Abstract:
The present invention discloses an LED driving circuit and controller, capable of maintaining an average value of a current flowing through an LED module at a predetermined current value. The LED driving circuit and controller compensates influences of any factors deviating the average value of the current flowing through the LED module by modulating a period of constant on time or constant off time, or by modulating the determining value(s) of current peak value and/or current valley value according to a difference between an actual average value and the preset current value.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 100109227, filed on Mar. 17, 2011. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to an LED driving circuit and an LED driving controller. 
     (2) Description of the Prior Art 
       FIG. 1  is a circuit diagram of a typical LED driving circuit. The LED driving circuit includes an inductance L, a transistor switch M 1 , a flywheel unit M 2 , an LED module  20 , a current detecting resistance Ri and a controller  10 . The controller  10  controls the transistor switch M 1  to be turned on/off by means of constant off-time control, so as to drive the LED module  20  lighting. The description of the detailed circuit operation is as following. 
     The controller  10  includes a comparator  2 , a SR flip-flop  6  and a constant off-time unit  4 . A non-inverting end of the comparator  2  receives a reference signal Vr and an inverting end thereof is coupled to the current detecting resistance Ri to receive a current detecting signal IFB. At the beginning of each cycle, the transistor switch M 1  is turned on and so an increasing current supplied by an input power source Vin flows through the inductance L, the LED module  20  to be grounded. When a level of the current detecting signal IFB is higher than that of the reference signal Vr, the comparator  2  generates a high-level signal to a set end S of the SR flip-flop  6 . Therefore, the SR flip-flop  6  is triggered to output a low-level signal at an inverting output end Q′ to cut the transistor switch M 1  off. Afterward, the current flowing through the LED module  20  freewheels through the flywheel unit M 2 , and an output end Q of the SR flip-flop  6  generates a high-level signal to the constant off-time unit  4 . After a predetermined off-time period, the constant off-time unit  4  generates a pulse signal to a reset end R of the SR flip-flop  6 , and so the SR flip-flop  6  generates a high-level signal at the inverting output end Q′ to turn on the transistor switch M 1  again for the next cycle. 
     In theory, the current of the LED module  20  is vibrated between a current peak value and a current valley value. However, the current valley value depends on the predetermined cut-off time period, and varies with a voltage of the input power source Vin. It results that an average value of the current is changed due to the input power source Vin, as same as an illumination and a color-temperature of the LED module  20 . Even the actual current peak value of the current the LED module  20  also varies because of time delay in circuit operation and the process error in the inductance value of the inductance L. 
     SUMMARY OF THE INVENTION 
     As mentioned above, the typical constant off time controller is utilized for driving the LED. The average value of current varies with the input voltage, the inductance value and etc. The LED driving circuit and the LED driving controller in the present invention adjust a determining level of the current peak/valley value or a constant on/off time period according to the actual average value of current and the predetermined current value, and so a deviation in the average value of current can be compensated to correct the average value of current to the predetermined current value. 
     To accomplish the aforementioned and other objects, the embodiment of the invention provides an LED driving circuit. The LED driving circuit comprises an LED module, an inductance, a flywheel unit, a switch module and an LED driving controller. The inductance is coupled to one end of the LED module. The flywheel unit is coupled to the LED module and the inductance for providing a current loop. The switch module is coupled to the LED module and controls a power from the input power source whether to supply to the LED module or not. The LED driving controller is utilized for cyclically turning the switch module on or off for a predetermined time period in response to a current detecting signal representing an amount of a current flowing through the LED module, wherein the predetermined time period is adjusted according to the current detecting signal to substantially stabilize an average value of the current flowing through the LED module at a predetermined current value. 
     The embodiment of the invention also provides an LED driving controller, utilized for controlling a switching converter circuit to supply a driving power to an LED module. The LED driving controller comprises a comparator unit, a switch control unit and a current corrective unit. The comparator unit generates a comparison result signal according to a current detecting signal representing an amount of a current flowing through the LED module. The switch control unit generates a switch control signal according to the comparison result signal and a predetermined time period to control a switch module of the switching converter circuit. The current corrective unit generates a current corrective signal in response to the current detecting signal to adjust the predetermined time period, thereby stabilizing an average value of the current flowing through the LED module at a predetermined current value. 
     The embodiment of the invention still also provides an LED driving controller, utilized for controlling a switching converter circuit to supply a driving power to an LED module. The LED driving controller comprises a hysteresis comparator unit, a switch control unit and a current corrective unit. The current corrective unit generates at least a reference signal according to a current detecting signal representing an amount of a current flowing through the LED module. The hysteresis comparator unit generates a comparison result signal according to the current detecting signal and the least a reference signal. The switch control unit generates a switch control signal in response to the comparison result signal to control a switch module of the switching converter circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be specified with reference to its preferred embodiment illustrated in the drawings, in which: 
         FIG. 1  is a circuit diagram of a typical LED driving circuit. 
