Load control device for a light-emitting diode light source

Abstract

A load control device for an electrical load is configured to operate in a normal mode and a burst mode to adjust the amount of power delivered to the electrical load. The load control device comprises a control circuit that operates in the normal mode to regulate an average magnitude of a load current conducted through the load between a maximum rated current and a minimum rated current. During the normal mode, the control circuit controls the operating period of a load regulation circuit between a high-end operating period and a low-end operating period. The control circuit operates in the burst mode to regulate the average magnitude of the load current below the minimum rated current. During the burst mode, the control circuit adjusts the low-end operating period to be less than or equal to a minimum on time of the load regulation circuit.

Claims

1. A circuit for controlling an intensity of a light-emitting diode (LED) light source, the circuit comprising: an LED drive circuit configured to control a magnitude of a load current conducted through the LED light source to control the intensity of the LED light source; a current sense circuit configured to generate a feedback signal that indicates the magnitude of the load current; and a control circuit configured to: generate at least one drive signal for controlling the LED drive circuit, the at least one drive signal characterized by an operating period and a duty cycle; adjust an on time of the at least one drive signal in response to the feedback signal to regulate the magnitude of the load current toward a target load current that ranges between a minimum rated current and a maximum rated current, the minimum rated current based on a minimum on time of the LED drive circuit; and adjust the operating period of the at least one drive signal to a low-end operating period by adjusting the on time of the at least one drive signal to a value less than or equal to the minimum on time while holding the duty cycle of the at least one drive signal constant.

2. The circuit of claim 1, wherein the control circuit is further configured to: operate in a first state and a second state on a periodic basis; control the LED drive circuit in the first state during a first time period in which the control circuit adjusts the on time of the at least one drive signal in response to the feedback signal to regulate a peak magnitude of the load current toward the target load current; control the LED drive circuit in the second state during a second time period in which the control circuit ceases adjustment of the on time of the at least one drive signal in response to the feedback signal; and adjust an average magnitude of the load current by adjusting lengths of the first and second time periods.

3. The circuit of claim 2, wherein the control circuit is configured to: operate in a normal mode to adjust the average magnitude of the load current above a minimum rated current of the LED drive circuit; and operate in a burst mode to adjust the average magnitude of the load current below the minimum rated current.

4. The circuit of claim 3, wherein the control circuit is further configured to: during the normal mode, maintain the operating period of the at least one drive signal at a high-end operating period after the target load current reaches a high-end transition value.

5. The circuit of claim 4, wherein the high-end transition value is equal to approximately a maximum rated current of the LED light source.

6. The circuit of claim 4, wherein the high-end transition value is less than a maximum rated current of the LED light source.

7. The circuit of claim 4, wherein the control circuit is configured to adjust the operating period of the at least one drive signal between the low-end operating period and the high-end operating period when the target load current is between a low-end transition value and the high-end transition value.

8. The circuit of claim 4, wherein the low-end operating period is shorter than the high-end operating period.

9. The circuit of claim 3, wherein the control circuit is further configured to: during the burst mode, maintain the peak magnitude of the load current at the minimum rated current.

10. The circuit of claim 3, wherein the control circuit is further configured to: during the normal mode, keep the length of the second time period at approximately zero seconds.

11. The circuit of claim 2, wherein the control circuit is further configured to: adjust the length of the first time period in first adjustment steps that are dependent on the low-end operating period.

12. The circuit of claim 11, wherein the control circuit is further configured to: adjust the length of the first time period in the first adjustment steps while keeping a length of the operating period constant.

13. The circuit of claim 2, wherein the control circuit is further configured to: adjust a ratio of the length of the first time period to the length of the second time period to adjust the average magnitude of the load current.

14. The circuit of claim 13, wherein the control circuit is further configured to: determine the target load current as a function of a target amount of power to be delivered to the LED light source; and adjust the ratio of the length of the first time period to the length of the second time period linearly with respect to the target amount of power.

15. The circuit of claim 1, wherein the control circuit is further configured to: periodically decrease the operating period of the at least one drive signal by a predetermined amount until the on time of the at least one drive signal is less than or equal to the minimum on time.

16. A method of controlling an intensity of a light-emitting diode (LED) light source, the method comprising: generating at least one drive signal for controlling an LED drive circuit to adjust a magnitude of a load current conducted through the LED light source to control the intensity of the LED light source, the at least one drive signal characterized by an operating period and a duty cycle; receiving a load current feedback signal that indicates the magnitude of the load current; adjusting an on time of the at least one drive signal in response to the feedback signal to regulate the magnitude of the load current toward a target load current that ranges between a minimum rated current and a maximum rated current, the minimum rated current based on a minimum on time of the LED drive circuit; and adjusting the operating period of the at least one drive signal to a low-end operating period by adjusting the on time of the at least one drive signal to a value less than or equal to the minimum on time while holding the duty cycle of the at least one drive signal constant.

