Patent Publication Number: US-11653431-B2

Title: Load control device for a light-emitting diode light source

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/001,050, filed on Aug. 24, 2020; which is a continuation of U.S. patent application Ser. No. 16/601,845, filed on Oct. 15, 2019, now U.S. Pat. No. 10,757,773 issued Aug. 25, 2020; which is a continuation of U.S. patent application Ser. No. 16/378,134, filed on Apr. 8, 2019 now U.S. Pat. No. 10,448,473 issued Oct. 15, 2019; which is a continuation of U.S. patent application Ser. No. 15/953,812, filed on Apr. 16, 2018 now U.S. Pat. No. 10,257,897 issued Apr. 9, 2019; which is a continuation of U.S. patent application Ser. No. 15/717,123, filed on Sep. 27, 2017, now U.S. Pat. No. 9,949,330 issued Apr. 17, 2018; which is a continuation of U.S. patent application Ser. No. 15/460,973, filed Mar. 16, 2017, now U.S. Pat. No. 9,814,112 issued Nov. 7, 2017; which is a continuation of U.S. patent application Ser. No. 15/291,308, filed Oct. 12, 2016, now U.S. Pat. No. 9,635,726 issued Apr. 25, 2017; which is a continuation of U.S. patent application Ser. No. 14/796,278, filed Jul. 10, 2015, now U.S. Pat. No. 9,497,817 issued Nov. 15, 2016; which is a continuation of U.S. patent application Ser. No. 14/290,584, filed May 29, 2014, now U.S. Pat. No. 9,113,521 issued Aug. 18, 2015; which claims the benefit of U.S. Provisional Patent Application No. 61/828,337, filed May 29, 2013, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Light-emitting diode (LED) light sources are often used in place of or as replacements for conventional incandescent, fluorescent, or halogen lamps, and the like. LED light sources may comprise a plurality of light-emitting diodes mounted on a single structure and provided in a suitable housing. LED light sources are typically more efficient and provide longer operational lives as compared to incandescent, fluorescent, and halogen lamps. In order to illuminate properly, an LED driver control device (i.e., an LED driver) may be coupled between a power source (e.g., an alternating-current (AC) source) and the LED light source for regulating the power supplied to the LED light source. The LED driver may regulate either the voltage provided to the LED light source to a particular value, the current supplied to the LED light source to a specific peak current value, or may regulate both the current and voltage. 
     LED light sources may comprise a plurality of individual LEDs that may be arranged in a series and parallel relationship. In other words, a plurality of LEDs may be arranged in a series string and a number of series strings may be arranged in parallel to achieve the desired light output. For example, five LEDs in a first series string each with a forward bias of approximately three volts (V) and each consuming approximately one watt of power (at 350 mA through the string) consume about 5 W. A second string of a series of five LEDs connected in parallel across the first string will result in a power consumption of 10 W with each string drawing 350 mA. Thus, an LED driver would supply 700 mA to the two strings of LEDs, and since each string has five LEDs, the output voltage provided by the LED driver would be about 15 volts. Additional strings of LEDs can be placed in parallel for additional light output, however, the LED driver should be operable to provide the necessary current. Alternatively, more LEDs can be placed in series on each string, and as a result, the LED driver should also be operable to provide the necessary voltage (e.g., 18 volts for a series of six LEDs). 
     LED light sources are typically rated to be driven via one of two different control techniques: a current load control technique or a voltage load control technique. An LED light source that is rated for the current load control technique is also characterized by a rated current (e.g., 350 milliamps) to which the peak magnitude of the current through the LED light source should be regulated to ensure that the LED light source is illuminated to the appropriate intensity and color. In contrast, an LED light source that is rated for the voltage load control technique is characterized by a rated voltage (e.g., 15 volts) to which the voltage across the LED light source should be regulated to ensure proper operation of the LED light source. Typically, each string of LEDs in an LED light source rated for the voltage load control technique includes a current balance regulation element to ensure that each of the parallel legs has the same impedance so that the same current is drawn in each parallel string. 
     In addition, it is known that the light output of an LED light source can be dimmed. Different methods of dimming LEDs include a pulse-width modulation (PWM) technique and a constant current reduction (CCR) technique. Pulse-width modulation dimming can be used for LED light sources that are controlled in either a current or voltage load control mode. In pulse-width modulation dimming, a pulsed signal with a varying duty cycle is supplied to the LED light source. If an LED light source is being controlled using the current load control technique, the peak current supplied to the LED light source is kept constant during an on time of the duty cycle of the pulsed signal. However, as the duty cycle of the pulsed signal varies, the average current supplied to the LED light source also varies, thereby varying the intensity of the light output of the LED light source. If the LED light source is being controlled using the voltage load control technique, the voltage supplied to the LED light source is kept constant during the on time of the duty cycle of the pulsed signal in order to achieve the desired target voltage level, and the duty cycle of the load voltage is varied in order to adjust the intensity of the light output. Constant current reduction dimming is typically only used when an LED light source is being controlled using the current load control technique. In constant current reduction dimming, current is continuously provided to the LED light source, however, the DC magnitude of the current provided to the LED light source is varied to thus adjust the intensity of the light output. 
     However, an LED light source may become instable or exhibit undesirable characteristics when dimmed to a low intensity level or when dimmed to off (i.e., 0% intensity). For example, when dimmed to a low intensity level or off, an LED light source may flicker, may exhibit inconsistent brightness or color across the individual LEDs of the LED light source, and/or may suddenly drop in intensity during the dimming procedure (e.g., from approximately 1% to off). For instance, when dimming an LED light source using the PWM technique, the on time of the duty cycle of the pulsed signal may reach a threshold where, if reduced any further, causes the LED light source to become instable or exhibit undesirable characteristics. Similarly, when dimming an LED light source using the CCR technique, the DC magnitude of the current provided to the LED light source may reach a threshold where, if reduced any further, causes the LED light source to become instable or exhibit undesirable characteristics. 
     SUMMARY 
     As described herein, a load control device for controlling (e.g., dimming) an intensity of a lighting load to a low intensity level and/or off is provided. The load control device may comprise a power converter circuit, a load regulation circuit, and/or a control circuit. The power converter circuit may be operable to receive a rectified AC voltage and to generate a DC bus voltage. The load regulation circuit may be operable to receive the DC bus voltage and to control a magnitude of a load current conducted through the lighting load, for example, using the DC bus voltage. The control circuit may be operatively coupled to the load regulation circuit for pulse width modulating and/or pulse frequency modulating the load current to control the intensity of the lighting load to a target intensity. The lighting load may comprise an LED light source. The load regulation circuit may comprise an LED drive circuit. 
     The control circuit may be configured to control the intensity of the lighting load by pulse width modulating the load current when the target intensity is above a predetermined threshold and control the intensity of the lighting load by pulse frequency modulating the load current when the target intensity is below the predetermined threshold. The predetermined threshold may be, for example, a low-end intensity (e.g., 1%). Pulse width modulating the load current may comprise maintaining a frequency of the load current constant and adjusting an on time of the load current. Pulse frequency modulating the load current may comprise maintaining the on time of the load current constant and adjusting the frequency of the load current. 
