Load Control Device for a Light-Emitting Diode Light Source

Abstract

A load control device for controlling a light source may be configured to control a target color temperature near a low-end intensity level to converge the color temperature towards one or more color temperatures. The light source may include first and second emitter circuits that emit light having first and second color temperatures, respectively. The load control device may control the first and second emitter circuits to cause a present color temperature of the cumulative light emitted by the light source to be controlled towards the target color temperature. When a target intensity level is less than a threshold intensity level, the control circuit may limit the target color temperature according to at least one limit curve that causes the target color temperature to converge towards at least one of the first or second color temperatures as the target intensity level decreases towards the low-end intensity level.

Description

BACKGROUND

A user environment, such as a residence or an office building for example, may be configured using various types of load control systems. A lighting control system may be used to control the lighting loads in the user environment. Each load control system may include various control devices, including input devices and load control devices. The load control devices may receive digital messages, which may include load control instructions, for controlling an electrical load from one or more of the load control devices. The load control devices may be capable of directly controlling an electrical load. The input devices may be capable of indirectly controlling the electrical load via the load control device. Examples of load control devices may include lighting control devices (e.g., a dimmer, a dimmer switch, an electronic switch, a ballast, or a light-emitting diode (LED) driver), a motorized window treatment, a temperature control device (e.g., a thermostat), an AC plug-in load control device, and/or the like. Examples of input devices may include remote control devices, occupancy sensors, daylight sensors, temperature sensors, and/or the like.

Lamps and displays using efficient light sources, such as light-emitting diodes (LED) light sources, for illumination are becoming increasingly popular in many different markets. LED light sources provide a number of advantages over traditional light sources, such as incandescent and fluorescent lamps. For example, LED light sources may have a lower power consumption and a longer lifetime than traditional light sources. In addition, the LED light sources may have no hazardous materials, and may provide additional specific advantages for different applications. When used for general illumination, LED light sources provide the opportunity to adjust the color (e.g., from white, to blue, to green, etc.) or the color temperature (e.g., from warm white to cool white) of the light emitted from the LED light sources to produce different lighting effects.

SUMMARY

As described herein, a load control device for controlling an amount of power delivered to a light source may be configured to control a target color temperature for the light source to converge the color temperature towards one or more color temperatures near a low-end intensity level. The light source may include a plurality of emitter circuits, each configured to emit light at a respective color temperature (e.g., a first emitter circuit configured to emit light having a first color temperature and a second emitter circuit configured to emit light having a second color temperature). The load control device may comprise a plurality of drive circuits, each configured to control a respective intensity level of each of the plurality of emitter circuits of the light source (e.g., a first drive circuit configured to control a first intensity level of the first emitter circuit and a second drive circuit configured to control a second intensity level of the second emitter circuit). The load control device may further comprise a control circuit configured to control the drive circuits to cause a present intensity level and a present color temperature of a cumulative light emitted by the light source to be controlled towards a target intensity level and a target color temperature, respectively. When the target intensity level is less than a threshold intensity level, the control circuit is configured to limit the target color temperature according to at least one limit curve that causes the target color temperature to converge towards at least one of a first color temperature or a second color temperature as the target intensity level decreases towards a low-end intensity level.

In addition, a method for controlling an amount of power delivered to a light source having a first emitter circuit configured to emit light having a first color temperature and a second emitter circuit configured to emit light having a second color temperature is disclosed herein. The method may comprise: (1) controlling a first drive circuit to control a first intensity level of the first emitter circuit; (2) controlling a second drive circuit to control a second intensity level of the second emitter circuit; (3) controlling the first and second drive circuits to cause a present intensity level and a present color temperature of a cumulative light emitted by the light source to be controlled towards a target intensity level and a target color temperature, respectively; and (4) when the target intensity level is less than a threshold intensity level, limiting the target color temperature according to at least one limit curve that causes the target color temperature to converge towards at least one of a first color temperature or a second color temperature as the target intensity level decreases towards a low-end intensity level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an example load control system, such as a light-emitting diode driver system.

FIG. 2 illustrates possible operating points that define color temperatures that may be achieved by the light-emitting diode driver system of FIG. 1 at the discrete intensity levels below the threshold intensity level.

FIGS. 3-5 illustrates multiple examples of limit curves for defining adjustment of a target color temperature of a light source when a target intensity level for the light source is less than a threshold intensity level.

FIG. 6 is a flowchart of an example procedure for adjusting a target color temperature of a light source.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of an example load control system, such as a light-emitting diode (LED) driver system 100. The LED driver system 100 may comprise a load control device, such as a driver module 120 (e.g., a dimming module), for controlling a light source 110. The LED driver system 100 may also comprise a power converter module 130 for powering the light source 110 and/or the driver module 120. For example, the light source 110 of the LED driver system 100 may comprise a plurality of emitter circuits, such as emitter circuits 111, 112 (e.g., LED circuits). Each of the emitter circuits 111, 112 may include one or more emitters.

The emitters of each emitter circuit 111, 112 may be electrically coupled together in series and/or parallel connection. As such, the emitters of each emitter circuit 111, 112 may be controlled in unison. The driver module 120 may control the emitter circuits 111, 112 to adjust an intensity level (e.g., lighting intensity level and/or brightness) and/or a color (e.g., a color temperature) of a cumulative light emitted by the light source 110. In some examples, the light source 110, the driver module 120, and the power converter module 130 may be separate devices (e.g., housed in separate enclosures and/or fixtures). Further, the light source 110, the driver module 120, and the power converter module 130 may be housed in a single enclosure, or some combination thereof (e.g., when the LED driver system 100 is a controllable light source). While FIG. 1 shows the light source 110 having the emitter circuits 111, 112, the light source 110 may also comprise more than two emitter circuits.

Each of the emitter circuits 111, 112 is shown in FIG. 1 as a single LED, but, as noted above, may each comprise a plurality of LEDs connected in series (e.g., a string or chain of LEDs), a plurality of LEDs connected in parallel, or a suitable combination thereof, depending on the particular lighting system. The emitter circuits 111, 112 may each represent a string of one or more LEDs, where the LEDs in each string are all configured to emit light at the same color temperature. The strings of LEDs represented by each of the emitter circuits 111, 112 may be configured to emit light at different color temperatures. Further, the emitter circuits of the light source 110 are not limited to LEDs, and in some examples, other technology, such as OLEDs may be used. For example, the light source 110 may comprise tape lighting (e.g., strip lighting) where each of emitter circuits 111, 112 is a separate string of LEDs, each string being of a respective, and different, color temperature for example. When the light source 110 is tape lighting, each tape of lighting may be mounted on the same piece of tape, mounted on different pieces of tape, housed separately, housed together in one housing, or some combination thereof.