17. The method of claim 16, further comprising: operating the LED drive circuit in a first state and a second state on a periodic basis; controlling the LED drive circuit in the first state during a first time period in which the on time of the at least one drive signal is adjusted in response to the feedback signal to regulate a peak magnitude of the load current toward the target load current; controlling the LED drive circuit in the second state during a second time period in which adjustment of the on time of the at least one drive signal in response to the feedback signal is ceased; and adjusting an average magnitude of the load current by adjusting lengths of the first and second time periods.

18. The method of claim 17, further comprising: operating in a normal mode to adjust the average magnitude of the load current above a minimum rated current of the LED drive circuit; and operating in a burst mode to adjust the average magnitude of the load current below the minimum rated current.

19. The method of claim 18, further comprising: when operating in the normal mode, maintaining the operating period of the at least one drive signal at a high-end operating period after the target load current reaches a high-end transition value.

20. The method of claim 17, further comprising: adjusting the length of the first time period in first adjustment steps that are dependent on the low-end operating period while keeping a length of the operating period constant.

21. The method of claim 17, wherein adjusting the average magnitude of the load current by adjusting lengths of the first and second time periods comprises: adjusting a ratio of the lengths of the first and second time periods to adjust the average magnitude of the load current.

22. The method of claim 17, further comprising: adjusting a duty cycle defining when the LED drive circuit operates in the first state and the second state to adjust the average magnitude of the load current; and when the duty cycle defining when the LED drive circuit operates in the first state and the second state is at a maximum duty cycle, adjusting the on time of the at least one drive signal to regulate the average magnitude of the load current toward the target load current.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) 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.

(3) 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.

(4) 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.

(5) FIG. 5 is a simplified schematic diagram of an isolated forward converter and a current sense circuit of an LED driver.

(6) FIG. 6 is an example diagram illustrating a magnetic core set of an energy-storage inductor of a forward converter.

(7) 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.

(8) 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.

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

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

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

(12) FIG. 12 is an example plot showing how a burst duty cycle may change as a result of including one additional inverter cycle in an active state period when an inverter circuit is operating under two different low-end frequencies.

(13) FIG. 13 is an example plot of an operating frequency of an LED driver as a function of a target intensity.

(14) FIG. 14 shows example plots illustrating the operation of a control circuit and an inverter circuit of an LED driver during a burst mode.

(15) FIG. 15 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.

(16) FIG. 16 is a simplified flowchart of an example low-end period adjustment procedure.

DETAILED DESCRIPTION

(17) 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 terminal N that are adapted to be coupled to an alternating-current (AC) power source (not shown).

(18) The LED driver 100 may comprise a radio-frequency (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.

(19) 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.

(20) 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.

(21) 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 low-end (i.e., minimum) intensity L.sub.LE (e.g., approximately 1-5%) and a high-end (i.e., maximum) intensity L.sub.HE (e.g., approximately 100%). 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.

(22) 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.

(23) The control circuit 150 may generate drive control signals V.sub.DRTVE1, 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 the magnitude of a load current I.sub.LOAD conducted through the LED light source 102, for example, to control the intensity of the LED light source 102 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 to adjust the magnitude of the load voltage V.sub.LOAD and/or the load current I.sub.LOAD.

(24) The current sense circuit 160 may receive a sense voltage V.sub.SENSE 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 and control the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 accordingly. For example, the control circuit 150 may control the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 to adjust a magnitude of the load current I.sub.LOAD to a target load current I.sub.TRGT to thus control the intensity of the LED light source 102 to the target intensity L.sub.TRGT (e.g., using a control loop).

(25) The load current I.sub.LOAD may be the current that is conducted through the LED light source 102. The target load current I.sub.TRGT may be the current that the control circuit 150 would ideally like to conduct through the LED light source 102 (e.g., based at least on the load current feedback signal V.sub.I-LOAD). The control circuit 150 may be limited to specific levels of granularity in which it can control the current conducted through the LED light source 102 (e.g., due to inverter cycle lengths, etc.), so the control circuit 150 may not always be able to achieve the target load current I.sub.TRGT. For example, FIGS. 2 and 13 illustrate the current conducted through an LED light source as a linear graph (at least in parts), and as such, illustrate the target load current I.sub.TRGT since the load current I.sub.LOAD itself may not actually follow a true linear path. Further, non-ideal reactions of the LED light source 102 (e.g., an overshoot in the load current I.sub.LOAD, for example, as shown in FIG. 14A) may cause the load current I.sub.LOAD to deviate from the target load current I.sub.TRGT. In the ideal situation, the load current I.sub.LOAD is approximately equal to the target load current I.sub.TRGT.

(26) 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.

(27) FIG. 2 is an example plot of the target load current I.sub.TRGT as a function of the target intensity L.sub.TRGT. The magnitude of the load current I.sub.LOAD may only be regulated to values between a maximum rated current I.sub.MAX and a minimum rated current I.sub.MIN, for example, due to hardware limitations of the load regulation circuit 140 and the control circuit 150. Thus, the target load current I.sub.TRGT may only be adjusted between the maximum rated current I.sub.MAX and the minimum rated current I.sub.MIN. When the target intensity I.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%), 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 is controlled to be equal to the target load current I.sub.TRGT. In 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. The control circuit 150 may adjust the target load current I.sub.TRGT between the maximum rated current I.sub.MAX and the minimum rated current I.sub.MIN in the normal mode, for example, as shown in FIG. 2.

