Abstract:
A light emitting diode driving circuit comprising: a current control unit, a current detection unit, and a driving control unit. The current control unit connects to the current detection unit and comprises a control end, a first input/output end, and a second input/output end to individually produce an electric potential detection signal. The light emitting diode driving circuit determines whether driving the light emitting diode is unusual or not according to the electric potential detection signals to decide to start a protect mechanism. Furthermore, the present invention can receive any types of dimming signals to adjust the light of the light emitting diode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a light emitting diode driving circuit and a controller thereof, and more particularly to a light emitting diode driving circuit and a controller thereof with dimming function and abnormality response. 
         [0003]    2. Description of Related Art 
         [0004]    Compared with the widely used fluorescent lamps, light emitting diode (LED) has the advantages of long lifetime, economical power consumption, low driving voltage, high security, environmental protection oriented, and etc. As to the liquid crystal display (LCD) industry, because LED as a light source of the backlight module has color saturation much higher than that of the present main-stream cold cathode fluorescent lamp (CCFL) and is completely conformed to the regulations of restriction of hazardous substance (ROHS). Therefore, the industry has devoted lots of efforts in research and development for expecting to use LED to take over the market of the other types of lighting sources. 
         [0005]      FIG. 1  shows a circuit diagram of a typical current balancing LED driving circuit. As shown, an alternative current (AC) input from an input power source AC is first converted into a direct current (DC) voltage VDD through an AC-to-DC converter  30 . Subsequently, the DC voltage VDD is converted into a driving voltage Vin through a DC-to-DC converter  40 , and the driving voltage Vin is then provided to the LED driving circuit. The AC-to-DC converter  30  has a bridge rectifier (not shown) for converting the AC input into the DC voltage VDD. Voltage level of the DC voltage VDD is then reduced to generate the driving voltage Vin by using a switched-mode power converting circuit (not shown) inside the DC-to-DC converter  40 . The LED driving circuit has a LED module  10  and a current balancer  20 . The LED module  10  is composed of a plurality of LEDs connected in parallel and in series. In order to have brightness of the LEDs of different LED strings being consistent, current flowing through each of the LED strings must be identical by using the current balancer  20 . The current balancer  20  equalizes current on the different LED strings by using the architecture of current mirror. The magnitude of current flowing through each of the LEDs can be controlled by the resistor R. 
         [0006]    The magnitude of current flowing through the LED can be represented by the function: 
         [0000]        I =( VDD−VG )/ R,    
         [0007]    where VG is the gate voltage of the semiconductor switch in the current balancer  20 . 
         [0008]    However, conventional current balancing LED driving circuit lacks an adequate protection mechanism. If any abnormality occurs in the circuit, such as short circuit, open circuit, operating range exceeded etc., unnecessary power loss or even circuit damage would be resulted because the current balancing LED driving circuit is operating. Besides, typical current balancing LED driving circuit does not have dimming function. It is hard to satisfy the requirements of some specific applications (e.g. back-light module). The application of the current balancing LED driving circuit is thus limited. 
       SUMMARY OF THE INVENTION 
       [0009]    In view of the above-discussed issues, the present invention discloses an LED driving circuit with dimming function and abnormality response. The LED driving circuit may determine if any error occurs when driving the LED based on the detection signal of the LED and is capable to activate the protection mechanism to stop the operation of the driving circuit or provide warning signals. In addition, the LED driving circuit is capable to control brightness of the LED based on any mode of dimming signals. 
         [0010]    To achieve the aforementioned advantages, an LED driving circuit comprising a conversion circuit, at least one current control unit, at least one current detection unit, and a driving control unit is provided in the present invention. The conversion circuit is coupled to an input power source for converting an input power from the input power source into a DC output signal to drive an LED module. Each of the current control units has a control pin, a first input/output (I/O) pin and a second I/O pin, and the first I/O pin is coupled to the LED module. Each of the current detecting modules is coupled to the second I/O pin of the respective current control unit to generate at least one detection signal. The driving control unit is coupled to the control pin and the first I/O pin of the current control unit, and also coupled to the current detection unit. The driving control unit is capable to adjust voltage level (i.e. high or low) of the control pin of the current control unit based on the detection signal so as to have the current flowing through the LED module stabilized around a preset current value. In case voltage level of one of the first I/O pins being higher than a preset level, the driving control unit cuts off the respective current control unit. 
