Patent Publication Number: US-9888544-B2

Title: Driving circuits and methods for controlling light source

Description:
RELATED APPLICATION 
     This application claims priority to Chinese Patent Application No. 201410834786.7, filed on Dec. 26, 2014, with the State Intellectual Property Office of the People&#39;s Republic of China, incorporated by reference in its entirety herein. 
     FIELD 
     The present teaching relates generally to light source driving circuits and in particular to driving circuits and methods for adjusting power supplied to loads based on ambient temperature. 
     BACKGROUND ART 
     Compared with traditional filament lights, LED (light emitting diode) light sources possess advantages such as environmental protection, energy savings, relatively high luminous efficiency, and long life. Thus, replacing filament lights with LED light sources is an inevitable trend based on current developments. LED bulbs are types of LED lights that are similar in shape and size to filament lights. An LED bulb includes an LED light source and a control chip. LED light sources require temperature controls as overheating will shorten their life. Yet the closed architecture of LED bulbs makes heat dissipation difficult. The high temperature may easily damage the control chip in the LED light source. Therefore, there exists a need to provide a solution to effectively manage the temperature of LED bulbs. 
     SUMMARY 
     In a first embodiment according to present invention, a light source driving circuit is disclosed. The light source driving circuit includes a power converter and a controller. The power converter is configured for receiving input voltage and providing output power to a load. The controller, coupled to the power converter, is configured for acquiring a first sensing signal indicative of an average current flowing through the load, generating a first temperature detecting signal indicative of ambient temperature of the light source driving circuit, and adjusting the average current flowing through the load based on the first temperature detecting signal and the first sensing signal. The controller decreases the average current of the load based on the first temperature detecting signal and the first sensing signal when the ambient temperature keeps rising after the ambient temperature rises above a first temperature threshold. 
     In a second embodiment according to the present invention, a controller for controlling ambient temperature of a light source driving circuit having a power converter is disclosed. The power converter is configured for receiving input voltage and supplying power to a load. The controller includes a sensing terminal, a compensation terminal and a driving terminal. The sensing terminal is configured for receiving a sensing signal indicative of an instant current flowing through the load. The compensation terminal is configured for generating an error signal based on the instant current sensing signal and a first temperature detecting signal indicative of the ambient temperature of the light source driving circuit. The driving terminal is configured for generating a driving signal based on the error signal to control the power converter, so as to adjust an average current flowing through the load. The driving signal decreases the average current flowing through the load when the ambient temperature keeps rising after the ambient temperature of the light source driving circuit rises above a first temperature threshold. 
     In a third embodiment of the present invention, a method for powering a load by a light source driving circuit having a power converter is disclosed. The method includes steps of: acquiring a first sensing signal indicative of an average current flowing through the load; generating a first temperature detecting signal indicative of an ambient temperature of the light source driving circuit; and adjusting the average current flowing through the load based on the first temperature detecting signal and the first sensing signal to make the average current flowing through the load inversely proportional to the ambient temperature of the light source driving circuit when the ambient temperature of the light source driving circuit is between a first temperature threshold and a second temperature threshold which is greater the first temperature threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of embodiments according to the invention will become apparent as the following detailed description proceeds, and upon reference to the drawings, where like numerals depict like elements, and in which: 
         FIG. 1  is a circuit diagram illustrating a light source driving circuit, in an embodiment according to the present disclosure. 
         FIG. 2A  is a schematic diagram of a controller in an embodiment according to the present disclosure. 
         FIG. 2B  is a waveform illustrating waveforms of signals received or generated by a controller in an embodiment according to the present disclosure. 
         FIG. 3  is a schematic diagram of a bandgap voltage generator in an embodiment according to the present disclosure. 
         FIG. 4  is a schematic diagram of an error signal generator in an embodiment according to the present disclosure. 
         FIG. 5  is a waveform illustrating a sensing signal IAVG which indicates an average current flowing through a light source as a function of ambient temperature in an embodiment according to the present disclosure. 
         FIG. 6  is a flowchart illustrating a method for controlling a light source driving circuit, in an embodiment according to the present disclosure. 
         FIG. 7  is a circuit diagram illustrating a light source driving circuit, in an embodiment according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments according to the present invention. While the invention will be described in conjunction with these embodiments, it will 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, which 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 will be recognized by one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
       FIG. 1  is a circuit diagram illustrating driving circuit  100  for driving a light source (e.g., a LED bulb), in accordance with an embodiment of the present disclosure. In the example of  FIG. 1 , the light source driving circuit  100  includes a rectifier  104 , a controller  110  and a power converter  120 . The rectifier  104  can be a bridge rectifier including diodes D 1 -D 4 . The rectifier  104  is configured to adjust a voltage of a power source  102 . The power converter  120  is configured to receive input power adjusted by the rectifier  104  and provide output power to a load (e.g., an LED string  130 ). The LED string  130  can be placed in a LED bulb. 