(28) 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. 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%), the control circuit 150 may be configured to operate the load regulation circuit 140 to set the burst duty cycle DC.sub.BURST equal to a maximum duty cycle DC.sub.MAX (e.g., 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 a burst mode to reduce the average magnitude I.sub.AVE of the load current I.sub.LOAD to be less the minimum rated current I.sub.MIN. For example, 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 to reduce the burst duty cycle DC.sub.BURST below the maximum duty cycle DC.sub.MAX. For example, the load regulation circuit 140 may adjust 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). For example, the peak magnitude I.sub.PK of the load current I.sub.LOAD may be equal to the minimum rated current I.sub.MIN during an active state of the burst mode.

(29) 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. As described herein, the control circuit 150 (e.g., via the load regulation circuit 140) may be configured to adjust the number of inverter cycles only by an integer number (i.e., by DC.sub.BURST-INTEGER) and not a fractional amount (i.e., DC.sub.BURST-FRACTIONAL). Therefore, the example plot of FIG. 3 may illustrate an ideal curve showing the 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 when the target intensity L.sub.TRGT is below the transition intensity L.sub.TRAN. Nonetheless, 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, for example, if the control circuit 150 is not configured to operate the burst duty cycle DC.sub.BURST at fractional amounts.

(30) 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.

(31) In the active state of the burst mode, the control circuit 150 may generate (e.g., actively generate) the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 to adjust the magnitude (e.g., the peak magnitude I.sub.PK) of the load current I.sub.LOAD, e.g., 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.

(32) In the inactive state of the burst mode, the control circuit 150 may freeze the control loop and may not generate the drive control signals V.sub.DRIVE1, V.sub.DRIVE2, for example, such that the magnitude of the load current I.sub.LOAD drops to approximately zero amps. While the control loop is frozen (e.g., in the inactive state), 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 load current feedback signal V.sub.I-LOAD (e.g., even though the control circuit 150 is not presently generating the drive signals V.sub.DRIVE1, V.sub.DRIVE2). For example, 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. Accordingly, when the control loop is unfrozen (e.g., when the control circuit 150 enters the active state), the control circuit 150 may continue to generate 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.

(33) 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, for example, when the target intensity L.sub.TRGT is below the transition intensity L.sub.TRAN. 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.

(34) 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.

(35) 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 generate the inverter voltage V.sub.INV at a constant operating frequency f.sub.OP (e.g., approximately 60-65 kHz) and thus a constant operating period T.sub.OP. However, the operating frequency f.sub.OP and/or operating period T.sub.OP may be adjusted under certain operating conditions. For example, the operating frequency f.sub.OP may be adjusted (e.g., increased or decreased) as the target intensity L.sub.TRGT of the LED light source 202 is adjusted towards the high-end intensity L.sub.HE (e.g., as shown in FIG. 13). For example, the operating frequency f.sub.OP may be adjusted (e.g., increased or decreased) as the target intensity L.sub.TRGT of the LED light source 202 is adjusted towards the transition intensity L.sub.TRAN. 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 the LED light source 202 towards the target intensity L.sub.TRGT.

(36) 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 previously mentioned, 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). For example, the minimum rated current I.sub.MIN may be chosen based on a minimum on-time T.sub.ON-MIN of the half-bridge inverter circuit of the forward converter 240. The value of the minimum on-time T.sub.ON-MIN may be set such that the on time of the half-bridge inverter circuit may be maintained within the hardware limitations of the forward converter. At the minimum rated current I.sub.MIN (e.g., at the transition intensity L.sub.TRAN), the inverter voltage V.sub.INV may be characterized by a low-end operating frequency f.sub.OP-LE and a low-end operating period T.sub.OP-LE.

(37) 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. The control circuit 150 may use power (e.g., a transition power) and/or current (e.g., a transition current) as a threshold to determine when to operate in the burst mode (e.g., instead of intensity). In the burst mode of operation, the control circuit 150 may be configured to switch the forward converter 240 between an active mode (e.g., in which the control circuit 150 actively generates 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 mode (e.g., in which the control circuit 150 freezes the control loop and does not generate the drive control signals V.sub.DRIVE1, V.sub.DRIVE2), for example, as shown in the state diagram of FIG. 4. In the burst mode, 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) and 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 normal mode and in the active state of the burst 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.

(38) The inverter voltage V.sub.INV is 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), such that a primary voltage V.sub.PRI is generated across the primary winding. The transformer 220 may be characterized by a turns ratio n.sub.TURNS (i.e., 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 C.sub.P1, C.sub.P2, C.sub.P3, 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), such that 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).

(39) 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 μF) 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 260 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 may be 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.

(40) 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. Each inner leg 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 (i.e., the magnetic core set 290 is partially gapped), such that the inner legs 296A, 296B are 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 extends for approximately 70% of the leg width w.sub.LEG of the inner leg 296A. In one or more embodiments, 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).