         [0011]    The present invention provides another LED driving circuit, which comprises a conversion circuit, at least one current control unit, at least one current detection unit, and a driving control unit. The conversion circuit is coupled to an input power source in order to convert an input power from the input power source into a DC output signal to drive an LED module. Each of the current control units has a control pin, a first I/O pin, and a second I/O pin, wherein the first I/O pin is coupled to the LED module. Each of the current detection units is coupled to the second I/O pin of the respective current control unit to generate at least one detection signal. The driving control unit is coupled to the control pin and the first I/O pin of the current control unit, and also coupled to the current detection unit. The driving control unit is capable to adjust voltage level of the control pin of the respective current control unit based on the detection signal so as to have current flowing through the LED module stabilized around a preset current value. The above-mentioned driving control unit is capable to generate a fault signal as voltage level of one of the first I/O pins is higher than a first preset level. 
         [0012]    The present invention also provides an LED driving circuit controller to control an LED driving circuit for driving an LED module. The LED driving circuit controller comprises a current adjusting unit and a signal processing unit. The current adjusting unit generates a current adjusting signal based on a current detection signal, which indicates magnitude of current flowing through the LED module, for controlling a power switch connected in series to the LED module to have voltage level of the current detection signal stabilized around a preset voltage value. The signal processing unit detects voltage level of the voltage signal from a coupling point between the power switch and the LED module, receives the current detection signal, and generates a fault signal when voltage level of the current detection signal lower than a first preset level over a preset period of time or voltage level of the voltage signal from the coupling point being higher than a second preset level. 
         [0013]    The Summary set out supra and the Detailed Descriptions discussed infra are for illustrative purposes only in order to further construe the scope of the present invention. Other objectives and advantages related to the present invention will be thoroughly explained in the subsequent descriptions and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a circuit diagram of a typical current balancing LED driving circuit; 
           [0015]      FIG. 2  is a circuit diagram of an LED driving circuit of a preferred embodiment according to the present invention; 
           [0016]      FIG. 3  is a circuit diagram of a driving control unit of an embodiment according to the present invention; 
           [0017]      FIG. 4  is a circuit diagram of a driving control unit of another embodiment according to the present invention; 
           [0018]      FIG. 5  is a circuit diagram of a driving control unit of yet another embodiment according to the present invention; and 
           [0019]      FIG. 6  is a circuit diagram of an LED driving circuit of another embodiment according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0020]    In present, the available Light Emitting Diode (LED) driving circuit with fixed voltage level to drive LED does not have dimming function and protection mechanism. The LED driving circuit according to the present invention not only provides protection mechanism against circuit abnormality but also performs dimming function based on a dimming signal. In addition, the LED driving circuit according to the present invention may achieve the feature of synchronous dimming on multiple LED groups by means of outputting dimming Pulse-Width-Modulation (PWM) signals. The technology of the present invention will be illustrated by the embodiments as follow. 
         [0021]    Refer to  FIG. 2 , a circuit diagram of an LED driving circuit of a preferred embodiment according to the present invention is shown. The LED driving circuit comprises a conversion circuit  200 , a current control unit  120 , a current detection unit  122 , and a driving control unit  130 . The conversion circuit  200  is coupled to an input power source AC in order to convert an input power from the input power source AC into a DC output signal Vout to drive an LED module  110 . The current control unit  120  has a semiconductor switch. In addition, the current control unit  120  has a control pin P 0 , a first I/O pin P 1 , and a second I/O pin P 2 . The first I/O pin P 1  of the current control unit  120  is coupled to the LED module  110  so that the current control unit  120  is connected to the LED module  110  in serial. The current detection unit  122  may be a resistor coupled to the second I/O pin P 2  of the current control unit  120  to generate a current detection signal S 1  based on the magnitude of the current flowing through the LED module  110 . The driving control unit  130  is coupled to the control pin P 0  and the first I/O pint P 1  of the current control unit  120 , and also coupled to the current detection unit  122 . The driving control unit  130  may adjust voltage level at the control pin P 0  of the current control unit  120  based on the detection signal S 1  so as to change the magnitude of equivalent resistance of the current control unit  120  to have the current flowing through the LED module  110  stabilized around a preset current value. 
