Abstract:
A light emitting diode (LED) driving circuit is provided. The LED driving circuit includes: at least one LED driving module, coupled to the at least one LED series, for driving the corresponding LED series; and a voltage regulating module, coupled to the at least one LED driving module, for providing a regulation signal according to an output signal from the at least one LED driving module, wherein an input voltage of the at least one LED series is regulated according to the regulation signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 100102472, filed in Taiwan, Republic of China on Jan. 24, 2011, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to light emitting diode (LED) driving technology, and in particular relates to voltage regulation technology for driving transistors in an LED circuit. 
     2. Description of the Related Art 
     Light Emitting diode (LED) driving chips are popularly used in display apparatuses such as LED televisions.  FIG. 1  is a schematic diagram showing the LED driving chip in the prior art. The LED driving chip  110  comprises a plurality of LED driving modules  112 . an LED series  130 , having a plurality of LEDs connected in series and between a voltage source Vsupply and an LED driving module  112 . The LED driving module  112  is used to drive the LED series  130 . 
     Note that, in the prior art, a variable resistor Rad disposed between the voltage source Vsupply and the LED series  130  for adjusting the voltage drop across the LED driving module  112 . Specifically, the components in  FIG. 1  satisfy the following equation: Vsupply=ILED×Rad+VLEDs+V 112 , where “ILED” denotes the current across the LED  130  and “VLEDs” denotes the voltage across the LED  130 . With a given specification for the LED  130 , both of the values of “ILED” and “VLEDs” are constant. Therefore, when the value of the variable resistor Rad is calibrated, the voltage drop V 112  across the LED driving module  112  will be accordingly calibrated. 
     However, the prior art fails to calibrate the voltage drop across the LED driving module  112  within a proper range automatically. Thus, the present invention provides a new circuit to solve this issue. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing the LED driving chip in the prior art. 
         FIG. 2  is a schematic diagram of an LED driving circuit according to an embodiment of the present invention. 
         FIG. 3  shows the characteristic curve of the driving transistor  212  for illustrating the ideal drain voltage of the driving transistor  212 . 
         FIG. 4A  is a schematic diagram of an LED driving circuit according to another embodiment of the present invention. 
         FIG. 4B  is a schematic diagram of the LED driving circuit according to yet another embodiment of the present invention. 
         FIG. 5  is a schematic diagram of the LED driving system  500  of the present invention. 
     
    
    
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
     The present invention provides a light emitting diode (LED) driving circuit. The LED driving circuit comprises: at least one LED driving module, coupled to the at least one LED series, for driving the corresponding LED series; and a voltage regulating module, coupled to the at least one LED driving module, for providing a regulation signal according to an output signal from the at least one LED driving module, wherein an input voltage of the at least one LED series is regulated according to the regulation signal. 
     The present invention also provides a light emitting diode (LED) driving and regulating system. The system comprises the LED driving module described above and a voltage source, providing a voltage to a plurality of LED series, and a plurality of LED driving modules, respectively, coupled to the plurality of the LED series, for driving the corresponding LED series, a controller, coupled to the voltage source, for sending a control signal to the voltage source to regulate the input voltage provided to the plurality of the LED series, wherein the plurality of the LED driving modules are coupled in series, and the LED driving modules of one stage receives and processes the output signal from the LED driving modules of a previous stage to provide an output signal, wherein the LED driving modules of the last stage sends its output signal to the controller for regulating the input voltage of the plurality of the LED series. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 2  is a schematic diagram of an LED driving circuit according to an embodiment of the present invention. The present invention achieves the purpose of automatically regulating the voltage of the LED driving circuit  200  by using feedback control. The LED driving circuit  200  comprises a plurality of LED driving modules  210  and a voltage regulating module  220 . Each of the LED driving modules  210  comprises a driving transistor  212  and an operational amplifier  214 , and the voltage regulating module  220  comprises a reference voltage generator  222  and a drain voltage comparator  224 . Specifically, the principal of the present invention is to regulate the drain voltage of the driving transistor  212  so that the driving transistor  212  works in a saturation region. Through this manner, the current on the driven LED series  230  will be stable and make the LED series  230  emit light constantly. In addition, to improve the efficiency of the driving module, the drain voltage of the driving transistor  212  has to be controlled to not overgrow so as to prevent the LED driving circuit  200  from unnecessary power loss (the power loss equals to the drain voltage times the current of the driving transistor  212 ). 
     The components of the present invention will be further discussed in detail in the following paragraphs. 