     For example, the control circuit may be configured to maintain the frequency of the load current at a normal pulse width modulation (PWM) frequency and adjust the on time of the load current between a maximum on time and a minimum on time when the target intensity is above the predetermined threshold, for example, when the target intensity is between a high-end intensity and the low-end intensity. The control circuit may be configured to maintain the on time of the load current at the minimum on time and adjust the frequency of the load current between the normal PWM frequency and a minimum PWM frequency when the target intensity is below the predetermined threshold, for example, when the target intensity is between the low-end intensity and a minimum intensity. The minimum intensity may be below (i.e., less than) the low-end intensity. The control circuit may be configured to maintain the frequency of the load current at the minimum PWM frequency and adjust the on time of the load current between the minimum on time and an ultra-low minimum on time when the target intensity is below the minimum intensity, for example, when the target intensity is between the minimum intensity and an ultra-low minimum intensity. For instance, the control circuit may dim the LED light source to off (i.e., the ultra-low minimum intensity may be 0% intensity). 
     The control circuit may be configured to dim the LED light source to off. For example, the control circuit may be configured to pulse width modulate the load current when the target intensity is below the minimum intensity, which is below the predetermined threshold (e.g., a low-end intensity). As such, the control circuit may be configured to control the intensity of the lighting load from the minimum intensity to off by pulse width modulating the load current. The control circuit may be configured to control the intensity of the lighting load from the predetermined threshold to off by pulse frequency modulating the load current. The control circuit may be configured to maintain a frequency of the load current constant, maintain an on time of the load current constant, and decrease a magnitude of the DC bus voltage when the target intensity is below the minimum intensity. For example, control circuit may control the intensity of the lighting load to off by decreasing the magnitude of the DC bus voltage. 
     The control circuit may be configured to control the intensity of the lighting load by pulse width modulating the load current when the target intensity is within a first intensity range and control the intensity of the lighting load by pulse frequency modulating the load current when the target intensity is within a second intensity range. The first intensity range may be greater than or less than the second intensity range. The control circuit may be configured to receive a command and control (e.g., dim) the intensity of the lighting load below the first intensity range and below the second intensity range to off. For example, the load control circuit may be configured to control the intensity of the lighting load below the second intensity range to off by pulse width modulating and/or pulse frequency modulating the load current. The load control circuit may be configured to control the intensity of the lighting load below the first intensity range and below the second intensity range to off by maintaining the frequency of the load current constant, maintaining the on time of the load current constant, and decreasing the magnitude of the DC bus voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram of an example system that comprises a light-emitting diode (LED) driver for controlling the intensity of an LED light source. 
         FIG.  2    is a block diagram of an example of an LED driver for controlling the intensity of an LED light source. 
         FIG.  3 A  is a schematic diagram of an example of a flyback converter and an LED drive circuit. 
         FIG.  3 B  is a schematic diagram showing an example the LED drive circuit of  FIG.  3 A . 
         FIG.  4 A  is a graph that illustrates an example of the relationship between an on-time T ON  of a load current of an LED driver and a target lighting intensity L TRGT  of an LED light source. 
         FIG.  4 B  is a graph that illustrates an example of the relationship between a frequency f LOAD  of a load current of an LED driver and a target lighting intensity L TRGT  of an LED light source. 
         FIG.  5 A  is a graph that illustrates an example of the relationship between an on-time T ON  of a load current of an LED driver and a target lighting intensity L TRGT  of an LED light source. 
         FIG.  5 B  is a graph that illustrates an example of the relationship between a frequency f LOAD  of a load current of an LED driver and a target lighting intensity L TRGT  of an LED light source. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a block diagram of an example system that comprises a light-emitting diode (LED) driver for controlling the intensity of an LED light source. A system  108  may comprise an alternating-current (AC) power source  104 , a dimmer switch  106 , an LED driver  100 , and/or an LED light source  102 . The LED driver  100  may control an intensity of the LED light source  102 . An example of the LED light source  102  may be an LED light engine. The LED light source  102  is shown as a plurality of LEDs connected in series but may comprise a single LED or a plurality of LEDs connected in series, parallel, or a suitable combination thereof, for example, depending on the particular lighting system. The LED light source  102  may comprise one or more organic light-emitting diodes (OLEDs). 
     The LED driver  100  may be coupled to the AC power source  104  via the dimmer switch  106 . The dimmer switch  106  may generate a phase-control signal V PC  (e.g., a dimmed-hot voltage). The dimmer switch  106  may provide the phase-control signal V PC  to the LED driver  100 . The dimmer switch  106  may comprise a bidirectional semiconductor switch (not shown), such as, for example, a triac or two anti-series-connected field-effect transistors (FETs), which may be coupled in series between the AC power source  104  and the LED driver  100 . The dimmer switch  106  may control the bidirectional semiconductor switch to be conductive for a conduction period T CON  each half-cycle of the AC power source  104  to generate the phase-control signal V PC . 
     The LED driver  100  may turn the LED light source  102  on and off in response to the conduction period T CON  of the phase-control signal V PC  received from the dimmer switch  106 . The LED driver  100  may adjust (i.e., dim) a present intensity L PRES  of the LED light source  102  to a target intensity L TRGT  in response to the phase-control signal V PC . The target intensity L TRGT  may range across a dimming range of the LED light source  102 . For example, the dimming range of the LED light source  102  may be between a low-end intensity L LE  (e.g., approximately 1%) and a high-end intensity L RE  (e.g., approximately 100%). The LED driver  100  may control the magnitude of a load current V LOAD  through the LED light source  102  and/or the magnitude of a load voltage V LOAD  across the LED light source. Accordingly, the LED driver  100  may control at least one of the load voltage V LOAD  across the LED light source  102  and the load current I LOAD  through the LED light source to control the amount of power delivered to the LED light source, for example, depending upon a mode of operation of the LED driver (e.g., as described herein). 
     The LED driver  100  may work with (i.e., control) a plurality of different LED light sources. For example, the LED driver  100  may work with LED lights sources that are rated to operate using different load control techniques, different dimming techniques, and/or different magnitudes of load current and/or voltage. The LED driver  100  may control the magnitude of the load current V LOAD  through the LED light source  102  and/or the load voltage V LOAD  across the LED light source using different modes of operation. For example, the LED driver  100  may use a current load control mode (i.e., for using the current load control technique) and/or a voltage load control mode (i.e., for using the voltage load control technique). The LED driver  100  may adjust the magnitude to which the LED driver  100  controls the load current I LOAD  through the LED light source  102  in the current load control mode. The LED driver  100  may adjust the magnitude to which the LED driver  100  controls the load voltage V LOAD  across the LED light source in the voltage load control mode. 
     When operating in the current load control mode, the LED driver  100  may control the intensity of the LED light source  102  using a PWM dimming mode (i.e., for using the PWM dimming technique), a CCR dimming mode (i.e., for using the CCR dimming technique), and/or a pulse frequency modulation (PFM) dimming mode (i.e., for using the PFM dimming technique). In the PWM dimming mode, the LED driver  100  may control the load current I LOAD  by altering the pulse duration of the load current I LOAD  and maintaining the frequency of the load current I LOAD  constant. In the PFM dimming mode, the LED driver  100  may control the load current I LOAD  by maintaining the pulse duration of the load current I LOAD  constant and altering the frequency of the load current I LOAD . In the CCR dimming mode, the LED driver  100  may control the load current I LOAD  by altering the DC magnitude of the current load current I LOAD . When operating in the voltage load control mode, the LED driver  100  may control the amount of power delivered to the LED light source  102  using the PWM dimming mode and/or the PFM dimming mode. The LED driver  100  may control the amount of power delivered to the LED light source  102  in response to a digital message, which may be received from a communication circuit, for example as described herein. 
