Patent Publication Number: US-10791603-B2

Title: Integrated circuit, dimmable light-emitting diode driving circuit and driving method

Description:
RELATED APPLICATIONS 
     This application claims the benefit of Chinese Patent Application No. 201811209486.4, filed on Oct. 17, 2018, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of power electronics, and more particularly to integrated circuits, dimmable light-emitting diode (LED) driving circuits, and associated driving methods. 
     BACKGROUND 
     A switched-mode power supply (SMPS), or a “switching” power supply, can include a power stage circuit and a control circuit. When there is an input voltage, the control circuit can consider internal parameters and external load changes, and may regulate the on/off times of the switch system in the power stage circuit. Switching power supplies have a wide variety of applications in modern electronics. For example, switching power supplies can be used to drive light-emitting diode (LED) loads. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of an example dimmable LED driving circuit. 
         FIG. 2  is a schematic block diagram of an example dimmable LED driving circuit, in accordance with embodiments of the present invention. 
         FIG. 3  is a schematic block diagram of a first example dimmable LED driving circuit, in accordance with embodiments of the present invention. 
         FIG. 4  is a waveform diagram of example operation of the example dimmable LED driving circuit of  FIG. 3 , in accordance with embodiments of the present invention. 
         FIG. 5  is a schematic block diagram of a second example dimmable LED driving circuit, in accordance with embodiments of the present invention. 
         FIG. 6  is a schematic block diagram of a third example dimmable LED driving circuit, in accordance with embodiments of the present invention. 
         FIG. 7  is a flow diagram of an example dimmable LED driving method, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference may now be made in detail to particular embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention may be described in conjunction with the preferred embodiments, it may be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it may be readily apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, processes, components, structures, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
     Light-emitting diode (LED) lighting is widely used in furniture, offices, outdoor lighting, stage lighting, and so on. The brightness of an LED load can be regulated with dimming technology, thereby expanding the applications of the LED lighting and improving user experience. The start-up time of an LED load may be related to a bus voltage, a duty ratio of the dimming signal, and an electrolytic capacitor connected in parallel with the LED load. However, when the electrolytic capacitor is relatively large in capacitance and the duty ratio of the dimming signal is relatively small, the start-up time of the LED load can become too long. 
     Referring now to  FIG. 1 , shown is a schematic block diagram of an example dimmable LED driving circuit. In this particular example, dimmable LED driving circuit  1  can include electrolytic capacitor C connected in parallel with an LED load, transistor Q, sampling resistor Rs, and current control loop circuit  11 . Current control loop circuit  11  can include dimming circuit  111 , error amplifier GM, and driving circuit  112 . Current control loop circuit  11  can regulate the current flowing through transistor Q according to dimming signal Ldim. Dimming signal Ldim may be a pulse-width modulation (PWM) signal or an analog dimming signal. After the dimmable LED driving circuit has started up, electrolytic capacitor C may be charged such that voltage Vc across electrolytic capacitor C reaches a driving voltage of the LED load, thereby driving the LED load to operate. As shown in  FIG. 1 , charging voltage Vc of electrolytic capacitor C may be calculated by: 
     
       
         
           
             Vc 
             = 
             
               
                 1 
                 c 
               
               * 
               
                 Vref 
                 Rs 
               
               * 
               t 
             
           
         
       
     
     For example, c is the capacitance of electrolytic capacitor C, Vref is a reference signal generated by dimming circuit  111  according to dimming signal Ldim, and t is the charging time of the electrolytic capacitor. It can be understood that reference signal Vref decreases as the duty ratio of dimming signal Ldim decreases, or reference signal Vref decreases per the amplitude of dimming signal Ldim. Therefore, a relatively small duty ratio of dimming signal Ldim can result in a relatively small current for charging electrolytic capacitor C, which may be generated after dimmable LED driving circuit  1  has started up. This can result in a relatively long time required for voltage Vc across electrolytic capacitor C to rise to a start-up voltage of the LED load. That is, the dimmable LED driving circuit may have a relatively long start-up time in this case. 
