Patent Publication Number: US-2013249437-A1

Title: Adaptive filter for led dimmer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/614,353 entitled “Adaptive Filter for LED Dimmer” to Xiaoyan Wang, et al., filed on Mar. 22, 2012, the contents of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to driving an LED (Light-Emitting Diode) lamp and, more specifically, to controlling dimming of the LED lamp. 
     2. Description of the Related Arts 
     LEDs are being adopted in a wide variety of electronics applications, for example, architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices, flashlights, etc. Compared to conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have significant advantages, including high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety. 
     The use of LEDs in lighting applications is expected to expand, as they provide significant advantages over incandescent lamps (light bulbs) in power efficiency (lumens per watt) and spectral quality. Furthermore, LED lamps represent lower environmental impact compared to fluorescent lighting systems (fluorescent ballast combined with fluorescent lamp) that may cause mercury contamination as a result of fluorescent lamp disposal. 
     However, conventional LED lamps cannot be direct replacements of incandescent lamps and dimmable fluorescent systems without modifications to current wiring and component infrastructure that have been built around incandescent light bulbs. This is because conventional incandescent lamps are voltage driven devices, while LEDs are current driven devices, requiring different techniques for controlling the intensity of their respective light outputs. 
       FIG. 1  illustrates a typical dimmer wiring configuration in conventional residential and commercial lighting applications. Predominantly, incandescent lamps operate off of alternating current (AC) systems. Specifically, a dimmer switch  10  is placed in series with an input voltage source  15  and the incandescent lamp  20 . The dimmer switch  10  receives a dimming input signal  25 , which sets the desired light output intensity of incandescent lamp  20 . Control of light intensity of the incandescent lamp  20  is achieved by adjusting the RMS voltage value of the lamp input voltage (V-RMS)  30  that is applied to incandescent lamp  20 . Dimming input signal  25  can either be provided manually (via a knob or slider switch) or via an automated lighting control system. 
     Many dimmer switches adjust the V-RMS by controlling the phase angle of the AC-input power that is applied to the incandescent lamp to dim the incandescent lamp.  FIGS. 2A ,  2 B, and  2 C illustrate typical lamp input voltage waveforms which are output to incandescent lamp  20 .  FIG. 2A  illustrates a typical lamp input voltage waveform  30  when no dimming switch  10  is present, or when the dimmer switch  10  is set to maximum light intensity and the voltage signal from the input voltage source  15  is unaffected by the dimmer switch  10 .  FIG. 2B  illustrates lamp input voltage  30  with a dimming effect based on leading edge phase angle modulation (i.e. a leading edge dimmer). In a leading edge dimmer, the dimmer switch  10  eliminates a section  32  having a period Td_off of the lamp input voltage  30  after the zero-crossings of the AC half-cycles and before the peaks. The input voltage  30  is unchanged during the period Td_on. As the dimming input signal  25  increases the desired dimming effect, the period Td_off of the eliminated section  32  increases, the period Td_on decreases, and the output light intensity decreases. For minimum dimming (maximum light intensity), the period Td_off of the eliminated section  32  becomes very small or zero. 
       FIG. 2C  illustrates lamp input voltage  30  with a dimming effect based on trailing edge phase angle modulation (i.e. a trailing edge dimmer). Trailing edge dimmer switches operate by removing trailing portions  34  of AC voltage half-cycles, after peaks and before zero-crossings. The input voltage  30  is unchanged during the period Td_on. Again, as the dimming input signal  25  increases the desired dimming effect, the period Td_off of the removed sections  34  increases, the period Td_on decreases, and the light intensity decreases. For minimum dimming (maximum light intensity) the period Td_off of the eliminated section  34  becomes very small or zero. 
     Controlling the phase angle is a very effective and simple way to adjust the RMS-voltage supplied to the incandescent bulb and provide dimming capabilities. However, conventional dimmer switches that control the phase angle of the input voltage are not directly compatible with conventional LED lamps, since LEDs, and thus LED lamps, are current driven devices. 