         [0022]    The above mentioned conversion circuit  200  can be a flyback power converter, a forward power converter, a pull-push power converter, a half-bridge power converter, a full-bridge power converter, or a DC-to-DC converter. In the present embodiment, a flyback power converter is shown as an example to illustrate the present invention. As shown, the flyback power converter comprises a transformer T, a switch set SW (in the flyback power converter, the switch set SW has one semiconductor switch, while in other types of power converters, such as half-bridge type and full-bridge type, the switch set may have a plurality of semiconductor switches), an output capacitor  216 , a voltage detection unit  230 , and a primary side control unit  220 . The transformer T has a primary side and a secondary side, wherein the primary side is coupled to the input power source AC via a bridge rectifier BD, the secondary side generates the DC output signal Vout through a rectifying unit  214 . The switch set SW is coupled to the primary side of the transformer T and is switched between conducting state and off state based on a control signal from the primary side control unit  220 . The output capacitor  216  is coupled to the secondary side of the transformer T for filtering out the noise from the DC output signal Vout so as to stabilize the voltage level of the DC output signal Vout. The voltage detection unit  230  is coupled to the secondary side of the transformer T for detecting the voltage level of the DC output signal Vout to generate a voltage detection signal. The voltage detection signal is then transferred to the primary side control unit  220  through an isolation unit  232 , which is capable to effectively isolate the primary side from the secondary side of the transformer T. The primary side control unit  220  identifies whether the DC output signal Vout is excessively high or over low based on the feedback voltage detection signal so as to adjust pulse width of the control signal to control the switch set SW, such that the DC output signal Vout can be stabilized around the preset voltage value. 
         [0023]    The primary side of the above-mentioned transformer T may have an input capacitor  212 , an initial driving circuit  222 , and a primary side auxiliary coil. The input capacitor  212  is used to stabilize the voltage level of a DC input voltage from the bridge rectifier BD. The initial driving circuit  222  has a resistor, a capacitor, and a diode. As the input power source AC is connected to the bridge rectifier BD, the capacitor begins to be charged via the resistor. When the voltage level in the capacitor reaches an enable level, the primary side control unit  220  starts to operate and control the switching of the switch set SW. Then, the primary side auxiliary coil charges the capacitor of the initial driving circuit  222  through the diode of the initial driving circuit  222  with the energy stored in the transformer T. Since the coil ratio between the primary side auxiliary coil and the secondary side coil is given, while the DC output signal Vout is stabilized around the preset voltage value, the voltage in the capacitor of the initial driving circuit  222  would be also stabilized around a fixed voltage value. In addition, the secondary side of the transformer T may have a secondary side auxiliary coil as well. For the same reason, since the coil ratio between the secondary side auxiliary coil and secondary side coil is also given, the voltage level charged to an auxiliary output capacitor  216 ′ via an auxiliary rectifying unit  214 ′ would be also stabilized around a fixed voltage value so as to provide a driving voltage VDD to drive the driving control unit  130 . 
