Patent Publication Number: US-9420654-B2

Title: Feedback control circuit and LED driving circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of China patent application serial no. 201310174923.4, filed on May 13, 2013. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of the specification. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a feedback control circuit and LED driving circuit. 
     (2) Description of the Prior Art 
     In general, the driving methods for LED can be classified into a constant voltage driving and a constant current driving. Due to the characteristics of the LED, the constant current driving method can optimize the luminous efficiency of LED, and it is the most popular method of driving LED. The common constant current driving method uses an error amplifier to adjust a driving voltage of an LED string through detecting a negative end voltage of the LED string. 
       FIG. 1  is a schematic diagram of a conventional an LED driving circuit with constant current controlling. The LED driving circuit comprises a boost converter circuit, a control circuit  10  and a current control circuit ILC for driving an LED module LD. The boost converter circuit comprises an inductance L, a capacitance C, a diode D and a transistor M. One end of the inductance L is coupled to an input voltage Vin and the other end thereof is coupled to a positive end of the diode D. A negative end of the diode D is coupled to the capacitance C to provide an output voltage Vout for driving the LED module LD. The transistor M is coupled to a connected point of the diode D and the inductance L, and is switched according to a control signal Sdrv to make an electric power from the input voltage Vin be stored in the inductance L and the capacitance C. A positive end of the LED module LD is coupled to the output voltage Vout, and the negative end thereof is coupled to the current control circuit ILC. The current control circuit ILC controls the current flowing through the LED module LD at a predetermined current value steadily. 
     The control circuit  10  comprises an error amplifier  1 , a compensation circuit  2 , a PWM comparator  3 , a logic circuit  4  and a driver circuit  5 . An inverting input end of the error amplifier  1  is coupled to a negative end of the LED module L for receiving a detection signal IFB, and a non-inverting input end thereof receives a reference level Vr. An output end of the error amplifier  1  is coupled to the compensation circuit  2  and generates an error compensated signal Scomp at the compensation circuit  2  according to the detection signal IFB and the reference level Vr. A non-inverting input end of the PWM comparator  3  receives the error compensated signal Scomp, and an inverting input end thereof receives a ramp signal and accordingly generates a PWM signal Spwm. The logic circuit  4  receives the PWM signal Spwm and accordingly generates a PWM control signal Sct. The driver circuit  5  receives the PWM control signal Sct and accordingly generates the control signal Sdrv to control the duty cycle of the transistor M for adjusting the output voltage Vout. 
       FIG. 2  is a schematic diagram of another conventional LED driving circuit, adapted to drive the plural of the LED strings of the backlight module of the LCD monitor lighting. The currents of the plural of the LED strings L 1 ˜LN are respectively controlled by current sources CS 1 ˜CSN. A backlight control circuit  20  comprises a minimum voltage selection circuit  21 , which is adapted to choose the minimum voltage among negative ends of all LED strings L 1 ˜LN, and transmit a minimum voltage signal to an error amplifier  13 . The error amplifier  13  controls a voltage supply circuit  11  according to the minimum voltage signal and a reference level Vr, and transforms an input voltage Vin into an output voltage Vout. 
     These control loop methods for driving LED require complicated loop compensation, which increases the difficulty in design. 
     SUMMARY OF THE INVENTION 
     For solving the aforementioned disadvantages of the prior art, the present invention provides a feedback control circuit and LED driving circuit to avoid a complicated pole-zero compensation and simultaneously achieves the high luminous efficiency of LED. 
     To accomplish the aforementioned and other objects, the present invention provides a feedback control circuit, adapted to control a converting circuit to execute a power transformation for driving an LED module, and the LED module has at least one LED string connected in parallel. The feedback control circuit comprises a detection circuit, a PWM circuit, a PWM logic control circuit and a PWM control circuit. The detection circuit is coupled to the least one LED string of the LED module and generates at least one detection signal is response to a state of the least one LED string. The PWM circuit comprises a capacitance, a charging circuit and a discharging circuit, and the charging circuit and the discharging circuit determine a capacitance voltage of the capacitance to increase, decrease or maintain according to a set of control signals. The PWM logic control circuit generates the set of the control signals in response to the compared results of a level of at least one detection signal with a high reference level and a low reference level, wherein the high reference level is higher than the low reference level. The PWM control circuit controls the converting circuit to execute the power transformation in response to the capacitance voltage of the capacitance. 