     In particular embodiments, when the voltage across the electrolytic capacitor is less than the start-up voltage of the LED load, the electrolytic capacitor may be additionally charged in order to reduce the time required for the voltage across the electrolytic capacitor to rise to the start-up voltage of the LED load. In this way the start-up speed of the dimmable LED driving circuit can be increased. In one embodiment, a dimmable LED driving circuit can include: (i) an electrolytic capacitor coupled in parallel to an output port of the dimmable LED driving circuit; and (ii) an auxiliary circuit configured to, when determining that a voltage across the electrolytic capacitor is less than a predetermined value, charge the electrolytic capacitor to reduce time required for the voltage across the electrolytic capacitor to rise to a start-up voltage of an LED load. In one embodiment, an integrated circuit for a dimmable LED driving circuit can include: (i) an electrolytic capacitor; (ii) a controlled current source; and (iii) an auxiliary circuit configured to, when determining that a voltage across the electrolytic capacitor is less than a start-up voltage of an LED load, regulate a current supplied by the controlled current source in order to charge the electrolytic capacitor. 
     Referring now to  FIG. 2 , shown is a schematic block diagram of an example dimmable LED driving circuit, in accordance with embodiments of the present invention. In this particular example, dimmable LED driving circuit  2  can include rectifier circuit  21 , electrolytic capacitor C′, transistor Q′, auxiliary circuit  22 , and current control loop circuit  23 . Rectifier circuit  21  can convert an alternating current (AC) input to a direct current (DC) output to direct current bus Bus. Electrolytic capacitor C′ can connect in parallel with the LED load between the output ends of dimmable LED driving circuit  2 . Auxiliary circuit  22  can, when determining that the voltage across electrolytic capacitor C′ is less than a predetermined value, charge electrolytic capacitor C′ in order to reduce the time required for the voltage across electrolytic capacitor C′ to rise to the start-up voltage of the LED load. Whether the voltage across electrolytic capacitor C′ is less than the predetermined value can be determined by detecting a bus voltage, or by detecting a voltage at either end of electrolytic capacitor C′. 
     In certain embodiments, auxiliary circuit  22  can be turned off when the voltage across electrolytic capacitor C′ rises to the predetermined value. For example, the predetermined value may be less than or equal to the start-up voltage of the LED load. Moreover, current control loop circuit  23  can, when the voltage across electrolytic capacitor C′ rises to the predetermined value, continuously charge electrolytic capacitor C′ until the voltage across electrolytic capacitor C′ rises to the start-up voltage of the LED load. This can activate the LED load and regulate the current flowing through the LED load according to a reference signal, in order to adjust the brightness of the LED load. In this way, the LED load can be activated stably and relatively quickly. 
     In particular embodiments, current control loop circuit  23  can charge electrolytic capacitor C′ when the voltage across electrolytic capacitor C′ is less than the start-up voltage of the LED load to activate the LED load, and regulate the current flowing through the LED load according to dimming signal Ldim. In addition, the reference signal can be set in accordance with the particular application. For example, the reference signal can be generated in accordance with dimming signal Ldim, and dimming signal Ldim may be a PWM dimming signal or an analog dimming signal. 
     In particular embodiments, when the voltage across the electrolytic capacitor is less than the start-up voltage of the LED load, the electrolytic capacitor may be additionally charged in order to reduce the time required for the voltage across the electrolytic capacitor to rise to the start-up voltage of the LED load, thereby increasing the start-up speed of the dimmable LED driving circuit. 