     SUMMARY OF THE INVENTION 
     An LED controller controls dimming of an LED lamp using adaptive filtering. The filter bandwidth adapts based on the operating condition, thus enabling the LED lamp to manage the tradeoff between noise rejection and response time based on the operating condition. 
     In one embodiment, the LED controller receives from a dimming switch, an input signal indicating a desired dimming amount for the LED lamp. The LED controller detects an operating condition of the LED lamp based on monitored characteristics of the LED lamp. The LED controller then dynamically adjusts bandwidth of an adaptive filter based on the detected operating condition. The adaptive filter is used to filter a dimming ratio signal indicating a proportion of power to provide to one or more LEDs to achieve the desired dimming amount. 
     In one embodiment, a power supply control state machine detects a state of the LED lamp as being in either a startup condition or a normal operating condition. Responsive to detecting the startup condition, the LED lamp is configured to have a first predetermined bandwidth. Responsive to detecting the normal operating condition, the adaptive filter is configured to have a second predetermined bandwidth narrower than the first predetermined bandwidth. 
     In one embodiment, the LED controller determines a rate of change in the desired dimming amount. Responsive to the rate of change being below a first threshold rate of change, the LED controller configures the adaptive filter to have a second predetermined bandwidth associated with a stable operating condition. Responsive to the rate of change exceeding the first threshold rate of change, the LED controller determines if the rate of change exceeds a second threshold. If the rate of change is below the second threshold, the LED controller configures the adaptive filter to have a third predetermined bandwidth corresponding to a normal active condition. The third predetermined bandwidth has a wider bandwidth than the second predetermined bandwidth. Otherwise, if the LED controller determines that the rate of change exceeds the second threshold, the LED controller configures the adaptive filter to have a fourth predetermined bandwidth corresponding to a rapid active condition. The fourth predetermined bandwidth is wider than the second and third predetermined bandwidths. 
     The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings and specification. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. 
         FIG. 1  is a block diagram illustrating a dimming system for incandescent lamps. 
         FIG. 2A  illustrates an example waveform produced by a dimming switch configured for no dimming. 
         FIG. 2B  illustrates an example waveform produced by a dimming switch configured for leading edge dimming. 
         FIG. 2C  illustrates an example waveform produced by a dimming switch configured for trailing edge dimming. 
         FIG. 3  illustrates a LED lamp circuit according to one embodiment of the present invention. 
         FIG. 4  is a block diagram illustrating an LED controller for controlling an LED lamp according to one embodiment of the present invention. 
         FIG. 5  illustrates a mapping curve between a phase modulation signal from a dimming switch and a dimming ratio signal for controlling LED dimming according to one embodiment of the present invention. 
         FIG. 6  is a flowchart illustrating a process for controlling an LED lamp according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The Figures (FIG.) and the following description relate to preferred embodiments of the present invention by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed invention. 
     Reference will now be made in detail to several embodiments of the present invention(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
     A dimming controller for an LED lamp controls dimming using an adaptive filter to reduce or eliminate perceivable flickering and to provide smooth transitions when actively changing the dimming level. The dimming controller detects whether the LED lamp is operating under a startup condition, an active condition (i.e., the dimming switch is being adjusted), or a stable condition (i.e., the dimming switch is not being adjusted). During stable conditions, the adaptive filter operates with a relatively narrow bandwidth to filter noise that may lead to perceivable flickering. During active and startup conditions, the adaptive mapping filter operates with a relatively wide bandwidth to provide a quick response to the dimmer switch. Embodiments of the LED lamp are described in further detail below. 
       FIG. 3  illustrates an LED lamp system including LED lamp  300  used with a dimmer switch  10  (e.g., a conventional dimmer switch). LED lamp  300  according to various embodiments can be a direct replacement of an incandescent lamp  20  in a conventional dimmer switch setting, such as the setting of  FIG. 1 . A dimmer switch  10  is coupled in series with AC input voltage source  15  and LED lamp  300 . Dimmer switch  10  controls dimming via phase modulation of an AC input voltage (e.g., via leading edge dimming or trailing edge dimming). Dimmer switch  10  receives a dimming input signal  25 , which is used to set the desired light output intensity of LED lamp  300 . Dimmer switch  10  receives AC input voltage signal  82  and adjusts the V-RMS value of lamp input voltage  84  in response to dimming input signal  25 . In other words, control of the light intensity outputted by LED lamp  300  is achieved by dimmer switch  10  adjusting the V-RMS value of the lamp input voltage  84  that is applied to LED lamp  300 . Dimming input signal  25  can either be provided manually (via a knob or slider switch, not shown herein) or via an automated lighting control system (not shown herein). LED lamp  300  controls the light output intensity to vary proportionally to the lamp input voltage  84 , exhibiting behavior similar to incandescent lamps. 