     The present invention also provides an LED driving circuit, adapted to drive a plural of the LED strings connected in parallel. The LED driving circuit comprises a converting circuit, a plural of the current control circuits and a feedback control circuit. The converting circuit, adapted to execute a power transformation for driving the plural of the LED strings. Each of the current control circuit has a current control end, coupled to the corresponding LED string of the plural of the LED strings for flowing through a corresponding LED string a predetermined current value. The feedback control circuit comprises a minimum voltage detection circuit, a PWM circuit, a PWM logic control circuit and a PWM control circuit. The minimum voltage detection circuit is coupled to the current control ends and generates a detection signal according to a minimum voltage among the current control ends. The PWM circuit comprises a capacitance, a charging circuit and a discharging circuit. The charging circuit and the discharging circuit determine a capacitance voltage of the capacitance to increase, decrease or maintain according to a set of control signals. The PWM logic control circuit generates the set of the control signals in responses to compared results of a level of the detection signal with a high reference level and a low reference level. The high reference level is higher than the low reference level. The PWM control circuit controls the converting circuit to execute the power transformation in response to the capacitance voltage of the capacitance. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. In order to make the features and the advantages of the invention comprehensible, exemplary embodiments accompanied with figures are described in detail below. 
    
    
     
       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 schematic diagram of a conventional an LED driving circuit with constant current controlling. 
         FIG. 2  is a schematic diagram of another conventional LED driving circuit. 
         FIG. 3  is a schematic diagram of an LED driving circuit according to a first preferred embodiment of the present invention. 
         FIG. 4  is a schematic diagram of a detection circuit according to a preferred embodiment of the present invention. 
         FIG. 5  is a schematic diagram of a compared result logic circuit according to a first preferred embodiment of the present invention. 
         FIG. 6  is a schematic diagram of an LED driving circuit according to a second preferred embodiment of the present invention. 
         FIG. 7  is a schematic diagram of a compared result logic circuit according to a second preferred embodiment of the present invention. 
         FIG. 8  is a schematic diagram of an LED driving circuit according to a third preferred embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawings. 
       FIG. 3  is a schematic diagram of an LED driving circuit according to a first preferred embodiment of the present invention. The LED driving circuit, adapted to drive a LED module LD which comprises an LED string, comprises a converting circuit  120 , a current control circuit ILC and a feedback control circuit  100 . The converting circuit  120  is coupled to an input voltage Vin, and transforms an electric power from the input voltage Vin into an output voltage Vout to drive the LED string of the LED module lighting. The current control circuit ILC has a current control end Ch coupled to the LED module LD for controlling the LED string flowing through a predetermined current value. 
     The feedback control circuit  100  comprises a detection circuit  102 , a PWM circuit, a PWM logic control circuit and a PWM control circuit  118 . The detection circuit  102  is coupled to the LED module LD. In the present embodiment, the detection circuit  102  is coupled to the current control end Ch for generating a detection signal Scs in response to a voltage or a current of the LED string. The PWM logic control circuit comprises a comparison circuit  105 , which compares a level of the detection signal Scs with a high reference level Vrh and a low reference level Vrl, and accordingly generates a high compared result signal SH and a low compared result signal SL. The high reference level Vrh is higher than the low reference level Vrl. The comparison circuit  105  comprises comparators  104  and  106 . A non-inverting input end of the comparator  104  receives the low reference level Vrl, and an inverting input end thereof receives the detection signal Scs, and accordingly generates the low compared result signal SL. When the level of the detection signal Scs is lower than the low reference level Vrl, the low compared result signal SL is at a high level. On the other hand, when the level of the detection signal Scs is higher than the low reference level Vrl, the low compared result signal SL is at a low level. A non-inverting input end of the comparator  106  receives the detection signal Scs, and an inverting input end thereof receives the high reference level Vrh, and accordingly generates the high compared result signal SH. When the level of the detection signal Scs is lower than the high reference level Vrh, the high compared result signal SH is at a low level. On the other hand, when the level of the detection signal Scs is higher than the high reference level Vrh, the high compared result signal SH is at a high level. The PWM logic control circuit further comprises a compared result logic circuit  110  and generates a set of control signals S 1  and S 2  to control the PWM circuit according to the compared result of the comparison circuit  105 , i.e., the high compared result signal SH and the low compared result signal SL. 