     Referring now to  FIG. 3 , shown is a schematic block diagram of a first example dimmable LED driving circuit, in accordance with embodiments of the present invention. In this particular example, dimmable LED driving circuit  3  can include rectifier circuit  31 , electrolytic capacitor C 1 , transistor Q 1 , resistor R 1 , auxiliary circuit  32 , and current control loop circuit  33 . Rectifier circuit  31  can convert the alternating current input to a direct current output to direct current bus Bus. Electrolytic capacitor C 1  can connect in parallel with the LED load between the output ends of dimmable LED driving circuit  3 . Transistor Q 1  can connect in series into a current loop of electrolytic capacitor C 1 . Auxiliary circuit  32  can, when the voltage across electrolytic capacitor C 1  is less than a predetermined value, control the current flowing through transistor Q 1  in order to charge electrolytic capacitor C 1 . 
     Current control loop circuit  33  can, when the voltage of electrolytic capacitor C 1  reaches the predetermined value, control the dimmable LED driving circuit to operate in a closed loop according to dimming signal Ldim 1 . The predetermined value may be less than or equal to the start-up voltage of the LED load. In this case, after dimmable LED driving circuit  3  has turned on, auxiliary circuit  32  can control transistor Q 1  to pre-charge electrolytic capacitor C 1 , and may be turned off after the voltage of electrolytic capacitor C 1  has reached the predetermined value. Current control loop circuit  33  can control transistor Q 1 , through the closed loop, to continuously charge electrolytic capacitor C 1  until the voltage of electrolytic capacitor C 1  reaches the start-up voltage of the LED load, such that the LED load starts working, and the current flowing through the LED load is regulated according to dimming signal Ldim 1 . 
     As shown in  FIG. 3 , auxiliary circuit  32  can include voltage sampling circuit  321 , voltage source Vk, comparator cmp 1 , voltage source Vclp, and switch S 1 . Voltage sampling circuit  321  can include resistors R 2  and R 3  for acquiring voltage sampling signal Vc 1  that represents the voltage across electrolytic capacitor C 1 . For example, the sampling point of voltage sampling circuit  321  may be at either end/terminal of electrolytic capacitor C 1 . That is, the sampling point may be at direct current bus Bus or at common node Dra of electrolytic capacitor C 1  and transistor Q 1 . For example, dimmable LED driving circuit  3  can also include diode D, which can connect between the output end of rectifier circuit  31  and electrolytic capacitor C 1 , in order to prevent a reverse current. The sampling point of voltage sampling circuit  321  can also be at the output end of the rectifier circuit. 
     When the sampling point is at direct current bus Bus, voltage sampling circuit  321  can connect between direct current bus Bus and ground. Comparator cmp 1  can compare reference value Vpre against voltage sampling signal Vc 1  that represents the voltage across electrolytic capacitor C 1 , in order to generate control signal Qpre for controlling switch S 1 . For example, reference value Vpre can correspond to the predetermined value. As shown in  FIG. 3 , reference value Vpre may be the voltage of voltage source Vk, and the predetermined value may be (R 2 +R 3 )Vk/R 2 . 
     When voltage sampling signal Vc 1  is less than reference value Vpre (e.g., the voltage across electrolytic capacitor C 1  is less than the predetermined value), comparator cmp 1  can activate control signal Qpre to control switch S 1  to be turned on, thereby controlling current iq 1  flowing through transistor Q 1  to be the predetermined pre-charge current. That is, electrolytic capacitor C 1  may be charged with the pre-charge current. The pre-charge current may be related to voltage source Vclp, and thus can be regulated by configuring voltage source Vclp according to particular applications. For example, auxiliary circuit  32  can also include inverter inv and switch S 2 . Switch S 2  can connect current control loop circuit  33 . Inverter inv can connect between the output of comparator cmp 1  and the control end of switch S 2 , and can control switch S 2  to be turned off when the voltage across electrolytic capacitor C 1  is less than the predetermined value, in order to disable control current control loop  33  circuit. 
     When voltage sampling signal Vc 1  reaches reference value Vpre (e.g., the voltage across electrolytic capacitor C 1  reaches the predetermined value), comparator cmp 1  may deactivate control signal Qpre to control the switch S 1  to be turned off and switch S 2  to be turned on. In this case, auxiliary circuit  32  may be controlled to be turned off, and current control loop circuit  33  can control the dimmable LED driving circuit to start to operate in a closed loop. 