Each of the emitter circuits 111, 112 may be configured to emit light at a color temperature (e.g., a different color temperature) that is along a black body locus. For example, the first emitter circuit 111 may represent a string of LEDs at a first color temperature T₁, and the second emitter circuit 112 may represent a string of LEDs at a second color temperature T₂. The first color temperature may be greater than the second color temperature. For example, the first color temperature may be a cool-white color temperature (e.g., such as approximately 3000 K) and the second color temperature may be a warm-white color temperature (e.g., such as approximately 1800 K). Although described in context of these color temperatures, the emitter circuits 111, 112 may be configured to emit light accordingly to any color temperature. Although described as comprising two emitter circuits, the LED driver system 100 may be include more or less than two emitter circuits that are each configured to emit light at different color temperatures (e.g., and that configured with the same or a different number of LEDs). In addition, the LED driver system 100 may include more than the two emitter circuits 111, 112, where several of the emitter circuits are at a first color temperature and one or more of the remaining emitter circuits are at a second different color temperature, for example. Further, as noted herein, each LED of each of the emitter circuits 111, 112 may be configured to emit light at nominal or rated color temperature, for example, as defined by ANSI C78.377-2011.

The power converter module 130 may include a power converter circuit 132, which may receive a source voltage, such as an AC mains line voltage V_AC, via a hot connection H and a neutral connection N. The power converter circuit 132 may generate a DC bus voltage V_BUS (e.g., approximately 15-50V) across a bus capacitor C_BUS. The power converter circuit 132 may comprise, for example, a boost converter, a buck converter, a buck-boost converter, a flyback converter, a single-ended primary-inductance converter (SEPIC), a Ćuk converter, or any other suitable power converter circuit for generating an appropriate bus voltage. The power converter circuit 132 may provide electrical isolation between the AC power source and the driver module 120 and/or the emitter circuits 111, 112. The power converter circuit 132 may also operate as a power factor correction (PFC) circuit to adjust the power factor of the LED driver system 100 towards a power factor of one. Although illustrated as connected to an AC power source (e.g., the AC mains line voltage V_AC), in other examples the LED driver system 100 may be coupled to a direct current (DC) power source. Here, the power converter module 130 may not be needed or may convert a DC source voltage of the DC power source to the DC bus voltage V_BUS (e.g., at a desired magnitude between approximately 15-50V).

The driver module 120 may comprise a plurality of drive circuits, such as LED drive circuits 121, 122 for controlling (e.g., individually controlling) an amount of power delivered to and an individual intensity level L_IND1, L_IND2 (e.g., lighting intensity level and/or luminous flux) of the light emitted by each of the respective emitter circuits 111, 112 of the light source 110. Both of the LED drive circuits 121, 122 may receive the bus voltage V_BUS (e.g., which may be generated by the power converter circuit 132). Each of the LED drive circuits 121, 122 may be configured to adjust (e.g., independently adjust), for example, a magnitude (e.g., an average magnitude) of a respective LED voltage V_LED1, V_LED2 produced across the respective emitter circuit 111, 112. For example, each of the LED drive circuits 121, 122 may be configured to pulse-width modulate the respective LED voltage V_LED1, V_LED2 for adjusting the individual intensity level L_IND1, L_IND2 of the light emitted by the respective emitter circuit 111, 112. In some examples, each of the LED drive circuits 121, 124 may receive the bus voltage V_BUS and may adjust magnitudes (e.g., average magnitudes) of respective LED currents I_LED1, I_LED2 conducted through the emitter circuits 111, 112. Each of the LED drive circuits 121, 122 may comprise a regulation circuit, such as a switching regulator (e.g., a buck converter) for controlling the magnitudes of the respective LED voltages V_LED1, V_LED2 and/or the respective LED currents I_LED1, I_LED2. While FIG. 1 shows the driver module 120 having the LED drive circuits 111, 112, the driver module 120 may also comprise more than two LED drive circuits. In some examples, the driver module 120 may comprise a plurality of LED drive circuits where each of the plurality of LED drive circuits is configured to control an individual intensity level of one of the plurality of emitter circuits of the light source 110.

The driver module 120 may comprise a control circuit 124 for controlling the LED drive circuits 121, 122 to control the individual intensity level L_IND1, L_IND2 of each of the emitter circuits 111, 112 of the light source 110. The control circuit 124 may comprise one or more of, for example, a microprocessor, a microcontroller, a programmable logic device (PLD), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or any other suitable processing device or controller. The control circuit 124 may be configured to control the LED drive circuits 121, 122 to control a present intensity level L_PRES (e.g., a present brightness) and/or a present color temperature T_PRES of a cumulative light emitted by the light source 110. For example, the control circuit 124 may be configured to control the present intensity level L_PRES of the cumulative light emitted by the light source 110 between a high-end intensity level L_HE (e.g., a maximum intensity level, such as approximately 100%) and a low-end intensity level L_LE (e.g., a minimum intensity level, such as approximately 0.1%-1.0%)). The control circuit 124 may be configured to generate one or more drive signals V_DR1, V_DR2 for controlling the respective LED drive circuits 121, 122. The control circuit 124 may be configured to generate each of the one or more drive signals V_DR1, V_DR2 at an operating frequency fop (e.g., approximately 2.05 kHz), such that each of the one or more drive signals V_DR1, V_DR2 are characterized by an operating period Top (e.g., approximately 488 usec). For example, the control circuit 124 may be configured to adjust an on-time T_ON1 of the first drive signal V_DR1 to adjust the individual intensity level L_IND1 of the first emitter circuit 111, and adjust an on-time T_ON2 of the second drive signal V_DR2 to adjust the individual intensity level L_IND2 of the second emitter circuit 112 (e.g., while maintaining the operating frequency fop and/or the operating period Top at constant values). The control circuit 124 may be configured to adjust (e.g., independently adjust) the on-time T_ON1 of the first drive signal V_DR1 and the on-time T_ON2 of the second drive signal V_DR2 to adjust the present intensity level L_PRES and/or the present color temperature T_PRES of the cumulative light emitted by the light source 110.

The control circuit 124 may be characterized by a minimum step size AT-ON (e.g., approximately 26 nsec) in the adjustment of the on-time T_ON1 of the first drive signal V_DR1 and the on-time T_ON2 of the second drive signal V_DR1, which may also establish a minimum on-time TON-MIN to which the control circuit 124 may adjust the on-time T_ON1 of the first drive signal V_DR1 and the on-time T_ON2 of the second drive signal V_DR2. The control circuit 124 may be configured to adjust the target intensity level L_TRGT of the light source 110 to a number NL (e.g., approximately 2000) of discrete intensity levels that are spaced apart (e.g., equally spaced apart) between the high-end intensity level L_HE and the low-end intensity level L_LE, which may result in an intensity level step size AL of approximately 0.05%.

The control circuit 124 may be configured to adjust the present color temperature T_PRES of the cumulative light emitted by the light source 110 by adjusting duty cycles d₁, d₂ of the respective drive signals V_DR1, V_DR2 during a cumulative on-time TON-c of both of the emitter circuits 111, 112. For example, the cumulative on-time TON-c may be the sum of the on-time T_ON1 of the first drive signal V_DR1 and the on-time T_ON2 of the second drive signal V_DR2, such that the duty cycles d₁, d₂ add up to 100%. The control circuit 124 may be configured to determine the duty cycles d₁, d₂ of the respective drive signals V_DR1, V_DR2 for controlling the present color temperature T_PRES of the cumulative light emitted by the light source 110 to the target color temperature T_TRGT. For example, the control circuit 124 may be configured to calculate the duty cycles d₁, d₂ of the respective drive signals V_DR1, V_DR2 based on the target color temperature T_TRGT.

