LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE

Abstract

A load control device for controlling the amount of power delivered to an electrical load is able to operate in a normal mode and a burst mode. The load control device may comprise a control circuit that activates an inverter circuit during active state periods and deactivates the inverter circuit during inactive state periods. The control circuit may operate in the normal mode to regulate an average magnitude of a load current conducted through the electrical load to be above a minimum rated current. The control circuit may operate in the burst mode to adjust the average magnitude of the load current to be below the minimum rated current. The control circuit may adjust the average magnitude of the load current by adjusting the length of the inactive state periods while holding the length of the active state periods constant.

Claims

1. A load control device for controlling an amount of power delivered to a lighting load, the load control device comprising: a load regulation circuit configured to control a magnitude of a load current conducted through the electrical load, the load regulation circuit comprising: an inverter circuit; and a control circuit to: receive a target intensity; compare the received target intensity with a high-end intensity setpoint, a low-end intensity setpoint, and a defined threshold intensity between the low-end intensity setpoint and the high-end intensity setpoint; for target intensities between a defined threshold intensity and a high-end intensity setpoint:  cause the inverter to operate in a NORMAL mode in which the inverter circuit transitions between an active state and an inactive state over each of a first plurality of periods, each of the first plurality of periods having a first duration; and  adjust the active state current supplied by the inverter circuit to deliver a target current to cause the lighting load to produce the received target intensity; and for target intensities between the defined threshold intensity a low-end intensity setpoint;  cause the inverter to operate in a BURST mode in which the inverter circuit maintains a constant current; and  cause the inverter circuit to transition between the active state and the inactive state over a second plurality of periods to deliver the target current to cause the lighting load to produce the received target intensity, each of the second plurality periods having a respective duration less than or equal to the first duration.

2. The load control device of claim 1 wherein to cause the inverter circuit to transition between the active state and the inactive state over the second plurality of periods, the control circuit to further: cause the inverter circuit to transition between the active state and the inactive state over a second plurality of periods, each of the second plurality periods having a respective duration.

3. The load control device of claim 2 wherein to cause the inverter circuit to transition between the active state and the inactive state over the second plurality of periods, the control circuit to further: cause the inverter circuit to transition between the active state and the inactive state over a second plurality of periods, each of the second plurality periods having a respective duration that is less than the first duration.

4. The load control device of claim 3 wherein to cause the inverter circuit to transition between the active state and the inactive state over a second plurality of periods, each of the second plurality periods having a respective duration that is less than the first duration, the control circuit to further: cause the inverter circuit to transition between the active state and the inactive state over each of the second plurality of periods, wherein the duration of each period included in at least a portion of the second plurality of periods decreases over successive periods.

5. The load control device of claim 4, the control circuit to further maintain a constant inverter duty cycle over each of the second plurality of periods.

6. The load control device of claim 5 wherein the control circuit to further: determine whether a duration of the active portion of one of the second plurality of periods is less than a defined minimum inverter ON time; and cease the decrease of the duration of subsequent second periods responsive to the determination, by the control circuitry, that the duration of the active portion of one of the second plurality of periods is less than the defined minimum inverter ON time.

7. The load control device of claim 1, further comprising: current sense circuit to provide a signal indicative of a magnitude of the load current to the control circuit.

8. A method of controlling operation of a light-emitting diode (LED) lighting device, the method comprising: receiving a target intensity by a control circuit included in an LED controller; and comparing, by the control circuit, the received target intensity with a high-end intensity setpoint, a low-end intensity setpoint, and a defined threshold intensity between the low-end intensity setpoint and the high-end intensity setpoint: for target intensities between a defined threshold intensity and a high-end intensity setpoint: causing an inverter circuitry included in an operatively coupled load regulation circuit to operate in a NORMAL mode in which the inverter circuit transitions between an active state and an inactive state over each of a first plurality of periods, each of the first plurality of periods having a first duration; and causing the load regulation circuit to adjust the active state current supplied to an LED lighting load to cause the LED lighting load to produce the received target intensity; and for target intensities between the defined threshold intensity a low-end intensity setpoint; causing the inverter circuit to operate in a BURST mode in which the inverter circuit transitions between the active state and the inactive state over each of a second plurality of periods, each of the second plurality periods having a respective duration less than or equal to the first duration; and causing the load regulation circuit to maintain a constant active state current supplied to the LED lighting load sufficient to cause the LED lighting load to produce the received target intensity.

9. The method of claim 8 wherein causing the inverter circuit to operate in the BURST mode in which the inverter circuit transitions between the active state and the inactive state over each of the second plurality of periods further comprises: causing, by the control circuit, the inverter circuit to transition between the active state and the inactive state over the second plurality of periods, each of the second plurality periods having a respective duration.

10. The method of claim 9 wherein causing the inverter circuit to transition between the active state and the inactive state over a second plurality of periods, further comprises: causing, by the control circuit, the inverter circuit to transition between the active state and the inactive state over a second plurality of periods, wherein a respective duration of each of the second plurality periods is less than the first duration.

11. The method of claim 10 wherein causing the inverter circuit to transition between the active state and the inactive state over a second plurality of periods, further comprises: causing, by the control circuit, the inverter circuit to transition between the active state and the inactive state over each of the second plurality of periods, wherein the duration of each period included in at least a portion of the second plurality of periods decreases over successive periods.

12. The method of claim 11, further comprising: maintaining, by the control circuit, a constant inverter duty cycle over each of the second plurality of periods.

13. The method of claim 12, further comprising: determining, by the control circuit, whether a duration of the active portion of one of the second plurality of periods is less than a defined minimum inverter ON time; and cease decreasing the duration of subsequent second periods responsive to the determination, by the control circuitry, that the duration of the active portion of one of the second plurality of periods is less than the defined minimum inverter ON time.

14. A non-transitory, machine-readable, storage device that includes instructions that, when executed by control circuitry disposed in a light-emitting diode (LED) controller couplable to an LED lighting device, cause the control circuitry to: receive a target intensity; and compare the received target intensity with a high-end intensity setpoint, a low-end intensity setpoint, and a defined threshold intensity between the low-end intensity setpoint and the high-end intensity setpoint: for target intensities between a defined threshold intensity and a high-end intensity setpoint: cause inverter circuitry included in an operatively coupled load regulation circuit to operate in a NORMAL mode in which the inverter circuit transitions between an active state and an inactive state over each of a first plurality of periods, each of the first plurality of periods having a first duration; and cause the load regulation circuit to adjust the active state current supplied to an LED lighting load to cause the LED lighting load to produce the received target intensity; and for target intensities between the defined threshold intensity a low-end intensity setpoint; cause the inverter circuit to operate in a BURST mode in which the inverter circuit transitions between the active state and the inactive state over each of a second plurality of periods, each of the second plurality periods having a respective duration less than or equal to the first duration; and cause the load regulation circuit to maintain a constant active state current supplied to the LED lighting load sufficient to cause the LED lighting load to produce the received target intensity.

15. The non-transitory, machine-readable, storage device of claim 14 wherein the instructions that cause the control circuit to cause the inverter circuit to operate in the BURST mode in which the inverter circuit transitions between the active state and the inactive state over each of the second plurality of periods further cause the control circuit to: causing, by the control circuit, the inverter circuit to transition between the active state and the inactive state over the second plurality of periods, each of the second plurality periods having a respective duration.

16. The non-transitory, machine-readable, storage device of claim 15 wherein the instructions that cause the control circuit to cause the inverter circuit to transition between the active state and the inactive state over a second plurality of periods, further cause the control circuit to: cause the inverter circuit to transition between the active state and the inactive state over a second plurality of periods, wherein a respective duration of each of the second plurality periods is less than the first duration.

17. The non-transitory, machine-readable, storage device of claim 16 wherein the instructions that cause the control circuit to cause the inverter circuit to transition between the active state and the inactive state over a second plurality of periods, further cause the control circuit to: cause the inverter circuit to transition between the active state and the inactive state over each of the second plurality of periods, wherein the duration of each period included in at least a portion of the second plurality of periods decreases over successive periods.

18. The non-transitory, machine-readable, storage device of claim 17 wherein the instructions, when executed by the control circuit, further cause the control circuit to: maintain a constant inverter duty cycle over each of the second plurality of periods.

19. The non-transitory, machine-readable, storage device of claim 18 wherein the instructions, when executed by the control circuit, further cause the control circuit to: determine whether a duration of the active portion of one of the second plurality of periods is less than a defined minimum inverter ON time; and cease the decrease of the duration of subsequent second periods responsive to the determination that the duration of the active portion of one of the second plurality of periods is less than the defined minimum inverter ON time.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a simplified block diagram of a light-emitting diode (LED) driver for controlling the intensity of an LED light source.