     In the example of  FIG. 1 , the power converter  120  is a buck converter. However, the power converter  120  disclosed herein can be another suitable converter (e.g., a boost converter or a buck-boost converter). The power converter  120  includes a capacitor  121 , a switch  122 , a diode  123 , a current monitor (e.g., a resistor  124 ), an energy storage unit  126  (e.g., two inductors  127  and  128  which are electromagnetically coupled with each other) and a capacitor  129 . A current to the energy storage unit  126  is controlled by the switch  122 . In the example of  FIG. 1 , the current to the energy storage unit  126  is the current flowing through the inductor  127 . The diode  123  is coupled between the switch  122  and the ground of the light source driving circuit  100 . The capacitor  129  is coupled to the LED string  130  in parallel. In one embodiment, the inductor  127  and the inductor  128  are electromagnetically coupled to each other and connected to a common node  125 . In the example of  FIG. 1 , the common node  125  is placed between the resistor  124  and the energy storage unit  126 . In another embodiment, the common node  125  can be placed between the switch  122  and the resistor  124 . The common node  125  provides the reference ground for the controller  110 . In one embodiment, the reference ground of the controller  110  is different with the ground of the light source driving circuit  100 . By turning on and off the switch  122 , a current flowing through the inductor  127  is adjusted, to adjust the power to the LED string  130 . The inductor  128  is configured to monitor the status of the inductor  127 . For example, the inductor  128  can generate a detecting signal AUX to detect whether the current flowing through the inductor  127  has decreased to a predetermined current value. 
     The resistor  124  has one end coupled to a node between the switch  122  and the cathode of the diode  123 , and the other end coupled to the inductor  127 . The resistor  124  is configured to provide a sensing signal ISEN (which may be referred to herein as the second sensing signal or as the instant current sensing signal). The sensing signal ISEN can indicate an instant current flowing through the LED string  130 , and an instant current flowing through the inductor  127  regardless of whether the switch  122  is turned on or is turned off. In other words, when the switch  122  is on or off, the resistor  124  can monitor the instant current flowing through the inductor  127  and the instant current flowing through the LED string  130 . 
     The controller  110  is configured to receive the sensing signal ISEN and adjust an average current flowing through the inductor  127  to a targeted current value by turning on or off the switch  122 . The capacitor  129  is configured to filter ripples of the current flowing through the LED string  130  and keep the current of the LED string  130  substantially steady and is equal to the average current flowing through the inductor  127 . Therefore, the current flowing through the LED string  130  can be adjusted to be equal to the targeted current value. Here “equal to the targeted current value” is achieved without consideration of the non-ideal condition of elements and the power delivered from the inductor  128  to the controller  110 . 
     In the example of  FIG. 1 , terminals of the controller  110  include a monitoring terminal ZCD, a ground terminal GND, a driving terminal DRV, a power terminal VDD, a sensing terminal CS and a compensation terminal COMP. The sensing terminal CS is coupled to the resistor  124  and is configured to receive the sensing signal ISEN indicative of the instant current flowing through the inductor  127  and the LED string  130 . The compensation terminal COMP is coupled to the reference ground of the controller  110  through a capacitor  113 . The compensation terminal COMP is configured to generate an error signal based on the sensing signal ISEN and a first temperature detecting signal indicating the ambient temperature of the light source driving circuit  100 . The driving terminal DRV is coupled to the switch  122  and is configured to generate a driving signal to control the power converter  120 . For example, the driving terminal DRV generates a pulse-width modulation signal PWM 1  to turn on or off the switch  122 , so as to adjust the average current flowing through the LED string  130 . The monitoring terminal ZCD is coupled to the inductor  128  and is configured to receive the detecting signal AUX indicating the status of the energy storage unit  126 , e.g., whether the current flowing through the inductor  127  decreases to the predetermined current value “0”. In one embodiment, the detecting signal AUX can further indicate whether the LED string  130  is in an open circuit state. The power terminal VDD is coupled to the inductor  128  and is configured to receive power from the inductor  128 . In the example of  FIG. 1 , the ground terminal GND is coupled to the common node  125  placed among the resistor  124 , the inductor  127  and the inductor  128 . The ground terminal GND is configured to provide reference ground of the controller  110 . 
