Patent Publication Number: US-2022240360-A1

Title: Load control device for controlling a driver for a lighting load

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/081,968, filed Oct. 27, 2020; which is a continuation of U.S. patent application Ser. No. 16/669,477, filed Oct. 30, 2019 (now U.S. Pat. No. 10,827,587), which is a continuation of U.S. patent application Ser. No. 16/183,565, filed Nov. 7, 2018 (now U.S. Pat. No. 10,492,255), which is a continuation of U.S. patent application Ser. No. 15/713,543, filed Sep. 22, 2017 (now U.S. Pat. No. 10,149,355), which claims priority to Provisional U.S. Patent Application No. 62/398,636, filed Sep. 23, 2016, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     A lighting source, such as a light-emitting diode (LED) light source, is typically driven by a load regulation device (e.g., such as an LED driver) in order to illuminate. A common control method for dimming an LED light source controlled by an LED driver is “zero-to-ten-volt” (0-10V) control, which is sometimes referred to as 1-10V control. A 0-10V LED driver receives power from an AC power source, with an external mechanical switch typically coupled between the AC power source and the 0-10V driver to provide a switched-hot voltage to the driver. Alternatively, the switched-hot voltage may be generated by an external power device (e.g., a power pack). The 0-10V driver controls the intensity of the connected LED light source in response to a 0-10V control signal received from a 0-10V control device (e.g., a 0-10V controller). Often, the 0-10V control device is mounted in an electrical wallbox and comprises an intensity adjustment actuator (e.g., a slider control). The 0-10 V control device regulates the direct-current (DC) voltage level of the 0-10V control signal provided to the driver between a substantially low voltage (e.g., zero to one volt) to a maximum voltage (e.g., approximately ten volts) in response to an actuation of the intensity adjustment actuator. For example, the 0-10V driver may control the intensity of the LED light source to a low-end intensity L LE  (e.g., approximately 0.1%-10%) when the DC voltage level of the 0-10V control signal is at the substantially low voltage (e.g., zero to one volt) and to a high-end intensity L HE  (e.g., approximately 100%) when the DC voltage level of the 0-10V control signal is at the maximum voltage (e.g., approximately ten volts). 
     To turn off the LED light source controlled by the 0-10V driver, power is removed from the 0-10V driver by, for example, controlling the switched-hot voltage to zero volts. The 0-10V control device may comprise a switching circuit for generating the switched-hot voltage. The switching circuit may include, for example, a mechanical air-gap switch, a relay, and/or a bidirectional semiconductor switch, such as a triac, one or more silicon-controlled rectifiers (SCRs), a field-effect transistor (FET) in a rectifier bridge, two FETs in anti-series connection, one or more insulated-gate bipolar junction transistors (IGBTs), or any suitable semiconductor switching circuit. In some cases, the 0-10V control device may be powered via the 0-10V control wires, for example, by drawing current from the 0-10V driver. Prior art 0-10V drivers typically source between 1-2 milliamperes of current, which the 0-10V control device may use to power itself. 
     Some 0-10 V drivers may be responsive to occupancy sensors, vacancy sensors, and/or daylight sensors. If the switched-hot voltage is controlled to zero volts to turn off the LED light source (e.g., by opening the switching circuit of the 0-10V control device or the power pack), the 0-10V driver will then be unpowered and unable to respond to the occupancy sensors, vacancy sensors, and/or daylight sensors. 
     Rather than removing power from an 0-10V driver to turn off the LED light source, the 0-10V driver may be controlled to an electronic off (e.g., standby) state in which the 0-10V driver remains powered, but turns off the LED light source. The 0-10V driver may be configured to change between an on state and the electronic off state in response to the 0-10V signal (e.g., using hysteresis). For example, during the on state, the 0-10V control device may be configured to adjust the DC voltage level of the 0-10V control signal between a minimum level (e.g., approximately 0.61-1.00 volts) and a maximum level (e.g., approximately ten volts) to adjust the intensity of the LED light source between the low-end intensity L LE  and the high-end intensity L H E, respectively. To control the 0-10V driver into the electronic off state, the 0-10V control device may be configured to adjust the DC voltage level of the 0-10V control signal to a standby level. For example, the 0-10V driver may be configured to change to the electronic off state when the DC voltage level of the 0-10V control signal drops below a falling threshold (e.g., approximately 0.6 V). The 0-10V driver may be configured to return to the on state (e.g., to turn on) when the DC voltage level of the 0-10V control signal rises above a rising threshold (e.g., approximately 1.0 V), after which the 0-10V driver may adjust the intensity of the LED light source between the low-end intensity L LE  and the high-end intensity L HE  as the 0-10V control signal ranges between the minimum level and the maximum level. 