       FIG.  2    is a block diagram of an example of an LED driver for controlling an LED light source. An LED driver  200  may comprise a radio-frequency (RFI) filter and rectifier circuit  215 , a buck-boost flyback converter  220 , a bus capacitor C BUS , an LED drive circuit  230 , a control circuit  240 , a power supply  250 , a phase-control input circuit  260 , memory  270 , and/or a communication circuit  280 . The LED driver  200  may be an example of the LED driver  100  of  FIG.  1   . As such, the LED driver  200  may be used within the system  108  of  FIG.  1   . The LED driver  200  may control an LED light source, such as the LED light source  102 . 
     The RFI filter and rectifier circuit  215  may receive the phase-control signal V PC  from a dimmer switch (e.g., the dimmer switch  106  of  FIG.  1   ). The RFI filter and rectifier circuit  215  may minimize the noise provided on an AC power source (e.g., the AC power source  104  of  FIG.  1   ). The RFI filter and rectifier circuit  215  may generate a rectified voltage V RECT . The buck-boost flyback converter  220  may receive the rectified voltage V RECT . The buck-boost flyback converter  220  may generate a variable direct-current (DC) bus voltage V BUS  across the bus capacitor C BUS . The buck-boost flyback converter  220  may provide electrical isolation between the AC power source and the LED light source  102 . The buck-boost flyback converter  220  may operate as a power factor correction (PFC) circuit to adjust the power factor of the LED driver  200  towards a power factor of one. The buck-boost flyback converter  220  may be a power converter circuit. Although illustrated as the buck-boost flyback converter  220 , the LED driver  200  may comprise any suitable power converter circuit for generating an appropriate bus voltage V BUS , such as, for example, a boost converter, a buck converter, a single-ended primary-inductor converter (SEPIC), a Ćuk converter, or other suitable power converter circuit. The bus voltage V BUS  may be characterized by some voltage ripple as the bus capacitor C BUS  periodically charges and discharges. 
     The LED drive circuit  230  may be a load regulation circuit. The LED drive circuit  230  may receive the bus voltage V BUS . The LED drive circuit  230  may control the amount of power delivered to the LED light source  102  so as to control the intensity of the LED light source  102 . The LED drive circuit  230  may comprise a controllable-impedance circuit, such as a linear regulator, for example, as described herein. The LED drive circuit  230  may comprise a switching regulator, such as a buck converter for example. Examples of various embodiments of LED drive circuits  230  are described in U.S. patent application Ser. No. 12/813,908, filed Jun. 11, 2010, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosure of which is hereby incorporated by reference. 
     The control circuit  240  may control the operation of the buck-boost flyback converter  220  and/or the LED drive circuit  230 . The control circuit  240  may comprise, for example, a controller or any other suitable processing device, such as, for example, a microcontroller, a programmable logic device (PLD), a microprocessor, an application specific integrated circuit (ASIC), or a field-programmable gate array (FPGA). The power supply  250  may receive the rectified voltage V RECT . The power supply  250  may generate a plurality of direct-current (DC) supply voltages for powering the circuitry of the LED driver  200 , for example, using the rectified voltage V RECT . For example, the power supply  250  may generate a first non-isolated supply voltage V CC1  (e.g., approximately 14 volts) for powering the control circuitry of the buck-boost flyback converter  220 , a second isolated supply voltage V CC2  (e.g., approximately 9 volts) for powering the control circuitry of the LED drive circuit  230 , and/or a third non-isolated supply voltage V CC3  (e.g., approximately 5 volts) for powering the control circuit  240 . 
     The control circuit  240  may be coupled to the phase-control input circuit  260 . The phase-control input circuit  260  may generate a target intensity control signal V TRGT . The target intensity control signal V TRGT  may comprise, for example, a square-wave signal having a duty cycle DC TRGT , which may be dependent upon the conduction period T CON  of the phase-control signal V PC  received from a dimmer switch (e.g., the dimmer switch  106  of  FIG.  1   ). The duty cycle DC TRGT  may be representative of the target intensity L TRGT  of the LED light source  102 . The target intensity control signal V TRGT  may comprise a DC voltage having a magnitude dependent upon the conduction period T CON  of the phase-control signal V PC , and thus representative of the target intensity L TRGT  of the LED light source  102 . 
     The control circuit  240  may be coupled to the memory  270 . The memory  270  may store the operational characteristics of the LED driver  200  (e.g., the load control mode, the dimming mode, the magnitude of the rated load voltage or current, and/or the like). The communication circuit  280  may be coupled to, for example, a wired communication link or a wireless communication link, such as a radio-frequency (RF) communication link or an infrared (IR) communication link. The control circuit  240  may update the target intensity L TRGT  of the LED light source  102  and/or the operational characteristics stored in the memory  270  in response to digital messages received via the communication circuit  280 . For example, the LED driver  200  may receive a full conduction AC waveform from the AC power source (i.e., not the phase-control signal V PC  from the dimmer switch) and may determine the target intensity L TRGT  for the LED light source  102  from the digital messages received via the communication circuit  280 . 
     The control circuit  240  may manage the operation of the buck-boost flyback converter  220  and/or the LED drive circuit  230  to control the intensity of the LED light source  102 . The control circuit  240  may receive a bus voltage feedback signal V BUS-FB , which may be representative of the magnitude of the bus voltage V BUS , from the buck-boost flyback converter  220 . The control circuit  240  may provide a bus voltage control signal V BUS-CNTL  to the buck-boost flyback converter  220  for controlling the magnitude of the bus voltage V BUS  to a target bus voltage V BUS-TRGT  (e.g., from approximately 8 volts to 60 volts). The LED drive circuit  230  may control a peak magnitude I PK  of the load current I LOAD  conducted through the LED light source  102  between a minimum load current I LOAD-MIN  and a maximum load current I LOAD-MAX  (e.g., when operating in the current load control mode), for example, in response to a peak current control signal V IPK  provided by the control circuit  240 . The control circuit  240  may receive a load current feedback signal V ILOAD , which is representative of an average magnitude I AVE  of the load current I LOAD  flowing through the LED light source  102 . The control circuit  240  may receive a regulator voltage feedback signal V REG-FB , which is representative of the magnitude of a regulator voltage V REG  (i.e., a controllable-impedance voltage) across the linear regulator of the LED drive circuit  230 , for example, as described herein. 
     The control circuit  240  may control the LED drive circuit  230  to control the amount of power delivered to the LED light source  102  using the current load control mode of operation and/or the voltage load control mode of operation. During the current load control mode, the LED drive circuit  230  may regulate the peak magnitude I PK  of the load current I LOAD  through the LED light source  102  to control the average magnitude I AVE  to a target load current I TRGT  in response to the load current feedback signal V ILOAD  (i.e., using closed loop control). The target load current I TRGT  may be stored in the memory  270 . The target load current I TRGT  may be programmed to be any specific magnitude depending upon the LED light source  102 . 
