Patent Publication Number: US-2012038283-A1

Title: Light emitting diode driving system and circuit thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The current disclosure relates to a light emitting diode (LED) driving system and circuit thereof. 
     2. Description of the Related Art 
     Liquid Crystal Display (LCD) is widely applied on current electronic products such as TVs, monitors, notebooks, mobile phones and PDAs. Therefore, the demand for backlight modules is growing. Since LEDs have such advantages as low power consumption, high brightness, small size and long life, most backlight modules of LCD use LEDs as the light source. 
     When applied on a backlight module, the LEDs are usually present as a set of serially connected LEDs which is called an LED chain, LED set or is LED bank. An LED circuit is usually used to provide fixed currents to many LED sets in order to turn on the LED sets and keep their brightness uniform. 
     A general DC-DC converter takes resistor to divide voltage to generate an analog feedback signal in order to create a fixed supply voltage applied on a plurality of LED sets and a driving circuit. If the supply voltage is larger than the needed voltage of the driving circuit and the plurality of LED sets, the driving circuit will consume the extra supply voltage and generate heat loss. In contrast, if the fixed voltage is smaller than the needed voltage of the driving circuit and the plurality of LED sets, the plurality of LED sets will lack brightness and have function disability. 
     Therefore, in order to decrease unnecessary heat loss of the driving circuit and provide enough supply voltage to drive a plurality of LED sets connected to the driving circuit, it is necessary to provide an LED array driving system that solves the aforementioned problems. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a light emitting diode (LED) driving circuit for controlling a direct current (DC) voltage converter to apply a regulated output voltage to an input terminal of an LED array having a plurality of LED sets, the LED driving circuit comprising a plurality of current sources connected to a plurality of output terminals of the LED array and configured to provide a current flowing through the plurality of LED sets, a comparison circuit connected to the plurality of output terminals and configured to generate a feedback signal representing the status of the LED array, and a current setup circuit configured to set the initial current value of the plurality of current sources, is wherein the DC voltage converter is configured to generate the regulated output voltage by taking into consideration the feedback signal. 
     In accordance with one embodiment of the present invention, a driving system for driving an LED array having a plurality of LED sets is configured to generate an regulated output voltage to an input terminal of the LED array, the driving system comprising an LED driving circuit which comprises a plurality of current sources connected to a plurality of output terminals of the LED array and configured to provide a current flowing through the plurality of LED sets, a comparison circuit connected to the plurality of output terminals and configured to generate a feedback signal representing a status of the LED array, and a DC voltage converting unit configured to generate the regulated output voltage of the driving system by taking into consideration the feedback signal. 
     In accordance with one embodiment of the present invention, an LED driving system for controlling a DC voltage converter to apply a regulated output voltage to an input terminal of an LED array having a plurality of LED sets, wherein the LED driving system comprises a first LED driving circuit connected to the DC voltage converter and at least one second LED driving circuit connected in series to the first LED driving circuit, and each of the first and second LED driving circuits comprises a plurality of current sources connected to a plurality of output terminals of a corresponding LED sets of the LED driving array and configured to provide a current flowing through the corresponding LED sets, a comparison circuit module connected to the plurality of output terminals of the corresponding LED sets of the LED driving array and an output terminal of a previous stage and configured to generate a feedback signal representing a status of the LED array, and a current setup circuit configured to set an initial current value flowing through the plurality of corresponding LED sets, wherein the DC voltage converter is configured to generate the regulated output voltage by taking into consideration the feedback signal of the first LED driving circuit. 
     In accordance with one embodiment of the present invention, an LED driving system for generating a regulated output voltage to an input terminal of an LED array having a plurality of LED sets, wherein the LED driving system comprises a DC voltage converting unit, a first LED driving circuit connected to the DC voltage converting unit and at least one second LED driving circuits connected in series to the first LED driving circuit, wherein each of the first and second LED driving circuits comprises a plurality of current sources connected to a plurality of output terminals of a corresponding LED sets of the LED driving array and configured to provide a current flowing through the corresponding LED sets, a comparison circuit module connected to the plurality of output terminals of the corresponding LED sets of the LED driving array and an output terminal of a previous stage and configured to generate a feedback signal representing the status of the LED array, and a current setup circuit configured to set an initial current value flowing through the plurality of corresponding LED sets, wherein the DC voltage converter is to regulate its output voltage by taking into consideration the feedback signal of the first LED driving circuit. 