[0010] FIG. 2 is an example plot of a target load current of the LED driver of FIG. 1 as a function of a target intensity.

[0011] FIG. 3 is an example plot of a burst duty cycle of the LED driver of FIG. 1 as a function of the target intensity.

[0012] FIG. 4 is an example state diagram illustrating the operation of a load regulation circuit of the LED driver of FIG. 1 when operating in a burst mode.

[0013] FIG. 5 is a simplified schematic diagram of an isolated forward converter and a current sense circuit of an LED driver.

[0014] FIG. 6 is an example diagram illustrating a magnetic core set of an energy-storage inductor of a forward converter.

[0015] FIG. 7 shows example waveforms illustrating the operation of a forward converter and a current sense circuit when the intensity of an LED light source is near a high-end intensity.

[0016] FIG. 8 shows example waveforms illustrating the operation of a forward converter and a current sense circuit when the intensity of an LED light source is near a low-end intensity.

[0017] FIG. 9 shows example waveforms illustrating the operation of a forward converter of an LED driver when operating in a burst mode.

[0018] FIG. 10 is a diagram of an example waveform illustrating a load current when a load regulation circuit is operating in a burst mode.

[0019] FIG. 11 is an example plot showing how a relative average light level may change as a function of the number of inverter cycles included in an active state period when a load regulation circuit is operating in a burst mode.

[0020] FIG. 12 shows example waveforms illustrating a load current when a load regulation circuit of an LED driver is operating in a burst mode.

[0021] FIG. 13 is an example of a plot relationship between a target load current and the lengths of an active state period and an inactive state period when a load regulation circuit of an LED driver is operating in a burst mode.

[0022] FIG. 14 is a simplified flowchart of an example procedure for operating a forward converter of an LED driver in a normal mode and a burst mode.

DETAILED DESCRIPTION

[0023] FIG. 1 is a simplified block diagram of a load control device, e.g., a light-emitting diode (LED) driver 100, for controlling the amount of power delivered to an electrical load, such as, an LED light source 102 (e.g., an LED light engine), and thus the intensity of the electrical load. 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 parallel or a suitable combination thereof, 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 comprise a hot terminal H and a neutral N. The terminals may be adapted to be coupled to an alternating-current (AC) power source (not shown).

[0024] The LED driver 100 may comprise a radio-frequency interference (RFI) filter circuit 110, a rectifier circuit 120, a boost converter 130, a load regulation circuit 140, a control circuit 150, a current sense circuit 160, a memory 170, a communication circuit 180, and/or a power supply 190. The RFI filter circuit 110 may minimize the noise provided on the AC mains. The rectifier circuit 120 may generate a rectified voltage V.sub.RECT.

[0025] The boost converter 130 may receive the rectified voltage V.sub.RECT and generate a boosted direct-current (DC) bus voltage V.sub.BUS across a bus capacitor C.sub.BUS The boost converter 130 may comprise any suitable power converter circuit for generating an appropriate bus voltage, such as, for example, a flyback converter, a single-ended primary-inductor converter (SEPIC), a Ćuk converter, or other suitable power converter circuit. The boost converter 130 may operate as a power factor correction (PFC) circuit to adjust the power factor of the LED driver 100 towards a power factor of one.

[0026] The load regulation circuit 140 may receive the bus voltage V.sub.BUS and control the amount of power delivered to the LED light source 102, for example, to control the intensity of the LED light source 102 between a high-end (e.g., maximum) intensity L.sub.HE (e.g., approximately 100%) and a low-end (e.g., minimum) intensity L.sub.LE (e.g., approximately 1-5% of the high-end intensity). An example of the load regulation circuit 140 may be an isolated, half-bridge forward converter. An example of the load control device (e.g., LED driver 100) comprising a forward converter is described in greater detail in commonly-assigned U.S. patent application Ser. No. 13/935,799, filed Jul. 5, 2013, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosure of which is hereby incorporated by reference. The load regulation circuit 140 may comprise, for example, a buck converter, a linear regulator, or any suitable LED drive circuit for adjusting the intensity of the LED light source 102.

[0027] The control circuit 150 may be configured to control the operation of the boost converter 130 and/or the load regulation circuit 140. An example of the control circuit 150 may be a controller. The control circuit 150 may comprise, for example, a digital 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 control circuit 150 may generate a bus voltage control signal V.sub.BUS-CNTL, which may be provided to the boost converter 130 for adjusting the magnitude of the bus voltage V.sub.BUS. The control circuit 150 may receive a bus voltage feedback control signal V.sub.BUS-FB from the boost converter 130, which may indicate the magnitude of the bus voltage V.sub.BUS.

[0028] The control circuit 150 may generate drive control signals V.sub.DRIVE1, V.sub.DRIVE2 The drive control signals V.sub.DRIVE1, V.sub.DRIVE2 may be provided to the load regulation circuit 140 for adjusting the magnitude of a load voltage V.sub.LOAD generated across the LED light source 102 and/or the magnitude of a load current I.sub.LOAD conducted through the LED light source 120. By controlling the load voltage V.sub.LOAD and/or the load current I.sub.LOAD, the control circuit may control the intensity of the LED light source 120 to a target intensity L.sub.TRGT. The control circuit 150 may adjust an operating frequency f.sub.OP and/or a duty cycle DC.sub.INV (e.g., an on time T.sub.ON) of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 in order to adjust the magnitude of the load voltage V.sub.LOAD and/or the load current I.sub.LOAD.

[0029] The current sense circuit 160 may receive a sense voltage V.sub.SENSE. The sense voltage V.sub.SENSE may be generated by the load regulation circuit 140. The sense voltage V.sub.SENSE may indicate the magnitude of the load current I.sub.LOAD. The current sense circuit 160 may receive a signal-chopper control signal V.sub.CHOP from the control circuit 150. The current sense circuit 160 may generate a load current feedback signal V.sub.I-LOAD, which may be a DC voltage indicating the average magnitude I.sub.AVE of the load current I.sub.LOAD. The control circuit 150 may receive the load current feedback signal V.sub.I-LOAD from the current sense circuit 160. The control circuit 150 may adjust the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 based on the load current feedback signal V.sub.I-LOAD so that the magnitude of the load current I.sub.LOAD may be adjusted towards a target load current I.sub.TRGT. For example, the control circuit 150 may set initial operating parameters for the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 (e.g., the operating frequency f.sub.OP and/or the duty cycle DC.sub.INV). The control circuit 150 may receive the load current feedback signal V.sub.I-LOAD indicating the effect of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2. Based on the indication, the control circuit 150 may adjust the operating parameters of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 to thus adjust the magnitude of the load current I.sub.LOAD towards a target load current I.sub.TRGT (e.g., using a control loop).

[0030] The load current I.sub.LOAD may be the current that is conducted through the LED light source 120. The target load current I.sub.TRGT may be the current that the control circuit 150 aims to conduct through the LED light source 120 (e.g., based at least on the load current feedback signal V.sub.I-LOAD). The load current I.sub.LOAD may be approximately equal to the target load current I.sub.TRGT but may not always match the target load current I.sub.TRGT. This may be because, for example, the control circuit 150 may have specific levels of granularity in which it can control the current conducted through the LED light source 120 (e.g., due to inverter cycle lengths, etc.). A person skilled in the art will appreciate that the figures shown herein (e.g., FIG. 2) that illustrate the current conducted through an LED light source as a linear graph (at least in parts) may represent the target load current I.sub.TRGT, since the load current I.sub.LOAD itself may not be exactly equal to the target load current I.sub.TRGT and may not actually follow a true linear path.

[0031] The control circuit 150 may be coupled to the memory 170. The memory 170 may store operational characteristics of the LED driver 100 (e.g., the target intensity L.sub.TRGT, the low-end intensity L.sub.LE, the high-end intensity L.sub.HE, etc.). The communication circuit 180 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 150 may be configured to update the target intensity L.sub.TRGT of the LED light source 102 and/or the operational characteristics stored in the memory 170 in response to digital messages received via the communication circuit 180. The LED driver 100 may be operable to receive a phase-control signal from a dimmer switch for determining the target intensity L.sub.TRGT for the LED light source 102. The power supply 190 may receive the rectified voltage V.sub.RECT and generate a direct-current (DC) supply voltage V.sub.CC for powering the circuitry of the LED driver 100.