     The driving transistor  212  of the present invention is used to drive a plurality of LEDs  230 , wherein the plurality of LED are connected in series (so called LED series  230 ) and between a voltage source  270  and the LED driving module  210 . The drain of the driving transistor  212  is connected to the LED series  230 . The operational amplifier  214  of the present invention has an output end coupled to the gate of the driving transistor  212 , and has an input end for receiving a driving voltage Vdr. 
     The reference voltage generator  222  in the voltage regulating module  220  is used to generate a reference voltage, which is used for comparison with the drain voltage of the driving transistor  212 . In the embodiment of  FIG. 2 , the reference voltage generator  222  is coupled to the positive input end of the operational amplifier  214 . The reference voltage generator  222  receives the driving voltage Vdr (which slightly equals to the drain voltage of the driving transistor  212 ) and then adds a voltage difference Vest to the driving voltage Vdr to generate a reference voltage which equals to the ideal drain voltage of the driving transistor  212 . 
       FIG. 3  shows the characteristic curve of the driving transistor  212  for illustrating the ideal drain voltage of the driving transistor  212 . The ideal drain voltage should be slightly greater than the lowest drain voltage Vth which makes the driving transistor  212  enter the saturation region so that the driving transistor  212  can have a stable current and the lowest power loss at the same time. Therefore, in an embodiment, if the reference voltage is the lowest drain voltage Vth, the voltage difference Vest should be set to be Vth−Vdr. 
     The drain voltage comparator  224  of the voltage regulating module is a multi-input comparator, which comprises a plurality of drain voltage input ends  241 , a reference voltage input end  242 , and a regulation signal output end  243 . Each drain voltage input end  241  of the drain voltage comparator  224  is coupled to and obtains an output signal (drain voltage) from the drain of a driving transistor  212  of one of the LED driving modules  210 . The reference voltage input end  242  of the drain voltage comparator  224  is coupled to the reference voltage generator  222  for receiving a reference voltage from the reference voltage generator  222 . 
     The drain voltage comparator  224  of the present invention can compare the drain voltage received by the drain voltage input end  241  with the reference voltage received by the reference voltage input end  242  and generate a regulation signal (the comparison result) to an external controller  250 . Then, the controller  250  regulates the voltage that the voltage source  270  provides to the LED  230  according to the regulation signal, and finally regulates the drain voltage Vds of the driving transistor  212 . For example, when a drain voltage of one of the driving transistors  212  is lower than the reference voltage (too low), the LED series  230  connected to this driving transistor will operate in an unstable state. On one hand, when detecting that the drain voltage Vds is too low, the drain voltage comparator  224  sends a regulation signal to the controller  250 , and the controller  250  regulates the output voltage of the voltage source  270  according to the regulation signal to make sure that all of the driving transistors  212  operate in the saturation region. On the other hand, when the drain voltage of the driving transistors  212  are all higher than the reference voltage (too high), the controller  250  reduces the voltage provided by the voltage source  270  and thus lowers the drain voltage Vds of all of the driving transistors  212  for limiting the power loss. 
       FIG. 4A  is a schematic diagram of an LED driving circuit according to another embodiment of the present invention. Similar to the LED driving circuit  200 , the LED driving circuit  400  in  FIG. 4A  comprises a plurality of LED driving modules  410  and a voltage regulating module  420 . Each of the LED driving modules  410  is used to drive the LED series  430 , and comprises a driving transistor  412  and an operational amplifier  414 . The voltage regulating module  420  comprises a reference voltage generator  422  and a drain voltage comparator  424 . In the drain voltage comparator  424  of  FIG. 4A , the drain voltage comparator  224  of the voltage regulating module  220  in  FIG. 2  is replaced by a combination of transistors Q 1 , Q 2  and Q 3 , where the transistors Q 1 , Q 2  and Q 3  form a plurality of current mirrors. Each drain voltage is inputted to the gate of the first transistor Q 1 , and the reference voltage Vest generated by the reference voltage generator  422  is coupled to the gate of the second transistor Q 2 . In addition, the regulation signal is the voltage of the source of the first transistor Q 1 , which will be inputted to an external controller  450 . The controller  450  regulates the voltage that the voltage source  470  provided to the LED  430  according to the regulation signal, thus regulating the drain voltage Vds of the driving transistor  412 . 