     In order to better understand the techniques, means, and effects of the current disclosure, the following detailed description and drawings are provided so that the purposes, features and aspects of the current disclosure can be thoroughly and concretely appreciated; however, the drawings are provided solely for reference and illustration and are not to be used to limit the current disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objectives and advantages of the present invention are illustrated in the following descriptions and with reference to the accompanying drawings in which: 
         FIG. 1  schematically illustrates one embodiment of the present invention with an LED driving circuit having a DC voltage converter; 
         FIG. 2  shows one embodiment of the present invention with circuit blocks of the DC voltage converter and the LED driving circuit; 
         FIG. 3  shows one embodiment of the present invention with circuit blocks of the DC voltage converter and the LED driving circuit; 
         FIG. 4  schematically illustrates one embodiment of the present invention with the structure of the LED driving system; 
         FIG. 5  shows circuit blocks of the LED driving system; 
         FIG. 6  shows another embodiment of the present invention with circuit blocks of the LED driving system; 
         FIG. 7  schematically illustrates one embodiment of the present invention with the LED driving system; 
         FIG. 8  shows circuit blocks of the LED driving circuit; 
         FIG. 9  shows circuit blocks of the comparison circuit module; 
         FIG. 10  schematically illustrates another embodiment of the present invention with LED driving system; 
         FIG. 11  shows circuit blocks of the LED driving circuit; and 
         FIG. 12  schematically illustrates another embodiment of the present invention with the LED driving system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention discloses an LED driving system and circuit thereof. 
       FIG. 1  schematically illustrates one embodiment of the present invention with an LED driving circuit  12  having a DC voltage converter. The LED driving circuit  12  is for controlling the DC voltage converter  10  to generate a regulated DC output voltage V REG, 1  to an input terminal of an LED array  14 . In current embodiment, the DC voltage converter  10  is a switch type voltage stabilizer and configured to generate a higher output voltage from an input power, such as a battery, to offer the LED array  14 . As shown in  FIG. 1 , the LED array  14  has a plurality of LED sets  141  and  142 , wherein each of the plurality of LED sets has a plurality of LED connected in series. The LED driving circuit  12  is configured to provide a plurality of fixed current sources (not shown) to the corresponding LED sets. 
       FIG. 2  shows one embodiment of the present invention with circuit blocks of the DC voltage converter and the LED driving circuit. As shown in  FIG. 2 , the LED driving circuit  12  comprises a comparison circuit  122  and a current setup circuit  124 . The comparison circuit  122  is connected to a plurality of output terminals OUT 1 -OUT n  of the LED array  14  and is configured to generate a feedback signal FB  1  representing a status of the LED array  14 . In accordance with one embodiment of the current invention, the comparison circuit  122  is a voltage comparator which is used to compare voltages of the plurality of output terminals OUT 1 -OUT n . When a voltage value of the output terminal OUT 1  is a minimum, the feedback signal FB 1  is the voltage value of the output terminal OUT 1 . In accordance with another embodiment of the current invention, the comparison circuit  122  is a current comparator which is used to compare currents flowing through each of the plurality of LED sets. When a current of the LED set  141  is a minimum, the feedback signal FB 1  is a voltage value of the output terminal OUT 1  of the LED set  141 . The current setup circuit  124  is configured to set a current value of the plurality of a fixed current source  123 . In the present embodiment, the current setup circuit  124  is configured to set the current value of the current source  123  by a resistor R 1 . 
     In operation, the LED connected in series of the plurality of LED sets, each has a slightly different voltage drop V F  so that the plurality of output terminals OUT 1 -OUT n  of the LED array  14  also each have a different voltage. If the DC voltage V REG, 1  from the DC voltage converter  10  is not is able to be regulated with total voltage drop of the plurality of LED sets, the extra voltage of the DC voltage V REG, 1  will be consumed by the driving circuit  12 , and causes power loss of the driving circuit. If the DC voltage V REG, 1  is insufficient, this will cause lack of brightness of the LED array  14  and even function disability of some of the plurality of LED sets. 
     Therefore, in accordance with one embodiment of the present invention, the feedback signal FB 1  is a minimum voltage of the plurality of output terminals OUT 1 -OUT n  of the LED array  14 , or the feedback signal FB 1  is a voltage value of one of the plurality of LED sets having a minimum current value of the LED array  14 . As shown in  FIG. 2 , the feedback signal FB 1  is transmitted to an input terminal of an error amplifier inside the DC voltage converter  10 . The error amplifier OP 1  is configured to amplify a voltage signal, and a voltage difference between the feedback signal FB 1  and a reference voltage V REF, 1  built-in the DC voltage converter  10 , to generate an output signal V ERR, 1 . Thus, by taking into consideration the output signal V ERR, 1 , the output voltage V REG, 1  of the DC voltage converter  10  can be configured to generate a proper voltage to the LED driving circuit  12 . 
