Patent Publication Number: US-2007114949-A1

Title: Pwm illumination control circuit with low visual noise for driving led

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a divisional application of U.S. application Ser. No. 10/708,212 filed on Feb. 17, 2003 which claims the priority benefit of Taiwan application serial no. 92134517, filed on Dec. 8, 2003. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an illumination control circuit. More particularly, the present invention relates to a pulse width modulation (PWM) illumination control circuit with low visual noise for driving a light-emitting diode (LED).  
      2. Description of the Related Art  
      In recent years, conventional cathode ray tubes (CRT) are gradually being replaced by liquid crystal displays (LCD) due to big improvements in the semiconductor manufacturing techniques. LCD has many advantages over CRT including lower power consumption, a lighter weight, a higher resolution, higher degree of color saturation and a longer service life. For these advantages, LCD is being widely used in many electronic products including digital cameras, notebook computers, desktop monitors, mobile phones, personal digital assistants (PDA), car television, global positioning systems (GPS), palm-top game player, electronic translators and even digital watches and so on.  
      In general, a liquid crystal display uses an array of light-emitting diodes (LED) driven by a simple driving circuit to serve as the light source. However, due to the special properties of an LED, brightness of the LED is not linearly related to the driving current. Furthermore, color of the LED may also vary according to the driving current. Hence, for a liquid crystal display that uses LED as a back light or illumination system, difficulties are often encountered when the illumination is varied by directly adjusting the driving current.  
      To avoid the difficulties of illumination adjustment through an amplitude variation of the driving current, a constant amplitude driving current is used with illumination adjustment achieved through a pulse width modulation (PWM) of the driving current. Ultimately, the LED is able to produce consistent emitting efficiency within a broad range.  
       FIG. 1  is a block diagram of a conventional pulse width modulation illumination control circuit.  FIG. 2  is a diagram showing the relationships between illumination control pulse signals and light-emitting diode driving current signals for the circuit in  FIG. 1 . In  FIG. 1 , an illumination control pulse signal Con that set the illumination of the light-emitting diode is sent to a DC/DC converter  110  to produce a light-emitting diode driving current signal Id for driving a light-emitting diode. The waveform diagrams (a), (b) and (c) in  FIG. 2  represent three different pulse width settings of the light-emitting diode driving current signals Id. For example, the light-emitting diode is at full illumination (100%) in  FIG. 2 ( a ), at 20% of the full illumination in  FIG. 2 ( b ) and at 50% of the full illumination in  FIG. 2 ( c ).  
      To prevent any perceived flickering in the light-emitting diode by the human eyes, the frequency of the illumination control pulse signal Con cannot be too low, typically above 200 Hz. In other words, the illumination control pulse signal Con must operate at a sufficiently high frequency so that the human eyes can retain a visual image and yet perceive a steady change of illumination without flickering. Obviously, these control signals may be implemented using a simple switching circuit that controls the on/off states of the entire DC/DC converter.  
      Because the frequency and duty cycle of the aforementioned illumination control pulse signal Con is set to be fixed according to the required illumination, interference with the vertical, horizontal scanning signals may occur when used as back light in a liquid crystal display. The difference in frequency between the back light and the video signals often leads to the so-called ‘fanning effect’, a watery wave pattern of the image on a display screen. In addition, the switching on or off of the DC/DC converter also leads to a significant loading on the power supply that provides power to the DC/DC converter. In other words, a ripple waveform synchronized with the illumination control pulse signal Con is also produced in the power supply. Once again, the ripple waveform may affect the video display signals leading to a flickering screen.  
      To prevent interference between the illumination control pulse signal Con and the vertical, horizontal scanning signal due to the frequency difference, the illumination control pulse signal Con and the horizontal scanning signals are synchronized to a frequency an integral multiple of each other. However, this arrangement will increase the production cost. To reduce the ripple waveform in the power supply, the frequency of the illumination control pulse signal Con can be increased. Yet, increasing the frequency of pulse signal Con leads to higher power consumption. With the demand of a larger display screen and a lesser visual noise, fabricating a light-emitting diode illuminated liquid crystal display with low noise and broad adjustable range of illumination is increasingly difficult.  
     SUMMARY OF THE INVENTION  
      Accordingly, one objective of the present invention is to provide a pulse width modulation (PWM) illumination control circuit with low visual noise for driving a light-emitting diode (LED). By varying the duty cycle or frequency of an illumination control pulse signal and maintaining an average duty cycle and frequency, visual noise interference due to pulse width modulation is reduced.  