     Since the falling threshold may be approximately 0.6 V, the DC voltage level of 0-10V control signal may be as low as 0.61 V when the 0-10V driver is being controlled to the low-end intensity L LE . This means that the DC voltage level of 0-10V control signal at the low-end intensity L LE  may be between the rising threshold and the falling threshold. If there is a momentary interruption of the power, such as a power outage or a manual switch-off of power to the 0-10V driver when the 0-10V driver is in the on state, and the DC voltage level of the 0-10V control signal is between the rising threshold and the falling threshold, the 0-10V driver may not turn back on when power is restored (e.g., re-applied) because the DC voltage level of 0-10V control signal will not be above the rising threshold. It is undesirable for a lighting load that is on to not turn back after a momentary power interruption. 
     SUMMARY 
     As described herein, a load control device for controlling an amount of power delivered to a lighting load may comprise a communication circuit configured to generate a control signal for controlling the amount of power delivered to the lighting load. The control signal may cause the lighting load to be turned on when the magnitude of the control signal rises above a threshold. The load control device may also comprise a control circuit configured to control the communication circuit to adjust the magnitude of the control signal so as to adjust an intensity of the lighting load between a low-end intensity and a high-end intensity. The magnitude of the control signal may be less than the threshold when the intensity of the lighting load is being controlled to the low-end intensity. When power has been applied to the lighting load, the control circuit may be configured to determine that a desired magnitude of the control signal is below the threshold, and increase the magnitude of the control signal to be equal to or greater than the threshold before decreasing the magnitude of the control signal to the desired magnitude. 
     The load control device described herein may include an intensity adjustment actuator and a potentiometer circuit responsive to the intensity adjustment actuator for determining the desired magnitude of the control signal. The load control device may further include a sense circuit configured to provide an indication of when power has been applied to the lighting load. Once power has been applied to the lighting load and the magnitude of the control signal is set to be equal to or greater than the threshold, the control circuit of the load control device may cause the magnitude of the control signal to be decreased to the desired magnitude over a first period of time. The control circuit may maintain the magnitude of the control signal constant at a level equal to or greater than the threshold for a second period of time before decreasing the magnitude of the control signal to the desired magnitude over the first period of time. 
     Other features and advantages of the present invention will become apparent from the following description of the invention that refers to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of an example 0-10V load control device. 
         FIG. 2  is a simplified flowchart of a voltage sense procedure that may be executed by a microprocessor of a control circuit of the load control device of  FIG. 1 . 
         FIG. 3  is a simplified diagram of example waveforms illustrating the operation of the load control device during the voltage sense procedure of  FIG. 2 . 
         FIG. 4  is a simplified block diagram of another example 0-10V load control device. 
         FIG. 5  is a simplified diagram of example waveforms illustrating the operation of a bump-up circuit of the load control device of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Described herein are examples of a load control system for controlling the amount of power delivered to an electrical load, such as a lighting load, and more particularly, of a wall-mounted load control device for controlling a load regulation device, such as an LED driver for an LED light source, via a control signal, such as a 0-10V control signal. 
       FIG. 1  is a simplified block diagram of an example 0-10V load control device  100 . The load control device  100  may comprise a hot terminal H adapted to be coupled to an AC power source  102  and a switched hot terminal SH adapted to be coupled to an electrical load. The electrical load may comprise a load regulation circuit for driving a lighting load, such as an LED driver  104  for controlling an LED light source  106 . In an example, the load control device  100  may comprise a neutral terminal N adapted to be coupled to the neutral side of the AC power source  102 . In another example, the load control device  100  may not require connection to the neutral side of the AC power source  102  via the neutral terminal N (e.g., the load control device may be a “two-wire” load control device). 