(41) FIG. 7 shows example waveforms illustrating the operation of a forward converter and a current sense circuit, for example, the forward converter 240 and the current sense circuit 260 shown in FIG. 5. For example, the forward converter 240 may generate the waveforms shown in FIG. 7 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 at different times (i.e., the FETs Q210, Q212 are 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 C.sub.P3 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 is developed across the capacitor. Accordingly, 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 (i.e., 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.

(42) When either of the high-side and low-side FETs Q210, Q212 is conductive, the magnitude of an output inductor current I.sub.L conducted by the output inductor L226 and 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.

(43) 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). However, a magnetizing current I.sub.MAG may continue to flow through the primary winding of the transformer 220 due to the magnetizing inductance L.sub.MAG of the transformer. When the target intensity L.sub.TRGT of the LED light source 202 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, for example, due to the parasitic capacitances C.sub.P1, C.sub.P2 of the FETs, the parasitic capacitance C.sub.P3 of the primary winding of the transformer 220, and/or any other parasitic capacitances of the circuit, such as, parasitic capacitances of the printed circuit board on which the forward converter 240 is mounted.

(44) 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. However, the magnetizing current I.sub.MAG (i.e., the reactive component of the primary current I.sub.PRI) may also flow through the sense resistor R222. The magnetizing current I.sub.MAG may change from a negative polarity to a positive polarity when the high-side FET Q210 is conductive, change from a positive polarity to a negative polarity when the low-side FET Q212 is conductive, and remain constant when the magnitude of the primary voltage V.sub.PRI is zero volts, for example, as shown in FIG. 7. The magnetizing current I.sub.MAG may have a maximum magnitude defined by the following equation:

(45) $I_{\mathrm{MAG}\text{-}\mathrm{MAX}}=\frac{V_{\mathrm{BUS}}.Math.T_{\mathrm{HC}}}{4.Math.L_{\mathrm{MAG}}}$
where T.sub.HC may be the half-cycle period of the inverter voltage V.sub.INV, i.e., T.sub.HC=T.sub.OP/2. As shown in FIG. 7, the areas 250, 252 are approximately equal, such that the average value of the magnitude of the magnetizing current I.sub.MAG is 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).

(46) The current sense circuit 260 may determine an average the primary current I.sub.PRIduring the positive cycles of the inverter voltage V.sub.INV, i.e., when 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, 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 when the high-side FET Q210 is conductive. Because the average value of the magnitude of the magnetizing current I.sub.MAG is 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 indicates the real component (e.g., only the real component) of the primary current I.sub.PRI during the on-time T.sub.ON.

(47) 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, such that the magnitude of the load current feedback signal V.sub.I-LOAD is the average value of the primary current I.sub.PRI and thus indicates 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 drives the signal-chopper control signal V.sub.CHOP high to render the transistor Q236 conductive. Accordingly, the control circuit 150 is able to accurately determine the average magnitude of the load current I.sub.LOAD from the magnitude of the load current feedback signal V.sub.I-LOAD since 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 are reduced or eliminated completely.

(48) As the target intensity L.sub.TRGT of the LED light source 202 is decreased towards the low-end intensity L.sub.LE and the on times T.sub.ON of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 get smaller, the parasitics of the load regulation circuit 240 (i.e., the parasitic capacitances C.sub.P1, C.sub.P2 of the FETs Q210, Q212, the parasitic capacitance C.sub.P3 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.

(49) 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). Accordingly, 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.

(50) FIG. 9 shows example waveforms illustrating the operation of a forward converter when operating in a burst mode (e.g., the forward converter 240 shown in FIG. 5). The inverter circuit of the forward converter 240 may generate the inverter voltage V.sub.INV during the active state (e.g., for length of an active state period T.sub.ACTIVE as shown in FIG. 9), for example, such that the magnitude of the load current I.sub.LOAD may be regulated to the minimum rated current I.sub.MIN. The inverter voltage V.sub.INV may not be generated during the inactive state (e.g., for an inactive state period T.sub.INACTIVE). The active state may begin on a periodic basis at 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. For example, 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), which in one example may be the minimum load current I.sub.MIN (e.g., I.sub.AVE=DC.sub.BURST.Math.I.sub.MIN).

(51) 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. For example, the burst mode period T.sub.BURST may be held constant and the length of the active state period T.sub.ACTIVE may be varied to adjust the duty cycle DC.sub.BURST, which in turn may vary the average magnitude I.sub.AVE of the load current I.sub.LOAD. For example, the active state period T.sub.ACTIVE may be held constant, and the length of burst mode period T.sub.BURST may be varied to adjust the burst duty cycle DC.sub.BURST, which in turn may vary the average magnitude I.sub.AVE of the load current I.sub.LOAD. Accordingly, 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, and 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. As described herein, the control circuit 150 may adjust the burst duty cycle DC.sub.BURST in response to the target intensity L.sub.TRGT using open loop control. 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).

(52) FIG. 10 is 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 240) 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 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.