     Current control loop circuit  33  can include dimming circuit  331 , error amplifier GM, and capacitor C 2 . When switch S 2  is controlled to be turned on (e.g., when current control loop circuit  33  is enabled), error amplifier GM, capacitor C 2 , resistor R 1 , and transistor Q 1  may form a controlled current source, which can be controlled by dimming signal Ldim 1  to regulate the current of the closed loop where electrolytic capacitor C 1  is located and/or the current of the closed loop where the LED load is located. Dimming circuit  331  can generate reference value Vref 1  based on dimming signal Ldim 1 . Dimming circuit  331  can generate reference value Vref 1  according to a predetermined dimming curve after receiving dimming signal Ldim 1 . The dimming curve may include a logarithmic dimming curve and a linear dimming curve, etc., which may be selected according to the particular application. 
     During the pre-charge phase of electrolytic capacitor C 1  (e.g., during the operation of auxiliary circuit  32 ), error amplifier GM can charge capacitor C 2  according to reference value Vref 1  and current sampling signal Vr 1 , which may represent the current flowing through transistor Q 1 . That is, during the pre-charging phase of electrolytic capacitor C 1 , voltage Vc 2  of capacitor C 2  may continuously increase, such that after switch S 2  is turned on, current control loop circuit  33  can control transistor Q 1  to be turned on immediately to continuously charge electrolytic capacitor C 1 . 
     That is, after the pre-charging phase ends, current control loop circuit  33  can control the output current of the controlled current source (e.g., including error amplifier GM, capacitor C 2 , resistor R 1 , and transistor Q 1 ) according to dimming signal Ldim 1 , to continuously charge electrolytic capacitor C 1 , until the voltage across electrolytic capacitor C 1  reaches the start-up voltage of the LED load, thereby activating the LED load. Then, current control loop circuit  33  can regulate the brightness of the LED load by regulating the current flowing through the LED load according to dimming signal Ldim 1 . 
     In particular embodiments, the dimmable LED driving circuit can include the dimming circuit in order to dim the LED load. When the voltage across the electrolytic capacitor is less than the start-up voltage of the LED load, the electrolytic capacitor may be additionally charged by the auxiliary circuit in order to reduce the time required for the voltage across the electrolytic capacitor to rise to the start-up voltage of the LED load, thereby increasing the start-up speed of the dimmable LED driving circuit. 
     Referring now to  FIG. 4 , shown is a waveform diagram of example operation of the example dimmable LED driving circuit of  FIG. 3 , in accordance with embodiments of the present invention. In this particular example, the voltage across electrolytic capacitor C 1  may be less than a predetermined value during time t 0  to t 1 , and the predetermined value may be slightly less than the start-up voltage of the LED load. When voltage sampling signal Vc 1 , which represents the voltage across electrolytic capacitor C 1 , is less than reference value Vpre, comparator cmp 1  may activate control signal Qpre to control switch S 1  to be turned on. When voltage Vdra of the point Dra is greater than 0 (e.g., direct current bus voltage Vbus of direct current bus Bus is greater than the voltage Vled of the LED load), transistor Q 1  may be turned on, and current iq 1  flowing through transistor Q 1  may be pre-charge current ipre. 
     That is, when control signal Qpre is active and voltage Vdra of the point Dra is greater than 0 during time t 0  to t 1 , electrolytic capacitor C 1  may be charged with the pre-charge current, such that the voltage across electrolytic capacitor C 1  quickly reaches the predetermined value, thereby increasing the start-up speed of the dimmable LED driving circuit. In addition, error amplifier GM can charge capacitor C 2  according to current sampling signal Vr 1 , which may represent the current flowing through transistor Q 1 , and reference value Vref 1 , during time t 0  to t 1 . Therefore, voltage Vc 2  of capacitor C 2  can gradually rise during time t 0  to t 1 . 