     Moreover, the DC voltage converter can be configured to receive an external voltage which is set up by users, and the DC voltage converter is configured to generate a regulated output voltage by taking into consideration a difference between the feedback signal and the external reference voltage.  FIG. 3  shows one embodiment of the present invention with circuit blocks of the DC voltage converter and the LED driving circuit. As shown in  FIG. 3 , the LED driving circuit can have a reference voltage generating unit  126 . The reference voltage generating unit  126  is configured to generate a reference voltage V REF, 2  to the DC voltage converter  10 ′. In one embodiment, the reference voltage V REF, 2  and the voltage V SET, 2  of the resistor R 2  are equal or proportional to each other, which means the reference voltage V REF, 2  is regulated with different currents flowing through the plurality of LED sets. 
       FIG. 4  schematically illustrates one embodiment of the present is invention with the structure of the LED driving system  40 . The driving system  40  is for driving an LED array  42 . As shown in  FIG. 4 , the LED array  42  has a plurality of LED sets  421  and  422 , wherein each of the plurality of LED sets has a plurality of LED connected in series. The LED driving system  40  is configured to generate an regulated output voltage V REG, 3  to an input terminal of the LED array  42 , and generate a plurality of fixed current sources (not shown) to the corresponding LED sets. 
       FIG. 5  shows circuit blocks of the LED driving system  40 . The driving system  40  has an LED driving circuit  402  and a DC voltage converting unit  404 . As shown in  FIG. 5 , the LED driving circuit  402  has a comparison circuit  4022  and a current setup circuit  4024 . The comparison circuit  4022  connected to the plurality of output terminals OUT 1 -OUT n  of the LED array  42  is configured to generate a feedback signal FB 3  representing a status of LED array  42 . 
     The DC voltage converting unit  404  can be configured to receive an external reference voltage V REF, 3  which is set up by users, and the DC voltage converter is configured to generate a regulated output voltage V REG, 3  by taking into consideration a voltage difference between the feedback signal FB 3  and the external reference voltage. In another one embodiment, as shown in  FIG. 5 , the LED driving circuit has a reference voltage generating unit  4026 . The reference voltage generating unit  4026  is configured to generate the reference voltage V REF, 3  to the DC voltage converting unit  404 . In another embodiment, the reference voltage V REF, 3  is a fixed value. In another embodiment, the value of the reference voltage V REF, 3  can be set up by a resistor R 4  having a same resistance as the resistor R 3 . The resistor R 3  is used to set up current flowing through the plurality of LED sets. The reference voltage V REF, 3  can be regulated with different currents flowing through the plurality of LED sets. 
     Similarly, the comparison circuit  4022  of the LED driving system  40  can be a voltage comparator which is configured to compare voltages of the plurality of output terminals OUT 1 -OUT n  of the LED array  42 . The is comparison circuit  4022  can also be a current comparator configured to compare a current value flowing through each of the LED sets. Therefore, the feedback signal FB 3  can be a minimum of voltages of the plurality of output terminals of the LED array  42 , or the feedback signal FB 3  can be a voltage value of the output terminal of the LED set having a minimum current value of the LED array  42 . Moreover, with the feedback signal FB 3  and the reference voltage V REF, 3 , the LED driving system  40  is configured to generate a regulated output voltage V REG, 3  by taking into consideration the total voltage drop V F  of the plurality of LED sets. 
       FIG. 6  shows another embodiment of the present invention indicating circuit blocks of the LED driving system  60 . The driving system  60  includes an LED driving circuit  62  and a DC voltage converting unit  64 . As shown in  FIG. 6 , the LED driving circuit  62  includes a comparison circuit  622  and a current setup circuit  624 . The comparison circuit  622  is configured to generate a feedback signal DFB representing a status of the LED array and the feedback signal DFB is a digital signal. The current setup circuit  624  is configured to set up a current value flowing through an LED array. 
     In accordance with one embodiment of the current invention, the comparison circuit  622  is a voltage comparator configured to compare a first initial value and voltages of the plurality of output terminals of an LED array. When any one of the voltage values of the plurality of output terminals is smaller than the first initial value, the feedback signal DFB stays on logic signal level  0 . When voltage values of all of the plurality of output terminals are larger than the first initial value, the feedback signal DFB will change to output logic signal level  1 . 