         [0024]    The driving control unit  130  has a signal processing unit  132  and a current adjusting unit  134 . Because current flowing through the LED module  110  will flow through the current detection unit  122  also, the current detection unit  122  is capable to generate the current detection signal S 1  for indicating the magnitude of the current flowing through the LED module  110 . The current adjusting unit  134  may be an error amplifier, which adjusts the output current adjusting signal by comparing the current detection signal S 1  with a first preset level Vref 1  to adjust the voltage level of the control pin of the current control unit  120  (i.e. the gate voltage of the semiconductor switch in the current control unit  120 ). In this way, equivalent resistance of the semiconductor switch can be adjusted to have the voltage level of the current detection signal S 1  stabilized around the first preset level Vref 1 , that is, the current flowing through the LED module  110  can be stabilized at a preset current value. The signal processing unit  132  receives the current detection signal S 1 , a control pin voltage signal S 3  of the current control unit  120 , a coupling point voltage signal S 2  (the voltage signal of the first I/O pin P 1  of the current control unit  120 ), a second preset level Vref 2 , a third preset level Vref 3 , a fourth preset level Vref 4 , an on/off signal ONOFF, a DC dimming signal PWMDC, and a clock signal CLOCK. The signal processing unit  132  determines whether any abnormality occurs in the LED driving circuit based on the above-mentioned signals, and if any abnormality has occurred, the signal processing unit  132  generates a fault signal FAULT. The fault signal FAULT may be transferred to the primary side control unit  220  as shown in  FIG. 2  for the primary side control unit  220  to decide whether to stop the operation of the LED driving circuit or not. In addition, the fault signal FAULT may be output to a microprocessor of the system, e.g. the image microprocessor of a liquid crystal display, for the microprocessor to determine the suitable process to be taken; or notify the user through an user interface component, e.g. a fault indicator, and let the user to decide whether to stop the operation of the LED driving circuit or not. The detailed operations of the driving control unit  130  and the internal circuit thereof are described below. 
         [0025]    Refer now to  FIG. 3 , a circuit diagram of a driving control unit of an embodiment according to the present invention is shown. The driving control unit  130  comprises a signal processing unit  132  and a current adjusting unit  134 . The signal processing unit  132  has four comparators  302 ,  304 ,  306 , and  308 , an AND gate  310 , an OR gate  312 , three time filtering units  314 ,  316 , and  318 , and a ramp signal generator  320 . The current adjusting unit  134  may be an error amplifier comparing the current detection signal S 1  and the first preset level Vref 1 . When the current detection signal S 1  is lower than the first preset level Vref 1 , the current adjusting unit  134  increases the voltage level of the output current adjusting signal, i.e. the gate voltage of the semiconductor switch in the current control unit  120 , so as to reduce the equivalent resistance of the semiconductor switch. The magnitude of the current flowing through the LED module  110  as well as the voltage level of the current detection signal S 1  is thus increased. On the other hand, when the current detection signal S 1  is higher than the first preset level Vref 1 , the current adjusting unit  134  reduces the voltage level of the output current adjusting signal to increase the equivalent resistance of the semiconductor switch. The magnitude of the current flowing through the LED module  110  as well as the voltage level of the current detection signal S 1  is thus decreased. By means of the aforementioned feedback control mechanism, it is possible to stabilize the current flowing through the LED module  110  around a preset current value. 
         [0026]    The comparator  304  in the signal processing unit  132  compares the current detection signal S 1  and the second preset level Vref 2 . The second preset level Vref 2  is lower than the first preset level Vref 1 . Under normal condition, the current detection signal S 1  is equal to the first preset level Vref 1 , and the comparator  304  outputs a low-level signal. Whereas, under abnormal conditions, such as open circuit in the LED driving circuit, the current passing through the LED module  110  cannot be adjusted to the preset current value. At this moment, the current detection signal S 1  would be lower than the second preset level Vref 2 , and the comparator  304  outputs a high-level signal indicating the circuit abnormality. Since the conduction of the LED driving circuit would be wrongly judged by the comparator  304  when the LED driving circuit is starting-up or during the dimming process, the signal processing unit  132  according to the present invention adopts a time filtering unit  316  and uses a clock signal CLOCK to determine whether the abnormal condition lasting over a preset period of time. If so, a signal which indicating the abnormality is sent to the OR gate  312 . In the present embodiment, the clock signal CLOCK is externally provided. However, in practice, the clock signal may be generated internally by the driving control unit  130 . 