The control circuit 124 may then determine the on-times T_ON1, T_ON2 for the respective drive signals V_DR1, V_DR2 to achieve the target intensity level L_TRGT and the target color temperature T_TRGT. For example, the control circuit 124 may calculate desired on-times T_ON1-D, T_ON2-D for the respective drive signals V_DR1, V_DR2 based on the target intensity level L_TRGT and the duty cycles d₁, d₂ as determined based on the target color temperature T_TRGT, e.g.,

$T_{\mathrm{ON1}-D}=d_1·L_{\mathrm{TRGT}}·T_{\mathrm{OP}},$ $\mathrm{and}$ $T_{\mathrm{ON}2-D}=d_2·L_{\mathrm{TRGT}}·T_{\mathrm{OP}}.$

The control circuit 124 may then determine each of the on-times T_ON1, T_ON2 for the respective drive signals V_DR1, V_DR2 by rounding each of the desired on-times T_ON1-D, T_ON2-D to the closest multiple of the minimum step size Δ_T-ON, e.g.,

$T_{\mathrm{ON}1}=\mathrm{ROUND}\left(T_{\mathrm{ON}1-D}\right)=\alpha ·{\Delta }_{T-\mathrm{ON}},$ $\mathrm{and}$ $T_{\mathrm{ON}2}=\mathrm{ROUND}\left(T_{\mathrm{ON}2-D}\right)=\beta ·{\Delta }_{T-\mathrm{ON}},$

wherein α and β are integer values. The control circuit 124 may then generate the drive signals V_DR1, V_DR2 using the respective on-times T_ON1, T_ON2 to control the cumulative light emitted by the light source 110 to the target intensity level L_TRGT and the target color temperature T_TRGT.

While not shown in FIG. 1, the LED drive circuits 121, 122 may generate one or more feedback signals that may be received by the control circuit 124 and may indicate magnitudes of respective operating characteristics (e.g., LED currents and/or luminous flux) of the respective emitter circuits 111, 112 of the light source 110. In addition, the driver module 120 may comprise one or more feedback circuits (not shown), which may be external to the LED drive circuits 121, 122 and may generate the one or more feedback signals that are received by the control circuit 124. The control circuit 124 may adjust the average magnitude of each of the LED voltages V_LED1, V_LED2 towards respective target voltages in response to the feedback signals. In some examples, the control circuit 124 may adjust each of the average magnitude of each of the LED currents I_LED1, I_LED2 towards respective target currents in response to the feedback signals.

The control circuit 124 may be configured to adjust (e.g., dim) the present intensity level L_PRES of the cumulative light emitted by the light source 110 towards a target intensity level L_TRGT (e.g., a target brightness), which may range across a dimming range of the controllable lighting device, e.g., between the low-end intensity level L_LE and the high-end intensity level L_HE. In some examples, the present intensity level L_PRES of each emitter (e.g., LED) may be dependent upon the magnitude of the LED voltages V_LED1, V_LED2 developed across and/or the LED currents I_LED1, I_LED2 conducted through the emitter circuits 111, 112. The control circuit 120 may be configured to adjust the present color temperature T_PRES of the cumulative light emitted by the light source 110 towards a target color temperature T_TRGT, which may range between a warm-white color temperature (e.g., approximately 1800 K) and/or a cool-white color temperature (e.g., approximately 3000 K). In some examples, the present color temperature T_PRES of the cumulative light emitted by the light source 110 may be dependent upon (e.g., a function of) the magnitude of the LED voltages V_LED1, V_LED2 across and/or the LED currents I_LED1, I_LED2 through the emitter circuit (e.g., and/or the intensity level of the light emitted by the emitter circuit), and the color temperature of each emitter circuit.

The LED driver system 100 may comprise a communication circuit 126 coupled to the control circuit 120. The communication circuit 126 may comprise a wired communication circuit. Alternatively or additionally, the communication circuit 126 may comprise a wireless communication circuit, such as, for example, a radio-frequency (RF) transceiver coupled to an antenna for transmitting and/or receiving RF signals. The wireless communication circuit may be an RF transmitter for transmitting RF signals, an RF receiver for receiving RF signals, or an infrared (IR) transmitter and/or receiver for transmitting and/or receiving IR signals. Alternatively or additionally, the communication circuit 126 may be coupled to the hot connection H and the neutral connection N of the LED driver system 100 for transmitting a control signal via the electrical wiring using, for example, a power-line carrier (PLC) communication technique. The control circuit 124 may be configured to receive and/or determine a commanded intensity level L_CMD and/or a commanded color temperature T_CMD from messages (e.g., digital messages) received via the communication circuit 126. The control circuit 124 may be configured to determine the target intensity level L_TRGT or the target color temperature T_TRGT for the light source 110 in response to the commanded intensity level L_CMD and/or the commanded color temperature T_CMD received via the messages. While not shown in FIG. 1, the driver module 120 may comprise a user interface having one or more actuators (e.g., buttons, sliders, etc.) for receiving user inputs, and the control circuit 124 may be configured to determine the target intensity level L_TRGT or the target color temperature T_TRGT for the light source 110 in response to actuation of the actuators of the user interface.

The LED driver system 100 may comprise a memory 128 configured to store operational characteristics (e.g., such as operational settings, control parameters, operating modes of the LED driver system 100, etc.), association information for associations with other devices, and/or instructions for controlling electrical loads. For example, the memory 128 may be configured to store the target intensity level L_TRGT, the target color temperature T_TRGT, the low-end intensity level L_LE, and/or the high-end intensity level L_HE. The memory 128 may be implemented as an external integrated circuit (IC) or as an internal circuit of the control circuit 124. The memory 128 may comprise a computer-readable storage media or machine-readable storage media that maintains computer-executable instructions for performing one or more procedure and/or functions as described herein. For example, the memory 128 may comprise computer-executable instructions or machine-readable instructions that when executed by the control circuit configure the control circuit to provide one or more portions of the procedures described herein. The control circuit 124 may access the instructions from the memory 128 for being executed to cause the control circuit 124 to operate as described herein, or to operate one or more other devices as described herein. The memory 128 may comprise computer-executable instructions for executing configuration software. For example, the operational characteristics and/or the association information stored in the memory 128 may be configured during a configuration procedure of the LED driver system 100.

The LED driver system 100 may comprise a power supply 129 that may receive the bus voltage V_BUS and generate a supply voltage V_CC for powering the control circuit 124 and other low-voltage circuitry of the LED driver system 100.

When the target intensity level L_TRGT (e.g., as determined from the commanded intensity level L_CMD as received via the communication circuit 126) is greater than (e.g., greater than or equal to) a threshold intensity level LTH (e.g., approximately 1%), the control circuit 124 may be configured to set the target intensity level L_TRGT equal (e.g., approximately equal) to the commanded intensity level L_CMD, and set the target color temperature T_TRGT equal (e.g., approximately equal) to the commanded color temperature L_TRGT. For example, the control circuit 124 may be configured to set the target intensity level L_TRGT to the one of the number NL of intensity levels between the high-end intensity level L_HE and the threshold intensity level LTH that is closest to the commanded intensity level L_CMD (e.g., by rounding the commanded intensity level L_CMD to the closest multiple of the intensity level step size Δ_L). In some examples, the threshold intensity level L_TH may be greater than or less than 1%.