      To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a low visual noise pulse width modulation illumination control circuit for controlling the illumination of light-emitting diodes inside a liquid crystal display. The low visual noise pulse width modulation illumination control circuit comprises an illumination control pulse generating unit and a DC/DC converter. The illumination control pulse-generating unit receives an illumination-adjusting signal. According to the illumination-adjusting signal, the illumination control pulse-generating unit generates an illumination control pulse signal having a duty cycle set to vary within a predetermined range. The DC/DC converter is coupled to the illumination control pulse-generating unit so that the illumination control pulse-generating unit can drive the light-emitting diodes according to the illumination control pulse signal.  
      In one embodiment of the invention, the illumination control pulse-generating unit of the low visual noise PWM illumination control circuit further comprises a noise generator, an analogue adder and a comparator. The noise generator generates a noise signal. The analogue adder is coupled to the noise generator for receiving the illumination-adjusting signal and the noise signal to produce a noise signal loaded illumination-adjusting signal. The comparator is coupled to the analogue adder for comparing the noise signal loaded illumination-adjusting signal with a triangular wave and producing the illumination control pulse signal.  
      In one embodiment of the invention, the noise signal level produced by the low visual noise PWM illumination control circuit can be adjusted.  
      The present invention also provides an alternative low visual noise pulse width modulation illumination control circuit for controlling the illumination of light-emitting diodes inside a liquid crystal display. The low visual noise pulse width modulation illumination control circuit comprises an illumination control pulse generating unit and a DC/DC converter. The illumination control pulse-generating unit receives an illumination-adjusting signal. According to the illumination-adjusting signal, the illumination control pulse-generating unit generates an illumination control pulse signal having a frequency set to vary within a predetermined range. The DC/DC converter is coupled to the illumination control pulse-generating unit so that the illumination control pulse-generating unit can drive the light-emitting diodes according to the illumination control pulse signal.  
      In one embodiment of the invention, the operations carried out by the illumination control pulse-generating unit of the low visual noise PWM illumination control circuit are implemented using a microprocessor.  
      In one embodiment of the invention, the phase of the illumination control pulse signal produced by the low visual noise PWM illumination control circuit also varies within a predetermined range.  
      Accordingly, the present invention provides a low visual noise PWM illumination control circuit for driving light-emitting diodes such that visual noise interference due to pulse width modulation is reduced by varying the duty cycle or frequency of an illumination control pulse signal and maintaining an average duty cycle and frequency. 
    
    
      It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.  
     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 following drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
       FIG. 1  is a block diagram of a conventional pulse width modulation illumination control circuit.  
       FIG. 2  is a diagram showing the relationships between illumination control pulse signals and light-emitting diode driving current signals for the circuit in  FIG. 1 .  
       FIG. 3  is a block diagram of a light-emitting diode low visual noise PWM illumination control circuit according to one preferred embodiment of the present invention.  
       FIG. 4  is a circuit diagram of an illumination control pulse-generating unit according to the preferred embodiment of the present invention.  
       FIG. 5  is a diagram showing the waveform of the illumination control pulse signal produced by the illumination control pulse-generating unit shown in  FIG. 4 .  
       FIG. 6  is a flow chart showing the steps for operating the illumination control pulse-generating unit according to the preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Reference will now be made in detail to the present preferred 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. 3  is a block diagram of a light-emitting diode low visual noise PWM illumination control circuit according to one preferred embodiment of the present invention. The low visual noise pulse width modulation (PWM) illumination control circuit  300  in  FIG. 3  is adapted to control the illumination level of light-emitting diodes (not shown) inside a liquid crystal display. The low visual noise PWM illumination control circuit  300  comprises an illumination control pulse-generating unit  310  and a DC/DC converter  320 . The illumination control pulse-generating unit  310  is used for receiving an illumination-adjusting signal Ref. According to the illumination-adjusting signal Ref, the illumination control pulse-generating unit  310  generates an illumination control pulse signal Con. To improve the visual noise interference of pulse width modulation, the duty cycle or frequency of the illumination control pulse signal Con is permitted to vary within a predetermined range. Hence, differential frequency interference between a fixed illumination control pulse signal Con and the vertical/horizontal scanning signal leading to the so-called ‘fanning effect’ with wavy lines on the display screen is prevented. In addition, the DC/DC converter  320  drives the light-emitting diodes according to the illumination control pulse signal Con generated by the illumination control pulse-generating unit  310 .  