     The PWM circuit comprises a capacitance Ccomp, a charging circuit Is and a discharging circuit Is′. The charging circuit Is is coupled to the capacitance Ccomp through a charging switch SW 1 , and the discharging circuit Is′ is coupled to the capacitance Ccomp through a discharging switch SW 2 . When the level of the detection signal Scs is between the high reference level Vrh and the low reference level Vrl, the compared result logic circuit  110  stops to generate the control signals S 1  and S 2  (i.e., the charging circuit Is and the discharging circuit Is′ respectively stop charging and discharging to the capacitance Ccomp). At this moment, a capacitance voltage of the capacitance Ccomp is maintained. When the level of the detection signal Scs is lower than the low reference level Vrl, the compared result logic circuit  110  generates the control signal S 1  to turn the charging switch SW 1  on, while turning the discharging switch SW 2  off. At this moment, the charging circuit Is charges the capacitance Ccomp to increase the capacitance voltage. When the level of the detection signal Scs is higher than the high reference level Vrh, the compared result logic circuit  110  generates the control signal S 2  to turn the discharging switch SW 2  on, while turning the charging switch SW 1  off. At this moment, the discharging circuit Is′ discharges the capacitance Ccomp to decrease the capacitance voltage. 
     In the embodiment of the present invention, a dimming control circuit  108  is additionally increased. The dimming control circuit  108  receives an external dimming signal PWM and generates a first dimming signal Sd 1  to the detection circuit  102 , or/and a second dimming signal Sd 2  to the compared result logic circuit  110 . The dimming signal PWM is used to control the PWM control circuit  118  of the feedback control circuit  100  or the current control circuit ILC to make the LED module LD periodically light and stop lighting for achieving the dimming function. However, the LED module LD periodically lights and stops lighting to make the voltage or the current of the LED module LD periodically vary. Therefore, a voltage of the current control end Ch also periodically varies when the LED module LD is driven to light, and moreover the voltage variations are different when the dimming signal PWM is used to control the PWM control circuit  118  and when the dimming signal PWM is used to control the current control circuit ILC. Such voltage variation would make the feedback control circuit  100  imprecisely control, and even erroneously operate. The dimming control circuit  108  separately controls the detection circuit  102  or/and the PWM logic control circuit in response to the dimming signal PWM to avoid the problems mentioned above. 
     The PWM control circuit  118  comprises a PWM comparator  112 , a logic circuit  114  and a driver circuit  16 . A non-inverting input end of the PWM comparator  112  receives the capacitance voltage of the capacitance Ccomp, an inverting input end thereof receives a ramp signal, and accordingly generates a PWM signal Spwm. The logic circuit  114  receives the PWM signal Spwm and accordingly generates a PWM control signal Sct. The driver circuit  116  receives the PWM control signal Set and accordingly generates a control signal Sdrv to control the converting circuit  120  for adjusting the output voltage Vout. When the capacitance voltage of the capacitance Ccomp increases, the duty cycle of the control signal Sdrv increases to raise the output voltage Vout. When the capacitance voltage of the capacitance Ccomp decreases, the duty cycle of the control signal Sdrv reduces to decrease the output voltage Vout. When the capacitance voltage of the capacitance Ccomp is maintained, the duty cycle of the control signal Sdrv is also maintained to make the changes of the output voltage Vout be relatively slow. 
       FIG. 4  is a schematic diagram of a detection circuit according to a preferred embodiment of the present invention. The detection circuit comprises an inverter  1022 , switches  1024  and  1026  and a detection capacitance  1028 . The detection circuit receives a first dimming signal Sd 1  generated by the dimming control circuit  108  of the aforementioned embodiment. The switch  1024  is coupled to the current control end Ch and the detection capacitance  1028 , and the switch  1026  is coupled to the detection capacitance  1028  and a common potential, herein the grounding. The inverter  1022  inverses the first dimming signal Sd 1  to make the switch  1024  and the switch  1026  not to be turned on simultaneously. When the dimming signal PWM represents that the LED module lights, the first dimming signal Sd 1  is at a high level to turn the switch  1024  on while the switch  1026  is turn-off. At this moment, the detection circuit samples a voltage of the current control end Ch by the detection capacitance  1028 . When the dimming signal PWM represents that the LED module stops lighting, the first dimming signal Sd 1  is at a low level to turn the switch  1024  off while the switch  1026  is turn-on. The sampled voltage of the detection capacitance  1028  is reset to be zero through the switch  1026 . By the control method mentioned above, the detection circuit operates between sampling state and non-sampling state in response to the dimming signal PWM. 