When the target intensity level L_TRGT is near the low-end intensity level L_LE (e.g., when the target intensity level L_TRGT is less than the threshold intensity level L_TH), the control circuit 124 may not be able to control the present color temperature T_PRES of the cumulative light emitted by the light source 110 to each of the color temperatures between the first color temperature T₁ and the second color temperature T₂ that are possible when the target intensity level L_TRGT is greater than (e.g., greater than or equal to) the threshold intensity level L_TH (e.g., due to the limitation of the minimum step size Δ_T-ON to which the control circuit 124 may adjust the on-time T_ON1 of the first drive signal V_DR1 and the on-time T_ON2 of the second drive signal V_DR2). FIG. 2 illustrates possible operating points 200 that define color temperatures that may be achieved at the discrete intensity levels below the threshold intensity level L_TH. As shown FIG. 2, the number of possible operating points 200 (e.g., color temperatures and corresponding intensity levels) that may be achieved at a specific intensity level decreases as the present intensity level L_PRES nears the low-end intensity level L_LE (e.g., approximately 0.1%). At the low-end intensity level L_LE, the control circuit 124 may be configured to control the present color temperature T_PRES of the light emitted by the light source 110 to the first color temperature T₁ by turning on only the first emitter circuit 111 and to the second color temperature T₂ by turning on only the second emitter circuit 112. In addition, at the low-end intensity level L_LE, the control circuit 124 may be configured to control the present color temperature T_PRES of the light emitted by the light source 110 to a third color temperature T₃ (e.g., a midpoint color temperature) between the first color temperature T₁ and the second temperature T₂. For example, the third color temperature T₃ may be approximately 2400 K. At the third color temperature T₃ at the low-end intensity level L_LE, the control circuit 124 may control both of the on-time T_ON1 of the first drive signal V_DR1 and the on-time T_ON2 of the second drive signal V_DR2 to the minimum on time TON-MIN. In other words, when commanded to go to the low-end intensity level L_LE, the control circuit 123 may be configured to control the cumulative light emitted by the light source 110 to only one of three color temperatures. In some examples, when commanded to go to the low-end intensity level L_LE, the control circuit 123 may be configured to control the cumulative light emitted by the light source 110 to only one of two color temperatures (e.g., the first color temperature T₁ and the second temperature T₂).

When the target intensity level L_TRGT is less than the threshold intensity level L_TH, the control circuit 124 may be configured to set the target intensity level L_TRGT equal (e.g., approximately equal) to the commanded intensity level L_CMD (e.g., in the same manner as set when the target intensity level L_TRGT is greater than the threshold intensity level L_TH). In addition, when the target intensity level L_TRGT is less than the threshold intensity level L_TH, the control circuit 124 may be configured to converge the target color temperature T_TRGT towards one or more of the possible color temperatures at the low-end intensity level L_LE (e.g., such as the first, second, and third color temperatures T₁, T₂, T₃) as the target intensity level L_TRGT decreases. For example, the control circuit 124 may be configured to limit the target color temperature T_TRGT according to one or more limit curves when the target intensity level L_TRGT is less than the threshold intensity level L_TH, which may allow the target color temperature T_TRGT to converge towards the one or more of the possible color temperatures at the low-end intensity level L_LE as the target intensity level L_TRGT decreases. The one or more of the possible color temperatures at the low-end intensity level L_LE to which the target color temperature T_TRGT may converge may be dependent upon the commanded color temperature T_CMD. Converging the target color temperature T_TRGT towards one or more color temperatures as the target intensity level L_TRGT decreases towards the low-end intensity level L_LE may cause the target color temperature T_TRGT to change by amounts that are less visibly perceptible and providing smooth dimming near the low-end intensity level L_LE.

FIG. 3 illustrates multiple examples of limit curves 300, 310 for defining adjustment of a target color temperature T_TRGT of a light source (e.g., the light source 110) when a target intensity level L_TRGT for the light source is less than a threshold intensity level L_TH. For example, the target color temperature T_TRGT of the light source may be adjusted by a control circuit of a driver module (e.g., such as the control circuit 124 of the driver module 120 of the LED driver system 100 shown in FIG. 1) according to the limit curves 300, 310 shown in FIG. 3 when the target intensity level L_TRGT is less than the threshold intensity level L_TH. As shown in FIG. 3, the control circuit may be configured to converge the target color temperature T_TRGT towards one of two color temperatures (e.g., the first color temperature T₁ and the second color temperature T₂) as the target intensity level L_TRGT decreases towards the low-end intensity level L_LE. The one of the two color temperatures to which the target color temperature T_TRGT may converge may depend upon the commanded color temperature T_CMD (e.g., depend upon whether the commanded color temperature T_CMD is greater than or less than a threshold color temperature T_TH). For example, the threshold color temperature T_TH may be approximately equal to the third color temperature T₃ (e.g., approximately 2400 K).

When the commanded color temperature T_CMD is greater than (e.g., greater than or equal to) the threshold color temperature T_TH, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the first limit curve 300. At a particular value of the target intensity level L_TRGT, the control circuit may not set the target color temperature T_TRGT to be less than the first limit curve 300 when the commanded color temperature T_CMD is greater than (e.g., greater than or equal to) the threshold color temperature T_TH. For example, the first limit curve 300 may define a linear relationship between the target color temperature T_TRGT and the target intensity level L_TRGT (e.g., a line) that extends from the third color temperature T₃ at the threshold intensity level L_TH to the first color temperature T₁ at the low-end intensity level L_LE. In some examples, the first limit curve 300 may define a non-linear relationship (e.g., having a curved shaped and/or an exponential shape) between the target color temperature T_TRGT and the target intensity level L_TRGT that extends from the third color temperature T₃ at the threshold intensity level L_TH to the first color temperature T₁ at the low-end intensity level L_LE. The first limit curve 300 may cause the target color temperature T_TRGT of the light source to converge toward the first color temperature T₁ as the target intensity level L_TRGT decreases towards the low-end intensity L_LE.

When the commanded color temperature T_CMD is less than the threshold color temperature T_TH, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the second limit curve 310. At a particular value of the target intensity level L_TRGT, the control circuit may not set the target color temperature T_TRGT to be greater than the second limit curve 310 when the commanded color temperature T_CMD is less than the threshold color temperature T_TH. For example, the second limit curve 310 may define a linear relationship between the target color temperature T_TRGT and the target intensity level L_TRGT (e.g., a line) that extends from the third color temperature T₃ at the threshold intensity level L_TH to the second color temperature T₂ at the low-end intensity level L_LE. In some examples, the second limit curve 310 may define a non-linear relationship (e.g., having a curved shaped and/or an exponential shape) between the target color temperature T_TRGT and the target intensity level L_TRGT that extends from the third color temperature T₃ at the threshold intensity level L_TH to the second color temperature T₂ at the low-end intensity level L_LE. The second limit curve 310 may cause the target color temperature T_TRGT of the light source to converge toward the second color temperature T₂ as the target intensity level L_TRGT decreases towards the low-end intensity L_LE.