     The switch  122  can be an N type metal oxide semiconductor field effect transistor (N type MOSFET). The state of the switch  122  is determined by the voltage difference between the gate voltage of the switch  122  and the voltage on the ground terminal (e.g., the voltage of the common node  125 ). Thus, the state of the switch  122  is determined by a pulse-width modulation signal PWM 1  output from the driving terminal DRV. When the switch  122  is turned on, the reference ground of the controller  110  is greater than the ground of light source driving circuit  100  which enables the circuit of the present invention to be applied to a power source with high voltage. 
     In operation, when the switch  122  is turned on, a current flows through the switch  122 , the resistor  124 , the inductor  127 , and the LED string  130  to the ground of the light source driving circuit  100 . When the switch  122  is turned off, a current flows through the resistor  124 , the inductor  127 , the LED string  130  and the diode  123 . The inductor  128 , coupled to the inductor  127 , is configured to detect the status of the inductor  127  (e.g., based on the detecting signal AUX to detect whether the current flowing through the inductor  127  has decreased to the predetermined current value). The controller  110  monitors the instant current flowing through the inductor  127  based on the detecting signal AUX and the sensing signal ISEN. The controller  110  controls the switch  122  by the pulse-width modulation signal PWM 1  to adjust the average current flowing through the inductor  127  to the targeted current value. Therefore, filtered by the capacitor  129 , the current flowing through the LED string  130  is equal to the targeted current as well. 
     In one embodiment, the controller  110  determines whether the LED string  130  is in the open circuit state based on the detecting signal AUX. If the LED string  130  is in the open circuit state, the voltage across the capacitor  129  increases. When the switch  122  is turned off, the voltage of the inductor  127  increases as the voltage of the detecting signal AUX increases. Accordingly, the current flowing through the monitoring terminal ZCD to the controller  110  increases. As such, the controller  110  can determine whether the LED string  130  is in the open circuit state based on the voltage of the detecting signal AUX and whether the current flowing to the controller  110  is greater than a current threshold. 
     The controller  110  determines whether the current flowing through the inductor  127  has decreased to the predetermined current value (e.g., dropped to 0) based on the detecting signal AUX. When the driving signal (e.g., the pulse-width modulation signal PWM 1 ) is in a first state (e.g., logic 1), the switch  122  is turned on and a current flows through the switch  122 , the resistor  124 , the inductor  127 , and the LED string  130  to the ground of the light source driving circuit  100 . Also, the current flowing through the inductor  127  starts to increase which causes the voltage of the sensing signal ISEN to increase. In one embodiment, when the switch  122  is turned on, the voltage of the detecting signal AUX is at a negative value. When the driving signal (e.g., the pulse-width modulation signal PWM 1 ) is in a second state (e.g., logic 0), the switch  122  is turned off and the voltage of the detecting signal AUX changes to a positive value. Also, a current flows through the resistor  124 , the inductor  127 , the LED string  130  and the diode  123 . In this case, the current flowing through the inductor  127  decreases which causes the voltage of the sensing signal ISEN to decrease. When the current flowing through the inductor  127  drops to the predetermined current value (e.g., drops to 0), a falling edge can be detected in the voltage of the detecting signal AUX. 
     The controller  110  determines whether the LED string  130  is in a short circuit condition based on the voltage of the power terminal VDD. If the LED string  130  is in a short circuit condition, then when the switch  122  is turned off, the voltage of the inductor  127  decreases. The voltage of the inductor  128  and the voltage at the power terminal VDD decrease accordingly. If the voltage of the power terminal VDD is less than a voltage threshold when the switch  122  is turned off, the controller  110  determines that the LED string  130  is in a short circuit condition. 
     The controller  110  controls the current flowing through the LED string  130  based on the ambient temperature of the light source driving circuit  100 . After the ambient temperature of the light source driving circuit  100  increases to a first temperature threshold TH 1  (e.g., 125° C.), if the ambient temperature keeps increasing, then the light source driving circuit  100  reduces the average current of the LED string  130  gradually. If the ambient temperature of the light source driving circuit  100  increases to a second temperature threshold TH 2  (e.g., 145° C.) which is higher than the first temperature threshold, which means an overheating situation exists, then the light source driving circuit  100  keeps the switch  122  off and decreases the average current of the LED string  130  to zero quickly. The following  FIG. 2A  to  FIG. 5  further illustrate how the controller  110  controls the current flowing through the LED string  130  based on the ambient temperature of the light source driving circuit  100 . 