(53) 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, referring to FIG. 9, 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 active state period 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 active state period T.sub.ACTIVE by predetermined time intervals that each correspond to the length of an inverter cycle of the inverter circuit of the load regulation circuit. For example, the adjustment to the active state period T.sub.ACTIVE may be made in one or more steps (e.g., with a substantially equal amount of adjustment in each step). The substantially equal amount of adjustment may be equal to, for example, a low-end operating period T.sub.OP-LE (e.g., approximately 12.8 microseconds). Therefore, the active state period T.sub.ACTIVE may be characterized by one or more inverter cycles and may be adjusted by adjusting the number of inverter cycles per active state period T.sub.ACTIVE. As such, the average magnitude I.sub.AVE of the load current I.sub.LOAD may be adjusted by a predetermined amount (e.g., starting at time t.sub.1 shown in FIG. 10) that corresponds, for example, to a change in load current I.sub.LOAD due to an increase or decrease of the number of inverter cycles per active state period T.sub.ACTIVE.

(54) One or more burst mode periods T.sub.BURST of the load regulation circuit may be characterized by active state periods T.sub.ACTIVE that comprise the same number of inverter cycles. In the example of FIG. 10, three burst mode periods T.sub.BURST 1002, 1004, 1006 may be characterized by equivalent active state periods T.sub.ACTIVE1 (e.g., active state periods T.sub.ACTIVE1 that have the same number of inverter cycles) and equivalent inactive state periods T.sub.INACTIVE. The active state period T.sub.ACTIVE2 of the burst mode period T.sub.BURST 1008 may be larger than the active state periods T.sub.ACTIVE1 of the other burst mode periods T.sub.BURST 1002, 1004, 1006 (e.g., by an additional inverter cycle). The inactive state period T.sub.INACTIVE2 of the burst mode period T.sub.BURST 1008 may be smaller than the inactive state period T.sub.INACTIVE1 (e.g., by one fewer inverter cycle). In other words, the active state period T.sub.ACTIVE2 during the burst mode period T.sub.BURST 1008 may be increased (e.g., by an additional inverter cycle) as compared to the active state periods T.sub.ACTIVE1 during the burst mode periods T.sub.BURST 1002, 1004, 1006. The inactive state period T.sub.ACTIVE2 during the burst mode period T.sub.BURST 1008 may be decreased (e.g., by one fewer inverter cycle) as compared to the inactive state periods T.sub.ACTIVE1 during the burst mode periods T.sub.BURST 1002, 1004, 1006. 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). The amount of increase in the average magnitude I.sub.AVE of the load current I.sub.LOAD may be in accordance with the additional length (e.g., in terms of inverter cycles) of the active state period T.sub.ACTIVE2 during the burst mode period T.sub.BURST 1008. Therefore, the control circuit may adjust (e.g., increase or decrease) the average magnitude I.sub.AVE of the load current I.sub.LOAD by adjusting the active state period T.sub.ACTIVE (e.g., by increments or decrements of one or more inverter cycles).

(55) A user's eyes may be more sensitive to changes in the relative light level at lower light intensities (e.g., closer to the low-end intensity L.sub.LE or when operating in the burst mode). FIG. 11 illustrates how the relative average light level of a lighting load may change as a function of the number N.sub.INV of inverter cycles included in the active state period T.sub.ACTIVE. 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 inverter circuit. As shown in FIG. 11, if the control circuit adjusts the length of the active state period T.sub.ACTIVE from four to five inverter cycles, the relative light level may change by approximately 25%. If the control circuit adjusts the length of the active state period T.sub.ACTIVE from five to six inverter cycles, the relative light level may change by approximately 20%. The control circuit may be configured to adjust the light intensity of the lighting load with fine granularity when the target intensity L.sub.TRGT is close to the low-end intensity L.sub.LE. Examples of a load control device capable of fine-tuning the light intensity of a lighting load in a low-end intensity range are described in greater detail in commonly-assigned U.S. Pat. No. 9,247,608, issued Jan. 26, 2016, and U.S. patent application Ser. No. 15/142,876, filed Apr. 29, 2016, both titled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosures of which are hereby incorporated by reference in their entireties.

(56) When the target intensity L.sub.TRGT is close to the low-end of the light intensity range, the inverter circuit may be controlled to operate at an adjusted low-end operating frequency f.sub.OP-LE-ADJ (or with an adjusted low-end operating period T.sub.OP-LE-ADJ). An example effect of applying such control may be illustrated by FIG. 12. As shown, when the inverter circuit is operating at a lower frequency f.sub.OP-LE1 (e.g., corresponding to a longer low-end operating period T.sub.OP-LE1), adjusting the length of the active state periods by one inverter cycle while keeping the burst operating period unchanged may cause the burst duty cycle to change between 50% and 40% (thus causing the light intensity of the lighting load to change accordingly). When the inverter circuit is operating at a higher frequency f.sub.OP-LE2 (e.g., corresponding to a shorter low-end operating period T.sub.OP-LE2), adjusting the length of the active state periods by one inverter cycle while keeping the burst operating period unchanged may cause the burst duty cycle to change between 50% and 43%. In other words, as the operating frequency of the inverter circuit increases, the ability of the control circuit to fine-tune the intensity of the lighting load may increase accordingly. Therefore, when the control circuit is operating in the burst mode and/or when the target intensity L.sub.TRGT of the lighting load is near the low-end of its intensity range (e.g., near the low-end transition intensity L.sub.TRAN, which may be approximately 5%), the control circuit may adjust the low-end operating frequency of the inverter circuit f.sub.OP-LE to an adjusted value (e.g., a higher value) such that fine-tuning of the intensity of the lighting load may be achieved, among other goals.