FIG. 4 illustrates multiple limit curves 410, 420, 430, 440 for defining adjustment of a target color temperature T_TRGT of a light source (e.g., the light source 110) when a target intensity level L_TRGT for the light source is less than a threshold intensity level L_TH. For example, the target color temperature T_TRGT of the light source may be adjusted by a control circuit of a driver module (e.g., such as the control circuit 124 of the driver module 120 of the LED driver system 100 shown in FIG. 1) according to the limit curves 410, 420, 430, 440 shown in FIG. 4 when the target intensity level L_TRGT is less than the threshold intensity level L_TH. As shown in FIG. 4, the control circuit may be configured to converge the target color temperature T_TRGT towards one of three color temperatures (e.g., the first color temperature TI, the second color temperature T₂, and the third color temperature T₃) as the target intensity level L_TRGT decreases towards the low-end intensity level L_LE. The one of the three color temperatures to which the target color temperature T_TRGT may converge may depend upon the commanded color temperature T_CMD (e.g., depend upon whether the commanded color temperature T_CMD is greater than or less than one of two threshold color temperatures T_TH1, T_TH2). For example, the first threshold color temperature T_TH1 may be approximately 2800 K and the second threshold color temperature T_TH2 may be approximately 2000 K.

When the commanded color temperature T_CMD is greater than (e.g., greater than or equal to) the first threshold color temperature T_TH1, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the first limit curve 410. At a particular value of the target intensity level L_TRGT, the control circuit may not set the target color temperature T_TRGT to be less than the first limit curve 410 when the commanded color temperature T_CMD is greater than (e.g., greater than or equal to) the first threshold color temperature T_TH1. For example, the first limit curve 410 may define a linear relationship between the target color temperature T_TRGT and the target intensity level L_TRGT (e.g., a line) that extends from approximately 2800 k at the threshold intensity level L_TH to the first color temperature T₁ at the low-end intensity level L_LE. In some examples, the first limit curve 410 may define a non-linear relationship (e.g., having a curved shaped and/or an exponential shape) between the target color temperature T_TRGT and the target intensity level L_TRGT that extends from approximately 2800 K at the threshold intensity level L_TH to the first color temperature T₁ at the low-end intensity level L_LE. The first limit curve 410 may cause the target color temperature T_TRGT of the light source to converge toward the first color temperature T₁ as the target intensity level L_TRGT decreases towards the low-end intensity L_LE.

When the commanded color temperature T_CMD is less than the second threshold color temperature T_TH2, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the fourth limit curve 440. At a particular value of the target intensity level L_TRGT, the control circuit may not set the target color temperature T_TRGT to be greater than the fourth limit curve 440 when the commanded color temperature T_CMD is less than second threshold color temperature T_TH2. For example, the fourth limit curve 440 may define a linear relationship between the target color temperature T_TRGT and the target intensity level L_TRGT (e.g., a line) that extends from approximately 2000 K at the threshold intensity level L_TH to the second color temperature T₂ at the low-end intensity level L_LE. In some examples, the fourth limit curve 440 may define a non-linear relationship (e.g., having a curved shaped and/or an exponential shape) between the target color temperature T_TRGT and the target intensity level L_TRGT that extends from the approximately 2000 K at the threshold intensity level L_TH to the second color temperature T₂ at the low-end intensity level L_LE. The fourth limit curve 440 may cause the target color temperature T_TRGT of the light source to converge toward the second color temperature T₂ as the target intensity level L_TRGT decreases towards the low-end intensity L_LE.

When the commanded color temperature T_CMD is between the first threshold color temperature T_TH1 and the second threshold color temperature T_TH2, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the second limit curve 420 and the third limit curve 430. The second limit curve 420 and the third limit curve 430 may cause the target color temperature T_TRGT of the light source to converge toward the third color temperature T₃ as the target intensity level L_TRGT decreases towards the low-end intensity L_LE.

When the commanded color temperature T_CMD is less than the first threshold color temperature T_TH1 and greater than (e.g., greater than or equal to) a third threshold color temperature T_TH3, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the second limit curve 420. For example, the third threshold color temperature T_TH3 may be approximately equal to the third color temperature T₃ (e.g., approximately 2400 K). At a particular value of the target intensity level L_TRGT, the control circuit may not set the target color temperature T_TRGT to be greater than the second limit curve 420 when the commanded color temperature T_CMD is less than first threshold color temperature T_TH1 and greater than (e.g., greater than or equal to) the third threshold color temperature T_TH3. For example, the second limit curve 420 may define a linear relationship between the target color temperature T_TRGT and the target intensity level L_TRGT (e.g., a line) that extends from approximately 2800 K at the threshold intensity level L_TH to the third color temperature T₃ at the low-end intensity level L_LE. In some examples, the second limit curve 420 may define a non-linear relationship (e.g., having a curved shaped and/or an exponential shape) between the target color temperature T_TRGT and the target intensity level L_TRGT that extends from the approximately 2800 K at the threshold intensity level L_TH to the third color temperature T₃ at the low-end intensity level L_LE.

When the commanded color temperature T_CMD is less than the third threshold color temperature T_TH3 and greater than (e.g., greater than or equal to) the second threshold color temperature T_TH2, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the third limit curve 430. At a particular value of the target intensity level L_TRGT, the control circuit may not set the target color temperature T_TRGT to be less than the third limit curve 430 when the commanded color temperature T_CMD is less than the third threshold color temperature T_TH3 and greater than (e.g., greater than or equal to) the second threshold color temperature T_TH2. For example, the third limit curve 430 may define a linear relationship between the target color temperature T_TRGT and the target intensity level L_TRGT (e.g., a line) that extends from approximately 2000 K at the threshold intensity level L_TH to the third color temperature T₃ at the low-end intensity level L_LE. In some examples, the third limit curve 430 may define a non-linear relationship (e.g., having a curved shaped and/or an exponential shape) between the target color temperature T_TRGT and the target intensity level L_TRGT that extends from approximately 2000 K at the threshold intensity level L_TH to the third color temperature T₃ at the low-end intensity level L_LE.

FIG. 5 illustrates multiple limit curves 510, 520, 530, 540 for defining adjustment of a target color temperature T_TRGT of a light source (e.g., the light source 110) when a target intensity level L_TRGT for the light source is less than a threshold intensity level L_TH. For example, the target color temperature T_TRGT of the light source may be adjusted by a control circuit of a driver module (e.g., such as the control circuit 124 of the driver module 120 of the LED driver system 100 shown in FIG. 1) according to the limit curves 510, 520, 530, 540 shown in FIG. 5 when the target intensity level L_TRGT is less than the threshold intensity level L_TH. As shown in FIG. 5, the control circuit may be configured to converge the target color temperature T_TRGT towards one of three color temperatures (e.g., the first color temperature T₁, the second color temperature T₂, and the third color temperature T₃) as the target intensity level L_TRGT decreases towards the low-end intensity level L_LE. The one of the three color temperatures to which the target color temperature T_TRGT may converge may depend upon the commanded color temperature T_CMD (e.g., depend upon whether the commanded color temperature T_CMD is greater than or less than one of two threshold color temperatures T_TH1, T_TH2). For example, the first threshold color temperature T_TH1 may be approximately 2800 K and the second threshold color temperature T_TH2 may be approximately 2000 K.