       FIG. 2A  illustrates a schematic diagram of the controller  110  in  FIG. 1 .  FIG. 2B  illustrates waveforms of signals generated or received by the controller in  FIG. 2A . Elements in  FIG. 2A  labeled the same in  FIG. 1  have similar functions.  FIG. 2A  is described in combination with  FIG. 1  and  FIG. 2B . In the example of  FIG. 2A , the controller  110  includes a start up and under voltage lockout circuit  201 , a filter  202 , a bandgap voltage generator  203 , a voltage scaling device  204 , a comparator  205 , an error signal generator  207 , a saw tooth signal generator  208 , a comparator  209 , a reset signal generator  210  and a pulse-width modulation signal generator  211 . 
     The start up and under voltage lockout circuit  201  is coupled to the power terminal VDD and is configured to selectively start one or more components of the controller  110  based on the power status. In one embodiment, if the voltage on the power terminal VDD is greater than a first predetermined voltage, then the start up and under voltage lockout circuit  201  starts all the components of the controller  110 . If the voltage on the power terminal VDD is less than a second predetermined voltage, then the start up and under voltage lockout circuit  201  shuts down all the components of the controller  110 . In one embodiment, the first predetermined voltage is greater than the second predetermined voltage. The power terminal VDD is configured to supply power to the controller  110 . The ground terminal GND is coupled to the reference ground of the controller  110 . 
     The filter  202  is coupled to the monitoring terminal CS and is configured to generate a sensing signal IAVG (which may be referred to herein as the first sensing signal or as the average current sensing signal). The sensing signal IAVG indicates an average current flowing through an energy storage unit  126  (e.g., the inductor  127 ) and an average current flowing through a load (e.g., the LED string  130 ). 
     The bandgap voltage generator  203  is coupled to the reference ground of the controller  100  through the ground terminal GND. The bandgap voltage generator  203  is configured to generate a bandgap voltage V BG  which is independent of temperature and to also generate a first temperature detecting signal VT 1  indicative of the ambient temperature of the light source driving circuit  100 . The error signal generator  207  is coupled to the bandgap voltage generator  203  and is configured to generate an error signal VEA based on the voltage difference between the first temperature detecting signal VT 1  and the sensing signal IAVG. More specifically, when the ambient temperature of the light source driving circuit  100  increases, the first temperature detecting signal VT 1  decreases accordingly. After the ambient environment of the light source driving circuit  100  gradually increases to a first temperature threshold TH 1  (e.g., 125° C.), if the ambient temperature keeps increasing, then the error signal generator  207  decreases the voltage of the error signal VEA on the compensation terminal COMP based on the first temperature detecting signal VT 1 , thereby decreasing the average current flowing through the load. The saw tooth signal generator  208  is configured to generate a saw tooth signal SAW. The comparator  209  is coupled to the error signal generator  207  and the saw tooth signal generator  208 . The comparator  209  is configured to compare the error signal VEA with the saw tooth signal SAW. The pulse-width modulation signal generator  211  is coupled to the comparator  209  and is configured to generate a driving signal (e.g., a pulse-width modulation signal PWM 1 ) to control the state of the switch  122  based on an output of the comparator  209 . 
     As discussed above, when the current flowing through the inductor  127  drops to the predetermined current value (e.g., drops to 0), a falling edge can be detected at the voltage of the detecting signal AUX. The reset signal generator  210  is coupled to the monitoring terminal ZCD and is configured to generate a reset signal RESET in response to the falling edge of the detecting signal AUX. The reset signal RESET affects the saw tooth signal generator  208  and the pulse-width modulation signal generator  211 . More specifically, the reset signal RESET can switch the driving signal (e.g., the pulse-width modulation signal PWM 1 ) to a first state (e.g., logic 1) to turn on the switch  122 . In one embodiment, the pulse-width modulation signal generator  211  is coupled to the reset signal generator  210 . The pulse-width modulation signal generator  211  is configured to generate the pulse-width modulation signal PWM 1  to control the state of the switch  122  through a driving terminal DRV based on the output of the comparator  209  and the reset signal RESET. 