       FIG. 4B  is a schematic diagram of the LED driving circuit according to yet another embodiment of the present invention. Similar to the LED driving circuit  200  in  FIG. 2 , the LED driving circuit  400  in  FIG. 4B  comprises a plurality of LED driving modules  410  and a voltage regulating module  420 . Each of the LED driving modules  410  is used to drive the LED series  430 , and comprises a driving transistor  412  and an operational amplifier  414 . The voltage regulating module  420  comprises a reference voltage generator  422  and a drain voltage comparator  424 . In the drain voltage comparator  424  of  FIG. 4A , the drain voltage comparator  224  of the voltage regulating module  220  is replaced by a combination of transistors Q 1 , Q 2  and Q 3 , where the transistors Q 1 , Q 2  and Q 3  form a plurality of current mirrors. The difference between embodiments in  FIG. 4A  and  FIG. 4   b  is that the regulation signal in  FIG. 4A  is the source voltage of the first transistor Q 1  while the regulation signal in  FIG. 4B  is the output of an OR gate  280 . The OR gate  280  in  FIG. 4B  comprises a plurality of input ends, respectively coupled to a drain of a first transistor Q 1  for inputting the drain voltage of the first transistor Q 1 . 
     In  FIG. 2 ,  FIG. 4A , and  FIG. 4B , the present invention further comprises an inverter  290 , which is coupled to the regulation signal for inverting the regulation signal. When the drain of the driving transistor  212 / 412  of the LED driving module  200 / 400  is higher than the reference voltage, the regulation signal is at a high level (H) and the output of the inverter  290  is at a low level (L); alternatively, when the drain of the driving transistor  212 / 412  of the LED driving module  200 / 400  is lower than the reference voltage, the regulation signal is at a low level (L) and the output of the inverter  290  is at a high level (H). 
     The present invention further provides an LED driving and regulating system, which uses feedback control to achieve the purpose of regulating the voltage.  FIG. 5  is a schematic diagram of the LED driving system  500  of the present invention. The LED driving and regulating system of the present invention comprises stages of LED driving modules  501 ˜ 503 , a voltage source  570  and a controller  550 . Each stage of the LED driving circuits  501 ˜ 503  can respectively drive and regulate the LED series  511 ˜ 513 . The LED driving circuit  501 ˜ 503 , for example, may be the LED driving module  200  of  FIG. 2 . Since the LED driving module  200  has been fully discussed previously, the detailed structure of the LED driving circuits  501 ˜ 503  will not be further described. In the present system, the voltage source  570  is used to provide a voltage, and the controller  570  is used to output a control signal to the voltage source  50  according to the regulation signal to regulate the input voltage of the LED series  511 ˜ 513 . In general, the LED driving circuits of the present invention can be respectively used in display chips. A single LED driving circuit can be used to control several LED series, but the number of the LED series has a limit. Therefore, for controlling a display which has a huge amount of LED series (for example, over 16 LED series), a plurality of LED driving circuits, as shown in  FIG. 5 , are required. 
     Please refer to  FIG. 2 ,  FIG. 4A ,  FIG. 4B , and  FIG. 5 . In a better embodiment, the voltage regulating module further comprises a double input OR gate  260 , which has a first input end  261  coupled to the output of the inverter  290  of the voltage regulating module, a second input end  262  coupled to the output of the double input OR gate of the voltage regulating module of the LED driving circuit of a previous state (not shown in these Figs.), and an output end  263  coupled to the input of the double input OR gate of the voltage regulating module of the LED driving circuit of a previous state of a next LED driving circuit (not shown in these Figs.) or the controller. 
     The LED driving circuits  501 ˜ 503  in  FIG. 5  are connected in series, where one of the LED driving circuit receives an output signal from the LED driving circuit of a previous state, and processes the signal with the logical unit  260  and  290  and then outputs the signal, and the LED driving circuit of the last stage sends the output signal to the controller  550  for regulating all of the input voltages of the LEDs  511 ˜ 513 . For example, the output of the double input OR gate of the LED driving circuit  502  in  FIG. 5  is coupled to the LED driving circuit  503  of a next stage, and the output of the double input OR gate of the LED driving circuit  503  is coupled to the controller  550 . In this embodiment, when the driving transistors of the LED driving circuits  501 ˜ 503  of every stage all enter the saturation region, the drain voltage comparator of each stage will output a high signal (H). The high signal is then inverted to a low signal (L) so that the double input OR gate of the LED driving circuit  503  of the last stage outputs a low signal, and thus the controller  550  lowers the output of the voltage source  570  to reduce the power loss of the driving transistors in each stage. On the contrary, when a driving transistor of an LED driving circuit of one stage does not operate in the saturation region, the drain voltage comparator in that stage will output a low signal. The low signal is then inverted to be a high signal so that the LED driving circuit  503  of the last stage outputs a high signal, and thus the controller  550  raises the voltage provided by the voltage source  570  to make sure that all of the driving transistors in every stage will be in saturation state. In an embodiment, the voltage source  570  is a DC to DC voltage converter, but the present invention should not be limited thereto. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.