According to the limit curves 510, 520, 530, 540 shown in FIG. 5, the control circuit may be configured to converge the target color temperature T_TRGT towards one of three color temperatures across a smaller range of the target intensity level L_TRGT as the target intensity level L_TRGT decreases towards the low-end intensity level L_LE (e.g., as compared to when the control circuit uses the limit curves 410, 420, 430, 440 shown in FIG. 4). Each of the limit curves 510, 520, 530, 540 may define a linear region between the threshold intensity level L_TH and an intermediate intensity level L_INT where the target color temperature T_TRGT is variable with respect to the target intensity level L_TRGT. When the target intensity level L_TRGT is less than the intermediate intensity level L_INT, the target color temperature T_TRGT may be held constant at one of the three color temperatures (e.g., the first color temperature T₁, the second color temperature T₂, or the third color temperature T₃).

When the commanded color temperature T_CMD is greater than (e.g., greater than or equal to) the first threshold color temperature T_TH1, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the first limit curve 510. At a particular value of the target intensity level L_TRGT, the control circuit may not set the target color temperature T_TRGT to be less than the first limit curve 510 when the commanded color temperature T_CMD is greater than (e.g., greater than or equal to) the first threshold color temperature T_TH1. For example, the first limit curve 510 may define a linear relationship between the target color temperature T_TRGT and the target intensity level L_TRGT (e.g., a line) that extends from approximately 2800 k at the threshold intensity level L_TH to the first color temperature T₁ at the intermediate intensity level L_INT. In some examples, the first limit curve 510 may define a non-linear relationship (e.g., having a curved shaped and/or an exponential shape) between the target color temperature T_TRGT and the target intensity level L_TRGT that extends from approximately 2800 K at the threshold intensity level L_TH to the first color temperature T₁ at the intermediate intensity level L_INT. The first limit curve 510 may cause the target color temperature T_TRGT of the light source to converge toward the first color temperature T₁ as the target intensity level L_TRGT decreases towards the intermediate intensity level L_INT. Below the intermediate intensity level L_INT, the first limit curve 510 may be equal to the first color temperature T₁.

When the commanded color temperature T_CMD is less than the second threshold color temperature T_TH2, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the fourth limit curve 540. At a particular value of the target intensity level L_TRGT, the control circuit may not set the target color temperature T_TRGT to be greater than the fourth limit curve 540 when the commanded color temperature T_CMD is less than second threshold color temperature T_TH2. For example, the fourth limit curve 540 may define a linear relationship between the target color temperature T_TRGT and the target intensity level L_TRGT (e.g., a line) that extends from approximately 2000 K at the threshold intensity level L_TH to the second color temperature T₂ at the intermediate intensity level L_INT. In some examples, the fourth limit curve 540 may define a non-linear relationship (e.g., having a curved shaped and/or an exponential shape) between the target color temperature T_TRGT and the target intensity level L_TRGT that extends from the approximately 2000 K at the threshold intensity level L_TH to the second color temperature T₂ at the intermediate intensity level L_INT. The fourth limit curve 540 may cause the target color temperature T_TRGT of the light source to converge toward the second color temperature T₂ as the target intensity level L_TRGT decreases towards the intermediate intensity level L_INT. Below the intermediate intensity level L_INT, the fourth limit curve 540 may be equal to the second color temperature T₂.

When the commanded color temperature T_CMD is between the first threshold color temperature T_TH1 and the second threshold color temperature T_TH2, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the second limit curve 520 and the third limit curve 530. The second limit curve 520 and the third limit curve 530 may cause the target color temperature T_TRGT of the light source to converge toward the third color temperature T₃ as the target intensity level L_TRGT decreases towards the intermediate intensity level L_INT.

When the commanded color temperature T_CMD is less than the first threshold color temperature T_TH1 and greater than (e.g., greater than or equal to) a third threshold color temperature T_TH3, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the second limit curve 520. For example, the third threshold color temperature T_TH3 may be approximately equal to the third color temperature T₃ (e.g., approximately 2400 K). At a particular value of the target intensity level L_TRGT, the control circuit may not set the target color temperature T_TRGT to be greater than the second limit curve 520 when the commanded color temperature T_CMD is less than first threshold color temperature T_TH1 and greater than (e.g., greater than or equal to) the third threshold color temperature T_TH3. For example, the second limit curve 420 may define a linear relationship between the target color temperature T_TRGT and the target intensity level L_TRGT (e.g., a line) that extends from approximately 2800 K at the threshold intensity level L_TH to the third color temperature T₃ at the intermediate intensity level L_INT. In some examples, the second limit curve 520 may define a non-linear relationship (e.g., having a curved shaped and/or an exponential shape) between the target color temperature T_TRGT and the target intensity level L_TRGT that extends from the approximately 2800 K at the threshold intensity level L_TH to the third color temperature T₃ at the intermediate intensity level L_INT. Below the intermediate intensity level L_INT, the second limit curve 520 may be equal to the third color temperature T₃.

When the commanded color temperature T_CMD is less than the third threshold color temperature T_TH3 and greater than (e.g., greater than or equal to) the second threshold color temperature T_TH2, the control circuit may be configured to limit the target color temperature T_TRGT of the light source according to the third limit curve 530. At a particular value of the target intensity level L_TRGT, the control circuit may not set the target color temperature T_TRGT to be less than the third limit curve 530 when the commanded color temperature T_CMD is less than the third threshold color temperature T_TH3 and greater than (e.g., greater than or equal to) the second threshold color temperature T_TH2. For example, the third limit curve 530 may define a linear relationship between the target color temperature T_TRGT and the target intensity level L_TRGT (e.g., a line) that extends from approximately 2000 K at the threshold intensity level L_TH to the third color temperature T₃ at the intermediate intensity level L_INT. In some examples, the third limit curve 420 may define a non-linear relationship (e.g., having a curved shaped and/or an exponential shape) between the target color temperature T_TRGT and the target intensity level L_TRGT that extends from approximately 2000 K at the threshold intensity level L_TH to the third color temperature T₃ at the intermediate intensity level L_INT. Below the intermediate intensity level L_INT, the third limit curve 530 may be equal to the third color temperature T₃.

FIG. 6 is a flowchart of an example procedure 600 for adjusting a target color temperature T_TRGT of a light source (e.g., the light source 110). The procedure 600 may be executed by a control circuit of a driver module (e.g., such as the control circuit 124 of the driver module 120 of the LED driver system 100 shown in FIG. 1). The light source may comprise a first emitter circuit (e.g., a first LED circuit) configured to emit light at a first color temperature T₁ and a second emitter circuit (e.g., a second LED circuit) configured to emit light at a second color temperature T₂. The control circuit may be configured to generate a first drive signal V_DR1 to control a first LED drive circuit for controlling an intensity level of the first emitter circuit and generate a second drive signal V_DR2 to control a second LED drive circuit for controlling an intensity level of the second emitter circuit. The control circuit may adjust the intensity levels of the first and second emitter circuits of the light source to control a present intensity level L_PRES and a present color temperature T_PRES of a cumulative light emitted by the light source. For example, the control circuit may execute the procedure 600 periodically at 610. In addition, the control circuit may execute the procedure 600 at 610 in response to receiving a message via a communication circuit (e.g., the communication circuit 126) and/or in response to an actuation of an actuator of the driver module.