     In some embodiments, the predetermined voltage may be set to be less than the start-up voltage of the LED load, such that when performing closed-loop control on the dimmable LED driving circuit, current control loop circuit  33  can continuously charge electrolytic capacitor C 1  until the voltage reaches the start-up voltage of the LED load, and can control the current iled of the LED load to remain stable after the LED load starts to operate normally, thereby improving the stability of the dimmable LED driving circuit at the start-up. 
     At time t 1 , the voltage across electrolytic capacitor C 1  can reach the predetermined value. At this time, reference value Vpre may not be greater than voltage sampling signal Vc 1 , such that control signal Qpre may be low, switch S 1  may be turned off, and switch S 2  can be turned on. That is, auxiliary circuit  32  can stop operating, and current control loop circuit  33  may start to perform closed-loop control on the dimmable LED driving circuit according to dimming signal Ldim 1 . Since the predetermined value is less than the start-up voltage of the LED load, current control loop circuit  33  can control transistor Q 1  to generate a current, in order to continuously charge the electrolytic capacitor. 
     At time t 2 , the voltage across electrolytic capacitor C 1  may reach the start-up voltage of the LED load, such that the LED load starts to work, thus completing the start-up process of the dimmable LED driving circuit. In certain embodiments, when the voltage across the electrolytic capacitor is less than the predetermined value, the electrolytic capacitor may be additionally charged by the auxiliary circuit in order to reduce the time required for the voltage across the electrolytic capacitor to rise to the start-up voltage of the LED load, thereby increasing the start-up speed of the dimmable LED driving circuit. 
     Referring now to  FIG. 5 , shown is a schematic block diagram of a second example dimmable LED driving circuit, in accordance with embodiments of the present invention. In this particular example, dimmable LED driving circuit  5  can include rectifier circuit  51 , electrolytic capacitor C 3 , transistors Q 2  and Q 3 , resistor R 4 , auxiliary circuit  52 , and current control loop circuit  53 . Rectifier circuit  51  can convert the alternating current input to a direct current output to direct current bus Bus. Electrolytic capacitor C 3  can connect in parallel with the LED load between the output ends of the dimmable LED driving circuit. Transistors Q 2  and Q 3  can connect in parallel into a current loop of electrolytic capacitor C 3 . Auxiliary circuit  52  can, when the voltage across electrolytic capacitor C 3  is less than a predetermined value, control the current flowing through transistor Q 3  to charge electrolytic capacitor C 3 . 
     Current control loop  53  can, when the voltage of electrolytic capacitor C 3  reaches the predetermined value, control the dimmable LED driving circuit to operate in a closed loop according to dimming signal Ldim 2 . For example, the predetermined value may be less than or equal to the start-up voltage of the LED load. When the predetermined value is less than the start-up voltage of the LED load, after dimmable LED driving circuit  5  is turned on, auxiliary circuit  52  can control transistor Q 3  to pre-charge electrolytic capacitor C 3 , and may be controlled be turned off after the voltage of electrolytic capacitor C 3  reaches the predetermined value. Current control loop circuit  53  can control transistor Q 2  through the closed loop, to continuously charge electrolytic capacitor C 3 , until the voltage of electrolytic capacitor C 3  reaches the start-up voltage of the LED load, and the current flowing through the LED load can be regulated according to dimming signal Ldim 2  after the LED load starts working. 