       FIG. 5  is a schematic diagram of a compared result logic circuit according to a first preferred embodiment of the present invention. The compared result logic circuit comprises NAND gates  1102  and  1104  and inverters  1106  and  1108 . The NAND gate  1102  receives the high compared result signal SH generated by the comparator  106  and the second dimming signal Sd 2  generated by the dimming control circuit  108 . The NAND gate  1104  receives the low compared result signal SL generated by the comparator  104  and the second dimming signal Sd 2  generated by the dimming control circuit  108 . 
     When the dimming signal PWM represents that the LED module lights, the second dimming signal Sd 2  is at a high level. If the detection signal Scs is higher than the high reference level Vrh, the high compared result signal SH is at the high level and the low compared result signal SL is at the low level. Thus, the control signal S 1  is at a low level and the control signal S 2  is at a high level. The discharging circuit Is′ discharges the capacitance Ccomp to decrease the capacitance voltage. If the detection signal Scs is lower than the low reference level Vrl, the high compared result signal SH is at the low level and the low compared result signal SL is at the high level. Thus, the control signal S 1  is at a high level and the control signal S 2  is at a low level. The charging circuit Is charges the capacitance Ccomp to increase the capacitance voltage. If the detection signal Scs is lower than the high reference level Vrh and is higher than the low reference level Vrl, the high compared result signal SH and the low compared result signal SL both are at the low level. Therefore, the control signals S 1  and S 2  both are at the low level. The charging circuit Is and the discharging circuit Is′ are respectively stopped charging and discharging the capacitance Ccomp to maintain the capacitance voltage. 
     When the dimming signal PWM represents that the LED module stops lighting, the second dimming signal Sd 2  is at the low level. At this moment, no matter what the levels of the high compared result signal SH and the low compared result signal SL are, the control signal S 1  and the control signal S 2  both are at the low level. Hence, the capacitance voltage of the capacitance Ccomp is maintained. 
       FIG. 6  is a schematic diagram of an LED driving circuit according to a second preferred embodiment of the present invention. The LED module LD of the present embodiment has a plural of LED strings. The plural of the current control circuits ILC 1 -ILCn respectively are coupled to corresponding one of a plural of the LED strings through the current control ends Ch 1 -Chn to control the current of the LED strings stabilizing at the predetermined current value. A detection circuit  202  of a feedback control circuit  200  has a plural of detection sub-circuits  2021 - 202   n , respectively coupled to corresponding one of the current control ends Ch 1 -Chn for generating detection signals Scs 1 -Scsn according to the state of the corresponding LED string. A comparison circuit  205  has a plural of comparison sub-circuits  2051 - 205   n , which receives the high reference level Vrh, the low reference level Vrl and the detection signal generated by the corresponding detection sub-circuit of the detection signals Scs 1 -Scsn. The plural of the comparison sub-circuits  2051 - 205   n  respectively generates high compared result signals SH 1 -SHn and low compared result signal SL 1 -SLn according to the compared results. A compared result logic circuit  210  receives the high compared result signals SH 1 -SHn and the low-compared result signals SL 1 -SLn. 
       FIG. 7  is a schematic diagram of a compared result logic circuit according to a second preferred embodiment of the present invention. The compared result logic circuit of the present embodiment additionally adds an AND gate  2102  and an OR gate  2104  on the basis of the compared result logic circuit shown in  FIG. 5 . When the dimming signal PWM represents that the LED module lights, the second dimming signal Sd 2  is at the high level. When any one of the low compared result signals SL 1 -SLn is at the high level, i.e., any one of the detection signals Scs 1 -Scsn is lower than the low reference level Vrl, the OR gate  2104  outputs a high level signal to the NAND gate  1104 . At this moment, the control signal S 1  is at a high level and the control signal S 2  is at a low level. The charging switch SW 1  is turned on and the discharging switch SW 2  is turned off. Therefore, the charging circuit Is charges the capacitance Ccomp to increase the capacitance voltage. When the high compared result signals SH 1 -SHn all are at the high level, i.e., the detection signals Scs 1 -Scsn all are higher than the high reference level Vrh, the AND gate  2102  outputs a high level signal to the NAND gate  1102 . At this moment, the control signal S 2  is at the high level and the control signal S 1  is at the low level. The charging switch SW 1  is turned off and the discharging switch SW 2  is turned on, and so the discharging circuit Is′ discharges the capacitance Ccomp to decrease the capacitance voltage. For the rest conditions, i.e., all detection signals Scs 1 -Scsn are higher than the low reference level Vrl, but not all of the above are higher than the high reference level Vrh, the control signal S 1  and the control signal S 2  are at the low level. At this moment, both the charging switch SW 1  and the discharging switch SW 2  are turned off, and the capacitance voltage of the capacitance Ccomp is maintained. 