     The load control device  100  may comprise first and second control terminals C 1 , C 2  adapted to be coupled to the LED driver  104  via a control wiring  108 . The LED driver  104  may be configured to control the power delivered to the LED light source  106 , and thus the intensity of the LED light source  106 , in response to a direct-current (DC) control signal V CS  received from the load control device  100  via the control wiring  108 . For example, the LED driver  104  may be configured to turn the LED light source  106  on and off, and/or to adjust the intensity of the LED light source  106  between a low-end (e.g., minimum) intensity L LE  and a high-end (e.g., maximum) intensity L HE  in response to the control signal V CS . The LED driver  104  may be configured to control the power delivered to the LED light source  106 , for example, by regulating the voltage generated across the LED light source  106  and/or regulating the current conducted through the LED light source  106 . Examples of an LED driver are described in greater detail in commonly-assigned U.S. Pat. No. 8,492,987, issued Jul. 23, 2013, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, and U.S. Pat. No. 9,232,574, issued Jan. 5, 2016, entitled FORWARD CONVERTER HAVING A PRIMARY-SIDE CURRENT SENSE CIRCUIT, the entire disclosures of which are hereby incorporated by reference. Although described as an LED light source driven by an LED driver, the electrical load referenced herein may comprise an electronic ballast for driving a fluorescent lamp. 
     The load control device  100  may comprise a switching circuit  110 , which may be electrically coupled in series between the hot terminal H and the switched hot terminal SH. The switching circuit  110  may be rendered conductive and non-conductive in response to actuations of an on/off actuator  112  (e.g., a toggle switch) to generate a switched-hot voltage V SH  at the switched hot terminal SH. The on/off actuator  112  may comprise a mechanical switch that is actuated by a slider control, for example, when the slider control reaches a minimum position (e.g., a “slide-to-off” slider control). 
     The load control device  100  may also include a driver communication circuit  114  that may comprise a current sink circuit adapted to sink current from the LED driver  104  via the control wiring  108 . The LED driver  104  may be configured to generate a link supply voltage (e.g., approximately 10 V) to allow the current sink circuit of the driver communication circuit  114  to generate the control signal V CS  on the control wiring  108 . The load control device  100  may comprise a power supply  116  coupled between the hot terminal H and the neutral terminal N for generating a DC supply voltage V CC  for powering the low-voltage circuitry of the load control device  100 . 
     The load control device  100  may comprise a control circuit  120  (e.g., a digital control circuit) configured to control the driver communication circuit  114  to generate the control signal V CS  for adjusting the intensity of the LED light source  106 . The control circuit  120  may include a microprocessor  122 . The control circuit  120  could also include any suitable controller or processing device, such as, for example, a programmable logic device (PLD), a microcontroller, an application specific integrated circuit (ASIC), or a field programmable gate array (FPGA). The microprocessor  122  may be configured to determine a desired light intensity L DE S for the LED light source  106  and a corresponding desired magnitude V DES  for the control signal V CS  in response to an intensity adjustment actuator  124  (e.g., a slider control). For example, the microprocessor  122  may be configured to receive a DC potentiometer wiper voltage V POT  from a potentiometer circuit  126 , which may be responsive to the intensity adjustment actuator  124 . The microprocessor  122  may be configured to control the magnitude of the control signal V CS  to the desired magnitude V DES  so as to adjust the intensity of the LED light source  106  to the desired light intensity L DES  (e.g., between a low-end intensity L LE  and a high-end intensity L HE ). 
     The microprocessor  122  of the control circuit  120  may generate a direct-current (DC) output signal V DC  and provide the output signal V DC  to the driver communication circuit  114 . For example, the microprocessor  122  may comprise a digital-to-analog converter (DAC) for generating the DC output signal V DC  that is received by the driver communication circuit  114  for generating the control signal V CS . The microprocessor  122  may adjust the magnitude of the control signal V CS  by adjusting the magnitude of the output signal V DC . The output signal V DC  may comprise a pulse-width modulated (PWM) signal or variable-frequency waveform, in response to which the driver communication circuit  114  may be configured to adjust the magnitude of the control signal V CS . The driver communication circuit  114  may comprise a current source circuit or a current source/sink circuit for generating the control signal V CS  in response to the output signal V DC . 
     The LED driver  104  may be controlled to an electronic off (e.g., standby) state during which the LED driver  104  may turn off the LED light source while control circuitry of the LED driver remains powered. The LED driver  104  may be configured to change between an on state and the electronic off state in response to the control signal V CS  (e.g., using hysteresis). For example, during the on state, the control circuit  120  may be configured to adjust the DC voltage level of the control signal V CS  between a low-end magnitude V LE  (e.g., approximately 0.9 volts) and a high-end magnitude V HE  (e.g., ten volts) to adjust the intensity of the LED light source  106  between the low-end intensity L LE  and the high-end intensity L HE , respectively. To control the LED driver  104  into the electronic off state, the control circuit  120  may be configured to adjust the DC voltage level of the control signal V CS  to a standby level. For example, the LED driver  104  may be configured to change to the electronic off state when the DC voltage level of the 0-10V control signal drops below a falling threshold V TH-FALLING  (e.g., approximately 0.6 V). The LED driver  104  may be configured to return to the on state (e.g., to turn on) when the DC voltage level of the control signal V CS  rises above a rising threshold V TH-RISING  (e.g., approximately 1.0 V), after which the LED driver  104  may adjust the intensity of the LED light source  106  between the low-end intensity L LE  and the high-end intensity L HE  as the control signal V CS  ranges between the low-end magnitude V LE  and the high-end magnitude V HE . 