     For example, auxiliary circuit  52  can include comparator cmp 2 , switch S 3 , and voltage source Vclp 1 . Voltage sampling signal Vc 3 , which may represent the voltage across electrolytic capacitor C 3 , and reference value Vpre 1 , can be input to comparator cmp 2 . For example, reference value Vpre 1  can correspond to the predetermined value. When voltage sampling signal Vc 3  is less than reference value Vpre 1  (e.g., the voltage across electrolytic capacitor C 3  is less than the predetermined value), comparator cmp 2  may activate control signal Qpre 1  to control switch S 3  to be turned on, thereby controlling transistor Q 3  to pre-charge electrolytic capacitor C 3  with pre-charge current ipre 1 . Pre-charge current ipre 1  may be related to voltage source Vclp 1 , and thus can be regulated by configuring the voltage of voltage source Vclp according to particular applications. When voltage sampling signal Vc 3 , which may represent the voltage across electrolytic capacitor C 3 , reaches reference value Vpre 1  (e.g., the voltage across electrolytic capacitor C 3  reaches the predetermined value), comparator cmp 2  may deactivate control signal Qpre 1  to control switch S 3  to be turned off, such that auxiliary circuit  52  may be turned off. 
     For example, while auxiliary circuit  52  charges electrolytic capacitor C 3  by controlling transistor Q 3  to be turned on, current control loop circuit  53  can control transistor Q 2  to generate a current, in order to charge electrolytic capacitor C 3 . Thus, the pre-charge process of electrolytic capacitor C 3  may be accelerated in certain embodiments, thereby further increasing the start-up speed of the dimmable LED driving circuit. 
     Current control loop  53  can include dimming circuit  531 , error amplifier GM 1 , and a capacitor C 4 . Error amplifier GM 1 , capacitor C 4 , resistor R 4  and transistor Q 2  may form a controlled current source, which may be controlled by dimming signal Ldim 2  to regulate the current on the closed loop where electrolytic capacitor C 3  is located and/or the current on the loop where the LED load is located. Dimming circuit  531  can generate reference value Vref 2  based on dimming signal Ldim 2 . Dimming circuit  531  may output reference value Vref 2  according to a predetermined dimming curve after receiving dimming signal Ldim 2 . The dimming curve may include a logarithmic dimming curve and a linear dimming curve, which may be selected according to different application scenarios. 
     After the pre-charging phase of electrolytic capacitor C 3  ends, current control loop circuit  53  can control the output current of the controlled current source (e.g., including error amplifier GM 1 , capacitor C 4 , resistor R 4 , and transistor Q 2 ) according to dimming signal Ldim 2 , in order to continuously charge electrolytic capacitor C 3 , until the voltage across electrolytic capacitor C 3  reaches the start-up voltage of the LED load, thereby activating the LED load. In particular embodiments, the pre-charging of the electrolytic capacitor and the closed-loop control of the dimmable LED driving circuit can be controlled by controlling different transistors, thereby further increasing the start-up speed of the dimmable LED driving circuit. 
     Referring now to  FIG. 6 , shown is a schematic block diagram of a third example dimmable LED driving circuit, in accordance with embodiments of the present invention. In this particular example, dimmable LED driving circuit  6  can include dimmer Triac, rectifier circuit  61 , diode D 1 , electrolytic capacitor C 5 , transistor Q 4 , resistor R 5 , auxiliary circuit  62 , and current control loop circuit  63 . Dimmer Triac can connect between the alternating current input end and the input end of rectifier circuit  61 , and may dim the LED load. For example, the dimmer may be a leading-edge phase-cut dimmer including a triac. Dimmer Triac has advantages of relatively small size, high withstand voltage, large capacity, strong function, fast response, high efficiency, and low cost. Dimming with a dimmer can make the dimmable LED driving circuit safer, more reliable, and more controllable. Diode D 1  may prevent a reverse current. Rectifier circuit  61  can convert the alternating current input to a direct current output to direct current bus Bus. Electrolytic capacitor C 5  can connect in parallel with the LED load between the output ends of dimmable LED driving circuit  6 . Transistor Q 3  can connect in series into a current loop of electrolytic capacitor C 5 . 
     Comparator cmp 3  can control switches S 4  and S 5  to be turned on or turned off by comparing voltage sampling signal Vc 5 , which may represent the voltage across electrolytic capacitor C 5 , and reference value Vpre 2 , thereby controlling the pre-charging phase and the normal operation phase of the dimmable LED driving circuit. For example, voltage sampling signal Vc 5  can be obtained by sampling the voltage at the output end of rectifier circuit  61 , or by sampling the voltage at either end of electrolytic capacitor C 5 . Reference value Vpre 2  may represent the predetermined value, which may be less than or equal to the start-up voltage of the LED load, and can be set according to the actual circuit structure and the parameters of each element and the sampling point of the voltage sampling signal. 