     To control the intensity of the LED light source  102  during the current load control mode, the control circuit  240  may control the LED drive circuit  230  to adjust the amount of power delivered to the LED light source  102  using the PWM dimming technique, the PFM dimming technique, and/or the CCR dimming technique. Using the PWM dimming technique, the control circuit  240  may control the peak magnitude I PK  of the load current I LOAD  through the LED light source  102  to the target load current I TRGT . Using the PWM dimming technique, the control circuit  240  may pulse-width modulate the load current I LOAD  to dim the LED light source  102  and achieve the target load current I TRGT . For example, the LED drive circuit  230  may control (i.e., adjust) a duty cycle DC ILOAD  of the load current I LOAD  in response to a duty cycle DC DIM  of a dimming control signal V DIM  provided by the control circuit  240 . Further, when using the PWM dimming technique, the LED drive circuit  230  may maintain a frequency f ILOAD  of the load current I LOAD  in response to a frequency f DIM  of the dimming control signal V DIM  provided by the control circuit  240 . The intensity of the LED light source  102  may be dependent upon the duty cycle DC ILOAD  and the frequency f ILOAD  of the pulse-width modulated load current I LOAD . 
     Using the PFM dimming technique, the control circuit  240  may control the peak magnitude I PK  of the load current I LOAD  through the LED light source  102  to the target load current I TRGT . Using the PFM dimming technique, the control circuit  240  may pulse frequency modulate the load current I LOAD  to dim the LED light source  102  and achieve the target load current I TRGT . For example, the LED drive circuit  230  may control (i.e., adjust) a frequency f ILOAD  of the load current I LOAD  in response to a frequency f DIM  of a dimming control signal V DIM  provided by the control circuit  240 . Further, when using the PFM dimming technique, the LED drive circuit  230  may maintain the duty cycle DC ILOAD  of the load current I LOAD  in response to a duty cycle DC DIM  of the dimming control signal V DIM  provided by the control circuit  240 . The intensity of the LED light source  102  may be dependent upon the duty cycle DC ILOAD  and the frequency f ILOAD  of the pulse-width modulated load current I LOAD . 
     Using the CCR technique, the control circuit  240  may not pulse-width modulate or pulse-frequency modulate the load current I LOAD . Using the CCR technique, the control circuit  240  may adjust the magnitude of the target load current I TRGT  so as to adjust the average magnitude I AVE  of the load current I LOAD  through the LED light source  102 . The average magnitude I AVE  of the load current I LOAD  through the LED light source  102  may be equal to the peak magnitude I PK  of the load current I LOAD  in the CCR dimming mode. 
     During the voltage load control mode, the LED drive circuit  230  may regulate the DC voltage of the load voltage V LOAD  across the LED light source  102  to a target load voltage V TRGT . The target load voltage V TRGT  may be stored in the memory  270 . The target load voltage V TRGT  may be programmed to be any specific magnitude depending upon the LED light source  102 . The control circuit  240  may dim the LED light source  102  using the PWM dimming technique and/or the PFM dimming technique during the voltage load control mode. For example, using the PWM dimming technique, the control circuit  240  may adjust a duty cycle DC VLOAD  of the load voltage V LOAD  in response to a duty cycle DC DIM  of the dimming control signal V DIM  to dim the LED light source  102 . Using the PFM dimming technique, the control circuit  240  may adjust the frequency f ILOAD  of the load voltage V LOAD  in response to a frequency f DIM  of the dimming control signal V DIM  to dim the LED light source  102 . An example of a configuration procedure for the LED driver  200  is described in greater detail in U.S. patent application Ser. No. 12/813,989, filed Jun. 11, 2010, entitled CONFIGURABLE LOAD CONTROL DEVICE FOR LIGHT-EMITTING DIODE LIGHT SOURCES, the entire disclosure of which is hereby incorporated by reference. 
       FIG.  3 A  is a schematic diagram of an example of a flyback converter and an LED drive circuit. A flyback converter  320  may comprise a flyback transformer  310 , a field-effect transistor (FET) Q 312 , a diode D 314 , a resistor R 316 , a resistor R 318 , a flyback control circuit  322 , a filter circuit  324 , an optocoupler circuit  326 , and/or a feedback resistor R 328 . An LED drive circuit  330  may comprise a regulation field-effect transistor (FET) Q 332 , a filter circuit  334 , an amplifier circuit  336 , a gate resistor R 338 , a feedback circuit  342 , a dimming FET Q 350 , a sample and hold circuit (SHC)  360 , and/or an overvoltage protection circuit  370 . The flyback converter  320  may be an example of the buck-boost flyback converter  220  of  FIG.  2   . The LED drive circuit  330  may be an example of the LED drive circuit  230  of  FIG.  2   . As such, the LED driver  100  of  FIG.  1    and/or the LED driver  200  of  FIG.  2    may comprise the flyback converter  320  and/or the LED drive circuit  330 . 
     The flyback transformer  310  may comprise a primary winding and a secondary winding. The primary winding may be coupled in series with the field-effect transistor (FET) Q 312 . Although illustrated as the field-effect transistor (FET) Q 312 , the primary winding of the flyback transformer  310  may be coupled in series with any flyback switching transistor or other suitable semiconductor switch. The secondary winding of the flyback transformer  310  may be coupled to the bus capacitor C BUS  via the diode D 314 . The bus voltage feedback signal V BUS-FB  may be generated by a voltage divider comprising the resistors R 316 , R 318  coupled across the bus capacitor C BUS    
     The flyback control circuit  322  may receive the bus voltage control signal V BUS-CNTL  from the control circuit  240 , for example, via the filter circuit  324  and the optocoupler circuit  326 . The filter circuit  324  and the optocoupler circuit  326  may provide electrical isolation between the flyback converter  320  and the control circuit  240 . The flyback control circuit  322  may comprise, for example, part number TDA 4863 , manufactured by Infineon Technologies. The filter circuit  324  may generate a filtered bus voltage control signal V BUS-F  using the bus voltage control signal V BUS-CNTL . For example, the filter circuit  324  may comprise a two-stage resistor-capacitor (RC) filter for generating the filtered bus voltage control signal V BUS-F . The filtered bus voltage control signal V BUS-F  may comprise a DC magnitude dependent upon the duty cycle DC BUS  of the bus voltage control signal V BUS-CNTL . The flyback control circuit  322  may receive a control signal representative of the current through the FET Q 312  from the feedback resistor R 328 , which is coupled in series with the FET Q 312 . 
     The flyback control circuit  322  may control the FET Q 312  to selectively conduct current through the flyback transformer  310  to generate the bus voltage V BUS . The flyback control circuit  322  may render the FET Q 312  conductive and non-conductive at a high frequency (e.g., approximately 150 kHz or less), for example, to control the magnitude of the bus voltage V BUS  in response to the DC magnitude of the filtered bus voltage control signal V BUS-F  and the magnitude of the current through the FET Q 312 . For example, the control circuit  240  may increase the duty cycle DC BUS  of the bus voltage control signal V BUS-CNTL  such that the DC magnitude of the filter bus voltage control signal V BUS-F  increases in order to decrease the magnitude of the bus voltage V BUS . The control circuit  240  may decrease the duty cycle DC BUS  of the bus voltage control signal V BUS-CNTL  to increase the magnitude of the bus voltage V BUS . The filter circuit  324  may provide a digital-to-analog conversion for the control circuit  240  (i.e., from the duty cycle DC BUS  of the bus voltage control signal V BUS-CNTL  to the DC magnitude of the filtered bus voltage control signal V BUS-CNTL ). The control circuit  240  may comprise a digital-to-analog converter (DAC) for generating (e.g., directly generating) the bus voltage control signal V BUS-CNTL  having an appropriate DC magnitude for controlling the magnitude of the bus voltage V BUS . 