At 612, the control circuit may receive a commanded intensity level L_CMD and/or a commanded color temperature T_CMD. For example, the control circuit may receive the commanded intensity level L_CMD and/or the commanded color temperature T_CMD in a message received via the communication circuit. At 614, the control circuit may determine a target intensity level L_TRGT in response to the commanded intensity level L_CMD. For example, the control circuit may be configured to control the first and second LED drive circuits to control the present intensity level L_PRES of the cumulative light emitted by the light source towards the target intensity level L_TRGT. The control circuit may be configured to set the target intensity level L_TRGT to the one of a number NL of intensity levels between a high-end intensity level L_HE and a low-end intensity level L_LE that is closest to the commanded intensity level L_CMD (e.g., by rounding the commanded intensity level L_CMD to the closest multiple of an intensity level step size Δ_L).

At 616, the control circuit may determine if the target intensity level L_TRGT is less than a threshold intensity level L_TH (e.g., is near the low-end intensity level L_LE). When the target intensity level L_TRGT is greater than (e.g., greater than or equal to) the threshold intensity level L_TH at 616, the control circuit may set the target color temperature T_TRGT equal to (e.g., approximately equal to) the commanded color temperature T_CMD at 618. When the target intensity level L_TRGT is less than the threshold intensity level L_TH at 616, the control circuit may select one or more limit curves based on the commanded color temperature L_CMD at 620. The control circuit may select, for example, one of the two limit curves 300, 310 shown in FIG. 3. For example, the control circuit may select one of the first limit curve 300 when the commanded color temperature L_CMD is greater than a threshold color temperature T_TH, and the second limit curve 310 when the commanded color temperature is less than the threshold color temperature T_TH. In addition, the control circuit may select, for example, one of the four limit curves 410, 420, 430, 440 shown in FIG. 4 and/or one of the four limit curves 510, 520, 530, 540 shown in FIG. 5.

At 622, the control circuit may determine if the commanded color temperature T_CMD exceeds one of the limit curves selected at 620. For example, the control circuit may determine that the commanded color temperature T_CMD exceeds one of the limit curves when the commanded color temperature is to the left of the select limit curves as shown in FIGS. 3-5. When the commanded color temperature L_CMD is greater than the threshold color temperature T_TH, the control circuit may determine, for example, that the commanded color temperature T_CMD exceeds the first limit curve 300 (e.g., as shown in FIG. 3) when the commanded color temperature L_CMD is less than the first limit curve 300 at the target intensity level L_TRGT. When the commanded color temperature L_CMD is less than the threshold color temperature T_TH, the control circuit may determine, for example, that the commanded color temperature T_CMD exceeds the second limit curve 310 (e.g., as shown in FIG. 3) when the commanded color temperature L_CMD is greater than the second limit curve 310 at the target intensity level L_TRGT. The control circuit may similarly determine that the commanded color temperature T_CMD has exceeded the limit curves 410-440 (e.g., as shown in FIG. 4) and the limit curves 510-540 (e.g., as shown in FIG. 5).

When the control circuit determines that the commanded color temperature T_CMD does exceeds the selected limit curve at 622, the control circuit may set the target color temperature T_TRGT equal to (e.g., approximately equal to) the commanded color temperature T_CMD at 618. When the control circuit determines that the commanded color temperature T_CMD exceeds the limit curve selected at 622, the control circuit may limit the target color temperature T_TRGT based on the limit curves at 624. For example, the control circuit may set the target color temperature T_TRGT equal to a value of a limited color temperature on the selected limit curve at the target intensity level L_TRGT (e.g., as determined at 614). When the commanded color temperature L_CMD is greater than the threshold color temperature T_TH, the control circuit may set the target color temperature T_TRGT equal to, for example, the value of the limited color temperature on the first limit curve 300 at the target intensity level L_TRGT. When the commanded color temperature L_CMD is less than the threshold color temperature T_TH, the control circuit may set the target color temperature T_TRGT equal to, for example, the value of the limited color temperature on the second limit curve 310 at the target intensity level L_TRGT. The control circuit may similarly set the target color temperature T_TRGT equal to values of limited color temperatures on the limit curves 410-440 (e.g., as shown in FIG. 4) and the limit curves 510-540 (e.g., as shown in FIG. 5).

After the target color temperature T_TRGT is set at 618 or 624, the control circuit may determine duty cycles di, d₂ of the respective drive signals V_DR1, V_DR2 for controlling the present color temperature T_PRES of the light emitted by the light source to the target color temperature T_TRGT at 626. For example, the control circuit 124 may be configured to calculate the duty cycles di, d₂ of the respective drive signals V_DR1, V_DR2 based on the target color temperature T_TRGT. At 628, the control circuit may calculate desired on-times T_ON1-D, T_ON2-D for the respective drive signals V_DR1, V_DR2 based on the target intensity level L_TRGT and the duty cycles d₁, d₂ as determined based on the target color temperature T_TRGT (e.g., T_ON1-D=d₁·L_TRGT·T_OP, and T_ON2-D=d₂·L_TRGT·T_OP). At 630, the control circuit may determine on-times T_ON1, T_ON2 for the respective drive signals V_DR1, V_DR2 by rounding each of the desired on-times T_ON1-D, T_ON2-D to the closest multiple of a minimum step size Δ_T-ON. Rounding to the closest multiple of the minimum step size Δ_T-ON may cause the control circuit to control the present color temperature T_PRES to the closest operating point (e.g., one of the operating points 200 shown in FIG. 2) to the target color temperature T_TRGT at the target intensity level L_TRGT. At 632, the control circuit may generate the drive signals V_DR1, V_DR2 using the respective on-times T_ON1, T_ON2 to control the cumulative light emitted by the light source to the target intensity level L_TRGT and the target color temperature T_TRGT.

Claims

1. A load control device for controlling an amount of power delivered to a light source having a first emitter circuit configured to emit light having a first color temperature and a second emitter circuit configured to emit light having a second color temperature, the load control device comprising: a first drive circuit configured to control a first intensity level of the first emitter circuit;a second drive circuit configured to control a second intensity level of the second emitter circuit; anda control circuit configured to control the first drive circuit and the second drive circuit to adjust the first intensity level of the first emitter circuit and the second intensity level of the second emitter circuit to cause a present intensity level and a present color temperature of a cumulative light emitted by the light source to be controlled towards a target intensity level and a target color temperature, respectively;wherein, when the target intensity level is less than a threshold intensity level, the control circuit is configured to limit the target color temperature according to at least one limit curve that causes the target color temperature to converge towards at least one of a first color temperature or a second color temperature as the target intensity level decreases towards a low-end intensity level.