[0032] FIG. 2 is an example plot of the target load current I.sub.TRGT as a function of the target intensity L.sub.TRGT. As shown, a linear relationship may exist between the target intensity L.sub.TRGT and the target load current I.sub.TRGT. That is, to achieve a higher target intensity, the control circuit 150 may increase the target load current I.sub.TRGT in proportion to the increase in the target intensity; to achieve a lower target intensity, the control circuit 150 may decrease the target load current I.sub.TRGT in proportion to the decrease in the target intensity. As the target load current I.sub.TRGT is being adjusted, the magnitude of the load current I.sub.LOAD may change accordingly. There may be limits, however, to how much the load current I.sub.LOAD may be adjusted. For example, the load current I.sub.LOAD may not be adjusted above a maximum rated current I.sub.MAX or below a minimum rated current I.sub.MIN (e.g., due to hardware limitations of the load regulation circuit 140 and/or the control circuit 150). Thus, the control circuit may be configured to adjust the target load current I.sub.TRGT between the maximum rated current I.sub.MAX and the minimum rated current I.sub.MIN so that the magnitude of the load current I.sub.LOAD may fall into in the same range. The maximum rated current I.sub.MAX may correspond to the high-end intensity L.sub.HE (e.g., approximately 100%). The minimum rated current I.sub.MIN may correspond to a transition intensity L.sub.TRAN (e.g., approximately 5% of the maximum intensity). Between the high-end intensity L.sub.HE and the transition intensity L.sub.TRAN, the control circuit 150 may operate the load regulation circuit 140 in a normal mode in which an average magnitude I.sub.AVE of the load current I.sub.LOAD may be controlled to be equal to (e.g., approximately equal to) the target load current I.sub.TRGT. During the normal mode, the control circuit 150 may adjust the average magnitude I.sub.AVE of the load current I.sub.LOAD to the target load current I.sub.TRGT in response to the load current feedback signal V.sub.I-LOAD (e.g., using closed loop control), for example. The control circuit 150 may apply various control techniques during the normal mode including, for example, a pulse-width modulation technique or a constant current reduction technique.

[0033] To adjust the average magnitude I.sub.AVE of the load current I.sub.LOAD to below the minimum rated current I.sub.MIN (and to thus adjust the target intensity L.sub.TRGT below the transition intensity L.sub.TRAN), the control circuit 150 may be configured to operate the load regulation circuit 140 in a burst mode. The burst mode may be characterized by a burst operating period that includes an active state period and an inactive state period. During the active state period, the control circuit 150 may be configured to regulate the load current I.sub.LOAD in ways similar to those in the normal mode. During the inactive state period, the control circuit 150 may be configured to stop regulating the load current I.sub.LOAD (e.g., to allow the load current I.sub.LOAD to drop to approximately zero). Although the active state and inactive state periods are described herein in association with the burst mode, a person skilled in the art will understand that the normal mode may also be characterized by an operating period that includes the active state period and the inactive state period, e.g., with both periods held constant and the inactive state period held at approximately zero. Examples of a load control device capable of operating in a burst mode and a normal mode are described in greater detail in commonly-assigned U.S. Pat. No. 9,247,608, issued Jan. 26, 2016, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosure of which is hereby incorporated by reference.

[0034] The ratio of the active state period to the burst operating period, e.g., T.sub.ACTIVE/T.sub.BURST, may represent a burst duty cycle DC.sub.BURST. The burst duty cycle DC.sub.BURST may be controlled, for example, between a maximum duty cycle DC.sub.MAX (e.g., approximately 100%) and a minimum duty cycle DC.sub.MIN (e.g., approximately 20%). The load current I.sub.LOAD may be adjusted towards the target current I.sub.TRGT (e.g., the minimum rated current I.sub.MIN) during the active state period of the burst mode. Setting the burst duty cycle DC.sub.BURST to a value less than the maximum duty cycle DC.sub.MAX may reduce the average magnitude I.sub.AVE of the load current I.sub.LOAD to below the minimum rated current I.sub.MIN.

[0035] FIG. 3 is an example plot of a burst duty cycle DC.sub.BURST (e.g., an ideal burst duty cycle DC.sub.BURST-IDEAL) as a function of the target intensity L.sub.TRGT. As described herein, when the target intensity L.sub.TRGT is between the high-end intensity L.sub.HE (e.g., approximately 100%) and a transition intensity L.sub.TRAN (e.g., approximately 5% of the maximum intensity), the control circuit 150 may be configured to operate the load regulation circuit 140 in the normal mode, e.g., by setting the burst duty cycle DC.sub.BURST to a maximum duty cycle DC.sub.MAX or approximately 100%. To adjust the target intensity L.sub.TRGT below the transition intensity L.sub.TRAN, the control circuit 150 may be configured to operate the load regulation circuit 140 in the burst mode, e.g., by adjusting the burst duty cycle DC.sub.BURST between the maximum duty cycle DC.sub.MAX (e.g., approximately 100%) and a minimum duty cycle DC.sub.MIN (e.g., approximately 20%). In the burst mode, a peak magnitude I.sub.PK of the load current I.sub.LOAD may be equal to the target current I.sub.TRGT (e.g., the minimum rated current I.sub.MIN) during an active state period of the burst mode.

[0036] With reference to FIG. 3, the burst duty cycle DC.sub.BURST may refer to an ideal burst duty cycle DC.sub.BURST-IDEAL, which may include an integer portion DC.sub.BURST-INTEGER and/or a fractional portion DC.sub.BURST-FRACTIONAL. The integer portion DC.sub.BURST-INTEGER may be characterized by the percentage of the ideal burst duty cycle DC.sub.BURST-IDEAL that includes complete inverter cycles (i.e., an integer value of inverter cycles). The fractional portion DC.sub.BURST-FRACTIONAL may be characterized by the percentage of the ideal burst duty cycle DC.sub.BURST-IDEAL that includes a fraction of an inverter cycle. In at least some cases, the control circuit 150 (e.g., via the load regulation circuit 140) may be configured to adjust the number of inverter cycles by an integer number (e.g., by DC.sub.BURST-INTEGER) and not a fractional amount (e.g., DC.sub.BURST-FRACTIONAL). Therefore, although the example plot of FIG. 3 illustrates an ideal curve showing continuous adjustment of the ideal burst duty cycle DC.sub.BURST-IDEAL from a maximum duty cycle DC.sub.MAX to a minimum duty cycle DC.sub.MIN, unless defined differently, burst duty cycle DC.sub.BURST may refer to the integer portion DC.sub.BURST-INTEGER of the ideal burst duty cycle DC.sub.BURST-IDEAL (e.g., if the control circuit 150 is not be configured to operate the burst duty cycle DC.sub.BURST at fractional amounts).

[0037] FIG. 4 is an example state diagram illustrating the operation of the load regulation circuit 140 in the burst mode. During the burst mode, the control circuit 150 may periodically control the load regulation circuit 140 into an active state and an inactive state, e.g., in dependence upon a burst duty cycle DC.sub.BURST and a burst mode period T.sub.BURST (e.g., approximately 4.4 milliseconds). For example, the active state period (T.sub.ACTIVE) may be equal to the burst duty cycle (DC.sub.BURST) times the burst mode period (T.sub.BURST) and the inactive state period (T.sub.INACTIVE) may be equal to one minus the burst duty cycle (DC.sub.BURST) times the burst mode period (T.sub.BURST). That is, T.sub.ACTIVE=DC.sub.BURST.Math.T.sub.BURST and T.sub.INACTIVE=(1−DC.sub.BURST).Math.T.sub.BURST.

[0038] In the active state of the burst mode, the control circuit 150 may be configured to generate the drive control signals V.sub.DRIVE1, V.sub.DRIVE2. The control circuit 150 may be further configured to adjust the operating frequency f.sub.OP and/or the duty cycle DC.sub.INV (e.g., an on time T.sub.ON) of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 in order to adjust the magnitude of the load current I.sub.LOAD. The control circuit 150 may be configured to make the adjustments using closed loop control. For example, in the active state of the burst mode, the control circuit 150 may generate the drive signals V.sub.DRIVE1, V.sub.DRIVE2 to adjust the magnitude of the load current I.sub.LOAD to be equal to a target load current I.sub.TRGT (e.g., the minimum rated current I.sub.MIN) in response to the load current feedback signal V.sub.I-LOAD.