     In accordance with another embodiment of the current invention, the comparison circuit  622  is a current comparator configured to compare a second initial value and currents flowing through each of the plurality of LED sets. When any one of the current values flowing through the plurality of LED sets is smaller than the second initial value, the feedback signal is DFB stays on logic signal level  0 . When the current values of all of the plurality of output terminals are larger than the second initial value, the feedback signal DFB will toggle to output logic signal level  1 . 
     As shown in  FIG. 6 , the DC voltage converting unit  64  includes an accumulator  642  and a DAC  644  connected to the accumulator  642 . When logic output of the comparison circuit  622  stays on logic signal level  0 , the accumulator  642  will count one at each time interval. This means the output signal of the accumulator  642  represents the time of the comparison circuit  622  staying on logical signal level  0 . The DAC  644  is configured to convert an output signal of the accumulator  642  to an analog signal VDAC. The output voltage V REG, 4  of the LED driving system  60  will be regulated by taking into consideration the analog signal VDAC. 
     In operation, when any one of voltage values of the plurality of output terminals of the LED array is smaller than the first initial value, or when any one of currents flowing through the plurality of LED sets is smaller than the second initial value, the corresponding LED set will have lack brightness or have function disability. Furthermore, the accumulator starts counting when the feedback signal DFB is at logic signal level  0 . The DAC  644  converts an output value of the accumulator  642  to an analog signal VDAC to increase the output voltage V REG, 4  of the LED driving system  60 . After a time interval, when any one of voltage values of the plurality of output terminals of the LED array is still smaller than the first initial value, or when any one of currents flowing through the plurality of LED sets is still smaller than the second initial value, the accumulator  642  will keep counting to increase the analog signal VDAC of the DAC  644 . Therefore, when the output voltage V REG, 4  goes up, the output voltage of the plurality of output terminals of the LED array goes up as well. If the feedback signal DFB keeps staying on logic signal level  0 , the output voltage V REG, 4  will keep increasing. When any one of voltage values of the plurality of output terminals of the LED array is larger than the first initial value or when the is current value of all of the plurality of output terminals of the LED array is larger than the second initial value, the feedback signal DFB toggles to output logic signal level  1 . When the feedback signal DFB toggles, the accumulator  642  stops counting and thus the output analog signal VDAC of the DAC  644  stays on the same voltage level. Meanwhile, the output voltage V REG, 4  stops increasing and each of the plurality of LED sets of the LED array stays on a proper driving voltage. In the driving applications of LED array, using digital type DC voltage converter having digital feedback signal input can save cost and improve efficiency compared with the conventional analog type DC voltage converter. 
     Besides, LED can be applied on any kinds of electronic display apparatus as light emitting source, ex. traffic signs or big-scale signage. Due to big-scale signage having more than ten thousands of LED, many of LED driving circuits connected in series are necessary to control the light emitting information of a plurality of LEDs. 
       FIG. 7  schematically illustrates one embodiment of the present invention with the LED driving system  70 . The LED driving system  70  is configured to control a DC voltage converter  72  to generate a regulated output voltage V REG, 5  to an input terminal of an LED array  74  having a plurality of LED sets  741  and  742 . As shown in  FIG. 7 , the LED driving system includes an LED driving circuit  702  connected to the DC voltage converter  72  and an LED driving circuit  704  serially connecting to the LED driving circuit  702 . 
       FIG. 8  shows circuit blocks of the LED driving circuit  702  and  704 . Each of the LED driving circuit includes a comparison circuit module  706  and a current setup circuit  707 . The comparison circuit module  706  is connected to a plurality of output terminals of the corresponding LED set of the LED array  74  and an output terminal of the previous stage. The current setup circuit  707  is to set up the current value flowing through the LED array  74  by a resistor R 6 .  FIG. 9  shows circuit blocks of the comparison circuit module. As shown in  FIG. 9 , the comparison circuit module  706  is includes a current comparison circuit  7062 , a voltage selecting circuit  7064  and a voltage comparison circuit  7066 . The current comparison circuit  7062  is configured to compare a current value of every corresponding LED set. The voltage selecting circuit  7064  selects voltage value of output terminal of the LED set having a minimum current value by taking into consideration output result of the current comparison circuit  7062 . The current comparison circuit  7066  is configured to compare a voltage value selected by the voltage selecting circuit  7064  and a voltage value of the output terminal of the previous LED driving circuit. The feedback signal of the previous LED driving circuit generates a minimum voltage as the feedback signal or the comparison circuit module  706  can compare a voltage value of the plurality of output terminals of the corresponding LED set and a feedback signal of the previous LED driving circuit to generate a minimum voltage as a feedback signal of the current LED driving circuit. 