       FIG.  3 B  is a schematic diagram of an example of the LED drive circuit of  FIG.  3 A . The LED drive circuit  330  may comprise the regulation field-effect transistor (FET) Q 332 , the filter circuit  334 , the amplifier circuit  336 , the gate resistor R 338 , the feedback circuit  342 , the dimming FET Q 350 , the sample and hold circuit  360 , and/or the overvoltage protection circuit  370 . The feedback circuit  342  may comprise a feedback resistor R 344 , a filter circuit  346 , and/or an amplifier circuit  348 . The sample and hold circuit  360  may comprise a FET Q 361 , a capacitor C 362 , a resistor R 363 , a resistor R 364 , a FET Q 365 , a resistor R 366 , and/or a resistor R 367 . The overvoltage protection circuit  370  may comprise a comparator U 371 , a resistor R 372 , a resistor R 373 , a resistor R 374 , a resistor R 375 , a filtering capacitor C 376 , a resistor R 378 , and/or a resistor R 379 . 
     The LED drive circuit  330  may comprise a linear regulator (i.e., a controllable-impedance circuit) including the regulation field-effect transistor (FET) Q 332  coupled in series with the LED light source  102  for conducting the load current I LOAD . Although illustrated as the FET Q 332 , the LED drive circuit  330  may comprise any power semiconductor switch coupled in series with the LED light source  102  for conducting the load current I LOAD . The regulation FET Q 332  may comprise a bipolar junction transistor (BJT), an insulated-gate bipolar transistor (IGBT), or any suitable transistor. The peak current control signal V IPK  (provided by the control circuit  240  may be coupled to the gate of the regulation FET Q 332  through the filter circuit  334 , the amplifier circuit  336 , and the gate resistor R 338 . The control circuit  240  may control the duty cycle DC IPK  of the peak current control signal V IPK  to control the peak magnitude I PK  of the load current I LOAD  conducted through the LED light source  102  to the target load current I TRGT . 
     The filter circuit  334  (e.g., a two-stage RC filter) may provide digital-to-analog conversion for the control circuit  240 , for example, by generating a filtered peak current control signal V IPK-F . The filtered peak current control signal V IPK-F  may have a DC magnitude dependent upon the duty cycle DC IPK  of the peak current control signal V IPK  and may be representative of the magnitude of the target load current I TRGT . The control circuit  240  may comprise a DAC for generating (e.g., directly generating) the peak current control signal V IPK  having an appropriate DC magnitude for controlling the peak magnitude I PK  of the load current I LOAD . The amplifier circuit  336  may generate an amplified peak current control signal V IPK-A . The amplifier circuit  336  may provide the amplified peak current control signal V IPK-A  to the gate of the regulation transistor Q 332  through the resistor R 338 , such that a drive signal at the gate of the regulation transistor Q 332 , e.g., a gate voltage V IPK-G , has a magnitude dependent upon the target load current I TRGT . The amplifier circuit  336  may comprise a standard non-inverting operational amplifier circuit having, for example, a gain α of approximately three. 
     The feedback resistor R 344  of the feedback circuit  342  may be coupled in series with the regulation FET Q 332 , for example, such that the voltage generated across the feedback resistor is representative of the magnitude of the load current I LOAD . For example, the feedback resistor R 344  may have a resistance of approximately 0.0375Ω. The filter circuit  346  (e.g., a two-stage RC filter) of the feedback circuit  342  may be coupled between the feedback resistor R 344  and the amplifier circuit  348  (e.g., a non-inverting operational amplifier circuit having a gain β of approximately 20). The amplifier circuit  348  may have a variable gain, which for example, may be controlled by the control circuit  240  and could range between approximately 1 and 1000. The amplifier circuit  348  may generate the load current feedback signal V ILOAD . The amplifier circuit  348  may provide the load current feedback signal V ILOAD  to the control circuit  240 . The load current feedback signal V ILOAD  may be representative of an average magnitude I AVE  of the load current I LOAD , e.g.,
 
 I   AVE   =V   ILOAD /(β· R   FB ),  (Equation 1)
 
wherein R FB  is the resistance of the feedback resistor R 344 . Examples of other feedback circuits for the LED drive circuit  330  are described in greater detail in U.S. patent application Ser. No. 12/814,026, filed Jun. 11, 2010, entitled CLOSED-LOOP LOAD CONTROL CIRCUIT HAVING A WIDE OUTPUT RANGE, the entire disclosure of which is hereby incorporated by reference.
 
     When operating in the current load control mode, the control circuit  240  may control the regulation FET Q 332  to operate in the linear region, such that the peak magnitude I PK  of the load current I LOAD  is dependent upon the DC magnitude of the gate voltage V IPK-g  at the gate of the regulation FET Q 332 . In other words, the regulation FET Q 332  may provide a controllable-impedance in series with the LED light source  102 . If the magnitude of the regulator voltage V REG  drops too low, the regulation FET Q 332  may be driven into the saturation region, such that the regulation FET Q 332  becomes fully conductive and the control circuit  240  is no longer able to control the peak magnitude I PK  of the load current I LOAD . Therefore, the control circuit  240  may adjust the magnitude of the bus voltage V BUS  to prevent the magnitude of the regulator voltage V REG  from dropping below a minimum regulator voltage threshold V REG-MIN  (e.g., approximately 0.4 volts). In addition, the control circuit  240  may adjust the magnitude of the bus voltage V BUS  to control the magnitude of the regulator voltage V REG  to be less a maximum regulator voltage threshold V REG-MAX  (e.g., approximately 0.6 volts), for example, to prevent the power dissipated in regulation FET Q 332  from becoming too large, thus increasing the total efficiency of the LED driver (e.g., the LED driver  100 , the LED driver  200 , and/or the like). Since the regulator voltage V REG  may have some ripple (e.g., which may be due to the ripple of the bus voltage V BUS ), the control circuit  240  may determine the minimum value of the regulator voltage V REG  during a period of time and to compare this minimum value of the regulator voltage V REG  to the regulator voltage threshold V REG-MIN  and the maximum regulator voltage threshold V REG-MAX . 
     When operating in the voltage load control mode, the control circuit  240  may drive the regulation FET Q 332  into the saturation region, for example, such that the magnitude of the load voltage V LOAD  is approximately equal to the magnitude of the bus voltage V BUS  (e.g., minus the small voltage drops due to the on-state drain-source resistance R DS-ON  of the FET regulation Q 332  and the resistance of the feedback resistor R 344 ). 
     The dimming FET Q 350  of the LED drive circuit  330  may be coupled between the gate of the regulation FET Q 332  and circuit common. The dimming control signal V DIM  from the control circuit  240  may be provided to the gate of the dimming FET Q 350 . When the dimming FET Q 350  is rendered conductive, the regulation FET Q 332  may be rendered non-conductive. When the dimming FET Q 350  is rendered non-conductive, the regulation FET Q 332  may be rendered conductive. 