     During the on state, the low-end magnitude V LE  of the control signal V CS  may be less than the rising threshold V TH-RISING . For example, the low-end magnitude V LE  of the control signal V CS  may be approximately 0.9 V while the rising threshold V TH-RISING  may be approximately 1.0 V. If the LED driver  104  and the load control device  100  temporarily lose power while the load control device is controlling the intensity of the LED light source  106  to the low-end intensity L LE , the magnitude of the control signal V CS  (e.g., the low-end magnitude V LE ) may not exceed the rising threshold V TH-RISING  when power is restored and the LED driver  104  may not turn on the LED light source  106 . Similarly, when the on/off actuator  112  is actuated to close the switching circuit  110  to turn the LED light source  106  on and the intensity adjustment actuator  122  is set to the low-end intensity L LE , the magnitude of the control signal V CS  may also not exceed the rising threshold V TH-RISING  and the LED driver  104  may not turn on the LED light source  106  when the LED driver  104  is switched on by the switching circuit  110 . 
     Accordingly, the control circuit  120  may be configured to at least temporarily increase the magnitude of the control signal V CS  when power is applied (e.g., initially applied or re-applied) to the lighting load  106  (i.e., to the LED driver  104 ). For example, the control circuit  120  may be configured to temporarily increase the magnitude of the control signal V CS  to be equal to or above the rising threshold V TH-RISING  when power is applied to the LED driver after an interruption and the desired magnitude V DE S for the control signal V CS  is initially less than the rising threshold V TH-RISING . The power interruption may be caused by a power outage or a manual switch-off of the on/off actuator  112 , for example. The control circuit  120  may comprise a voltage sense circuit  128  configured to generate a voltage sense signal V SENSE  that may indicate when power has been applied to the LED driver  104 . For example, the voltage sense circuit  128  may be coupled between the switched hot terminal SH and the neutral terminal N to receive the switched-hot voltage V SH  as shown in  FIG. 1 . The voltage sense circuit  128  may be configured to drive the voltage sense signal V SENSE  high towards the supply voltage V CC  when the magnitude of the switched-hot voltage V SH  rises above a voltage sense threshold V TH-SENSE  (e.g., the voltage sense circuit  128  may comprise a comparator circuit). In an example, the control circuit  120  may be configured to determine that power has just been applied to the LED driver  104  in response to detecting a rising edge of the voltage sense signal V SENSE . In another example, the voltage sense signal V SENSE  may simply be a scaled version of the switched-hot voltage V SH  (e.g., the voltage sense circuit  128  may comprise a scaling circuit, such as a resistive divider), and the control circuit  120  may be configured to sample the voltage sense signal V SENSE  and compare the sampled magnitude to the voltage sense threshold V TH-SENSE  to determine when power has just been applied (or re-applied) to the LED driver  104 . 
     While the switching circuit  110  and the on/off actuator  112  is shown in  FIG. 1  as integral with the load control device  100 , the switching circuit and/or the on/off actuator  112  could be external to the load control device  100  (e.g., the switching circuit could be included in an external light switch or an external switching power pack). In addition, the switching circuit  110  could comprise a relay controlled by the microprocessor  122  and the on/off actuator  112  could comprise a low-voltage switch (e.g., a mechanical tactile switch) for generating a low-voltage signal that is received by the microprocessor  122 . The microprocessor  122  may be configured to detect that the low-voltage switch has been actuated, close the relay, and temporarily increase the magnitude of the control signal V CS  (e.g., without the need for the voltage sense circuit  128 ). 
     As described herein, power being applied (e.g., initially applied or re-applied) to the lighting load  106  may occur when power is restored after a temporary power interruption (e.g., by an electrical utility company), when the switching circuit  110  is closed, and/or when an external switching circuit (e.g., in a light switch or a switching power pack) coupled in series between the AC power source  102  and the lighting load  106  is closed. One of ordinary skill in the art will recognize that there are other ways that power may be applied to a lighting load. 