     When the dimming signal PWM represents that the LED module stops lighting, the second dimming signal Sd 2  is at the low level. At this moment, whether what the levels of the high compared result signals SH 1 -SHn and the low compared result signals SL 1 -SLn are, the control signal S 1  and the control signal S 2  both are at the low level. Thus, the capacitance voltage of the capacitance Ccomp is maintained. 
     Referring to  FIG. 6 , a PWM control circuit  218  receives the ramp signal and the capacitance voltage of the capacitance Ccomp and accordingly generates the control signal Sdrv to control a converting circuit  220  for adjusting the output voltage Vout. 
       FIG. 8  is a schematic diagram of an LED driving circuit according to a third preferred embodiment of the present invention. The present embodiment replaces the detection circuit with a minimum voltage detection circuit  302 . The minimum detection circuit  302  is coupled to the current control ends Ch 1 -Chn and compares the voltages of the current control ends Ch 1 -Chn with each other, and accordingly outputs a minimum voltage signal Ch_min according to the minimum voltage among the voltages of the current control ends Ch 1 -Chn. A comparison circuit  305  receives the low reference level Vrl, the high reference level Vrh and the minimum voltage signal Ch_min. When the minimum voltage signal Ch_min is higher than the high reference level Vrh, the comparison circuit  305  outputs the high compared result signal SH with high level and the low compared result signal SL with low level. At this moment, a compared result logic circuit  310  outputs the control signal S 2  with high level and the control signal S 1  with low level to turn on the discharging switch SW 2  and turn off the charging switch SW 1 . Thus, the discharging circuit Is′ discharges the capacitance Ccomp for reducing the capacitance voltage. When the minimum voltage signal Ch_min is lower than the low reference level Vrl, the comparison circuit  305  outputs the high compared signal SH with low level and the low compared result signal SL with high level. At this moment, the compared result logic circuit  310  outputs the control signal S 2  with low level and the control signal S 1  with high level to turn on the charging switch SW 1  and turn off the discharging switch SW 2 . Thus, the charging circuit Is charges the capacitance Ccomp for increasing the capacitance voltage. When the minimum voltage signal Ch_min is lower than the high reference level Vrh and is higher than the low reference level Vrl, the comparison circuit  305  outputs the high compared result signal SH and the low-compared result signal SL both with low level. At this moment, the compared result logic circuit  310  outputs the control signals S 1  and S 2  both with low level to turn off the charging switch SW 1  and discharging switch SW 2 . Hence, the capacitance voltage of the capacitance Ccomp is maintained. 
     In addition, the dimming control circuits  108  and  208  in the above embodiments generate the first dimming signal Sd 1  and the second dimming signal Sd 2  according to the dimming signal PWM. It may have a phase difference or a delay time between the first dimming signal Sd 1  and the second dimming signal Sd 2  to control the circuits which respond to the dimming signal PWM in a sequence. According to the practical application, the dimming control circuit  108  and  208  may be omitted. In a feedback control circuit  300  of the present embodiment, the dimming control circuit is omitted, and so the minimum voltage detection circuit  302  and the compared result logic circuit  310  directly receives the dimming signal PWM. Hence, the minimum voltage detection circuit  302  operates between sampling state and non-sampling state in response to the dimming signal PWM. The PWM control circuit  318  receives the ramp signal and the capacitance voltage of the capacitance Ccomp and accordingly generates the control signal Sdrv to control a converting circuit  320  for adjusting the output voltage Vout. 
     The feedback control circuit of the present invention executes changing, discharging and maintaining for the capacitance Ccomp through the above control loop to make the output voltage Vout not be modulated according to only a compared result of a reference level and the feedback signal. Therefore, the output voltage Vout is dynamically switched between being modulated and being maintained at a very low frequency. The feedback control circuit replaces the conventional error amplifier with the digital logic circuit. 
     Furthermore, the feedback control circuit of the present invention adds the controlling of the dimming signal PWM to the LED driving circuit. Under the control of a duty cycle of the dimming signal PWM lower than 100%, the feedback control circuit is switched between the sampling and the non-sampling states. Namely, when the dimming signal PWM represents that the LED module lihgts, the voltage of the capacitance Ccomp is adjusted normally according to the sampled result. On the other hand, when the dimming signal PWM represents that the LED module stops lighting, the voltage of the capacitance Ccomp is maintained. Thus, the LED driving circuit of the present invention is suitable to applications, such as the backlight of the LCD monitor, the illumination. 
     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.