(57) The operating frequency f.sub.OP-LE of the inverter circuit near the low-end intensity (e.g., whether or not the inverter circuit is controlled to operate in the burst mode) may be adjusted based on a minimum on time of the inverter circuit. As described herein, during the active state of the burst mode, the control circuit may be configured to adjust the on-time T.sub.ON of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 to control the peak magnitude I.sub.PK of the load current I.sub.LOAD to the minimum rated current I.sub.MIN using closed loop control (e.g., in response to the load current feedback signal V.sub.I-LOAD). The value of the low-end operating frequency may be chosen to ensure that the control circuit does not adjust the on-time T.sub.ON of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 below the minimum on-time T.sub.ON-MIN. For example, the low-end operating frequency f.sub.OP may be calculated by assuming worst-case operating conditions and component tolerances and stored in memory in the LED driver. Since the LED driver may be configured to drive a plurality of different LED light sources (e.g., manufactured by a plurality of different manufacturers) and/or adjust the magnitude of the load current I.sub.LOAD and the magnitude of the load voltage V.sub.LOAD to a plurality of different magnitudes, the value of the on-time T.sub.ON during the active state of the burst mode may be greater than the minimum on-time T.sub.ON-MIN for many installations. If the value of the on-time T.sub.ON near the low-end intensity (e.g., during the active state of the burst mode) is too large, steps in the intensity of the LED light source may be visible to a user when the target intensity L.sub.TRGT is adjusted near the low-end intensity (e.g., during the burst mode).

(58) Accordingly, when operating near the low-end intensity (e.g., in the burst mode), the control circuit may be configured to minimize the on-time T.sub.ON of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 until the minimum on-time T.sub.ON-MIN is achieved. For example, the control circuit may be configured to periodically adjust the low-end operating period T.sub.OP-LE (e.g., decreasing the low-end operating period T.sub.OP-LE or increasing the low-end operating frequency f.sub.OP-LE) while maintaining the duty cycle of the inverter circuit constant, until the on-time T.sub.ON of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 is equal to or slightly below the minimum on-time T.sub.ON-MIN. The control circuit may be configured to store the adjusted low-end operating period T.sub.OP-LE-ADJ and/or the adjusted low-end operating frequency f.sub.OP-LE-ADJ in memory. Subsequently, the adjusted low-end operating period T.sub.OP-LE-ADJ and/or the adjusted low-end operating frequency f.sub.OP-LE-ADJ may be used as the low-end operating period T.sub.OP-LE and/or low-end operating frequency f.sub.OP-LE when the target intensity L.sub.TRGT is close to the low-end of the light intensity range (e.g., during burst mode). The stored adjusted low-end operating period T.sub.OP-LE and/or adjusted low-end operating frequency f.sub.OP-LE-ADJ may also be used during the normal mode. For example, during the normal mode, the control circuit may adjust the operating frequency f.sub.op of the inverter circuit between the adjusted low-end operating frequency f.sub.OP-LE-ADJ and a high-end operating frequency f.sub.OP-HE. The operating frequency f.sub.OP may be adjusted as a function (e.g., as a linear function) of the target intensity L.sub.TRGT according to an adjusted operating frequency plot 1300 (e.g., as shown in FIG. 13).

(59) FIG. 13 is an example plot of the operating frequency f.sub.OP of the inverter circuit as a function of the target intensity L.sub.TRGT. As shown, the low-end operating frequency of the inverter circuit may be controlled from a default low-end operating frequency towards an adjusted low-end operating frequency f.sub.OP-LE-ADJ (e.g., approximately 58 kHz) when the target intensity L.sub.TRGT is near or below a low-end transition value L.sub.TRAN-LOW and/or when the target load current is near or below a low-end transition value I.sub.TRAN-LOW. The low-end transition intensity L.sub.TRAN-LOW may or may not be the same as the low-end transition intensity L.sub.TRAN described herein. For example, the low-end transition intensity L.sub.TRAN-LOW may be greater than the low-end transition intensity L.sub.TRAN. Similarly, the low-end transition current I.sub.TRAN-LOW may or may not be the same as the minimum rated current I.sub.MIN described herein. For example, the low-end transition current I.sub.TRAN-LOW may be greater than the minimum rated current I.sub.MIN. The operating frequency of the inverter circuit may be adjusted (e.g., decreased linearly) as the target intensity L.sub.TRGT (or target load current I.sub.TRGT) is adjusted towards the high-end intensity L.sub.HE (or the maximum rated current I.sub.MAX). The operating frequency may be adjusted to a high-end operating frequency f.sub.OP-HE (e.g., approximately 32 kHz) when the target intensity L.sub.TRGT reaches a high-end transition value L.sub.TRAN-HIGH (or when the target load current I.sub.TRGT reaches a high-end transition value I.sub.TRAN-HIGH). The high-end transition value for the target intensity may be less than or equal to the maximum intensity L.sub.HE (e.g., 100%) of the lighting load. The high-end transition value for the target load current may be less than or equal to the maximum rated current I.sub.MAX of the lighting load.