       FIG. 4  is a circuit diagram of an illumination control pulse-generating unit according to the preferred embodiment of the present invention. As shown in  FIG. 4 , the illumination control pulse-generating unit  400  comprises a noise generator  410 , an analogue adder  420  and a comparator  430 . The noise generator  410  further comprises a resistor  411  and an amplifier  421  electrically connected together and the analogue adder  420  further comprises a plurality of resistors  422 ,  423 ,  425  and an amplifier  421  electrically connected together. The noise generator  410  outputs a noise signal Nos after the amplifier  412  inside the noise generator  410  amplifies the thermal noise produced by the resistor  411 . The noise signal Nos is transmitted to the analogue adder circuit  420  such that the noise signal Nos and an illumination-adjusting signal Ref originally set to control the output duty cycle of the DC voltage are summed together to produce a noise signal loaded illumination-adjusting signal Ref. The resistor  422  is a variable resistor so that the level of the noise signal Nos loaded on the illumination-adjusting signal Ref can be adjusted. The noise signal loaded illumination-adjusting signal Ref is transmitted to the comparator  430  where the signal is compared with a triangular wave Tri to produce an illumination control pulse signal Con having a duty cycle that varies within the acceptable noise signal level as shown in  FIG. 5 .  
      As shown in  FIG. 5 , although the duty cycle of the illumination control pulse signal Con varies on each transient moment of each cycle, the average power of the noise is zero. Hence, the average duty cycle of the entire circuit after adding the noise is identical to one without adding any noise. In other words, the illumination of the light-emitting diodes after adding noise to the circuit is identical to the illumination without adding any noise to the circuit.  
       FIG. 6  is a flow chart showing the steps for operating the illumination control pulse-generating unit according to the present invention. When the illumination control pulse-generating unit  310  as shown in  FIG. 3  is implemented using a microprocessor, the steps in  FIG. 6  can be carried out to produce an illumination control pulse signal Con with a variable frequency so that visual noise interference due to pulse width modulation is reduced.  
      Assuming that the illumination control pulse signal Con in  FIG. 3  has a frequency F=1/T, where T is the cycle of the illumination control pulse signal Con, n illumination control pulse signals Con with different cycle time such as T0, T1, T2, . . . , Tn−1 such that (T0+T1+T2+ . . . +Tn−1)/n=T can be designed. Furthermore, the n illumination control pulse signals Con with different cycle time can be permuted to form a queue before turning each signal out sequentially. For example, if sequence 0 is {T0, T1, T2, . . . , Tn−1}, sequence 1 is {T0, T2, . . . ,} and so on, the n illumination control pulse signals Con with different cycle time may be arranged to form a list of K different non-repeating sequence including sequence 0, sequence 1, sequence 2, . . . , sequence K−1. Thereafter, the steps depicted in  FIG. 6  can be executed using the microprocessor so that illumination control pulse signals Con each having a different frequency are sequentially output. The operating steps of a digitally operated illumination control pulse-generating unit with a low visual noise level are explained as follows.  
      In step S 610 , variables I, J are set to 0. Thereafter, in step S 620 , the I th  illumination control pulse signal cycle in sequence J and the received illumination-adjusting signal are combined to produce an illumination control pulse signal. In step S 630 , a 1 is added to the variable I in preparation for retrieving the next illumination control pulse signal cycle in sequence J. In step S 640 , the value of I is checked to determine whether it is equal to n. When the value of I is not equal to n, the operation returns to step S 620 . However, if the value of I is equal to n, step S 650  is executed to reset I to 0 and add 1 to the value of J in preparation for retrieving the first illumination control pulse signal cycle of the next sequence. Thereafter, step S 660  is executed to determine whether the value of J is equal to K. When the value of J is not equal to K, the operation returns to step S 620 . On the other hand, if the value of J is equal to K, step S 670  is executed to reset the value of J to 0 and return the operation to step S 620 .  
      The steps carried out in aforementioned description assumes the existence of K sequences. However, anyone familiar with the technique may understand that the operation is greatly simplified when K is 1. In addition, the phase of the illumination control pulse signal generated in step S 620  can be set to vary within a predetermined range so that an illumination control pulse signal with a wider frequency range is produced.  
      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 cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.