         [0027]    The comparator  302  in the signal processing unit  132  compares the third preset level Vref 3  with the coupling point voltage signal S 2 , i.e. the voltage signal of the first I/O pin P 1  of the current control unit  120 . Under normal operating condition, the coupling point voltage signal S 2  falls in a Safe Operating Area (SOA) such that the coupling point voltage signal S 2  is lower than the third preset level Vref 3 . The consideration concerning the setting of the third preset level Vref 3  is to build a limitation for preventing the coupling point voltage signal S 2  from exceeding the tolerable voltage level of the current control unit  120  or resulting undesirable reduction in conversion efficiency. The third preset level Vref 3  can be generated inside the driving control unit  130  or externally supplied. For example, as the current control unit  120  is designed to withstand 30-volt voltage, the third preset level Vref 3  can be set at 25 volts, or as the DC output signal Vout is set at 30 volts, the third preset level Vref 3  can be set at 6 volts under the consideration of conversion efficiency over 80%. In case a short circuit happened in the LED module  110 , for example, an LED in the LED module  110  is fail, results in the reduction of voltage drop across the LED module  110 , the potential of the coupling point voltage signal S 2  is increased to exceed the third preset level Vref 3 . Then, the comparator  302  outputs a high-level signal representing the abnormal condition to the OR gate  312 . It is noted that there may be two or more reference levels set in the comparison of the coupling point voltage signal S 2  under different circumstances, such as the voltage level of 6 volts and 25 volts in the above-mentioned example. 
         [0028]    The signal processing unit  306  in the signal processing unit  132  compares the fourth preset level Vref 4  with the control pin voltage signal S 3  of the current control unit  120  when the on/off signal ONOFF is high (which indicates the conducting state). The on/off signal ONOFF may be an enable signal or a burst dimming signal. In addition, the enable signal and the burst dimming signal may be input to the same pin of the signal processing unit  132 , and judged by a time filtering unit  314 . That is, if the on/off signal ONOFF is maintained at the same level over a predetermined period of time, the on/off signal ONOFF is judged as the enable signal, otherwise the on/off signal ONOFF should be the burst dimming signal. When the on/off signal ONOFF is low (which indicates the off state), the current adjusting unit  134  may cut off the current control unit  120 . When the on/off signal ONOFF is high to indicate the conducting state, the current adjusting unit  134  resumes its operation to control the current control unit  120  to have the current flowing through the LED module  110  stabilized around a preset current value. Thereby, the LED driving circuit in accordance with the present invention can provide dimming function. The detailed operation of the comparator  306  is mentioned below. 
         [0029]    Under normal operating condition, the semiconductor switch of the current control unit  120  operates in the linear region, therefore the equivalent resistance of the current control unit  120  can be adjusted by the potential of the control pin P 0 . However, when the on/off signal ONOFF is low to indicate the off state, the current adjusting signal provided by the current adjusting unit  134  would be low to cut off the current control unit  120 . At this time, the control pin voltage signal S 3  would be wrongly determined. To prevent this problem, the comparator  306  stops operating when the on/off signal ONOFF is low, and detects the voltage level of the current adjusting signal when the on/off signal ONOFF is not low. In addition, in case the current control unit  120  is damaged to result in short circuit, the control pin voltage signal S 3  cannot be used to control the magnitude of the current flowing through the LED module  110 . At this time, voltage level of the control pin voltage signal S 3  of the current control unit  120  would be lower than the fourth preset level Vref 4 . Then, the comparator  306  would output a high-level signal indicating the abnormal condition. To avoid any possible wrong determination made by the comparator  306  during the transition of the on/off signal ONOFF, the time filtering unit  318  is employed. 
         [0030]    When the OR gate  312  receives the signal indicating abnormality from any one of the comparators  302 ,  304  and  306 , a fault signal FAULT is generated to indicate the occurrence of abnormal circuit operation. Furthermore, when the abnormality occurs and a fault signal FAULT is generated, the AND gate  310  may shutdown the driving control unit  130  based on the received the fault signal FAULT, as shown in  FIG. 3 , or the fault signal FAULT is merely used to notify microprocessors in the system or the user as described above. Certainly, in practical, other adequate processes after the fault signal FAULT has been generated can be performed based on the conditions of abnormality. 