         FIG. 2  is a circuit diagram of an LED driving circuit in accordance with a first exemplary embodiment of the present invention. 
         FIG. 3  is a circuit diagram of an LED driving circuit in accordance with a second exemplary embodiment of the present invention. 
         FIG. 4  is a circuit diagram of an LED driving circuit in accordance with a third exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  is a circuit diagram of an LED driving circuit in accordance with a first exemplary embodiment of the present invention. The LED driving circuit comprises a switching converter circuit and an LED driving controller  100 . The LED driving controller  100  controls the switching converter circuit to supply a driving power to an LED module  120 . The switching converter circuit includes an inductance L, a switch module SW and a flywheel unit D. One end of the inductance L is coupled to an input power source Vin, the other end thereof is coupled to a positive end of the LED module  120 . One end of the switch module SW is coupled to a negative end of the LED module  120 , and another end of the switch module SW is coupled to the ground. A positive end of the flywheel unit D is coupled to the negative end of the LED module  120 , and a negative end of the flywheel unit D is coupled to the input power source Vin to provide a freewheeling current loop for a current of the inductance L. 
     In the present embodiment, the LED driving controller  100  is a constant off time controller, comprising a comparator unit  102 , a SR flip-flop  106 , a constant off time unit  104 , a switch control unit and a current corrective unit  108 . A current detecting resistance Ri is coupled to the switch module SW and generates a current detecting signal IFB representing an amount of a current of the LED module  120  during the period of the switch module SW being turned on. The comparator unit  102  generates a comparison result signal according to the current detecting signal IFB and a reference signal Vr, which represents a predetermined current value. A set end S of the SR flip-flop  106  is coupled to an output end of the comparator unit  102 , a reset end R and an output end Q thereof are coupled to the constant off time unit  104 , and an inverting output end Q′ is coupled to a controlled end of the switch module SW. At the beginning of each cycle, the LED driving controller  100  turns the switch module SW on and so an increasing current from the input power source Vin flows through the inductance L, the LED module  120 , the switch module SW, the current detecting resistance Ri to ground. When the current increases to have a level of the current detecting signal IFB be higher than a level of the reference signal Vr, the comparator unit  102  generates a high-level signal to the SR flip-flop  106 . Then, the SR flip-flop  106  turns off the switch module SW and triggers the constant off time unit  104  to start counting time. The constant off time unit  104  generates a pulse signal to reset the SR flip-flop  106  when counting a predetermined time period, so that the SR flip-flop  106  turns on the switch module SW again for next cycle. The predetermined time period depends on an application circuit. An applicable predetermined time period is to operate the inductance L operates in the continuous current mode, i.e., the current of the inductance L is still above the zero current when the switch module SW is turned off. 
     The current corrective unit  108  determines a difference between a predetermined current value and an average value of the current, which is calculated according to the current detecting signal IFB, and accordingly generates a current corrective signal Ic to adjust the predetermined time period of the constant off time unit  104 . Thereby, the average value of the current flowing through the LED module  120  is substantially stabilized at the predetermined current value. In the present embodiment, the current detecting resistance Ri detects the current only when the switch module SW is turned on. The current corrective unit  108  is coupled to the comparator unit  102  and the SR flip-flop  106 , and determines a turned-on timing of the switch module SW. During that the switch module SW is turned on, the current corrective unit  108  determines whether the difference of the average value of the current and the predetermined current according to the current detecting signal IFB and the reference signal. If there is the difference, the current corrective unit  108  adjusts the predetermined time period. For example, when the average value of the current value is higher than the predetermined current value, the current corrective unit  108  lengthens the predetermined time period until that the average value is equal to the predetermined current value. On the other hand, when the average value is lower than the predetermined current value, the current corrective unit  108  shortens the predetermined time period until that the current average value is equal to the predetermined current value. Thus, the LED driving circuit of the present invention can be applicable for the full voltage range of 90-264V and has an advantage of compensating process errors of components and time delay in circuit operation resulted in deviation of the average value of the current flowing through the LED module  120 . 
     The embodiment mentioned above detects the peak value of the current and determines the period of the off time of the switch module according to the average value of the current to ensure that the current flowing through the LED module is within a suitable operation range and further the life spin of the LED module is extended. Of course, the present invention can alternatively detects a valley value of the current and determine a period of an on time of the switch module according to the average value of the current. The detailed description is described below. 