     One example of a dimmer switch  10  is described in U.S. Pat. No. 7,936,132. In one embodiment, dimmer switch  10  employs phase angle switching to adjust the lamp input voltage  82  by using a TRIAC circuit (not shown). A TRIAC is a bidirectional device that can conduct current in either direction when it is triggered, or turned on. Once triggered, the TRIAC continues to conduct until the current drops below a certain threshold, called a holding current. For the internal timing of a TRIAC dimmer to function properly, current must be drawn from dimmer switch  10  at certain times. In one embodiment, LED lamp  300  is configured to draw current from dimmer switch  10  in a manner that allows the internal circuitry of dimmer switch  10  to function properly. 
     LED lamp  300  controls dimming of LEDs to achieve desired dimming based on the dimming input signal  25 . LED lamp  300  controls dimming in a manner that reduces or eliminates perceivable flickering of the LEDs throughout the dimming range, and will cause the LED brightness to respond quickly and smoothly when dimmer input signal  25  to dimmer switch  10  is adjusted. 
     An embodiment of LED lamp  300  includes input filters  304 , bridge rectifier  306 , boost circuit  320 , LED controller  310 , current regulator  330 , and one or more LEDs  302 . 
     Input filters  304  (e.g., RL filters) filter lamp input voltage  84 . These filters  304  help to reduce noise by limiting electromagnetic interference (EMI) and in-rush current. Bridge rectifier  306  generates a rectified input voltage  112  from the filtered lamp input voltage  84 . Boost circuit  320  is an AC-DC converter that receives rectified input voltage  112  and generates a DC boosted voltage V BOOST  based on boost control signal BDRV from LED controller  310 . Boost circuit  320  also provides sensed input voltage VINS and sensed boost current signal BISNS to LED controller  310 . These signals are used by LED controller  310  to efficiently control boost circuit  320  as will be described below. 
     In one embodiment, boost circuit  320  comprises resistors R 1 , R 2 , and Rb, inductor Lb, diodes D 1  and D 2 , switch Qb, input capacitor C 1 , and output capacitor C 2 . Inductor Lb stores power from rectified input voltage  112  (stored across capacitor C 1 ) when switch Qb turns on and releases power to current regulator  330  (stored across capacitor C 2 ) when boost switch Qb turns off. Diode D 1  is coupled in series between inductor Lb and current regulator  330 , and provides the power from boost inductor Lb to power converter  330  when switch Qb turns off. Boost resistor Rb is coupled in series with boost switch Qb and dissipates power from inductor Lb when switch Qb turns on. Diode D 2  is coupled between rectified input voltage  112  and current regulator  330 , and charges capacitor C 2  when V BOOST  is lower than the rectified input voltage  112  (generally during startup conditions). Resistors R 1  and R 2  form a voltage divider to provide sense voltage VINS representing rectified input voltage  112 . Furthermore, a feedback voltage BISNS across resistor Rb represents sensed current through Qb. Switch Qb is controlled by switch control signal BDRV to control the base current of Qb in a manner that efficiently delivers supply voltage V BOOST  to power converter  330  based on the feedback signals BISNS, VINS. Boost transistor Qb is shown as a bipolar junction transistor (BJT), but in other embodiments drive transistor Qb can be a metal oxide semiconductor field effect transistor (MOSFET). 
     Current regulator  330  receives the boosted DC voltage V BOOST  from the boost circuit  320  and a control signal FDRV from LED controller  310 . Based on these signals, current regulator  330  regulates current through LEDs  302 . Current regulator  330  may employ pulse-width-modulation (PWM) and/or constant current control to achieve the target light output intensity for the LEDs  302 . In one embodiment, current regulator  330  comprises a flyback converter to regulate current through the LEDs  302 . 