     In one embodiment, a duty-cycle of the pulse-width modulation signal PWM 1  is determined by the error signal VEA, while the voltage of the error signal VEA is adjusted by the error signal generator  207  based on the first temperature detecting signal VT 1 . As discussed above, when the ambient temperature of the light source driving circuit  100  increases, the voltage of the first temperature detecting signal VT 1  decreases accordingly. After the ambient environment of the light source driving circuit  100  gradually increases to the first temperature threshold TH 1  (e.g., 125° C.), if the ambient temperature keeps increasing, the error signal generator  207  decreases the voltage of the error signal VEA based on the first temperature detecting signal VT 1 , so as to decrease the duty cycle of the pulse-width modulation signal PWM 1 . Accordingly, the average current flowing through the inductor  127  and the average current flowing through the LED string  130  decrease, which slows down or stops the increase of the ambient temperature of the light source driving circuit  100 . As such, damage to the LED string  130  and the inner or peripheral components of the light source driving circuit  100  caused be overheating can be avoided. While the present invention is described in an example of decreasing the duty cycle of the pulse-width modulation signal PWM 1 , it will be understood that it is not intended to limit the present invention. On the contrary, if the ambient temperature of the light source driving circuit  100  decreases within a certain range (e.g., 145° C.-125° C.), then the error signal generator  207  can increase the voltage of the error signal VEA based on the first temperature detecting signal VT 1 , so as to increase the duty cycle of the pulse-width modulation signal PWM 1 . Accordingly, the average current flowing through the inductor  127  and the average current flowing through the LED string  130  increase. 
     In one embodiment, the reset signal RESET is a pulse signal with a fixed frequency. In other embodiments, the reset signal RESET is a pulse signal which is configured to maintain an off time of the switch  122  as a constant. For example, in the example of  FIG. 2B , the reset signal RESET can maintain the logic 0 state time T OFF  of the pulse-width modulation signal PWM 1  constant. 
     In response to the reset signal RESET, the pulse-width modulation signal generator  211  generates the pulse-width modulation signal PWM 1  having a first state (e.g., logic 1) to turn on the switch  122 . When the switch  122  is turned on, a current flows through the switch  122 , the resistor  124 , the inductor  127 , and the LED string  130  to the ground of the light source driving circuit  100 . The saw tooth signal SAW generated by the saw tooth signal generator  208  starts to increase from an initial level INI in response to a pulse of the reset signal RESET. When the voltage of the saw tooth signal SAW increases to the voltage of the error signal VEA, the pulse-width modulation signal generator  211  generates the pulse-width modulation signal PWM 1  having a second state (e.g., logic 0) to turn off the switch  122 . The saw tooth signal SAW is reset to the initial level INI until the next pulse of the reset signal RESET is received by the saw tooth signal generator  208 . The saw tooth signal SAW starts to increase from the initial level INI again when the next pulse in the reset signal RESET arrives. 
     The bandgap voltage generator  203  further generates a second temperature detecting signal VT 2  indicating the ambient temperature of the light source driving circuit  100 . The voltage scaling device  204  is coupled to the bandgap voltage generator  203  and is configured to convert the bandgap voltage V BG  to a suitable reference voltage V BG ′ (e.g., by decreasing the bandgap voltage V BG  in proportion). The comparator  205  is coupled to the bandgap voltage generator  203  and the voltage scaling device  204 . The comparator  205  is configured to compare the second temperature detecting signal VT 2  with the reference voltage V BG ′ to generate an overheating signal OTP. The overheating signal OTP can indicate whether an overheating situation is occurring. More specifically, when the ambient temperature of the light source driving circuit  100  is less than or equal to a second temperature threshold (e.g., 145° C.), the voltage of the second temperature detecting signal VT 2  is greater than the reference voltage V BG ′. Thus, the comparator  205  generates the overheating signal OTP having a first state (e.g., logic 1) which enables the pulse-width modulation signal generator  211  to generate a driving signal (e.g., the pulse-width modulation signal PWM 1 ) to turn on a switch  122  at a terminal DRV under control of a comparator  209  or/and a reset signal generator  210 . When the ambient temperature of the light source driving circuit  100  is greater than the second temperature threshold (e.g., 145° C., indication of overheating), the voltage of the second temperature detecting signal VT 2  is less than the reference voltage V BG ′. Thus, the comparator  205  generates the overheating signal OTP having a second state (e.g., logic 0) which enables the pulse-width modulation signal generator  211  to maintain the driving signal (e.g., the pulse-width modulation signal PWM 1 ) at a particular state (e.g., logic 0) to keep the switch  122  off, so as to cut down the current flowing through the LED string  130 . As such, damage to the LED string  130  and the inner or peripheral components of the light source driving circuit  100  caused by overheating can be avoided. 
       FIG. 3  is a schematic diagram of the bandgap voltage generator  203  in  FIG. 2A . Elements labeled the same in  FIG. 2A  have similar functions.  FIG. 3  is described in combination with  FIG. 2A . The bandgap voltage generator  203  includes resistors R 1 -R 4 , transistors Q 1  and Q 2  and an operational amplifier  301 . The resistor R 1  and the transistor Q 1  form a first voltage stabilizing circuit. The resistor R 2 , the resistor R 3 , the resistor R 4  and the transistor Q 2  form a second voltage stabilizing circuit. As shown in  FIG. 3 , the first voltage stabilizing circuit and the second voltage stabilizing circuit are coupled to two input terminals of the operational amplifier  301  and generate a bandgap voltage V BG  which is independent of an ambient temperature of a light source driving circuit  100  at an output terminal of the operational amplifier  301 . The bandgap voltage V BG  is provided to a common node between the voltage scaling device  204  and the resistor R 2 . 