     While using the PWM dimming technique during the current load control mode, the control circuit  240  may adjust the duty cycle DC DIM  of the dimming control signal V DIM  (e.g., to adjust the length of an on time t ON  that the regulation FET Q 332  is conductive) to control when the regulation FET Q 332  conducts the load current I LOAD  and to control the intensity of the LED light source  102 . For example, the control circuit  240  may generate the dimming control signal V DIM  using a constant frequency f DIM  (e.g., approximately in the range of 500-550 Hz), such that the on time t ON  of the dimming control signal V DIM  is dependent upon the duty cycle DC DIM , i.e.,
 
 t   ON =(1 −DC   DIM )/ f   DIM .  (Equation 2)
 
As the duty cycle DC DIM  of the dimming control signal V DIM  increases, the duty cycle DC ITRGT , DC VTRGT  of the corresponding load current I LOAD  or load voltage V LOAD  decreases, and vice versa.
 
     While using the PFM dimming technique during the current load control mode, the control circuit  240  may adjust the frequency f DIM  of the dimming control signal V DIM  to control the frequency at which the regulation FET Q 332  conducts the load current I LOAD  and to control the intensity of the LED light source  102 . For example, the control circuit  240  may generate the dimming control signal V DIM  using a constant on time t ON , such that the frequency f DIM  of the dimming control signal V DIM  is dependent upon the duty cycle DC DIM , i.e.,
 
 f   DIM =(1− DC   DIM )/ t   ON .  (Equation 3)
 
As the duty cycle DC DIM  of the dimming control signal V DIM  increases, the duty cycle DC ITRGT , DC VTRGT  of the corresponding load current I LOAD  or load voltage V LOAD  decreases, and vice versa.
 
     When using the PWM dimming technique and/or the PFM dimming technique in the current load control mode, the control circuit  240  may control the peak magnitude I PK  of the load current I LOAD  in response to the load current feedback signal V ILOAD  to maintain the average magnitude I AVE  of the load current I LOAD  constant (i.e., at the target lamp current I TRGT ). The control circuit  240  may calculate the peak magnitude I PK  of the load current I LOAD  from the load current feedback signal V ILOAD  and the duty cycle DC DIM  of the dimming control signal V DIM , i.e.,
 
 I   PK   =I   AVE /(1− DC   DIM ).  (Equation 4)
 
The load current feedback signal V ILOAD  may be representative of the average magnitude I AVE  of the load current I LOAD . When using the CCR dimming technique during the current load control mode, the control circuit  240  may maintain the duty cycle DC DIM  of the dimming control signal V DIM  at a high-end dimming duty cycle DC HE  (e.g., approximately 0%, such that the FET Q 332  is always conductive) and/or may adjust the target load current I TRGT  (e.g., via the duty cycle DC IPK  of the peak current control signal V IPK ) to control the intensity of the LED light source  102 .
 
     The regulator voltage feedback signal V REG-FB  may be generated by the sample and hold circuit  360  of the LED drive circuit  330 . The regulator voltage feedback signal V REG-FB  may be representative of the regulator voltage V REG  generated across the series combination of the regulation FET Q 332  and the feedback resistor R 344  when the regulation FET Q 332  is conducting the load current I LOAD . The FET Q 361  of the sample and hold circuit  360  may be coupled to the junction of the LED light source  102  and the regulation FET Q 332 . Although illustrated as the FET Q 361 , the sample and hold circuit  360  may include any sampling transistor. When the FET Q 361  is rendered conductive, the capacitor C 362  may charge to approximately the magnitude of the regulator voltage V REG  through the resistor R 363 . The capacitor C 362  may have a capacitance of approximately 1 μF. The resistor R 363  may have a resistance of approximately 10Ω. The capacitor C 362  may be coupled to the control circuit  240  through the resistor R 364  for providing the regulator voltage feedback signal V REG-FB  to the control circuit  240 . The resistor R 364  may have a resistance of approximately 12.1 kΩ. The gate of the FET Q 361  may be coupled to circuit common through the FET Q 365  and to the second isolated supply voltage V CC2  through the resistor R 366 . The resistor R 366  may have a resistance of approximately 20 kΩ. The gate of the second FET Q 365  may be coupled to the third non-isolated supply voltage V CC3  through the resistor R 367 . The resistor R 367  may have a resistance of approximately 10 kΩ. 
     The control circuit  240  may generate a sample and hold control signal V SH  that is operatively coupled to the control input (i.e., the gate) of the FET Q 365  of the sample and hold circuit  360 . The sample and hold control signal V SH  may be coupled to the FET Q 365  to render the FET Q 361  conductive and non-conductive to controllably charge the capacitor C 362  to the magnitude of the regulator voltage V REG . For example, when using the PWM dimming mode and/or the PFM dimming mode, the control circuit  240  may render the FET Q 361  conductive during an on time t ON  (e.g., each on time t ON ) of the dimming control signal V DIM  (i.e., when the dimming FET Q 350  is non-conductive and the regulation FET Q 332  is conductive). When the FET Q 361  is rendered conductive during the on time t ON  of the dimming control signal V DIM , the regulator voltage feedback signal V REG-FB  may be representative of the magnitude of the regulator voltage V REG  when the regulation FET Q 332  is conducting the load current I LOAD . When the control circuit  240  is using the CCR dimming mode, the FET Q 361  may be rendered conductive at all times. 
     The overvoltage protection circuit  370  of the LED drive circuit  330  may be responsive to the magnitude of the bus voltage V BUS  and/or the magnitude of the regulator feedback voltage V REG-FB . The difference between the magnitudes of the bus voltage V BUS  and the regulator feedback voltage V REG-FB  may be representative of the magnitude of the load voltage V LOAD  across the LED light source  102 . The comparator U 371  of the overvoltage protection circuit  370  may have an output coupled to the gate of the regulation FET Q 332  for rendering the FET non-conductive if the load voltage V LOAD  exceeds an overvoltage threshold. The overvoltage protection circuit  370  may comprise a resistor divider that includes the resistors R 372 , R 373 . The resistor divider that includes the resistors R 372 , R 373  may receive the regulator feedback voltage V REG-FB . The junction of the resistors R 372 , R 373  may be coupled to the non-inverting input of the comparator U 371  through the resistor R 374 . The non-inverting input may be coupled to the third non-isolated supply voltage V CC3  through the resistor R 375  and/or to circuit common through the filtering capacitor C 376 . The filtering capacitor C 376  may have a capacitance of approximately 10 μF. 
     The overvoltage protection circuit may comprise a resistor divider that includes the resistors  3478 ,  379 . The resistor divider that includes resistors R 378 , R 379  may be coupled between the bus voltage V BUS  and circuit common. The junction of the resistors R 378 , R 379  may be coupled to the inverting input of the comparator U 371 , such that, for example, the magnitude of the voltage at the non-inverting input of the comparator U 371  may be responsive to the regulator feedback voltage V REG-FB  and/or such that the magnitude of the voltage at the inverting input of the comparator U 371  may be responsive to the bus voltage V BUS . The comparator U 371  may operate to render the regulation FET Q 332  non-conductive if the difference between the magnitudes of the bus voltage V BUS  and the regulator feedback voltage V REG-FB  exceeds the overvoltage threshold. 
     The resistances of the resistors R 372 , R 373 , R 374 , R 375 , R 378 , R 379  of the overvoltage protection circuit  370  may be determined such that the voltage at the non-inverting input of the comparator U 371  is proportional to the magnitude of the regulator feedback voltage V REG-FB . Accordingly, the magnitude of the bus voltage V BUS  that may cause the voltage at the inverting input of the comparator U 371  to exceed the voltage at the non-inverting input increases in proportional to the magnitude of the regulator feedback voltage V REG-FB , such that the overvoltage threshold that the load voltage V LOAD  exceeds to render the regulation FET Q 332  non-conductive remains approximately constant as the magnitude of the regulator feedback voltage V REG-FB  changes. The resistances of the resistors R 375 , R 374  may be greater than the resistances of the resistors R 372 , R 373  to avoid loading the regulator feedback voltage V REG-FB . 