     In one embodiment, current regulator  330  comprises a transformer T 1 , a switch Q 1  (e.g., a MOSFET transistor), resistors Rs, R 3 , and R 4 , diode D 3 , and output capacitor C 3 . 
     Transformer T 1  stores power from boosted voltage Vboost when switch Qc turns on and releases power to LEDs  302  (stored across capacitor C 3 ) via diode D 3  when switch Qc turns off. Resistors R 3 , R 4  are arranged in a voltage divider configuration to produce feedback voltage FVSNS representing voltage across the auxiliary winding of transformer T 1 . Voltage FISNS represents the primary current through the primary winding of T 1  in the form of a voltage across sense resistor Rs. Based on the feedback signals FVSNS, FISNS, LED controller  310  generates drive signal FDRV to control switching of current regulator  320  to efficiently control power to the LEDs  302 . 
     In one embodiment, LED controller  310  controls current regulator  330  to achieve constant current operation in which substantially constant current is maintained through the LEDs  302 . The output current through the LEDs  302  is proportional to the product of the peak voltage on the current sensing resistor Rs (represented by FISNS) and the reset time of transformer T 1 . The reset time of the transformer T 1  is the time between when switch Q 1  is turned off and the falling edge of the transformer auxiliary voltage (represented by FSNS). In one embodiment, LED controller  310  uses constant current control to implement peak current switching to limit primary current Ip through primary winding of T 1  by sensing the voltage FISNS and to turn off the switch Q 1  when FISNS exceeds a threshold value Vipeak. LED controller  310  also samples voltage FVSNS at the end of each switching cycle of current regulator  330  to measure the reset time of the transformer T 1 . Constant current regulation is maintained by adjusting the threshold value Vipeak in inverse proportion to the measured reset time of the transformer T 1  in the previous cycle. An example embodiment of constant current control operation is described in more detail in U.S. Pat. No. 7,443,700 entitled “On-time Control for Constant Current Mode in a Flyback Power Supply”, issued on Oct. 28, 2008. 
     LED Controller 
     The LED controller  310  receives sensed input voltage VINS and the various feedback signals BISNS, FISNS, FVSNS, and generates output control signals BDRV and FDRV to control switching of boost circuit  320  and current regulator  330  respectively in order to achieve desired dimming of LEDs  302 . Furthermore, LED controller  310  operates to control switching in order to reduce or eliminate perceivable flickering and to ensure the perceived LED brightness will respond quickly and smoothly to the dimming switch. 
       FIG. 4  illustrates an example embodiment of an LED controller  310 . LED controller  310  comprises dimmer detection unit  410 , dimming processing unit  420 , boost circuit and current regulator control unit  430 , power supply control state machine  440 , and bandwidth control module  450 . 
     Dimmer detection unit  410  detects an amount of dimming to apply to the LEDs  302  based on VINS representing the dimmer switch output. In one embodiment, dimmer detection unit  410  comprises a phase detector  415  that receives the sensed input voltage VINS representing the rectified dimmer voltage and generates a dimming phase signal phase_out representing an amount of phase modulation (if any) detected in the sensed input voltage VINS (e.g., between 0 and 100%). 
     Dimming processing unit  420  receives the dimming phase signal phase_out and generates a dimming ratio signal dim_out representing a dimming ratio for controlling brightness of the LEDs. For example, in one embodiment, the dimming ratio represents a fraction of power to deliver to LEDs  302  to achieve the desired dimming. Thus, when the dimming ratio=1, LED controller  310  produces control signals to control boost circuit  320  and current regulator  330  such that 100% of the available power is outputted to LEDs  302 . When the dimming ratio=0.1, LED controller  310  produces control signals to output 10% of the available power to LEDs  302 . In one embodiment, dimming processing unit  420  maps phase_out to dim_out such that LED lamp  300  will behave similarly to an incandescent lamp in its luminosity response to dimming control signal  25 . Thus, for example, if the dimmer switch  10  is set to 50% dimming, LED controller  310  will control current through LEDs  302  such that the LEDs output 50% of their maximum output luminosity. Furthermore, dimming processing unit  420  adaptively filters the dimming ratio signal depending on the operating conditions in order to provide a smooth response without substantial perceivable flickering as will be described below. The dimming ratio signal dim_out is used by boost circuit and current regulator control unit  430  to generate appropriate drive signals BDRV, FDRV to achieve desired dimming based on feedback signals BISNS, FISNS, FVSNS. 
