the invention may be used with many modifications of form, structure, arrangement, proportions, materials, elements, and components and otherwise, used in the practice of the invention, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims and their legal equivalents, and not limited to the foregoing description.