     A voltage of a common node B between the resistor R 3  and the resistor R 4  is labeled as a first temperature detecting signal VT 1  indicating the ambient temperature of the light source driving circuit  100 . As shown in  FIG. 3 , the voltage of the first temperature detecting signal VT 1  equals the sum of a voltage V BE  between the base and the emitter of the transistor Q 2  and the voltage of the resistor R 4 . That is, the voltage of VT 1  equals V BE +r4*I 1 , where r4 is the resistance of the resistor R 4 , and I 1  is the value of a current flowing through the resistor R 4 . The voltage V BE  has a negative temperature characteristic which means the voltage V BE  decreases with an increase of the ambient temperature. Therefore, the first temperature detecting signal VT 1  in the second voltage stabilizing circuit decreases as the ambient temperature increases. As discussed above, in the controller  110  of the light source driving circuit  100 , an error signal generator  207  adjusts a voltage of an error signal VEA based on the first temperature detecting signal VT 1 . After the ambient temperature of the light source driving circuit  100  increases to a first temperature threshold TH 1  (e.g., 125° C.), if the ambient temperature continues increasing, the first temperature detecting signal VT 1  decreases. Then the error signal generator  207  decreases the voltage of the error signal VEA to decrease a duty cycle of a pulse-width modulation signal PWM 1 , so as to decrease an average current flowing through a LED string  130 . 
     Also, a voltage of a common node A between the resistor R 2  and the resistor R 3  is labeled as a second temperature detecting signal VT 2  indicating the ambient temperature of the light source driving circuit  100 . The voltage scaling device  204  is coupled to the bandgap voltage generator  203  and is configured to convert the bandgap voltage V BG  to a reference voltage V BG ′ (e.g., by decreasing the bandgap voltage V BG  in proportion). The second temperature detecting signal VT 2  and the reference voltage V BG ′ are outputted to two input terminals of a comparator  205  respectively to generate an overheating signal OTP. When the ambient temperature of the light source driving circuit  100  is less than or equal to a second temperature threshold (e.g., 145° C.), the voltage of the second temperature detecting signal VT 2  is greater than the reference voltage V BG ′. Thus, the comparator  205  generates the overheating signal OTP having a first state (e.g., logic 1) under control of a comparator  209  or/and a reset signal generator  210 . The overheating signal OTP having the first state enables the pulse-width modulation signal generator  211  to generate a driving signal (e.g., the pulse-width modulation signal PWM 1 ) at the terminal DRV to control a switch  122 . When the ambient temperature of the light source driving circuit  100  is greater than the second temperature threshold (e.g., 145° C., indication of overheating), the voltage of the second temperature detecting signal VT 2  is less than the reference voltage V BG ′. Thus, the comparator  205  generates the overheating signal OTP having a second state (e.g., logic 0) which enables the pulse-width modulation signal generator  211  to maintain the driving signal (e.g., the pulse-width modulation signal PWM 1 ) at a particular state (e.g., logic 0) to keep the switch  122  in the off state, so as to cut off the current flowing through the LED string  130 . 
     The structure of the bandgap voltage generator  203  discussed above is just one possible embodiment and is for illustrative purposes, and does not limit the present invention to its use. On the contrary, the present invention is intended to cover other suitable alternatives, modifications and equivalents of the bandgap voltage generator  203 . 
       FIG. 4  is a schematic diagram of an error signal generator  207  in  FIG. 2A .  FIG. 4  is described in combination with  FIG. 2A . The error signal generator  207  includes an operational amplifier  401 , an operational amplifier  403 , a current mirror  405 , a current mirror  407 , a current mirror  409 , an operational amplifier  411 , an operational amplifier  413 , and resistors R 5 -R 7 . In steady operation, a signal at the positive terminal of an ideal operational amplifier is the same as the one at the negative terminal. Thus, as shown in  FIG. 4 , a voltage V C  at the common mode C is equal to a reference voltage REF 1  (a predetermined constant value). A voltage V D  at the common mode D is equal to the voltage of a first temperature detecting signal VT 1 . The voltage of the first temperature detecting signal VT 1  decreases as the ambient temperature of a light source driving circuit  100  increases. Before the ambient temperature of the light source driving circuit  100  increases to a first temperature threshold TH 1  (e.g., 125□), the voltage V D  is greater than the voltage V C . Thus, the decrease of the voltage of the first temperature detecting signal VT 1  will not cause an increase in a current I 2 . After the ambient temperature of the light source driving circuit  100  increases to the first temperature threshold TH 1  (e.g., 125□), the voltage V D  is less than the voltage V C . As the voltage difference across the resistor R 5  increases, a current I 2  flowing through the resistor R 5  increases accordingly. The current mirror copies the current flowing through an input path to an output path. Thus, due to the current mirror  405  and the current mirror  407 , a current I 3  and a current I 4  increase when the ambient temperature rises. 