[0039] In the inactive state of the burst mode, the control circuit 150 may let the magnitude of the load current I.sub.LOAD drop to approximately zero amps, e.g., by freezing the control loop and/or not generating the drive control signals V.sub.DRIVE1, V.sub.DRIVE2. While the control loop is frozen (e.g., in the inactive state), the control circuit 150 may stop responding to the load current feedback signal V.sub.I-LOAD (e.g., the control circuit 150 may not adjust the values of the operating frequency f.sub.OP and/or the duty cycle DC.sub.INV in response to the feedback signal). The control circuit 150 may store the present duty cycle DC.sub.INV (e.g., the present on time T.sub.ON) of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 in the memory 170 prior to (e.g., immediately prior to) freezing the control loop. When the control loop is unfrozen (e.g., when the control circuit 150 enters the active state), the control circuit 150 may resuming generating the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 using the operating frequency f.sub.OP and/or the duty cycle DC.sub.INV from the previous active state.

[0040] The control circuit 150 may be configured to adjust the burst duty cycle DC.sub.BURST using an open loop control. For example, the control circuit 150 may be configured to adjust the burst duty cycle DC.sub.BURST as a function of the target intensity L.sub.TRGT when the target intensity L.sub.TRGT is below the transition intensity L.sub.TRAN For example, the control circuit 150 may be configured to linearly decrease the burst duty cycle DC.sub.BURST as the target intensity L.sub.TRGT is decreased below the transition intensity L.sub.TRAN (e.g., as shown in FIG. 3), while the target load current I.sub.TRGT is held constant at the minimum rated current I.sub.MIN (e.g., as shown in FIG. 2). Since the control circuit 150 changes between the active state and the inactive state in dependence upon the burst duty cycle DC.sub.BURST and the burst mode period T.sub.BURST (e.g., as shown in the state diagram of FIG. 4), the average magnitude I.sub.AVE of the load current I.sub.LOAD may be a function of the burst duty cycle DC.sub.BURST (e.g., I.sub.AVE=DC.sub.BURST.Math.I.sub.MIN). During the burst mode, the peak magnitude I.sub.PK of the load current I.sub.LOAD may be equal to the minimum rated current I.sub.MIN, but the average magnitude I.sub.AVE of the load current I.sub.LOAD may be less than the minimum rated current I.sub.MIN.

[0041] FIG. 5 is a simplified schematic diagram of a forward converter 240 and a current sense circuit 260 of an LED driver (e.g., the LED driver 100 shown in FIG. 1). The forward converter 240 may be an example of the load regulation circuit 140 of the LED driver 100 shown in FIG. 1. The current sense circuit 260 may be an example of the current sense circuit 160 of the LED driver 100 shown in FIG. 1.

[0042] The forward converter 240 may comprise a half-bridge inverter circuit having two field effect transistors (FETs) Q210, Q212 for generating a high-frequency inverter voltage V.sub.INV from the bus voltage V.sub.BUS. The FETs Q210, Q212 may be rendered conductive and non-conductive in response to the drive control signals V.sub.DRIVE1, V.sub.DRIVE2. The drive control signals V.sub.DRIVE1, V.sub.DRIVE2 may be received from the control circuit 150. The drive control signals V.sub.DRIVE1, V.sub.DRIVE2 may be coupled to the gates of the respective FETs Q210, Q212 via a gate drive circuit 214 (e.g., which may comprise part number L6382DTR, manufactured by ST Microelectronics). The control circuit 150 may be configured to generate the inverter voltage V.sub.INV at an operating frequency f.sub.OP (e.g., approximately 60-65 kHz) and thus an operating period T.sub.OP. The control circuit 150 may be configured to adjust the operating frequency f.sub.OP under certain operating conditions. The control circuit 150 may be configured to adjust a duty cycle DC.sub.INV of the inverter voltage V.sub.INV to control the intensity of an LED light source 202 towards the target intensity L.sub.TRGT.

[0043] In a normal mode of operation, when the target intensity L.sub.TRGT of the LED light source 202 is between the high-end intensity L.sub.HE and the transition intensity L.sub.TRAN, the control circuit 150 may adjust the duty cycle DC.sub.INV of the inverter voltage V.sub.INV to adjust the magnitude (e.g., the average magnitude I.sub.AVE) of the load current I.sub.LOAD towards the target load current I.sub.TRGT. As described herein, the magnitude of the load current I.sub.LOAD may vary between the maximum rated current I.sub.MAX and the minimum rated current I.sub.MIN (e.g., as shown in FIG. 2). At the minimum rated current I.sub.MIN and/or the transition intensity L.sub.TRAN, the inverter voltage V.sub.INV may be characterized by a transition (e.g., from a normal mode to a burst mode) operating frequency f.sub.OP-T, a transition operating period T.sub.OP-T, and a transition duty cycle DC.sub.INV-T.

[0044] When the target intensity L.sub.TRGT of the LED light source 202 is below the transition intensity L.sub.TRAN, the control circuit 150 may be configured to operate the forward converter 240 in a burst mode of operation. In addition to or in lieu of using target intensity as a threshold for determining when to operate in the burst mode, the control circuit 150 may use power (e.g., a transition power) and/or current (e.g., a transition current) as the threshold. In the burst mode of operation, the control circuit 150 may be configured to switch the forward converter 240 between an active state (e.g., in which the control circuit 150 may actively generate the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 to regulate the peak magnitude I.sub.PK of the load current I.sub.LOAD to be equal to the minimum rated current I.sub.MIN) and an inactive state (e.g., in which the control circuit 150 may freeze the control loop and does not generate the drive control signals V.sub.DRIVE1, V.sub.DRIVE2) FIG. 4 shows a state diagram illustrating the transmission between the two states. The control circuit 150 may change the forward converter 240 between the active state and the inactive state in dependence upon a burst duty cycle DC.sub.BURST and a burst mode period T.sub.BURST (e.g., as shown in FIG. 4). The control circuit 150 may adjust the burst duty cycle DC.sub.BURST as a function of the target intensity L.sub.TRGT, which is below the transition intensity L.sub.TRAN (e.g., as shown in FIG. 3). In the active state of the burst mode (as well as in the normal mode), the forward converter 240 may be characterized by a turn-on time T.sub.TURN-ON and a turn-off time T.sub.TURN-OFF. The turn-on time T.sub.TURN-ON may be a time period from when the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 are driven until the respective FET Q210, Q212 is rendered conductive. The turn-off time T.sub.TURN-OFF may be a time period from when the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 are driven until the respective FET Q210, Q212 is rendered non-conductive.

[0045] The inverter voltage V.sub.INV may be coupled to the primary winding of a transformer 220 through a DC-blocking capacitor C216 (e.g., which may have a capacitance of approximately 0.047 μf). A primary voltage V.sub.PRI may be generated across the primary winding. The transformer 220 may be characterized by a turns ratio n.sub.TURNS (e.g., N.sub.1/N.sub.2), which may be approximately 115:29. A sense voltage V.sub.SENSE may be generated across a sense resistor R222, which may be coupled in series with the primary winding of the transformer 220. The FETs Q210, Q212 and the primary winding of the transformer 220 may be characterized by parasitic capacitances CP.sub.1, CP.sub.2, CP.sub.3, respectively. The secondary winding of the transformer 220 may generate a secondary voltage. The secondary voltage may be coupled to the AC terminals of a full-wave diode rectifier bridge 224 for rectifying the secondary voltage generated across the secondary winding. The positive DC terminal of the rectifier bridge 224 may be coupled to the LED light source 202 through an output energy-storage inductor L226 (e.g., which may have an inductance of approximately 10 mH). The load voltage V.sub.LOAD may be generated across an output capacitor C228 (e.g., which may have a capacitance of approximately 3 μF).

[0046] The current sense circuit 260 may comprise an averaging circuit for producing the load current feedback signal V.sub.I-LOAD. The averaging circuit may comprise a low-pass filter comprising a capacitor C230 (e.g., which may have a capacitance of approximately 0.066 uF) and a resistor R232 (e.g., which may have a resistance of approximately 3.32 kΩ). The low-pass filter may receive the sense voltage V.sub.SENSE via a resistor R234 (e.g., which may have a resistance of approximately 1 kΩ). The current sense circuit 160 may comprise a transistor Q236 (e.g., a FET as shown in FIG. 5) coupled between the junction of the resistors R232, R234 and circuit common. The gate of the transistor Q236 may be coupled to circuit common through a resistor R238 (e.g., which may have a resistance of approximately 22 KΩ). The gate of the transistor Q236 may receive the signal-chopper control signal V.sub.CHOP from the control circuit 150. An example of the current sense circuit 260 is described in greater detail in commonly-assigned U.S. patent application Ser. No. 13/834,153, filed Mar. 15, 2013, entitled FORWARD CONVERTER HAVING A PRIMARY-SIDE CURRENT SENSE CIRCUIT, the entire disclosure of which is hereby incorporated by reference.