       FIG. 3  is a block diagram showing the LED driving circuit in accordance with a second exemplary embodiment of the present invention. The LED driving circuit includes a switching converter circuit and an LED driving controller  200 . The LED driving controller  200  controls the switching converter circuit to supply a driving power to an LED module  220 . The switching converter circuit comprises an inductance L, a switch module SW and a flywheel unit D. One end of the switch module SW is coupled to an input power source Vin, and the other end thereof is couple to one end of the inductance L. The other end of the inductance L is coupled to a positive end of the LED module  220 , and a negative end of the LED module  220  is coupled to the ground through a current detecting resistance Ri. A positive of the flywheel unit D is coupled to the negative end of the LED module  220 , and a negative end of the flywheel unit D is coupled to a node between the inductance L and the switch module SW to supply the current of the inductance L a freewheeling path. 
     In the present embodiment, the LED driving controller  200  is a constant on time controller, comprising a comparator unit  202 , a SR flip-flop  206 , a constant on time unit  204  and a current corrective unit  208 . Compared with the embodiment shown in  FIG. 2 , the present embodiment can detect a current of the LED module  220  when the switch module SW is turned on and turned off, i.e., full time. Thus, the LED driving controller  200  exactly determines the average value of the current flowing through the LED module  220 . The comparator unit  202  generates a comparison result signal according to the current detecting signal IFB generated by the current detecting resistance Ri and a reference signal Vr. A set end S of the SR flip-flop  206  is coupled to an output end of the comparator unit  202 , an output end Q thereof is coupled to the constant on time unit  204  and a controlled end of the switch module SW, a reset end R thereof is coupled to an output end of the constant on time unit  204 . At the beginning of each cycle, the current of the LED module  220  is lower and so the comparator unit  202  generates a high-level signal. Therefore, the RS flip-flop  206  generates a high-level signal at the output end Q to turn the switch module SW on and so an increasing current from the input power source Vin flows through the switch module SW, the inductance L, the LED module  220 , the current detecting resistance Ri to ground. The constant on time unit  204  starts counting time when receiving the high-level signal of the output end Q of the SR flip-flop  206 . The constant on time unit  204  generates a pulse signal to reset the RS flip-flop  206  after counting the predetermined time period. At this time, the SR flip-flop  206  cuts the switch module SW off and then the current of the inductance L freewheels through the flywheel unit D. When the current lowers to cause the comparator unit  202  to generate the high-level signal, the switch module SW is turned on again for next cycle. The current of the inductance L all keeps above a current valley value (represented by the reference signal Vr), so that the inductance L is operated in the continuous current mode. 
     In the present embodiment, the current corrective unit  208  is an error amplifier, which receives the current detecting signal IFB and the reference signal Vr for determining a difference of the average value of the current and the predetermined current value. Accordingly, the current corrective unit  208  generates a current corrective signal Ic to adjust the predetermined time period of the constant on time unit  204 . When the average value is higher than the predetermined current value, the current corrective unit  208  shortens the predetermined time period until that the average value is equal to the predetermined current value. On the other hand, when the average value is lower than the predetermined current value, the current corrective unit  208  lengthens the predetermined time period until that the current average value is equal to the predetermined current value. Thus, any factor of influencing the current can be dynamically compensated and so the average value of the current flowing through the LED module  220  is substantially stabilized at the predetermined current value. 
     The two embodiments mentioned above illustrate that the present invention is applied to the LED driving circuits with the constant off time control (for detecting the current peak value) and the constant on time control (for detecting the current valley value). Besides, the present invention can be applied to a LED driving circuit with a ripple mode control, which detects both the peak and valley value of the current of the LED module to control the switch module.  FIG. 4  is a circuit diagram showing the LED driving circuit in accordance with a third exemplary embodiment of the present invention. The LED driving circuit comprises a switching converter circuit and an LED driving controller  300 . The LED driving controller  300  controls the switching converter circuit to supply a driving power to an LED module  320 . The switching converter circuit includes an inductance L, a switch module SW and a flywheel unit D. One end of the switch module SW is coupled to an input power source Vin, and another end of the switch module SW is coupled to one end of the inductance L. The other end of the inductance L is coupled to a positive end of the LED module  320 . A negative end of the LED module  320  is coupled to, a positive end of the flywheel unit D to form a flywheeling path for the current of the inductance L. A current detecting resistance Ri is coupled to the other end of the inductance L and the ground, and generates a current detecting signal IFB representing an amount of the current flowing through the LED module  320 . 