       FIG.  4 A  is a graph that illustrates an example of the relationship between an on-time T ON  of a load current of an LED driver (e.g., the LED driver  100  of  FIG.  1   , the LED driver  200  of  FIG.  2   , and/or the like) and a target lighting intensity L TRGT  of an LED light source (e.g., the LED light source  102  and/or the like).  FIG.  4 B  is a graph that illustrates an example of the relationship between a frequency f LOAD  of a load current of an LED driver (e.g., the LED driver  100  of  FIG.  1   , the LED driver  200  of  FIG.  2   , and/or the like) and a target lighting intensity L TRGT  of an LED light source (e.g., the LED light source  102  and/or the like). 
       FIG.  5 A  is a graph that illustrates an example of the relationship between an on-time T ON  of a load current of an LED driver (e.g., the LED driver  100  of  FIG.  1   , the LED driver  200  of  FIG.  2   , and/or the like) and a target lighting intensity L TRGT  of an LED light source (e.g., the LED light source  102  and/or the like).  FIG.  5 B  is a graph that illustrates an example of the relationship between a frequency f LOAD  of a load current of an LED driver (e.g., the LED driver  100  of  FIG.  1   , the LED driver  200  of  FIG.  2   , and/or the like) and a target lighting intensity L TRGT  of an LED light source (e.g., the LED light source  102  and/or the like). One or more of the embodiments described with relation to  FIGS.  4 A,  4 B,  5 A , and/or  5 B may be performed by an LED driver (e.g., the LED driver  100  of  FIG.  1   , the LED driver  200  of  FIG.  2   , and/or the like) using a current control mode and/or a voltage control mode. 
     The control circuit  240  may be configured to control the intensity of the LED light source  102  by pulse width modulating the load current I LOAD  when the target intensity is above a predetermined threshold and control the intensity of the LED light source  102  by pulse frequency modulating the load current I LOAD  when the target intensity is below the predetermined threshold. The predetermined threshold may be, for example, a low-end intensity L LE  (e.g., 1%) as shown in  FIGS.  4 A- 4 B  and  FIGS.  5 A- 5 B . Pulse width modulating the load current I LOAD  may comprise maintaining a frequency f LOAD  of the load current I LOAD  constant and adjusting an on time T ON  of the load current I LOAD . Pulse frequency modulating the load current I LOAD  may comprise maintaining the on time T ON  of the load current I LOAD  constant and adjusting the frequency f LOAD  of the load current I LOAD . 
     When the LED driver is operating in the PWM dimming mode, the control circuit  240  may adjust the duty cycle DC ILOAD  of the pulse-width modulated load current I LOAD  to dim the LED light source  102  between the high-end intensity L HE  (e.g., approximately 100%) and the low-end intensity L LE  (e.g., approximately 1%) in response to the phase-control signal V PC . For example, the control circuit  240  may render the dimming FET Q 350  conductive for an on time T ON  and non-conductive for an off time T off  during a period (e.g., each period) T PWM  of the pulse-width modulated load current I LOAD . The control circuit  240  may hold a frequency f LOAD  of the pulse-width modulated load current I LOAD  constant at a normal PWM frequency f NORM  (e.g., approximately in the range of 500-550 Hz) and may adjust the length of the on time T ON  to dim the LED light source  102  between the high-end intensity L HE  and the low-end intensity L LE , for example, as shown in  FIGS.  4 A- 4 B  and  FIGS.  5 A- 5 B . For example, the length of the on time T ON  may be controlled between a maximum on time T MAX  (e.g., approximately 1.8 msec) corresponding to the high-end intensity L HE  (e.g., the duty cycle DC ILOAD  may equal approximately 100%) of the LED light source  102  and a minimum on time T MIN  (e.g., approximately 18 μsec) corresponding to the low-end intensity L LE  (e.g., the duty cycle DC ILOAD  may equal approximately 1%) of the LED light source  102 . 
     The LED driver may adjust (e.g., fade) the intensity of the LED light source  102  from the present intensity L PRES  to off (e.g., 0%) over a fade time period T FADE . When fading the intensity of the LED light source  102  to off, the control circuit  240  may adjust the intensity of the LED light source  102  below the low-end intensity L LE  (e.g., 1%), for example, to a minimum intensity L MIN , to an ultra-low minimum intensity L MIN-UL , and/or to off. Hardware limitations of the control circuit  240  (e.g., a minimum pulse width that may be generated by the control circuit) may prevent the length of the on time T ON  of the pulse-width modulated load current I LOAD  from being adjusted below the minimum on time T MIN , for example, when the frequency f LOAD  of the pulse-width modulated load current I LOAD  is at the normal PWM frequency f NOR . 
     The control circuit  240  may adjust the intensity of the LED light source  102  below the low-end intensity L LE  by pulse frequency modulating the load current I LOAD . For example, the control circuit  240  may adjust the intensity of the LED light source  102  below the low-end intensity L LE  to the minimum intensity L MIN  by the maintaining the length of the on time T ON  constant at the minimum on time T MIN  and decreasing the frequency f LOAD  of the pulse-width modulated load current I LOAD , for example, as shown in  FIGS.  4 A- 4 B and  5 A- 5 B . The control circuit  240  may decrease the intensity of the LED light source  102  from the low-end intensity L LE  to the minimum intensity L MIN  (e.g., approximately 0.1%) by decreasing the frequency f LOAD  from the normal PWM frequency f NORM  to a minimum PWM frequency f LORD  (e.g., approximately 120 Hz). As such, the control circuit  240  may adjust the intensity of the LED light source  102  by adjusting the length of the on time T ON  and maintaining the frequency f LOAD  when the target intensity L TRGT  is greater than the low-end intensity L LE , and by adjusting the frequency f LOAD  and maintaining the on time T ON  when the target intensity L TRGT  is less than the low-end intensity L LE . In one or more embodiments, the control circuit  240  may decrease the intensity of the LED light source  102  from the low-end intensity L LE  to off by decreasing the frequency f LORD , for example, from the normal PWM frequency f NORM  to the minimum PWM frequency f MIN . 
     The control circuit  240  may control the intensity of the LED light source  102  below the minimum intensity L MIN  to an ultra-low minimum intensity L MIN-UL , for example, as shown in  FIGS.  5 A- 5 B . The control circuit  240  may control the intensity of the LED light source  102  below the minimum intensity L MIN  to an ultra-low minimum intensity L MIN-UL  by pulse width modulating the load current. For example, the control circuit  240  may maintain the frequency f LOAD  of the load current I LOAD  constant at the minimum PWM frequency f MIN  (e.g., approximately 120 Hz) and decrease the on time T ON  below the minimum on time T MIN . For example, the control circuit  240  may decrease the on time T ON  from the minimum on time T MIN  to an ultra-low minimum on time T MIN-UL  while maintaining the frequency f LOAD  constant at the minimum PWM frequency f LORD  to control the intensity of the LED light source  102  below the minimum intensity L MIN  to an ultra-low minimum intensity L MIN-UL . For instance, the control circuit may dim the LED light source to off by pulse width modulating the load current (i.e., the ultra-low minimum intensity may be 0% intensity). In such examples, the minimum PWM frequency may be decreased below 120 Hz. The control circuit  240  may decrease the on time T ON  until the hardware limitations of the control circuit  240  prevent the on time T ON  from being decreased any further. 