       FIG. 2  is a simplified flowchart of a voltage sense procedure  200  that may be executed by the microprocessor  122  of the control circuit  120  of the load control device  100 .  FIG. 3  is a simplified diagram of example waveforms illustrating the operation of the load control device  100  during the voltage sense procedure  200 . The voltage sense procedure  200  may begin when the microprocessor  122  detects a rising edge of the voltage sense signal V SENSE  at step  210  indicating that power has just been applied to the LED driver  104  (e.g., as shown at time t RISING  in  FIG. 3 ), e.g., after an interruption. The microprocessor  122  may then determine the desired magnitude V DES  for the control signal V CS  (e.g., using the desired light intensity L DE S determined from the intensity adjustment actuator  122 ) at step  212 . If the desired magnitude V DES  is not less than the rising threshold V TH-RISING  at step  214 , the microprocessor  122  may set the magnitude of the control signal V CS  to the desired magnitude V DES  at step  216 , before the voltage sense procedure  200  exits. If the desired magnitude V DES  is less than the rising threshold V TH-RISING  at step  214 , the microprocessor  122  may set the magnitude of the control signal V CS  to be equal to the rising threshold V TH-RISING  plus an offset voltage V OFFSET  at step  218 . For example, the offset voltage V OFFSET  may be sized to ensure that the magnitude of the control signal V CS  is greater than the rising threshold V TH-RISING  when power is applied to the LED driver  104  (e.g., as shown at time t RISING  in  FIG. 3 ) so as to drive the LED driver  104  to the on state. The microprocessor  122  may then fade (e.g., adjust) the magnitude of the control signal V CS  to the desired magnitude V DES  (e.g., the low-end magnitude V LE  as shown in  FIG. 3 ) over a first period of time T FADE , which may be approximately 0.5-1 second, at step  220 , before the voltage sense procedure  200  exits. While not shown in  FIG. 2 , the microprocessor  122  may hold the magnitude of the control signal V CS  equal to the rising threshold V TH-RISING  plus the offset voltage V OFFSET  for a second period of time before beginning to fade the magnitude of the control signal V CS  to the desired magnitude V DES  over the period of time T FADE . 
     The operation of the control circuit  120  in response to the application of power to the LED driver  104  may be controllable and/or programmable. For example, the control circuit  120  may be configured to adjust the magnitude of the offset voltage V OFFSET , and/or the length of the first and/or the second time period (e.g., the period of time T FADE ), in response to an external input (e.g., a programming input). The external input may be received, for example, from an actuation of the intensity adjustment actuator  124  and/or the on/off actuator  112 , an actuation of one or more programming buttons (not shown), an actuation of one or more separate programming potentiometers (not shown), and/or one or more messages received via a communication circuit (not shown). 
       FIG. 4  is a simplified block diagram of another example of a 0-10V load control device  300 . The load control device  300  may be configured to control an amount of power delivered to an electrical load. The electrical load may include, e.g., a load regulation circuit for driving the electrical load, such as an LED driver  304  for controlling an LED light source  306 . The load control device  300  may or may not be electrically coupled in series between an AC power source  302  and the electrical load. The load control device  300  may comprise first and second control terminals C 1 , C 2  adapted to be coupled to the LED driver  304  via a control wiring  308 . The load control device  300  may comprise a communication circuit configured to generate a control signal for controlling power delivered to the LED lighting load  306 . The load control device  300  may include, for example, a current sink circuit  310  electrically coupled to the control terminals C 1 , C 2  for sinking current from the LED driver  104  via the control wiring  108 . The current sink circuit  310  may be configured to generate a DC control signal V CS  for controlling the LED driver  304  to turn the LED light source  306  on and off, and to adjust the intensity of the LED light source  306  when the LED light source  306  is on. 
     The load control device  300  may comprise a control circuit  320  (e.g., an analog control circuit) configured to control the current sink circuit  310  to generate the control signal V CS  for turning the LED light source  306  on and off, and for adjusting the intensity of the LED light source  306 . The control circuit  320  may comprise a potentiometer circuit  322  for generating a DC output signal V DC  in response to an intensity adjustment actuator  324  (such as, e.g., a slider control, a thumbwheel, or a knob). The potentiometer circuit  322  may provide the DC output signal V DC  to the current sink circuit  310  for controlling the magnitude of the control signal V CS  to a desired magnitude V DES  so as to adjust the intensity of the LED light source  306  to a desired light intensity L DES  (e.g., between a low-end intensity L LE  and a high-end intensity L HE ). 