(60) As the target intensity L.sub.TRGT is controlled between the high-end intensity L.sub.HE of the lighting load, the operating frequency f.sub.OP of the inverter circuit may be adjusted (e.g., gradually decreased) towards the high-end operating frequency f.sub.OP-HE. The operating period of the inverter circuit may be adjusted (e.g., gradually increased) accordingly. The adjustment to the operating frequency may be performed as a function of the target intensity L.sub.TRGT (or the target load current I.sub.TRGT). For example, as the target intensity L.sub.TRGT or target load current I.sub.TRGT increases, the operating frequency of the inverter circuit may be decreased proportionally (e.g., as a linear function of the target intensity L.sub.TRGT or the target load current I.sub.TRGT). The operating frequency may reach the high-end operating frequency f.sub.OP-HE once the target intensity L.sub.TRGT or target load current I.sub.TRGT reaches the high-end transition values described herein. The high-end transition value(s) may be predetermined (e.g., determined during system configuration and stored in memory). For example, the high-end transition value(s) may correspond to the maximum intensity (e.g., 100%) or the maximum rated current of the lighting load. Alternatively, the high-end transition value(s) may be set to be less than the maximum intensity (e.g., to 90%) or less than the maximum rated current of the lighting load.

(61) Although the example plot in FIG. 13 shows that the operating frequency f.sub.OP is adjusted to and maintained at the adjusted low-end operating frequency f.sub.OP-LE-ADJ when the target intensity L.sub.TRGT is equal to or less than the low-end transition value L.sub.TRAN-LOW, the scope of the present disclosure is not limited to only such an implementation. In certain embodiments, the control circuit may be configured to continue to adjust the low-end operating frequency after the target intensity L.sub.TRGT is adjusted below the low-end transition value L.sub.TRAN-LOW. For example, the control circuit may be configured to adjust the low-end operating frequency as a function (e.g., a linear function) of the target intensity L.sub.TRGT even when the target intensity L.sub.TRGT is adjusted below the low-end transition value L.sub.TRAN-LOW. In other words, the control circuit may be configured to adjust the operating frequency of the inverter circuit as a function (e.g., a linear function) of the target intensity L.sub.TRGT so long as the target intensity L.sub.TRGT is less than the high-end transition value L.sub.TRAN-HIGH. Further, although the example plot in FIG. 13 shows that the adjusted low-end operating frequency f.sub.OP-LE-ADJ is higher than the high-end operating frequency f.sub.OP-HE, the reverse may be true in some embodiments. In other words, the adjusted low-end operating frequency f.sub.OP-LE-ADJ may be lower than the high-end operating frequency f.sub.OP-HE in some embodiments, and the control circuit may be configured to increase the operating frequency of the inverter circuit as the target intensity L.sub.TRGT is adjusted from the low-end transition value L.sub.TRAN-LOW to the high-end transition value L.sub.TRAN-HIGH.

(62) FIG. 14 shows example plots illustrating the operation of a control circuit and an inverter circuit of an LED driver (e.g., the control circuit 150 and the inverter circuit of the forward converter 240), for example during the burst mode, to minimize the on-time T.sub.ON of the drive control signals V.sub.DRIVE1, V.sub.DRIVE2 until the minimum on time T.sub.ON-MIN is achieved. The target intensity L.sub.TRGT may be adjusted in response to digital messages received via a communication circuit (e.g., the communication circuit 180). After the target intensity L.sub.TRGT is controlled below the transition intensity L.sub.TRAN (e.g., at time t.sub.1 of FIG. 14 and/or when the control circuit begins to operate the inverter circuit in the burst mode), the on-time T.sub.ON may be greater than the minimum on-time T.sub.ON-MIN. The control circuit may decrease the low-end operating period T.sub.OP-LE (e.g., at time t.sub.2) by a predetermined amount ΔT.sub.OP (and thus increase the low-end operating frequency f.sub.OP-LE). The control circuit may decrease the low-end operating period T.sub.OP-LE while maintaining the duty cycle of the inverter circuit constant. The predetermined amount ΔT.sub.OP may be approximately 42 nanoseconds, for example. The control circuit may then determine whether the on-time T.sub.ON is less than or equal to the minimum on-time T.sub.ON-MIN. In an example, the control circuit may wait for a wait period T.sub.WAIT (e.g., approximately ten seconds) before checking to determine if the on-time T.sub.ON is less than or equal to the minimum on-time T.sub.ON-MIN (e.g., at time t.sub.3). If the on-time T.sub.ON is still greater than the minimum on-time T.sub.ON-MIN at time t3, the control circuit may once again decrease the low-end operating period T.sub.OP-LE by the predetermined amount ΔT.sub.OP (e.g., at time t.sub.3). As shown in FIG. 14, after this decrease in the low-end operating period T.sub.OP-LE, the on-time T.sub.ON may decrease below the minimum on-time T.sub.ON-MIN. As described herein, the value of the minimum on-time T.sub.ON-MIN may be predetermined (e.g., set during configuration and stored in memory) such that the on time of the inverter circuit may be maintained within the hardware limits of the relevant circuitry. Once the on-time T.sub.ON is decreased to or below the minimum on-time T.sub.ON-MIN, the control circuit may cease reducing the low-end operating period T.sub.OP-LE. The control circuit may store the final, adjusted value for the low-end operating period T.sub.OP-LE (and/or the final, adjusted value for the low-end operating frequency f.sub.OP-LE) in memory.

