Abstract:
Techniques pertaining to driving LEDs on multiple branches of a circuit are disclosed. According to one aspect of the present invention, an LED drive circuit includes a boost circuit configured for receiving an input voltage and providing an output voltage according to a control signal, a selector configured for alternatively selecting one of the feedback signals as an output feedback signal and switching on a corresponding branch circuit, and a pulse-width modulation (PWM) controller configured for generating a pulse-width modulation signal as the control signal for the boost circuit according to the output feedback signal of the selector, essentially to match respective currents in the multiple branches.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is related to the area of\LED (light-emitting diode) illumination, more particularly, to techniques for driving LEDs on multiple branches of a circuit. 
         [0003]    2. Description of Related Art 
         [0004]    In a prior art system, an LED drive IC with inductor-based step-up topology can drive only LEDs on one branch circuit, where all LEDs are serially connected to maintain the same current flowing through. In this case, the current difference among all LEDs is zero and current matching between LEDs is perfect. 
         [0005]    Nowadays, with display screen size increasing, more and more LEDs are needed to provide uniform backlight for a LCD screen. Some conventional drive ICs are developed to drive WLEDs (white light-emitting diode) on several branch circuits by neglecting the difference of the forward-bias voltage between WLEDs, as shown in  FIG. 1 . If all WLEDs have exactly the same forward-bias voltage with the same current, the current through each branch circuit will be matched. Unfortunately, it is not the case in reality. With 3˜5 serially connected WLED in one branch, the total difference of voltage drop under a certain current will be big, sometimes up to 1V. It will lead to serious current mismatch between WLED branches if every branch is driven by the same voltage supply. 
         [0006]    Some conventional design employs an internal current regulation loop to realize matching among LED branches as shown in  FIG. 2 . However, this current regulation circuit consumes additional power and leads to efficiency loss. To realize the matching, the difference of LED forward-bias voltage between two branches must be consumed on two NMOS field effect transistors MN 1  or MN 2 . This power consumption will be transferred to heat. Sometimes this heat is so high and could cause IC reliability problems. Furthermore, this additional current regulation circuit introduces stability issues as well, needing much more design efforts. 
         [0007]    Thus, improved techniques are needed for driving multiple branches of LEDs having matched currents going therethrough. 
       SUMMARY OF THE INVENTION 
       [0008]    This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments. Simplifications or omissions in this section as well as in the abstract or the title of this description may be made to avoid obscuring the purpose of this section, the abstract and the title. Such simplifications or omissions are not intended to limit the scope of the present invention. 
         [0009]    In general, the present invention is related to techniques for driving LEDs on multiple branches of a circuit. According to one aspect of the present invention, an LED drive circuit includes a boost circuit configured for receiving an input voltage and providing an output voltage according to a control signal, a selector configured for alternatively selecting one of the feedback signals as an output feedback signal and switching on the corresponding branch circuit, and a pulse-width modulation (PWM) controller configured for generating a pulse-width modulation signal as the control signal for the boost circuit according to the output feedback signal of the selector, essentially to match respective currents in the multiple branches. 
         [0010]    The present invention may be used to drive LEDs used for illumination. Depending on implementation, the present invention may be implemented as a method, a circuit and a part of a system. According to one embodiment, the present invention is a method for driving LEDs on multiple branches of a circuit, the method comprises: generating an output voltage by regulating an input voltage according to a control signal; sampling a current in each branch of LEDs to produce a feedback signal; alternatively selecting one from the feedback signals of the multi-branches as an output feedback signal and switching on corresponding branch of LEDs; and generating a pulse-width modulation signal as the control signal according to the output feedback signal. 
         [0011]    Other objects, features, and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0013]      FIG. 1  is a block diagram showing one conventional LED drive circuit; 
           [0014]      FIG. 2  is a block diagram showing another conventional LED drive circuit; 
           [0015]      FIG. 3  is a block diagram showing a multi-branch LED drive circuit according to one embodiment of the present invention; 
           [0016]      FIG. 4  is a circuit diagram showing a feedback signal selector of the multi-branch LED drive circuit shown in  FIG. 3 ; 
           [0017]      FIG. 5  is a circuit diagram showing a pulse-width modulation controller of the multi-branch LED drive circuit shown in  FIG. 3 ; 
           [0018]      FIG. 6  is a timing diagram showing an operation of the feedback signal selector shown in  FIG. 4 ; and 
           [0019]      FIG. 7A  and  FIG. 7B  are a timing diagram showing an operation of the pulse-width modulation controller shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The detailed description of the present invention is presented largely in terms of procedures, steps, logic blocks, processing, or other symbolic representations that directly or indirectly resemble the operations of devices or systems contemplated in the present invention. These descriptions and representations are typically used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. 
         [0021]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the order of blocks in process flowcharts or diagrams or the use of sequence numbers representing one or more embodiments of the invention do not inherently indicate any particular order nor imply any limitations in the invention. 
         [0022]    Embodiments of the present invention are discussed herein with reference to  FIGS. 3-7 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only as the invention extends beyond these limited embodiments. 
         [0023]    Referring to  FIG. 3 , there shows a diagram  100  in which a LED drive circuit is employed to drive LEDs on two branches  20  according to one embodiment of the present invention. The LED drive circuit comprises a boost circuit  10 , a feedback signal selector  30  and a pulse-width modulation (PWM) controller  40  to drive a plurality of LEDs on multiple branches  20  (only two are shown as an example). 