2. The load control device of claim 1, wherein the control circuit is configured to: receive a commanded color temperature for controlling a cumulative light emitted by the light source;when the commanded color temperature is greater than a threshold color temperature, limit the target color temperature according to a first limit curve that causes the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level; andwhen the commanded color temperature is less than the threshold color temperature, limit the target color temperature according to a second limit curve that causes the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level.

3. The load control device of claim 2, wherein the control circuit is configured to: cause the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level by setting the target color temperature equal to a value of a first limited color temperature on the first limit curve at the target intensity level when the commanded color temperature exceeds the first limit curve; andcause the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level by setting the target color temperature equal to a value of a second limited color temperature on the second limit curve at the target intensity level when the commanded color temperature exceeds the second limit curve.

4. The load control device of claim 3, wherein the control circuit is configured to: set the target color temperature equal to the commanded color temperature when the commanded color temperature is greater than the threshold color temperature and the commanded color temperature does not exceed the first limit curve; andset the target color temperature equal to the commanded color temperature when the commanded color temperature is less than the threshold color temperature and the commanded color temperature does not exceed the second limit curve.

5. The load control device of claim 3, wherein the control circuit is configured to: determine that the commanded color temperature exceeds the first limit curve when the commanded color temperature is greater than the threshold color temperature and the commanded color temperature is less than the first limit curve at the target intensity level; anddetermine that the commanded color temperature exceeds the second limit curve when the commanded color temperature is less than the threshold color temperature and the commanded color temperature is less than the second limit curve at the target intensity level.

6. The load control device of claim 2, further comprising: a communication circuit configured to receive messages;wherein the control circuit is configured to receive a message including the commanded color temperature.

7. The load control device of claim 1, wherein the control circuit is configured to: receive a commanded color temperature for controlling a cumulative light emitted by the light source;when the commanded color temperature is greater than a first threshold color temperature, limit the target color temperature according to a first limit curve that causes the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level;when the commanded color temperature is less than a second threshold color temperature, limit the target color temperature according to a second limit curve that causes the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level; andwhen the commanded color temperature is between the first and second threshold color temperatures, limit the target color temperature according to a third and fourth limit curves that cause the target color temperature to converge towards a third color temperature as the target intensity level decreases towards the low-end intensity level, wherein the third color temperature is between the first and second color temperatures.

8. The load control device of claim 7, wherein the control circuit is configured to: cause the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level by setting the target color temperature equal to the value of the first limited color temperature on the first limit curve at the target intensity level when the commanded color temperature exceeds the first limit curve;cause the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level by setting the target color temperature equal to the value of the second limited color temperature on the second limit curve at the target intensity level when the commanded color temperature exceeds the second limit curve; andcause the target color temperature to converge towards the third color temperature as the target intensity level decreases towards the low-end intensity level by setting the target color temperature equal to a value of a third limited color temperature on the third limit curve at the target intensity level when the commanded color temperature is greater than the third color temperature and the commanded color temperature exceeds the third limit curve, and setting the target color temperature equal to a value of a fourth limited color temperature on the fourth limit curve at the target intensity level when the commanded color temperature is less than the third color temperature and the commanded color temperature exceeds the fourth limit curve.

9. The load control device of claim 7, wherein the control circuit is configured to cause the target color temperature to converge to the first, second, and third color temperatures when the target intensity level is equal to an intermediate intensity level that is less than the threshold intensity level and greater than the low-end intensity level.

10. The load control device of claim 1, wherein control circuit configured to: generate first and second drive signals for controlling the first and second drive circuits, respectively;determine respective on-times for the first and second drive signals in response to the target intensity level and the target color temperature; andround the respective on-times for the first and second drive signals to multiples of a minimum step size at which the control circuit may adjust the respective on-times of the first and second drive signals.

11. A method for controlling an amount of power delivered to a light source having a first emitter circuit configured to emit light having a first color temperature and a second emitter circuit configured to emit light having a second color temperature, the method comprising: controlling a first drive circuit to control a first intensity level of the first emitter circuit;controlling a second drive circuit to control a second intensity level of the second emitter circuit;controlling the first and second drive circuits to cause a present intensity level and a present color temperature of a cumulative light emitted by the light source to be controlled towards a target intensity level and a target color temperature, respectively; andwhen the target intensity level is less than a threshold intensity level, limiting the target color temperature according to at least one limit curve that causes the target color temperature to converge towards at least one of a first color temperature or a second color temperature as the target intensity level decreases towards a low-end intensity level.

12. The method of claim 11, further comprising: receiving a commanded color temperature for controlling a cumulative light emitted by the light source.

13. The method of claim 12, wherein limiting the target color temperature further comprises: when the commanded color temperature is greater than a threshold color temperature, limiting the target color temperature according to a first limit curve that causes the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level; andwhen the commanded color temperature is less than the threshold color temperature, limiting the target color temperature according to a second limit curve that causes the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level.

14. The method of claim 13, wherein limiting the target color temperature according to the first limit curve further comprises setting the target color temperature equal to a value of a first limited color temperature on the first limit curve at the target intensity level when the commanded color temperature exceeds the first limit curve; and wherein limiting the target color temperature according to the second limit curve further comprises setting the target color temperature equal to a value of a second limited color temperature on the second limit curve at the target intensity level when the commanded color temperature exceeds the second limit curve.

15. The method of claim 14, further comprising: setting the target color temperature equal to the commanded color temperature when the commanded color temperature is greater than the threshold color temperature and the commanded color temperature does not exceed the first limit curve; andsetting the target color temperature equal to the commanded color temperature when the commanded color temperature is less than the threshold color temperature and the commanded color temperature does not exceed the second limit curve.

16. The method of claim 14, further comprising: determining that the commanded color temperature exceeds the first limit curve when the commanded color temperature is greater than the threshold color temperature and the commanded color temperature is less than the first limit curve at the target intensity level; anddetermining that the commanded color temperature exceeds the second limit curve when the commanded color temperature is less than the threshold color temperature and the commanded color temperature is less than the second limit curve at the target intensity level.

17. The method of claim 12, wherein limiting the target color temperature further comprises: when the commanded color temperature is greater than a first threshold color temperature, limiting the target color temperature according to a first limit curve that causes the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level;when the commanded color temperature is less than a second threshold color temperature, limiting the target color temperature according to a second limit curve that causes the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level; andwhen the commanded color temperature is between the first and second threshold color temperatures, limiting the target color temperature according to a third and fourth limit curves that cause the target color temperature to converge towards a third color temperature as the target intensity level decreases towards the low-end intensity level, wherein the third color temperature is between the first and second color temperatures.

18. The method of claim 17, further comprising: causing the target color temperature to converge to the first, second, and third color temperatures when the target intensity level is equal to an intermediate intensity level that is less than the threshold intensity level and greater than the low-end intensity level.

19. The method of claim 12, further comprising: receiving a message including the commanded color temperature.

20. The method of claim 11, further comprising: generating first and second drive signals for controlling the first and second drive circuits, respectively;determining respective on-times for the first and second drive signals in response to the target intensity level and the target color temperature; androunding the respective on-times for the first and second drive signals to multiples of a minimum step size at which the control circuit may adjust the respective on-times of the first and second drive signals.

21-40. (canceled)