[0047] FIG. 6 is an example diagram illustrating a magnetic core set 290 of an energy-storage inductor (e.g., the output energy-storage inductor L226 of the forward converter 240 shown in FIG. 5). The magnetic core set 290 may comprise two E-cores 292A, 292B, and may comprise part number PC40EE16-Z, manufactured by TDK Corporation. The E-cores 292A, 292B may comprise respective outer legs 294A, 294B and inner legs 296A, 296B. The inner legs 296A, 296B may be characterized by a width w.sub.LEG (e.g., approximately 4 mm). The inner leg 296A of the first E-core 292A may comprise a partial gap 298A (e.g., the magnetic core set 290 may be partially-gapped), such that the inner legs 296A, 296B may be spaced apart by a gap distance d.sub.GAP (e.g., approximately 0.5 mm). The partial gap 298A may extend for a gap width w.sub.GAP (e.g., approximately 2.8 mm) such that the partial gap 298A may extend for approximately 70% of the leg width w.sub.LEG of the inner leg 296A. Either or both of the inner legs 296A, 296B may comprise partial gaps. The partially-gapped magnetic core set 290 (e.g., as shown in FIG. 6) may allow the output energy-storage inductor L226 of the forward converter 240 (e.g., shown in FIG. 5) to maintain continuous current at low load conditions (e.g., near the low-end intensity L.sub.LE).

[0048] FIG. 7 shows example waveforms illustrating the operation of a forward converter (e.g., the forward converter 240) and a current sense circuit (e.g., the current sense circuit 260). The forward converter 240 may generate the waveforms shown in FIG. 7, for example, when operating in the normal mode and in the active state of the burst mode as described herein. As shown in FIG. 7, a control circuit (e.g., the control circuit 150) may drive the respective drive control signals V.sub.DRIVE1, V.sub.DRIVE2 high to approximately the supply voltage V.sub.CC to render the respective FETs Q210, Q212 conductive for an on time T.sub.ON. The FETs Q210, Q212 may be rendered conductive at different times. When the high-side FET Q210 is conductive, the primary winding of the transformer 220 may conduct a primary current I.sub.PRI to circuit common through the capacitor C216 and sense resistor R222. After (e.g., immediately after) the high-side FET Q210 is rendered conductive (at time t.sub.1 in FIG. 7), the primary current I.sub.PRI may conduct a short high-magnitude pulse of current due to the parasitic capacitance CP.sub.3 of the transformer 220 as shown in FIG. 7. While the high-side FET Q210 is conductive, the capacitor C216 may charge, such that a voltage having a magnitude of approximately half of the magnitude of the bus voltage V.sub.BUS may be developed across the capacitor. The magnitude of the primary voltage V.sub.PRI across the primary winding of the transformer 220 may be equal to approximately half of the magnitude of the bus voltage V.sub.BUS (e.g., V.sub.BUS/2). When the low-side FET Q212 is conductive, the primary winding of the transformer 220 may conduct the primary current I.sub.PRI in an opposite direction and the capacitor C216 may be coupled across the primary winding, such that the primary voltage V.sub.PRI may have a negative polarity with a magnitude equal to approximately half of the magnitude of the bus voltage V.sub.BUS.

[0049] When either of the high-side and low-side FETs Q210, Q212 are conductive, the magnitude of an output inductor current I.sub.L conducted by the output inductor L226 and/or the magnitude of the load voltage V.sub.LOAD across the LED light source 202 may increase with respect to time. The magnitude of the primary current I.sub.PRI may increase with respect to time while the FETs Q210, Q212 are conductive (e.g., after an initial current spike). When the FETs Q210, Q212 are non-conductive, the output inductor current I.sub.L and the load voltage V.sub.LOAD may decrease in magnitude with respective to time. The output inductor current I.sub.L may be characterized by a peak magnitude I.sub.L-PK (and an average magnitude I.sub.L-AVG, for example, as shown in FIG. 7. The control circuit 150 may increase and/or decrease the on times T.sub.ON of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 (e.g., and the duty cycle DC.sub.INV of the inverter voltage V.sub.INV) to respectively increase and decrease the average magnitude I.sub.L-AVG of the output inductor current I.sub.L, and thus respectively increase and decrease the intensity of the LED light source 202.

[0050] When the FETs Q210, Q212 are rendered non-conductive, the magnitude of the primary current I.sub.PRI may drop toward zero amps (e.g., as shown at time t.sub.2 in FIG. 7 when the high-side FET Q210 is rendered non-conductive). A magnetizing current I.sub.MAG may continue to flow through the primary winding of the transformer 220, for example, due to the magnetizing inductance L.sub.MAG of the transformer. When the target intensity L.sub.TRGT of the LED light source 102 is near the low-end intensity L.sub.LE, the magnitude of the primary current I.sub.PRI may oscillate after either of the FETs Q210, Q212 is rendered non-conductive. The oscillation may be caused by the parasitic capacitances CP.sub.1, CP.sub.2 of the FETs, the parasitic capacitance CP.sub.3 of the primary winding of the transformer 220, and/or any other parasitic capacitances of the circuit (e.g., such as the parasitic capacitances of the printed circuit board on which the forward converter 240 is mounted).

[0051] The real component of the primary current I.sub.PRI may indicate the magnitude of the secondary current I.sub.SEC and thus the intensity of the LED light source 202. The magnetizing current I.sub.MAG (e.g., the reactive component of the primary current I.sub.PRI) may flow through the sense resistor R222. When the high-side FET Q210 is conductive, the magnetizing current I.sub.MAG may change from a negative polarity to a positive polarity. When the low-side FET Q210 is conductive, the magnetizing current I.sub.MAG may change from a positive polarity to a negative polarity. When the magnitude of the primary voltage V.sub.PRI is zero volts, the magnetizing current I.sub.MAG may remain constant, for example, as shown in FIG. 7. The magnetizing current I.sub.MAG may have a maximum magnitude defined by the following equation:

[00001]$I_{\mathrm{MAG}-\mathrm{MAX}}=\frac{V_{\mathrm{BUS}}.Math.T_{\mathrm{HC}}}{4.Math.L_{\mathrm{MAG}}}$

[0052] where T.sub.HC may be the half-cycle period of the inverter voltage V.sub.INV, e.g., T.sub.HC=T.sub.OP/2. As shown in FIG. 7, the areas 250, 252 may be approximately equal, such that the average value of the magnitude of the magnetizing current I.sub.MAG may be zero during the period of time when the magnitude of the primary voltage V.sub.PRI is greater than approximately zero volts (e.g., during the on time T.sub.ON as shown in FIG. 7).

[0053] The current sense circuit 260 may determine an average of the primary current I.sub.PRI during the positive cycles of the inverter voltage V.sub.INV, e.g., when the high-side FET Q210 is conductive. As described herein, the high-side FET Q210 may be conductive during the on time T.sub.ON. The load current feedback signal V.sub.I-LOAD, which may be generated by the current sense circuit 260, may have a DC magnitude that is the average value of the primary current I.sub.PRI (e.g., when the high-side FET Q210 is conductive). Because the average value of the magnitude of the magnetizing current I.sub.MAG may be approximately zero during the period of time that the high-side FET Q210 is conductive (e.g., during the on time T.sub.ON), the load current feedback signal V.sub.I-LOAD generated by the current sense circuit may indicate the real component (e.g., only the real component) of the primary current I.sub.PRI (e.g., during the on time T.sub.ON).

[0054] When the high-side FET Q210 is rendered conductive, the control circuit 150 may drive the signal-chopper control signal V.sub.CHOP low towards circuit common to render the transistor Q236 of the current sense circuit 260 non-conductive for a signal-chopper time T.sub.CHOP The signal-chopper time T.sub.CHOP may be approximately equal to the on time T.sub.ON of the high-side FET Q210, for example, as shown in FIG. 7. The capacitor C230 may charge from the sense voltage V.sub.SENSE through the resistors R232, R234 while the signal-chopper control signal V.sub.CHOP is low. The magnitude of the load current feedback signal V.sub.I-LOAD may be the average value of the primary current I.sub.PRI and may indicate the real component of the primary current during the time when the high-side FET Q210 is conductive. When the high-side FET Q210 is not conductive, the control circuit 150 may drive the signal-chopper control signal V.sub.CHOP high to render the transistor Q236 conductive. Accordingly, the control circuit 150 may be able to determine the average magnitude of the load current I.sub.LOAD from the magnitude of the load current feedback signal V.sub.I-LOAD, at least partially because the effects of the magnetizing current I.sub.MAG and the oscillations of the primary current I.sub.PRI on the magnitude of the load current feedback signal V.sub.I-LOAD may be reduced or eliminated.