         [0024]    The boost circuit  10  is configured for receiving an input voltage VIN and providing an output voltage VBST according to a control signal from the PWM controller  40 . Each of the LED branches is connected between the output voltage VBST and a ground reference and comprises a plurality of LEDs D 11 -D 14  or D 21 -D 24 . A feedback resistor R 1  or R 2 , and a branch switch MN 1  or MN 2  aew connected in series. Each LED branch provides a feedback signal FB 1  or FB 2  representative of current I 1  or I 2  flowing therein via the feedback resistor R 1  or R 2 . Each LED branch selectively turns on or off when a switch signal is applied on the branch switch MN 1  or MN 2 . The feedback signal selector  30  is configured for alternatively selecting one from the two feedback signals FB 1  and FB 2  as an output feedback signal FB and providing a switch-on signal to the branch switch MN 1  or MN 2  of a corresponding branch simultaneously. The PWM controller  40  is configured for generating a PWM signal according to the output feedback signal FB of the feedback signal selector  30  and outputting the PWM signal as the control signal to the boost circuit  10 . 
         [0025]    In operation, the multi-branch LED drive circuit can regulate a current in each LED branch. To take the two branches as an example, at one time, the first branch is turned on and the second branch is off. The LED current I 1  in the first branch is accurately regulated to be Vref/R 1 . In another time, the first branch is turned off and the second branch is turned on. The current I 2  in the second branch is accurately regulated to be Vref/R 2 . The values of the feedback resistors R 1  and R 2  can be accurately matched using either on-chip resistors or external resistors. The matching between resistors can be easily controlled within 0.1%. Thus, the currents in the multi-branches are matched accurately. 
         [0026]    The boost circuit  10  includes an inductor L 1 , a diode D 1 , a capacitor C 1  and a boost switch. The inductor L 1 , the diode D 1 , the capacitor C 1  are connected in series between the input voltage VIN and the ground reference, wherein a first terminal of the inductor L 1  is coupled to the input voltage VIN, a positive terminal of the diode D 1  is coupled to a second terminal of the inductor L 1 . The boost switch is connected between the second terminal of the inductor L 1  and the ground reference and provided for selectively coupling the second terminal of the inductor L 1  to the ground reference according to the control signal from the PWM controller  40 . An intermediate node between the capacitor C 1  and the diode D 1  provide the output voltage VBST for the multi-branches of LEDs. 
         [0027]      FIG. 4  shows a circuit diagram of the feedback signal selector  30  available for the multi-branch LED drive circuit according to one embodiment of the present invention. The feedback signal selector  30  comprises a first selecting unit  32  and a second selecting unit  34 . The first selecting unit  32  generates a first switch signal ON_ 1  to the branch switch MN 1  and a second switch signal ON_ 1  to the branch switch MN 2  according to an enable signal ENA thereof so that the branch switches MN 1  and MN 2  are switched on alternatively. The second selecting unit  34  comprises a first transmission gate receiving the feedback signal FB 1  and outputting the feedback signal FB 1  when the first switch signal is on, a second transmission gate receiving the feedback signal FB 2  and outputting the feedback signal FB 2  when the second switch signal is on. 
         [0028]    The first selecting unit  32  comprises a D trigger, a first AND gate AND 1  and a second AND gate AND 2 . The D trigger has an input terminal d, an output terminal q, an inverted output terminal qb, a clock signal terminal ck, a reset terminal r. One input terminal of the first AND gate AND 1  is coupled to the inverted output terminal qb and the input terminal d of the D trigger. One input terminal of the second AND gate AND 2  is coupled to the output terminal q of the D trigger. The other terminal of the first AND gate AND 1  is coupled to the enable signal ENA, the other terminal of the second AND gate AND 2  is coupled to the enable signal ENA. The enable signal ENA is connected to the clock signal terminal ck. A UV signal is coupled to the reset terminal r. The first AND gate AND 1  outputs the first switch signal ON_ 1  via an output terminal thereof, and the second AND gate AND 2  outputs the second switch signal ON_ 1  via an output terminal thereof. 
         [0029]      FIG. 6  is a timing diagram showing an operation of the first selecting unit. When a rising edge of the enable signal ENA comes, the output terminal q of the D trigger outputs high level to the input terminal of the first AND gate AND 1 , the other input terminal of the first AND gate AND 1  is high level, so the first AND gate AND 1  output high level as the first switch signal ON_ 1 , the inverted output terminal qb of the D trigger outputs low level to the input terminal of the second AND gate AND 2 , the other input terminal of the second AND gate AND 2  is high level, so the second AND gate AND 2  output low level as the second switch signal ON_ 2 . When a fall edge of the enable signal ENA comes subsequently, the second AND gate AND 2  output low level as the second switch signal ON_ 2 , and the first AND gate AND 1  output low level as the first switch signal ON_ 1 . When a next rising edge of the enable signal comes, the output terminal q of the D trigger outputs low level, the inverted terminal qb of the D trigger outputs high level, so the second AND gate AND 2  output high level as the second switch signal ON_ 2 , and the first AND gate AND 1  output low level as the first switch signal ON_ 1 . Repeats the above operations, the feedback signal selector  30  can alternatively select one from the two feedback signals FB 1  and FB 2  as the output feedback signal FB and provide a switch-on signal to the branch switch MN 1  or MN 2  of corresponding branch simultaneously. 
         [0030]      FIG. 5  is a circuit diagram showing the PWM controller  40  of the multi-branch LED drive circuit. Referring to  FIG. 5 , the PWM controller  40  comprises an error amplifier  42 , an oscillator  46 , a PWM comparator  44  and a logic drive unit  48 . 
         [0031]    The oscillator  46  is used to generate a triangle wave signal. The triangle wave signal is coupled to a negative input of the PWM comparator  44 . A negative input of the error amplifier  42  is coupled to the output of the feedback signal selector  30  for receiving the output feedback signal FB. A positive input of the error amplifier  42  is coupled to a reference port for receiving a reference voltage Vref. The error amplifier  42  is used to compare the feedback signal FB and the reference voltage Vref and amplify the difference and subsequently output the error signal CMP. The PWM comparator  44  is used to compare the triangle wave signal and the error signal CMP to generate a square wave signal PWMO with a certain duty cycle. Referring to  FIG. 7A  and  FIG. 7B , when an electric potential of the triangle signal is higher than that of the error signal CMP, the PWM comparator  44  output a low level; otherwise, the PWM comparator  44  output a high level. The square wave signal PWMO is sent to the logic drive unit  48 . The logic drive unit  48  drives the boost switch to turn on or off according to the square wave signal. 
         [0032]    For the boost circuit shown in  FIG. 3 , the output voltage VBST and the input voltage satisfy the following equation. 
         [0000]    
       