     Similarly, a voltage V E  at the common node E, which is coupled to one end of the resistor R 6 , is equal to a reference voltage REF 2  (a predetermined constant value). The other end of the resistor R 6  is coupled to the reference ground of a controller  110 . Thus, the voltage difference on the resistor R 6  is constant. A current I 5  flowing through the resistor R 5  is constant as well. Also, because of the current mirror  409 , a current I 6  is constant. Thus, in a condition where the current I 6  is constant while the current I 4  increases, a voltage of an error signal VEA outputted at a compensation terminal COMP decreases. As discussed above, decreasing the voltage of the error signal VEA can cause an average current flowing through a LED string  130  to decrease, and the voltage of the sensing signal IAVG also decreases. With the influence of the operational amplifier  413 , the voltage difference V G  of the resistor R 7  decreases which leads to decreasing the current I 7  flowing through the resistor R 7  to achieve a current balance. Because of the decrease of the average current flowing through the LED string  130 , the rise of the ambient temperature of the light source driving circuit  100  is slowed down or stopped. As such, the ambient temperature of the light source driving circuit  100  is under effective control. 
       FIG. 5  is a waveform illustrating the sensing signal IAVG in  FIG. 4  versus ambient temperature. In the examples of  FIG. 4  and  FIG. 5 , according to the circuit structure of an error signal generator  207 , before the ambient temperature of a light source driving circuit  100  increases to a first temperature threshold TH 1  (e.g., 125° C.), voltage V D  is greater than the voltage V C . Thus, decreasing the voltage of a first temperature detecting signal VT 1  does not cause a change to the voltage of the sensing signal IAVG. In other words, when the ambient temperature of the light source driving circuit  100  is less than the first temperature threshold TH 1 , a controller  100  maintains an average current flowing through a LED string  130  at a predetermined value such that the average current does not vary with the ambient temperature. After the ambient temperature of the light source driving circuit  100  increases to the first temperature threshold (e.g., 125° C.), if the ambient temperature continues rising, the error signal generator  207  decreases a voltage of an error signal VEA based on the first temperature detecting signal VT 1  and the sensing signal IAVG (the voltage of the error signal VEA is inversely proportional to the ambient temperature), so as to decrease a duty cycle of a pulse-width modulation signal PWM 1 . Thus, the average current flowing through the inductor  127  decreases. The voltage of the sensing signal IAVG decreases accordingly. As such, the rise of the ambient temperature of the light source driving circuit  100  is slowed down or stopped. For example, if the ambient temperature of the light source driving circuit  100  is between the first temperature threshold TH 1  and a second temperature threshold TH 2 , when the ambient temperature of the light source driving circuit  100  increases from a first temperature T 1  to a second temperature T 2 , the average current flowing through the LED string  130  decreases from a first current to a second current under control of the controller  110 . Therefore, the voltage of the sensing signal IAVG drops from a first level IAVG 1  to a second level IAVG 2 . The second level IAVG 2  can be greater than 0. In other words, when the ambient temperature is between the first temperature threshold TH 1  and the second temperature threshold TH 2 , the controller  110  adjusts the average current flowing through the LED strings  130  to make the average current inversely proportional to the ambient temperature. After the ambient temperature of the light source driving circuit  100  exceeds a second temperature threshold TH 2  (e.g., 145° C.), the comparator  205  in the controller  110  generates an overheating signal OTP having a second state (e.g., logic 0) which enables the pulse-width modulation signal generator  211  to maintain a driving signal (e.g., the pulse-width modulation signal PWM 1 ) at a predetermined state (e.g., logic 0) at the driving terminal DRV to keep the switch  122  off, so as to cut off a current flowing through the LED string  130 . Accordingly, the voltage of the sensing signal IAVG decreases rapidly (e.g., drops to 0). 
     The waveform shown in  FIG. 5  is not intend to limit the present invention. On the contrary, the present invention covers various kinds of error signal generators that may lead to different waveforms for the sensing signal IAVG. 