     The LED driving controller  300  includes a hysteresis comparator unit  302 , a SR flip-flop  306 , an inverter  304  and a current corrective unit  308 . The current corrective unit  308  generates a reference signal Vr according to the current detecting signal IFB. The hysteresis comparator unit  302  generates a comparison result signal according to the current detecting signal IFB and the reference signal Vr. The current corrective unit  308  may generate two reference signals as hysteresis reference levels for different hysteresis comparator unit  302  which needs two reference level to perform the operation of hysteresis comparing. In the present embodiment, the hysteresis comparator unit  302  determines higher and lower hysteresis level according to the reference signal Vr and a hysteresis range. Thus, the current of the inductance L is vibrated with a current range above zero, i.e., the inductance L is operated in the continuous current mode. The current corrective unit  308  may be an error amplifier. In the present embodiment, take an integral circuit as the current corrective unit  308  to determine the average value of the current for example. The current corrective unit  308  described below also be applied to the two embodiments mentioned above for replacing the error amplifier. 
     The current corrective unit  308 , comprising a comparator  301 , a charging unit, a discharge unit and an integral unit  303 , is utilized for generating the reference signal Vr. The integral unit  303  may be a capacitance or a circuit with integral function. The charging unit has a first current source I 1 , which is coupled to the integral unit  303 , utilized to supply a basic charging current charging for the integral unit  303 . The discharge unit has a second current source I 2  and a switch  305 . The second current source I 2  is coupled to the integral unit  303  through the switch  305 , and provides a discharge current for discharging the integral unit  303 . Wherein, the current of the first current source I 1  is smaller than the current of the second current source I 2 . An inverting end of the comparator  301  receives a reference basic signal Vr′, and a non-inverting end thereof receives the current detecting signal IFB. Accordingly, the comparator  301  controls the switch  305  to be turned on/off. When a level of the current detecting signal IFB is lower than that of the reference basic signal Vr′, the comparator  301  outputs a low-level signal to turn the switch  305  off. At this time, the integral unit  303  is charged by the first current source I 1  to increase a voltage of the integral unit  303 . When the level of the current detecting signal IFB is higher than the level of the reference basic signal Vr′, the comparator  301  outputs a high-level signal to turn on the switch  305  and so the second current source I 2  discharges the integral unit  303 . Because the current of the first current source I 1  is smaller than the current of the second current source I 2 , the voltage of the integral unit  303  is reduced. Thus, when the average value of the current detecting signal IFB is higher than the reference basic signal Vr′, the current corrective unit  308  lowers the level of the reference signal Vr. On the other hand, when the average value of the current detecting signal IFB is lower than the reference basic signal Vr′, the current corrective unit  308  increases the level of the reference signal Vr. The hysteresis comparator unit  302  receives the current detecting signal IFB and the reference signal Vr to accordingly generate a comparison result signal. A set end S of the SR flip-flop  306  is coupled to an output end of the hysteresis comparator unit  302 , and a reset end R of the SR flip-flop  306  is coupled to the output end of the hysteresis comparator unit  302  through the inverter  304 . An output end Q of the SR flip-flop  306  is coupled to a controlled end of the switch module SW. At the beginning of each cycle, the current of the LED module  320  is lower and so the hysteresis comparator unit  302  generates a high-level signal. At this time, the output end Q of the SR flip-flop  306  generates a high-level signal to turn the switch module SW on. Then, an increasing current of the input power source Vin flows through the switch module SW, the LED module  320 , the inductance L, the current detecting resistance Ri to the ground. When the current of the LED module  320  increases to cause the hysteresis comparator unit  302  generating a low-level signal, the SR flip-flop  306  turns the switch module SW off and the current of the inductance L freewheels through the flywheel unit D. On the other hand, when the current of the LED module  320  decreases to cause the hysteresis comparator unit  302  generating the high-level signal, the SR flip-flop  306  turns the switch module SW on again for next cycle. 
     The current corrective unit  308  may be applied to the LED driving controller  100  shown in  FIG. 2  to generate the reference signal Vr received by the comparator unit  102  to replace the current corrective unit  108 . Therefore, the LED driving controller  100  with the current corrective unit  308  can stabilize the average value of the current flowing through the LED module  120  substantially at the predetermined current value by adjusting the reference signal Vr. Similarly, the current corrective unit  308  may be applied to the LED driving controller  200  shown in  FIG. 3  to generate the reference signal Vr received by the comparator unit  202  to replace the current corrective unit  208  for stabilizing the average value of the current flowing through the LED module  220  substantially at the predetermined current value. 
     While the preferred embodiments of the present invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the present invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the present invention.