[0055] As the target intensity L.sub.TRGT of the LED light source 202 is decreased towards the low-end intensity L.sub.LE and/or the on times T.sub.ON of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 get smaller, the parasitic of the load regulation circuit 140 (e.g., the parasitic capacitances CP.sub.1, C.sub.P2 of the FETs Q210, Q212, the parasitic capacitance CP.sub.3 of the primary winding of the transformer 220, and/or other parasitic capacitances of the circuit) may cause the magnitude of the primary voltage V.sub.PRI to slowly decrease towards zero volts after the FETs Q210, Q212 are rendered non-conductive.

[0056] FIG. 8 shows example waveforms illustrating the operation of a forward converter and a current sense circuit (e.g., the forward converter 240 and the current sense circuit 260) when the target intensity L.sub.TRGT is near the low-end intensity L.sub.LE, and when the forward converter 240 is operating in the normal mode and the active state of the burst mode. The gradual drop off in the magnitude of the primary voltage V.sub.PRI may allow the primary winding of the transformer 220 to continue to conduct the primary current I.sub.PRI, such that the transformer 220 may continue to deliver power to the secondary winding after the FETs Q210, Q212 are rendered non-conductive, for example, as shown in FIG. 8. The magnetizing current I.sub.MAG may continue to increase in magnitude after the on time T.sub.ON of the drive control signal V.sub.DRIVE1 (e.g., and/or the drive control signal V.sub.DRIVE2). The control circuit 150 may increase the signal-chopper time T.sub.CHOP to be greater than the on time T.sub.ON. For example, the control circuit 150 may increase the signal-chopper time T.sub.CHOP (e.g., during which the signal-chopper control signal V.sub.CHOP is low) by an offset time T.sub.OS when the target intensity L.sub.TRGT of the LED light source 202 is near the low-end intensity L.sub.LE.

[0057] FIG. 9 shows example waveforms illustrating the operation of a forward converter (e.g., the forward converter 240 shown in FIG. 5) when operating in a burst mode. The inverter circuit of the forward converter 240 may generate the inverter voltage V.sub.INV during an active state (e.g., for the duration of an active state period T.sub.ACTIVE) A purpose of the inverter voltage V.sub.INV may be to regulate the magnitude of the load current I.sub.LOAD to the minimum rated current I.sub.MIN during the active state period. During an inactive state period, the inverter voltage V.sub.INV may be reduced to zero (e.g., not generated). The forward converter may enter the active state on a periodic basis with an interval approximately equal to a burst mode period T.sub.BURST (e.g., approximately 4.4 milliseconds). The active state period T.sub.ACTIVE and inactive state period T.sub.INACTIVE may be characterized by durations that are dependent upon a burst duty cycle DC.sub.BURST, e.g., T.sub.ACTIVE=DC.sub.BURST.Math.T.sub.BURST and T.sub.INACTIVE=(1−DC.sub.BURST).Math.T.sub.BURST. The average magnitude I.sub.AVE of the load current I.sub.LOAD may be dependent on the burst duty cycle DC.sub.BURST. For example, the average magnitude I.sub.AVE of the load current I.sub.LOAD may be equal to the burst duty cycle DC.sub.BURST times the load current I.sub.LOAD (e.g., I.sub.AVE=DC.sub.BURST.Math.I.sub.LOAD). When the load current I.sub.LOAD is equal to the minimum load current I.sub.MIN, the average magnitude I.sub.AVE of the load current I.sub.LOAD may be equal to I.sub.AVE=DC.sub.BURST.Math.I.sub.MIN.

[0058] The burst duty cycle DC.sub.BURST may be controlled to adjust the average magnitude I.sub.AVE of the load current I.sub.LOAD. The burst duty cycle DC.sub.BURST may be controlled in different ways. For example, the burst duty cycle DC.sub.BURST may be controlled by holding the burst mode period T.sub.BURST constant and varying the length of the active state period T.sub.ACTIVE The burst duty cycle DC.sub.BURST may also be controlled by holding the active state period T.sub.ACTIVE constant and varying the length of the inactive state period T.sub.INACTIVE (and thus varying the length of the burst mode period T.sub.BURST). As the burst duty cycle DC.sub.BURST is increased, the average magnitude I.sub.AVE of the load current I.sub.LOAD may increase. As the burst duty cycle DC.sub.BURST is decreased, the average magnitude I.sub.AVE of the load current I.sub.LOAD may decrease. The control circuit 150 may be configured to adjust the burst duty cycle DC.sub.BURST using open loop control (e.g., in response to the target intensity L.sub.TRGT). The control circuit 150 may be configured to adjust the burst duty cycle DC.sub.BURST using closed loop control (e.g., in response to the load current feedback signal V.sub.I-LOAD).

[0059] FIG. 10 shows a diagram of an example waveform 1000 illustrating the load current I.sub.LOAD when a load regulation circuit (e.g., the load regulation circuit 140) is operating in a burst mode, for example, as the target intensity L.sub.TRGT of a light source (e.g., the LED light source 202) is being increased (e.g., from the low-end intensity L.sub.LE). A control circuit (e.g., the control circuit 150 of the LED driver 100 shown in FIG. 1 and/or the control circuit 150 controlling the forward converter 240 and the current sense circuit 260 shown in FIG. 5) may adjust the length of the active state period T.sub.ACTIVE of the burst mode period T.sub.BURST by adjusting the burst duty cycle DC.sub.BURST. Adjusting the length of the active state period T.sub.ACTIVE may adjust the average magnitude I.sub.AVE of the load current I.sub.LOAD, and in turn the intensity of the light source.

[0060] The active state period T.sub.ACTIVE of the load current I.sub.LOAD may have a length that is dependent upon the length of an inverter cycle of the inverter circuit of the load regulation circuit (e.g., the operating period T.sub.OP). For example, the active state period T.sub.ACTIVE may comprise six inverter cycles, and as such, may have a length that is equal to the duration of the six inverter cycles. The control circuit may adjust (e.g., increase or decrease) the length of the active state periods T.sub.ACTIVE by adjusting the number of inverter cycles in the active state period T.sub.ACTIVE. As such, the control circuit may adjust the length of the active state periods T.sub.ACTIVE by predetermined increments/decrements, e.g., with each increment/decrement corresponding to approximately the length of an inverter cycle (e.g., such as the transition operating period T.sub.OP-T, which may be approximately 12.8 microseconds). Since the average magnitude I.sub.AVE of the load current I.sub.LOAD may depend on the active state period T.sub.ACTIVE, the average magnitude I.sub.AVE may also be adjusted by a predetermined increment/decrement that corresponds to a change in the load current I.sub.LOAD resulting from the addition or removal of an inverter cycle per active state period T.sub.ACTIVE.

[0061] FIG. 10 shows four burst mode periods T.sub.BURST 1002, 1004, 1006, 1008 with equivalent length. The first three burst mode periods T.sub.BURST 1002, 1004, 1006 may be characterized by equivalent active state periods T.sub.ACTIVE(e.g., with the same number of inverter cycles) and equivalent inactive state periods T.sub.INACTIVE. The fourth burst mode period T.sub.BURST 1008 may be characterized by an active state period T.sub.ACTIVE2 that is larger than the active state periods T.sub.ACTIVE1 (e.g., by one more inverter cycle), and an inactive state period T.sub.INACTIVE2 that is smaller than the inactive state period T.sub.INACTIVE1 (e.g., by one fewer inverter cycle). The larger active state period T.sub.ACTIVE2 and smaller inactive state period T.sub.INACTIVE2 may result in a larger duty cycle and a corresponding larger average magnitude I.sub.AVE of the load current I.sub.LOAD (e.g., as shown during burst mode period 1008). As the average magnitude I.sub.AVE of the load current I.sub.LOAD increases, the intensity of the light source may increase accordingly. Hence, as shown in FIG. 10, by adding inverter cycles to or removing inverter cycles from the active state periods T.sub.ACTIVE while maintaining the length of the burst mode periods T.sub.BURST, the control circuit may adjust the average magnitude I.sub.AVE of the load current I.sub.LOAD. Such adjustments to only the active state periods T.sub.ACTIVE, however, may cause changes in the intensity of the lighting load that are perceptible to the user, e.g., when the target intensity is equal to or below the transition intensity L.sub.TRAN.