         [0031]    In addition to the above-mentioned burst dimming signal, the dimming signal may be a DC dimming signal too. As shown in  FIG. 3 , when a DC dimming signal PWMDC is provided, the comparator  308  compares the DC dimming signal PWMDC with a ramp signal generated by the ramp signal generated  320  to output a pulse-width-modulation signal PWMOUT. The pulse-width-modulation signal PWMOUT can be output to a driving control unit of another LED driving circuits as the on/off signal ONOFF to achieve the purpose of synchronous dimming. Whereas, if no such requirement, the pulse-width-modulation signal PWMOUT can be received by the original driving control unit  130  as the on/off signal ONOFF to periodically cut off the current control unit  120  to achieve the dimming function. In practice, the output end for outputting the pulse-width-modulation signal PWMOUT and the input end for inputting the on/off signal ONOFF of the driving control unit  130  can be packaged as two different pins. When the dimming signal is a DC dimming signal and no synchronous requirement, it may simply connect these two pins. 
         [0032]    Refer now to  FIG. 4 , a circuit diagram of a driving control unit of another embodiment according to the present invention is shown. Comparing the embodiment depicted in  FIG. 4  with the embodiment in  FIG. 3 , the major difference there between lies in the current control unit  120 . The current control unit  120  in  FIG. 4  comprises a current mirror unit  124  and a selection unit  126 . The current mirror unit  124  consists of a plurality of semiconductor switches, and each semiconductor switch has a control end, a first I/O end and a second I/O end. The control ends of these semiconductor switches are coupled to each other to form the control pin P 0  of the current control unit  120 . The first I/O ends of these semiconductor switches are coupled to the LED module  110 . The second I/O ends of these semiconductor switches are coupled to the current detection unit  122  to form the second I/O pin P 2  of the current control unit  120 . The selection unit  126  is coupled to the first I/O ends of these semiconductor switches of the current mirror unit  124  and selectively outputs one of the voltage level signals on these first I/O ends as the coupling point voltage signal S 2  to the driving control unit  130 . The selecting unit  126  may comprise a plurality of diodes. Positive ends of these diodes are coupled to the respective first I/O ends of the semiconductor switches in the current mirror unit  124 , while negative ends of these diodes are coupled with each other to form the first I/O pin P 1  of the current control unit  120 . In this way, the selection unit  126  may output the voltage level signal with highest potential among the first I/O ends of these semiconductor switches. That is, when abnormality occurs to cause voltage level of first I/O end of a certain semiconductor switch abnormally rising, the driving control unit  130  may determine the abnormality and output the fault signal FAULT. Thereby, the driving control unit according to the present invention can drive the LED module with multiple LED series. 
         [0033]    Refer now to  FIG. 5 , a circuit diagram of a driving control unit of yet another embodiment according to the present invention is shown. Compared with the embodiment in  FIG. 4 , the signal processing unit  132  of the driving control unit  130  in  FIG. 5  is composed of a plurality of signal processors  132 a,  132 b,  132 c operating individually. In  FIG. 5 , the components having the identical function in compared with that in the embodiment shown in  FIG. 3  are given the same reference number and added label a, b, c for grouping the components. Since the operations of the components in each group follow the description of  FIG. 3 , only the operations concerning the relationship between these groups are illustrated. In the present embodiment, in order to avoid simultaneous switching of the semiconductor switches  120   a ,  120   b ,  120   c  to cause greater voltage ripple, a phase splitter  136  is used to generate pulse-width-modulation signals PWMa, PWMb, PWMc based on the delay signal Delay to control the current adjusting units  134   a ,  134   b ,  134   c , respectively, such that there exists a time interval between the switching time of these semiconductor switches  120   a ,  120   b ,  120   c . The delay signal Delay may be an external signal or generated inside the driving control unit  130 . When any one of the signal processors  132   a ,  132   b ,  132   c  determines that abnormality occurs in the corresponding LED