(63) FIGS. 15 and 16 are simplified flowcharts of example procedures that may be executed by a control circuit of a load control device (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). FIG. 15 is a simplified flowchart of an example target intensity procedure 1500 that may be executed by the control circuit, e.g., when the target intensity L.sub.TRGT is adjusted at 1510 (e.g., in response to digital messages received via the communication circuit 180). The control circuit may determine if it is operating the forward converter near or below the transition intensity L.sub.TRAN-LOW (or L.sub.TRGT<L.sub.TRAN-LOW) and/or in the burst mode at 1512. If the control circuit determines that it is not operating the forward converter below the transition intensity L.sub.TRAN-LOW or in the burst mode (e.g., but rather in the normal mode), then the control circuit may determine and set the operating frequency f.sub.OP as a function of the target intensity L.sub.TRGT at 1514 (e.g., as shown in FIG. 13). The control circuit may then determine and set the target load current I.sub.TRGT as a function of the target intensity L.sub.TRGT at 1516 (e.g., as shown in FIG. 2), and/or set the burst duty cycle DC.sub.BURST equal to a maximum duty cycle DC.sub.MAX (e.g., approximately 100%) at 1518 (e.g., as shown in FIG. 3), before the target intensity procedure 1500 exits.

(64) If the control circuit determines that it is operating the forward converter in the burst mode and/or that the target intensity L.sub.TRGT is near or below the transition intensity L.sub.TRAN-LOW (e.g., L.sub.TRGT<L.sub.TRAN-LOW), then the control circuit may set the operating frequency f.sub.OP to the low-end operating frequency f.sub.OP-LE at step 1520 and may set the target load current I.sub.TRGT to a minimum value (e.g., to the minimum rated current I.sub.MIN) at 1522 (e.g., as shown in FIG. 2). The control circuit may then determine and set the burst duty cycle DC.sub.BURST (if the control circuit is operating in the burst mode) as a function of the target intensity L.sub.TRGT at 1524 (e.g., using open loop control as shown in FIG. 3), and the control circuit may exit the target intensity procedure 1500.

(65) FIG. 16 is a simplified flowchart of an example low-end period adjustment procedure 1600 that may be executed by the control circuit (e.g., periodically at every ten seconds) at 1610. If the target intensity L.sub.TRGT is below the transition intensity L.sub.TRAN-LOW and/or the control circuit is operating in the burst mode at 1612, the control circuit may determine if the present value for the on-time T.sub.ON is less than or equal to the minimum on-time T.sub.ON-MIN (e.g., approximately 500 microseconds) at 1614. If not, the control circuit may decrease the low-end operating period T.sub.OP-LE by a predetermined amount ΔT.sub.OP at 1616 (e.g., while holding the duty cycle of the inverter circuit constant) and store the new value for the low-end operating period T.sub.OP-LE in memory at 1618, before the example low-end period adjustment procedure 1600 exits. The control circuit may continue to periodically execute the example low-end period adjustment procedure 1600 (e.g., at every ten seconds) to decrease the low-end operating period T.sub.OP-LE by the predetermined amount ΔT.sub.OP at 1616 until the on-time T.sub.ON is determined to be less than or equal to the minimum on-time T.sub.ON-MIN at 1614.

(66) The control circuit may adjust the low-end operating period T.sub.OP-LE using the low-end period adjustment procedure 1600 in addition to providing fine-tune adjustment of the intensity of the lighting load. For example, the control circuit may be configured to operate in the burst mode when the target intensity L.sub.TRGT is below the transition intensity L.sub.TRAN and adjust the lengths of the active state period T.sub.ACTIVE and/or the inactive state period T.sub.INACTIVE at the adjusted low-end operating frequency in order to fine-tune the intensity of the lighting load. Although the disclosure herein describes the low-end operating period adjustment procedure 1600 in the context of burst mode, the procedure may be executed even if the control circuit is not configured to operate in the burst mode.

(67) 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, from 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.

(68) 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, an 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, a controllable circuit breaker, or other switching device for turning electrical loads or appliances on and off; a plug-in load control device, a controllable electrical receptacle, or a 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 a computer monitor; a power supply; an audio system or an 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.