[0062] FIG. 11 illustrates how the relative average light intensity of a light source may change as a function of the number N.sub.INV of inverter cycles included in an active state period T.sub.ACTIVE if the control circuit only adjusts the active state periods T.sub.ACTIVE during the burst mode. As described herein, T.sub.ACTIVE may be expressed as T.sub.ACTIVE=N.sub.INV.Math.T.sub.OP-LE, wherein T.sub.OP-LE may represent a low-end operating period of the relevant inverter circuit. As shown in FIG. 11, if the control circuit adjusts the length of the active state periods T.sub.ACTIVE from four to five inverter cycles, the relative light intensity may change by approximately 25%. If the control circuit adjusts the length of the active state periods T.sub.ACTIVE from five to six inverter cycles, the relative light intensity may change by approximately 20%.

[0063] Fine tuning of the light level or light intensity of the lighting load may be achieved by configuring the control circuit to adjust (e.g., increase or decrease) the length of the inactive state periods T.sub.INACTIVE in the burst mode. Adjustments to the length of the inactive state periods T.sub.INACTIVE may be made between adjusting the length of the active state periods T.sub.ACTIVE Adjustments to the length of the inactive state periods T.sub.INACTIVE may also be made while adjusting the length of the active state periods T.sub.ACTIVE. The adjustments to the inactive state periods T.sub.INACTIVE may be made in one or more steps with respective adjustment amounts. The respective adjustment amounts may be substantially equal to or different from each other. The respective adjustment amounts may be determined such that an adjustment made to the inactive state periods will cause a same or smaller change to the light intensity (e.g., a smaller change relative to a specific light intensity level) than an adjustment to active state periods (e.g., by one inverter cycle) would have caused had the inactive state periods not been changed. In an example, one or more of the respective adjustment amounts made to the inactive state periods may be smaller than an adjustment amount made to the active state periods. In an example, the respective adjustment amounts made to the inactive state periods may not be smaller than the adjustment amount made to the active state periods, but the changes caused by the respective inactive adjustment amounts to the relative light intensity may still be smaller than the change caused by the active state adjustment amount. The control circuit may adjust the length of the inactive state periods T.sub.INACTIVE as a function of the target intensity L.sub.TRGT of the lighting load.

[0064] FIG. 12 shows example waveforms 1210-1280 illustrating the load current I.sub.LOAD when a load regulation circuit (e.g., the load regulation circuit 140) is controlled (e.g., by the control circuit 150) to operate in the burst mode. More specifically, the illustrated example shows that the control circuit may adjust the target intensity L.sub.TRGT of the light source (e.g., the LED light source 202) by first adjusting the length of the inactive state periods and then adjusting the length of the active state periods. By using the control technique shown in FIG. 12, the control circuit may accomplish fine dimming of the lighting load.

[0065] As shown in FIG. 12, the control circuit may control the load current I.sub.LOAD to have a default burst mode period T.sub.BURST-DEF (e.g., as shown in waveform 1210). For example, the default burst mode period T.sub.BURST-DEF may be approximately 800 microseconds to correspond to a frequency of approximately 1.25 kHz. The inverter circuit comprised in the load regulation circuit may be characterized by an operating frequency f.sub.OP-BURST (e.g., approximately 25 kHz) and an operating period T.sub.OP-BURST (e.g., approximately 40 microseconds). The control circuit may adjust the length of the inactive state periods T.sub.INACTIVE gradually, for example, between adjusting the length of the active state periods T.sub.ACTIVE. The adjustment to the length of the inactive state periods T.sub.INACTIVE may be made in one or more steps (e.g., over one or more adjacent or separate burst mode periods) with respective inactive state adjustment amounts Δ.sub.INACTIVE. The respective inactive state adjustment amounts may be substantially the same for each step or may be different for different steps, so long as the adjustments may allow fine tuning of the light intensity of the lighting load. For example, the inactive-state adjustment amount Δ.sub.INACTIVE may be equal to a percentage (e.g., approximately 1%) of the default burst mode period T.sub.BURST-DEF (e.g., approximately 8 microseconds).

[0066] The control circuit may adjust the length of the inactive state periods T.sub.INACTIVE (e.g., by the inactive-state adjustment amount Δ.sub.INACTIVE each time) while maintaining the length of the active state period T.sub.ACTIVE constant (as shown in waveforms 1210-1260 in FIG. 12). When the length of the inactive state periods T.sub.INACTIVE has been adjusted by a threshold amount (e.g., a maximum adjustment amount Δ.sub.INACTIVE-MAX, as shown in waveform 1260), the control circuit may adjust the length of the active state periods T.sub.ACTIVE by an active state adjustment amount Δ.sub.ACTIVE (e.g., by one additional inverter cycle length) in a succeeding burst mode period, for example. The control circuit may adjust the length of the inactive state periods (e.g., in the same succeeding burst mode period) such that the length of the burst mode period T.sub.BURST may revert back to that of the default burst mode period T.sub.BURST-DEF, and the length of the inactive state periods T.sub.INACTIVE may be equal to the difference between the default burst mode period T.sub.BURST-DEF and the present length of the active state periods T.sub.ACTIVE (as shown in waveform 1270 of FIG. 12). The control circuit may then go back to adjusting the length of the inactive state periods T.sub.INACTIVE as described herein until the length of the inactive state periods T.sub.INACTIVE has once again been adjusted by the maximum adjustment amount Δ.sub.INACTIVE-MAX. At that point, the control circuit may adjust the length of the active state periods T.sub.ACTIVE and/or the length of the inactive state periods T.sub.INACTIVE such that the burst mode period T.sub.BURST may again be adjusted back to the default burst mode period T.sub.BURST-DEF. Eventually, the burst duty cycle DC.sub.BURST may reach approximately 100% (e.g., as shown in waveform 1280) and the light intensity of the lighting load may reach the transition intensity L.sub.TRAN Beyond that point, the control circuit may begin adjusting the target load current I.sub.TRGT in the normal mode (e.g., via PWM or CCR).

[0067] As described herein, the user's eyes may be more sensitive to changes in the relative light level of the lighting load when the light level is low (e.g., below to the transition intensity L.sub.TRAN). The maximum adjustment amount Δ.sub.INACTIVE-MAX for the inactive state periods T.sub.INACTIVE may be sized to reduce perceptible changes in the relative light level of the lighting load. For example, if the lengths of the active state periods T.sub.ACTIVE and the inactive state periods T.sub.INACTIVE are both adjusted (e.g., between waveforms 1260 and 1270 in FIG. 12), a properly sized maximum adjustment amount Δ.sub.INACTIVE-MAX may enable a smooth transition from a current intensity level into the next intensity level. The maximum adjustment amount Δ.sub.INACTIVE-MAX may be determined as a function of the present length of the active state period T.sub.ACTIVE (e.g., the number of inverter cycles included in the active state period T.sub.ACTIVE). The determination may be made by calculating a value for the maximum adjustment amount Δ.sub.INACTIVE-MAX in real-time or by retrieving a predetermined value from memory (e.g., from a lookup table). In an example, when the active state period T.sub.ACTIVE presently includes four inverter cycles, the maximum adjustment amount Δ.sub.INACTIVE-MAX may be approximately equal to the change in the relative light level when the length of the active state period T.sub.ACTIVE changes from four to five inverter cycles (e.g., 25% as shown in FIG. 11). In another example, the maximum adjustment amount Δ.sub.INACTIVE-MAX may be approximately equal to the burst operating period T.sub.OP-BURST of the inverter circuit (e.g., approximately 40 microseconds). The control circuit may store the value of the maximum adjustment amount Δ.sub.INACTIVE-MAX in memory (e.g., in a lookup table)

[0068] FIG. 13 shows two example plot relationships depicting how a target light intensity of the lighting load may change in accordance with changes in the lengths of the active and inactive state periods when a load regulation circuit (e.g., the load regulation circuit 140) is controlled (e.g., by the control circuit 150) to operate in the burst mode. Plot 1300 shows an example relationship between the length of the inactive state period T.sub.INACTIVE and the target intensity L.sub.TRGT of the lighting load. Plot 1310 shows an example relationship between the length of the active state period T.sub.ACTIVE and the target intensity L.sub.TRGT of the lighting load. The length of the active state period T.sub.ACTIVE may be expressed in time terms or in terms of the number of inverter cycles N.sub.INV included in the active state period T.sub.ACTIVE, for example.