         
           
             
               
                 
                   VBST 
                   = 
                   
                     
                       1 
                       
                         1 
                         - 
                         D 
                       
                     
                     · 
                     VIN 
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0033]    D is the duty cycle of the square wave signal PWMO, wherein 0&lt;D&lt;1. So, if the duty cycle D is increased, it may cause the output voltage VBST increase. Then, the current of each LED branch increase to result the feedback signal FB 1  or FB 2  increase too. The negative feedback regulation continues until the electric potential of the feedback signal FB is equal to the reference voltage. 
         [0034]    Thus, the regulated current I 1  of the first LED branch may satisfy the following equation. 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     1 
                   
                   = 
                   
                     
                       
                         V 
                         
                           FB 
                            
                           
                               
                           
                            
                           1 
                         
                       
                       
                         R 
                         1 
                       
                     
                     = 
                     
                       
                         V 
                         Ref 
                       
                       
                         R 
                         1 
                       
                     
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0035]    Accordingly, the current I 2  of the second LED branch may satisfy the following equation. 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     2 
                   
                   = 
                   
                     
                       
                         V 
                         
                           FB 
                            
                           
                               
                           
                            
                           2 
                         
                       
                       
                         R 
                         2 
                       
                     
                     = 
                     
                       
                         V 
                         Ref 
                       
                       
                         R 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
         [0036]    Thus, if R 2 =R 1 , the current I 2  of the second LED branch will equal to the current I 1  of the first LED branch accurately. 
         [0037]    The present invention has been described in sufficient details with a certain degree of particularity. It is understood to those skilled in the art that the present disclosure of embodiments has been made by way of examples only and that numerous changes in the arrangement and combination of parts may be resorted without departing from the spirit and scope of the invention as claimed. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description of embodiments.