     The control circuit  240  may be configured to dim the LED light source  102  to off. The control circuit  240  may be configured to control the intensity of the LED light source  102  from the predetermined threshold to off by pulse frequency modulating the load current I LOAD . The control circuit  240  may be configured to pulse width modulate the load current I LOAD  when the target intensity is below the minimum intensity L MIN . As such, the control circuit  240  may be configured to control the intensity of the LED light source  102  from the minimum intensity L MIN  to off by pulse width modulating the load current I LOAD . 
     In one or more embodiments, the control circuit  240  may control the intensity of the LED light source  102  by decreasing the magnitude of the DC bus voltage V BUS . For example, the control circuit  240  may be configured to control the intensity of the LED light source  102  below the minimum intensity level L MIN  by decreasing the magnitude of the DC bus voltage V BUS . The control circuit may be configured to maintain a frequency f LOAD  of the load current I LOAD  constant (e.g., at the minimum PWM frequency f MIN ), maintain an on time T ON  of the load current I LOAD  constant (e.g., at the minimum on time T MIN  or at the ultra-low minimum on time T MIN-UL ), and decrease a magnitude of the DC bus voltage V BUS  when the target intensity L TRGT  is below the minimum intensity L MIN . For example, control circuit may control the intensity of the LED light source  102  to off by decreasing the magnitude of the DC bus voltage V BUS . 
     The high-end intensity L HE  may be approximately 100%. The low-end intensity L LE  may be approximately 1%. The minimum intensity L MIN  may be approximately in the range of 0.1-1%. The ultra-low minimum intensity L MIN-UL  may be approximately in the range of 0-0.1%. For example, the ultra-low minimum intensity L MIN-UL  may be 0% (i.e., off). The maximum on time T MAX  may be approximately 1.8 msec. The minimum on time T MIN  may be approximately 18 μsec. The ultra-low minimum on time T MIN-U  may be approximately 1 μsec. The normal PWM frequency f NORM  may be approximately in the range of 500-550 Hz. The minimum PWM frequency f MIN  may be approximately in the range of 120-150 Hz. 
     Although illustrated in  FIGS.  4 A- 4 B  and  FIGS.  5 A- 5 B  as controlling the length of the on time T ON  of the load current I LOAD  between the high-end intensity L HE  and the minimum intensity L MIN  and controlling the frequency f LOAD  of the load current I LOAD  between the minimum intensity L MIN  and the ultra-low minimum intensity L MIN-UL , the control circuit  240  may be configured to control the intensity of the LED light source  102  by pulse width modulating the load current I LOAD  when the target intensity is within a first intensity range and control the intensity of the LED light source  102  by pulse frequency modulating the load current I LOAD  when the target intensity is within a second intensity range. The first intensity range may be greater than or less than the second intensity range. Further, the control circuit  240  may be configured to control the intensity of the LED light source  102  by pulse width modulating the load current I LOAD  when the target intensity is within a third intensity range. The third intensity range may be below a known operating range of the LED light source  102 . As such, the control circuit  240  may control the LED light source  102  by adjusting a first parameter (e.g., on time T ON  of the load current I LOAD ) to a control point that produces a known, reliable response of the LED light source  102  (e.g., the low-end intensity L LE ), adjusting a second parameter (e.g., the frequency f LOAD  of the load current I LOAD ) to a second control point that may or may not produce a known reliable response of the LED light source  102 , and adjusting the first parameter past the second control point, which may produce an unknown and potentially unreliable response of the LED light source  102 . However, this may be acceptable because the control circuit  240  may be fading the LED light source  102  to off. 
     The control circuit  240  may be configured to receive a command and control (e.g., dim) the intensity of the LED light source  102  below the first intensity range and below the second intensity range to off. For example, the load control circuit may be configured to control the intensity of the LED light source  102  below the second intensity range to off by pulse width modulating and/or pulse frequency modulating the load current I LOAD . The load control circuit may be configured to control the intensity of the LED light source  102  below the first intensity range and below the second intensity range to off by maintaining the frequency f LOAD  of the load current I LOAD  constant, maintaining the on time T ON  of the load current I LOAD  constant, and decreasing the magnitude of the DC bus voltage V BUS . 
     The control circuit  240  may control the length of the on time T ON  and/or the frequency f LOAD  of the load current I LOAD  to adjust the intensity of the LED light source  102  between the minimum intensity L MIN  (e.g., 0.1%) and the high-end intensity L HE  (e.g., 100%) during, for example, normal operation of the LED driver (i.e., not only when the LED driver is fading the intensity of the LED light source to off). 
     One or more of the embodiments described herein (e.g., as performed by a load control device) may be used to decrease the intensity of a lighting load and/or increase the intensity of the lighting load. For example, one or more embodiments described herein may be used to adjust the intensity of the lighting load from on to off, off to on, from a higher intensity to a lower intensity, and/or from a lower intensity to a higher intensity. For example, although described as adjusting the intensity of the LED light source  102  from the present intensity L PRES  to off (e.g., 0%), the LED driver may adjust (e.g., fade) the intensity of the LED light source  102  from off (e.g., 0%) to a target intensity L TRGT  (e.g., an intensity between an ultra-low minimum intensity L MIN-IL  and a high-end intensity L HE ) to over the fade time period T FADE  (e.g., in accordance with  FIGS.  4 A,  4 B,  5 A , and/or  5 B). 
     Although described with reference to an LED driver, one or more embodiments described herein may be used with other load control devices. For example, one or more of the embodiments described herein may be performed by a variety of load control devices that are configured to control of a variety of electrical load types, such as, for example, a LED driver for driving an LED light source (e.g., an LED light engine); a screw-in luminaire including a dimmer circuit and an incandescent or halogen lamp; a screw-in luminaire including a ballast and a compact fluorescent lamp; a screw-in luminaire including an LED driver and an LED light source; a dimming circuit for controlling the intensity of an incandescent lamp, a halogen lamp, an electronic low-voltage lighting load, a magnetic low-voltage lighting load, or another type of lighting load; an electronic switch, controllable circuit breaker, or other switching device for turning electrical loads or appliances on and off; a plug-in load control device, controllable electrical receptacle, or controllable power strip for controlling one or more plug-in electrical loads (e.g., coffee pots, space heaters, other home appliances, and the like); a motor control unit for controlling a motor load (e.g., a ceiling fan or an exhaust fan); a drive unit for controlling a motorized window treatment or a projection screen; motorized interior or exterior shutters; a thermostat for a heating and/or cooling system; a temperature control device for controlling a heating, ventilation, and air conditioning (HVAC) system; an air conditioner; a compressor; an electric baseboard heater controller; a controllable damper; a humidity control unit; a dehumidifier; a water heater; a pool pump; a refrigerator; a freezer; a television or computer monitor; a power supply; an audio system or amplifier; a generator; an electric charger, such as an electric vehicle charger; and an alternative energy controller (e.g., a solar, wind, or thermal energy controller). A single control circuit may be coupled to and/or adapted to control multiple types of electrical loads in a load control system.