[0069] As described herein, the control circuit (e.g., the control circuit 150 of the LED driver 100 shown in FIG. 1 and/or the control circuit 150 controlling the forward converter 240 and the current sense circuit 260 shown in FIG. 5) may determine the magnitude of the target load current I.sub.TRGT and/or the burst duty cycle DC.sub.BURST based on the target intensity L.sub.TRGT. The control circuit may determine the target intensity L.sub.TRGT, for example, via a digital message received via the communication circuit 180, via a phase-control signal received from a dimmer switch, and/or the like. The target intensity L.sub.TRGT may be constant or may be changing (e.g., fading) from one intensity level to another. The control circuit may determine the length of the active state period T.sub.ACTIVE based on the target intensity L.sub.TRGT. After determining the length of the active state period T.sub.ACTIVE, the control circuit may determine the length of the inactive state period T.sub.INACTIVE that may be used with the present active state period T.sub.ACTIVE such that the light source may be driven to the target intensity L.sub.TRGT. The control circuit may determine the lengths of the active state period T.sub.ACTIVE and/or the inactive state period T.sub.INACTIVE by calculating the values in real-time and/or retrieving the values from memory (e.g., via a lookup table or the like).

[0070] Referring to FIG. 13, if the control circuit determines that the target intensity L.sub.TRGT falls within the range 1321, then the control circuit may determine to set the burst mode period to a default burst mode period (e.g., such as T.sub.BURST-DEF, which may be approximately 800 microseconds) and the active state period T.sub.ACTIVE to a minimum active state period T.sub.ACTIVE-MIN (e.g., including four inverter cycles). The control circuit may determine to set the inactive state period T.sub.INACTIVE according to the profile 1341, which may range from a maximum inactive state period T.sub.INACTIVE-MAX to a minimum inactive state period T.sub.MIN1. The maximum inactive state period T.sub.INACTIVE-MAX may be determined based on the length of the present burst operating period (e.g., the default burst mode period T.sub.BURST-DEF) and/or the length of the present active state period T.sub.ACTIVE-MIN. The minimum inactive state period T.sub.MIN1 may be determined based on the maximum inactive state adjustment amount Δ.sub.INACTIVE-MAX, which may in turn be dependent upon the length of the present active state period T.sub.ACTIVE-MIN. The gradient of the profile 1341 may be determined based on the size of an inactive state adjustment step (e.g., such as the inactive state adjustment amount Δ.sub.INACTIVE), which, may be equal to a percentage (e.g., approximately 1%) of the default burst mode period T.sub.BURST-DEF, for example. As noted herein, the control circuit may determine the lengths of the active state period T.sub.ACTIVE and/or the inactive state period T.sub.INACTIVE by calculating the values in real-time and/or retrieving the values from memory.

[0071] If the control circuit determines that the target intensity L.sub.TRGT falls within the range 1322, then the control circuit may determine to set the active state period T.sub.ACTIVE to 1332. The active state period 1332 may be greater than the minimum active state period T.sub.ACTIVE-MIN. For example, the active state period 1332 may include one more inverter cycle than the minimum active state period T.sub.ACTIVE-MIN. The control circuit may determine to set the inactive state period T.sub.INACTIVE according to the profile 1342. In an example, the starting point of the profile 1342 may be dependent upon the length of the present burst cycle period (e.g., the default burst cycle period T.sub.BURST-DEF) and the length of the present active state period 1332. The ending point of the profile 1342 may be dependent upon the maximum inactive state adjustment amount Δ.sub.INACTIVE-MAX, which may in turn be dependent upon the length of the present active state period 1332. The gradient of the profile 1342 may be determined based on the size of an inactive-state adjustment step (e.g., such as the inactive-state adjustment amount Δ.sub.INACTIVE), which, as noted herein, may be equal to a percentage (e.g., approximately 1%) of the default burst mode period T.sub.BURST-DEF. Similarly, if the control circuit determines that the target intensity L.sub.TRGT falls within one of the target intensity ranges 1323-1327, then the control circuit may determine to set the active state period T.sub.ACTIVE to one of 1333-1337 and determine to set the inactive state period T.sub.INACTIVE according to one of the profiles 1343-1347, respectively.

[0072] The profiles 1341-1347 may be linear or non-linear, and may be continuous (e.g., as shown in FIG. 13) or comprise discrete steps. The minimum inactive state periods for the profiles 1341-1347 may be dependent upon the present maximum adjustment amount Δ.sub.INACTIVE-MAX, which may in turn be dependent upon the length of the respective active state period T.sub.ACTIVE. The maximum adjustment amount Δ.sub.INACTIVE-MAX of the inactive state period T.sub.INACTIVE may be sized to reduce perceptible changes in the relative light level of the lighting load. In an example, the profiles 1341-1347 may be configured such that when the lengths of the active state period T.sub.ACTIVE and the inactive state period T.sub.INACTIVE are both adjusted (e.g., between waveforms 1260 and 1270 as shown in FIG. 12), the waveform characterized by the greater target intensity may generate a greater light output of the lighting load. In such an example, there may be slight steps up in the actual light output of the lighting load when the lengths of the active state period T.sub.ACTIVE and the inactive state period T.sub.INACTIVE are both adjusted (e.g., between waveforms 1260 and 1270 as shown in FIG. 12).

[0073] The graphs 1300, 1310 may represent a portion of the target intensity range between the low-end intensity L.sub.LE and the transition intensity L.sub.TRAN or the entire target intensity range between the low-end intensity L.sub.LE and the transition intensity L.sub.TRAN More or less than seven active state periods T.sub.ACTIVE (e.g., T.sub.ACTIVE-MIN through 1337) may be provided between the low-end intensity L.sub.LE and the transition intensity L.sub.TRAN.

[0074] FIG. 14 illustrates an example target intensity procedure 1400 that may be executed by the control circuit described herein (e.g., the control circuit 150 of the LED driver 100 shown in FIG. 1 and/or the control circuit 150 controlling the forward converter 240 and the current sense circuit 260 shown in FIG. 5). For example, the target intensity procedure 1400 may be executed when the target intensity L.sub.TRGT is adjusted at 1410 (e.g., in response to digital messages received via the communication circuit 180). The control circuit may determine if it is operating the load regulation circuit in the burst mode at 1412 (e.g., the target intensity L.sub.TRGT is between the low-end intensity L.sub.LE and the transition intensity L.sub.TRAN, or L.sub.LE≤L.sub.TRGT≤L.sub.TRAN). If the control circuit determines that it is not operating the load regulation circuit in the burst mode (e.g., but rather in the normal mode), then the control circuit may determine and set the target load current I.sub.TRGT as a function of the target intensity L.sub.TRGT at 1414 (e.g., as shown in FIG. 2). The control circuit may then set the burst duty cycle DC.sub.BURST equal to a maximum duty cycle DC.sub.MAX (e.g., approximately 100%) at 1415 (e.g., as shown in FIG. 3), and the control circuit may exit the target intensity procedure 1400.

[0075] If the control circuit determines that it is operating the load regulation circuit in the burst mode at 1412 (e.g., the target intensity L.sub.TRGT is below the transition intensity L.sub.TRAN, or L.sub.TRGT<L.sub.TRAN), then the control circuit may determine the lengths of the active state period T.sub.ACTIVE and/or the inactive state period T.sub.INACTIVE for one or more burst mode periods T.sub.BURST (e.g., using open loop control) at 1418. For example, the control circuit may determine target lengths of the active state period T.sub.ACTIVE and the inactive state period T.sub.INACTIVE that correspond to the target intensity L.sub.TRGT. The control circuit may then determine the lengths of the active state period T.sub.ACTIVE and/or the inactive state period T.sub.INACTIVE for one or more burst mode periods. As described herein, the length of the inactive state period may be gradually adjusted (e.g., gradually increased or decreased) in one or more burst mode periods until a maximum amount of adjustment is reached. The length of the active state period may then be adjusted in a subsequent burst mode period. The determination process may be repeated in the manner described herein until the target lengths of the active state period T.sub.ACTIVE and inactive state period T.sub.INACTIVE are achieved.

[0076] The control circuit may perform the foregoing process by calculating the relevant values in real-time or retrieving the values from memory (e.g., via a lookup table or the like). The control circuit may set the lengths of the active state period T.sub.ACTIVE and/or the inactive state period T.sub.INACTIVE for the one or more burst mode periods T.sub.BURST at 1420, and the control circuit may exit the target intensity procedure 1400. As described herein, the control circuit may adjust the active state period T.sub.ACTIVE and/or the inactive state period T.sub.INACTIVE as a function of the target intensity L.sub.TRGT using open loop control. Other ways to adjust the active state period T.sub.ACTIVE and/or the inactive state period T.sub.INACTIVE may be employed, including, for example, using closed loop control (e.g., in response to the load current feedback signal V.sub.I-LOAD).

[0077] 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, one or more of the embodiments described herein (e.g., as performed by a load control device) may be used to fade the intensity of a light source from on to off (e.g., the low-end intensity L.sub.LE may be equal to 0%) and/or to fade the intensity of the light source from off to on.

[0078] 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.