       FIG. 6  is a flowchart  600  illustrating a method for controlling a light source driving circuit (such as the light source driving circuit  100  for driving the LED string  130 ), in accordance with an embodiment of the present disclosure.  FIG. 6  is described in combination with  FIG. 1 - FIG. 5 . 
     In block  602 , a sensing signal IAVG which indicates an average current flowing through a load (e.g., the LED string  130 ) is acquired. The sensing signal IAVG can be acquired based on a sensing signal ISEN indicative of an instant current flowing through the load and an instant current flowing through an energy storage unit. 
     In block of  604 , a first temperature detecting signal VT 1  indicative of an ambient temperature of the light source driving circuit  100  is generated. More specifically, the bandgap voltage generator  203  in the  FIG. 3  generates the first temperature detecting signal VT 1 . With an increase in the ambient temperature of the light source driving circuit  100 , a voltage of the first temperature detecting signal VT 1  decreases. 
     In block of  606 , if the ambient temperature of the light source driving circuit is between a first temperature threshold TH 1  and a second temperature threshold TH 2 , the average current flowing through the load is adjusted based on the first temperature detecting signal VT 1  and the sensing signal IAVG, so that the average current is inversely proportional to the ambient temperature. More specifically, an error signal generator  207  generates an error signal VEA based on the sensing signal IAVG and the first temperature detecting signal VT 1 . After the ambient temperature of the light source driving circuit  100  rises to the first temperature threshold TH 1  (e.g., 125° C.), if the ambient temperature continues to increase, then the error signal generator  207  decreases a voltage of the error signal VEA on an compensation terminal COMP based on the sensing signal IAVG and the first temperature detecting signal VT 1 . A pulse-width modulation signal generator  211  adjusts the average current flowing through the load based on the error signal VEA. In one embodiment, the pulse-width modulation signal generator  211  adjusts the average current flowing through the load by adjusting a duty cycle of a pulse-width modulation signal PWM 1  based on the error signal VEA. For example, after the ambient temperature of the light source driving circuit  100  rises to the first temperature threshold (e.g., 125° C.), if the ambient temperature keeps increasing, the error signal generator  207  decreases the voltage of the error signal VEA based on the first temperature detecting signal VT 1 , so as to decrease the duty cycle of the pulse-width modulation signal PWM 1 . Therefore, the average current flowing through the inductor  127  and the average current flowing through the load decrease. The rise of the average temperature of the light source driving circuit  100  is slowed down or stopped. 
     In an embodiment, the method further includes: if the ambient temperature of the light source driving circuit  100  is greater than the second temperature threshold TH 2  (e.g., 145° C.), an overheating signal OTP having a second state (e.g., logic 0) is generated based on a second temperature detecting signal VT 2 , which enables the pulse-width modulation signal generator  211  to maintain a driving signal (e.g., the pulse-width modulation signal PWM 1 ) at a terminal DRV at a particular state (e.g., logic 0) to turn off the switch  122 . Therefore, the current flowing through the LED string  130  is cut off. 
       FIG. 7  is a circuit diagram illustrating a light source driving circuit  700 , in accordance with another embodiment of the present disclosure. Elements labeled the same as in  FIG. 1  have similar functions. The light source driving circuit  700  is similar with the light source driving circuit  100  except for the structure of the power converter  120 . In the example of  FIG. 7 , the energy storage unit  126  only includes an inductor  127 . The inductor  127  has one end coupled to a monitoring terminal ZCD for providing a detecting signal AUX. The detecting signal AUX indicates whether a current flowing through the inductor  127  has decreased to a predetermined current value. In an embodiment, the light source driving circuit  700  includes a voltage level shifter such as a Zener diode D 5  coupled between the inductor  127  and a controller  110 . The Zener diode D 5  is configured to act as a bias voltage level shifter to add bias voltage to the controller  110 , so as to provide suitable power from the inductor  127  to the controller  110  through a power terminal VDD. In another embodiment, the Zener diode D 5  can be replaced by another kind of element, such as a resistor. In another embodiment, the light source driving circuit  700  does not include a voltage level shifter. 
     Advantageously, the light source driving circuit, the controller and the control method of the present invention can help effectively avoid the damage to LED strings and the inner or peripheral components of the light source driving circuit caused by overheating. 
     While the foregoing description and drawings represent embodiments of the present invention, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the principles of the present invention as defined in the accompanying claims. One skilled in the art will appreciate that the invention may be used with many modifications of form, structure, arrangement, proportions, materials, elements, and components and otherwise, used in the practice of the invention, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims and their legal equivalents, and not limited to the foregoing description.