driving circuit, such as any one of the control pins of the semiconductor switches  120   a ,  120   b ,  120   c  is lower than the fourth preset level Vref 4 ; any one of the second I/O pins is persistently lower than a second preset level Vref 2  over a preset period of time; or voltage level on any one of the first I/O pins is higher than a third preset level Vref 3 , the driving control unit  130  may cut off the semiconductor switch  120   a ,  120   b ,  120   c  of the corresponded current control unit to shut down the operations of the abnormal LED driving circuit or cut off all the semiconductor switches  120   a ,  120   b ,  120   c  of the current control units. In addition, the driving control unit  130  may further send a fault signal FAULT through the OR gate  138 . The on/off signal ONOFF received by the phase dividing unit  136  may be a burst dimming signal or a DC dimming signal PWMDC. When the DC dimming signal PWMDC is provided, the DC dimming signal PWMDC can be converted into the burst dimming signal by any one of the signal processors  132   a ,  132   b ,  132   c , e.g. the signal processor  132   c  shown in  FIG. 5 . The phase splitter  136  also outputs a pulse-width-modulation signal PWMOUT for the purpose of synchronization with other circuits. Furthermore, the LED modules  110   a ,  110   b ,  110   c  may use identical LEDs or different LEDs. For example, the LED modules  110   a ,  110   b ,  110   c  may be a red light LED module, a green light LED module, and a blue light LED module, respectively. If the three LED modules  110   a ,  110   b ,  110   c  are identical, the first preset level Vref 1  received by the current adjusting units  134   a ,  134   b ,  134   c  can be the same, while in case of different LED modules, the first preset levels Vref 1  corresponding to the three current adjusting units  134   a ,  134   b ,  134   c  may be different. 
         [0034]    Refer now to  FIG. 6 , a circuit diagram of an LED driving circuit of another embodiment according to the present invention is shown. Compared with the aforementioned embodiments, the conversion circuit  200 ′ in the LED driving circuit of  FIG. 6  is a DC-to-DC step-up conversion circuit with step-up ratio (output voltage/input voltage) depending on the duty cycle of the control signal of a PWM controller, rather than on the coil ratio as in the case of a transformer. The conversion circuit  200 ′ in  FIG. 6  comprises a converting unit  210 ′, a conversion control unit (i.e. the PWM controller)  220 ′, and a voltage detection unit  230 ′. The converting unit  210 ′ has an inductor, a rectifying unit, a capacitor, and a switch. The inductor is coupled to a DC input power source Vdc. The switch is coupled to the inductor and is switching between conducting state and off state based on the control signal generated by the conversion control unit  220 ′. One end of the rectifying unit is coupled to the coupling point between the inductor and the switch, and the other end is coupled to the LED module  110 . The capacitor is coupled to the coupling point between the rectifying unit and the LED module  110  in order to provide the DC output signal Vout. The voltage detection unit  230 ′ is coupled to one of the two ends of the LED module  110 , and generates a voltage detection signal based on the voltage level of the coupled end of the LED module  110 . The conversion control unit  220 ′ generates the control signal based on the voltage detection signal to have the voltage level at the end of the LED module  110  detected by the voltage detection unit  230 ′ stabilized around a preset voltage value. As illustrated in  FIG. 6 , the voltage detection unit  230 ′ is coupled to the negative end of the LED module  110  which is also the first I/O pin P 1  of the current control unit  120 . Thereby, the voltage level on the first I/O pin P 1  can be stabilized at a preset voltage value to ensure conversion efficiency of the LED driving circuit. 
         [0035]    The illustrations set out supra discuss merely the detailed descriptions and drawings of the preferred embodiments according to the present invention, rather than for restricting the present invention thereto. The scope of the present invention should be delineated by the subsequent claims, and all embodiments conforming to the spirit of the present invention as well as analogous variations thereof are deemed to be encompassed by the scope of the present invention. All changes or modifications that those skilled in the art can conveniently think of in the field of the present invention are deemed to be embraced within the scope defined by the following claims.