     When voltage sampling signal Vc 5  is less than reference value Vpre 2  (e.g., the voltage across electrolytic capacitor C 5  is less than the predetermined value), switch S 4  may be turned on, and switch S 5  may be turned off. Auxiliary circuit  62  can charge electrolytic capacitor C 5  with a predetermined pre-charge current. The predetermined pre-charge current may be set by setting the voltage of voltage source Vclp 2 . Further, the predetermined value may be less than or equal to the start-up voltage of the LED load. When voltage sampling signal Vc 5  reaches reference value Vpre 2  (e.g., the voltage across electrolytic capacitor C 5  reaches the predetermined value), switch S 4  may be turned off, and switch S 5  may be turned on. Current control loop circuit  63  can perform closed-loop control according to reference value Vre 3 , such that the current flowing through the LED load may be a current corresponding to reference value Vre 3 . 
     In particular embodiments, the dimmable LED driving circuit can include the dimmer to dim the LED load. When the voltage across the electrolytic capacitor is less than the start-up voltage of the LED load, the electrolytic capacitor may be additionally charged by the auxiliary circuit to reduce the time required for the voltage across the electrolytic capacitor reaches the start-up voltage of the LED load, thereby increasing the start-up speed of the dimmable LED driving circuit. Also, the additional current generated by the auxiliary circuit can speed up the startup of the silicon-controlled dimmer, thereby improving the efficiency of the circuit. 
     In one embodiment, a method of controlling a dimmable LED driving circuit, can include: (i) detecting a voltage across an electrolytic capacitor in the LED driving circuit; (ii) determining whether the voltage across the electrolytic capacitor is less than a predetermined value; and (iii) charging the electrolytic capacitor by an auxiliary circuit when the voltage across the electrolytic capacitor is less than the predetermined value, in order to reduce time required for the voltage across the electrolytic capacitor to rise to a start-up voltage of an LED load. 
     Referring now to  FIG. 7 , shown is a flow diagram of an example dimmable LED driving method, in accordance with embodiments of the present invention. In this particular example, at S 100 , whether the voltage across the electrolytic capacitor is less than a predetermined value may be determined by detecting a bus voltage of the dimmable LED driving circuit, or by detecting a voltage at either end of the electrolytic capacitor. At S 200 , the electrolytic capacitor may be charged by an auxiliary circuit if the voltage across the electrolytic capacitor is less than the predetermined value, in order to reduce the time required for the voltage across the electrolytic capacitor rising to the start-up voltage. For example, the predetermined value may be less than or equal to a start-up voltage of an LED load. 
     At S 300 , if the voltage across the electrolytic capacitor rises to the predetermined value, the auxiliary circuit may be turned off. Further, the electrolytic capacitor may be continuously charged by a current control loop circuit when the voltage across the electrolytic capacitor rises to the predetermined value, and the current control loop circuit can regulate a current flowing through the LED load when the voltage across the electrolytic capacitor rises to the start-up voltage. Further, the electrolytic capacitor may be charged by the current control loop circuit when the voltage across the electrolytic capacitor is less than the start-up voltage, and the current control loop circuit can regulate the current flowing through the LED load when the voltage across the electrolytic capacitor rises to the start-up voltage. 
     In particular embodiments, when the voltage across the electrolytic capacitor is less than the start-up voltage of the LED load, the electrolytic capacitor may be additionally charged by the auxiliary circuit in order to reduce the time required for the voltage across the electrolytic capacitor rising to the start-up voltage of the LED load, thereby increasing the start-up speed of the dimmable LED driving circuit. 
     The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with modifications as are suited to particular use(s) contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.