     In operation, each of the plurality of LED driving circuits of the LED driving system  70  will generate a feedback signal which represents a minimum of voltages of the plurality of input terminals of the LED driving circuit. The plurality of input terminals includes an output terminal of the previous LED driving circuit and the plurality of corresponding output terminals. Therefore, a feedback signal of the LED driving circuit  702  represents a minimum of voltages of all output terminals of the LED array  74 . The feedback signal is finally transmitted to the DC voltage converter  72  to regulate output voltage V REG, 5 . 
     Similarly, the comparison circuit module  706  can include a reference voltage generating unit to generate a reference voltage. The DC voltage converter can be configured to generate a regulated output voltage V REG, 5  by taking into consideration a difference between the reference voltage and the feedback signal of the first LED driving circuit  702 . The reference voltage can be a fixed value or regulated with current value flowing through the LED array  74 . 
       FIG. 10  schematically illustrates another embodiment of the present invention with LED driving system  90 . The LED driving system  90  is configured to generate a regulated output voltage V REG, 6  to an input terminal of an LED array  92 , wherein the LED array  92  includes a plurality of LED sets  921  and  922 . As shown in  FIG. 10 , the LED driving system  90  includes a DC voltage converting unit  902 , an LED driving circuit  904  connected to the DC voltage converting unit and an LED driving circuit  906  serially connecting to the LED driving circuit  904 . 
       FIG. 11  shows circuit blocks of the LED driving circuit  904  and  906 . Each of LED driving circuits includes a comparison circuit module  907  and a current setup circuit  908 . The comparison circuit module  907  is connected to the plurality of output terminals of the corresponding LED set of the LED array  74  and an output terminal of the previous stage. The current setup circuit  908  sets up a current value flowing through the LED array  92  by a resistor R 7 . 
     Similarly, the comparison circuit module is configured to generate a feedback signal representing a status of the LED array  92 . Since each of the LED driving circuits is serially connected, a feedback signal of the LED driving circuit  904  represents a minimum of voltages of the plurality of output terminals of the LED array  92  or a minimum of currents of the plurality of LED sets of the LED array  92 . The DC voltage converting unit  902  regulates the output voltage V REG, 6  of the driving system  90  by taking into consideration the feedback signal of the LED driving circuit  904 . The comparison circuit module  904  can include a reference voltage generating unit to generate a reference voltage. Therefore, the DC voltage converting unit  902  can be configured to generate a regulated output voltage V REG, 6  by taking into consideration a difference between the reference voltage and the feedback signal of the LED driving circuit. The reference voltage can be regulated by different resistances. 
     In accordance with another embodiment of the current invention, a feedback signal of the LED driving circuit  904  is a digital signal.  FIG. 12  is schematically illustrates another embodiment of the present invention indicating the LED driving system  90 . As shown in  FIG. 12 , each of the LED driving circuits includes a comparison circuit module  910 , a logic circuit  912  and a current setup circuit  914 . The DC voltage converting unit  902  includes an accumulator  9022  and a DAC  9044 . 
     In accordance with one embodiment of the present invention, the comparison circuit module  910  is a voltage comparison circuit. In LED driving circuit  904 , the comparison circuit module  910  compares voltages of the plurality of output terminals of the corresponding LED sets. When any one of the voltages of the plurality of output terminals is smaller than the first initial value, the logic circuit  912 , such as an AND gate, will output logic signal level  0 , which means any one of the voltages of the plurality of output terminals of the LED array  92  is smaller than the first initial value. The accumulator  9022  keeps counting and then an analog signal outputted by the DAC  9024  increases to increase output a voltage V REG, 6  of the DC voltage converting unit  902 . 
     When all of the voltage values of the plurality of output terminals of the LED array  92  are larger than the first initial value, the logic circuit  912  of the LED driving circuit  904  will toggle to output logic signal level  1 . When the logic circuit  912  toggles, the accumulator  9022  stops counting, the output voltage V REG, 6  of the DC voltage converting unit  902  stays on a proper voltage level to drive the LED array  92 . 
     Although the present invention and its objectives have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, many of the processes discussed above can be implemented using different methodologies, replaced by other processes, or both. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the is specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, which perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized by taking into consideration the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.