     The LED driver  304  may be controlled to an electronic off (e.g., standby) state during which the LED driver  304  may turn off the LED light source while control circuitry of the LED driver remains powered (e.g., in a similar manner as the LED driver  104  shown in  FIG. 1 ). The LED driver  304  may be configured to change between an on state and the electronic off state in response to the control signal V CS  (e.g., using hysteresis). For example, the LED driver  304  may be configured to change to the on state (i.e., to turn on) when the DC voltage level of the control signal V CS  rises above a rising threshold V TH-RISING  (e.g., approximately 1.0 V). 
     As with the load control device  100  of  FIG. 1 , the low-end magnitude V LE  of the control signal V CS  generated by the load control device  300  may be less than the rising threshold V TH-RISING . In some cases (such as, e.g., when a desired magnitude of the control signal V CS  is less than the rising threshold V TH-RISING  when power is applied to the LED light source  306 ), the control circuit  320  may be configured to temporarily increase the magnitude of the control signal V CS  to be equal to or greater than the rising threshold V TH-RISING  before decreasing the magnitude of the control signal V CS  to the desired magnitude. The control circuit  320  may be configured to determine when power is applied to the LED light source  306  (e.g., to the LED driver  304 ) in response to the magnitude (e.g., a change of the magnitude) of the control signal V CS  generated by the current sink circuit  310 . For example, the LED driver  304  may be configured to generate a link supply voltage to allow the current sink circuit  310  to generate the control signal V CS  on the control wiring  308 . As such, the magnitude of the control signal V CS  may indicate when the LED driver  304  is powered. For example, when the LED driver  304  is unpowered, the magnitude of the control signal V CS  may drop to approximately zero volts. When power is restored, the magnitude of the control signal V CS  may rise back to the level before power was lost. The control circuit  320  may be configured to determine that power has been lost and re-applied based on the changes (e.g., the drop and rise) in the magnitude of the control signal V CS . 
     The control circuit  320  may comprise a bump-up circuit  330  for temporarily increasing the magnitude of the control signal V CS  (e.g., when power is applied to the LED driver  304 ).  FIG. 5  is a simplified diagram of example waveforms illustrating the operation of the bump-up circuit  330  of the load control device  300 . The bump-up circuit  330  may comprise a bipolar junction transistor Q 332  that may in turn include a collector coupled to an anode of a capacitor C 334 , the series combination of which is coupled between the first control terminal C 1  and the wiper of the potentiometer circuit  322  (e.g., the DC output signal V DC  of the control circuit  320 ), with an emitter of the transistor Q 332  coupled to the first control terminal C 1 , and a cathode of the capacitor C 334  coupled to the wiper of the potentiometer circuit  322  and the current sink circuit  310 . The bump-up circuit  330  may further comprise two resistors R 336 , R 338  coupled in series between the first and second control terminals C 1 , C 2 . The tie point of the resistors R 336 , R 338  may be coupled to the base of the transistor Q 332 . 
     When the magnitude of the control signal V CS  is approximately zero volts (e.g., the LED driver  304  is unpowered), the transistor Q 332  may be non-conductive and the capacitor C 334  may be uncharged. After power is applied to the LED driver  304 , the LED driver  304  may begin to generate the link supply voltage (e.g., across the resistors R 334 , R 336  of the bump-up circuit  330 ). Once the voltage across the resistor R 334  exceeds the rated emitter-base voltage of the transistor Q 332 , the transistor may become conductive. When the transistor Q 332  first becomes conductive, the capacitor C 334  may be uncharged, and thus the transistor Q 332  may pull the magnitude of the DC output signal V DC  up towards the magnitude at the first control terminal C 1 . This may cause the current sink circuit  310  to temporarily increase the magnitude of the control signal V CS  to be greater than the rising threshold V TH-RISING  (e.g., by an offset voltage V OFFSET ). As the capacitor C 334  charges, the magnitude of the DC output signal V DC  may continue to fall until the capacitor C 334  is fully charged and the magnitude of the DC output signal V DC  has returned to the level determined by the potentiometer circuit  322  and the intensity adjustment actuator  324  (e.g., the low-end magnitude V LE  as shown in  FIG. 5 ). The bump-up circuit  330  may be configured to temporarily increase the magnitude of the control signal V CS  for a bump-up period T BUMP  (e.g., as shown in  FIG. 5 ).