     In one embodiment, the dimming processing unit  420  comprises a dimming curve mapping unit  422  and a mapping filter  424 . Dimming curve mapping unit  422  maps the dimming phase signal phase_out to a pre-filtered dimming ratio signal  426  based on the luminosity curve of LED lamp  300 . An embodiment of the mapping performed by dimming curve mapping unit  422  is illustrated in  FIG. 5 . As can be seen in  FIG. 5 , the mapping is non-linear and is steeper in the mid-range (i.e., a small change in phase modulation will cause a large change in the fraction of power delivered to LEDs  302  to achieve the desired dimming), and flatter at the extremes (i.e., a large change in phase modulation will cause only a small change or no change in the fraction of power delivered to LEDs  302  to achieve the desired dimming). 
     Referring again to  FIG. 4 , pre-filtered dimming ratio  426  is filtered by mapping filter  424  which has a bandwidth that is adaptively adjusted based on the current operating conditions of LED lamp  300 . The bandwidth of the mapping filter  424  is related to the group delay, which defines the latency through filter  424  and therefore controls the response time of filter  424  to changing characteristics of pre-filtered dimming ratio  426 . A narrower bandwidth of mapping filter  424  will have larger group delay through mapping filter  424 . For example, in one embodiment of mapping filter  424 , the group delay is reduced by half when the bandwidth is increase by a factor of two. Based on the operating condition, it may be desirable to vary the tradeoff between noise rejection (i.e., narrower filter bandwidth) and response time (i.e., wider filter bandwidth). 
     In one embodiment, bandwidth control unit  450  detects the operating condition of LED lamp  300  and configures the bandwidth of mapping filter  424  based on the detected condition. For example, in one embodiment, the bandwidth control unit  450  detects if the LED lamp  300  is operating in one of the following conditions: (1) stable operation (when dimmer switch  10  is not being adjusted), (2) active operation (when dimmer switch  10  is being adjusted), (3) startup operation (the condition when the power is turned on). Bandwidth control unit  450  detects the operating condition based on startup signal  452  received from power supply control state machine  440 , pre-filtered dimming signal  426 , and dimming output signal dim_out using techniques described in further detail below. 
     During startup conditions, LED controller  310  has no initial knowledge about the dimmer phase condition. Therefore, during startup conditions, a low DC group delay is desired. Thus, mapping filter  424  is configured with a relatively wide bandwidth (e.g., a first predetermined bandwidth) to enable controller  310  to quickly determine the dimmer phase angle and deliver the appropriate LED brightness. 
     During stable operation, response time is less important than noise rejection because the dimmer switch is not being adjusted. During stable operation, low-frequency noise may be present that could lead to perceivable flickering of the LED output if not properly filtered. A human eye is particularly sensitive to the effects of LED current ripple in the low frequency range (around 50 Hz or lower). For example, such noise may be present based switching noise from: (1) the boost circuit  320  and current regulator  330  (operating, for example, in a 1 KHz to 240 kHz frequency range); (2) from AC line surge and sag; and/or (3) from ON/OFF switching of dimmer switch. To effectively reduce or eliminate this noise, the filter is configured with a relatively narrow bandwidth (e.g., a second predetermined bandwidth) during stable dimming conditions. 
     During active dimmer operation, the filter operates to smooth the dimming ratio change so that as the dimmer is adjusted, there is a smooth response in LED brightness. As seen in  FIG. 5  the mapping from the dimmer phase phase_out to the dimmer ratio dim_out is non-linear and has a lower slope at the low and high ends of the range than in the middle of the range. Thus, the mapping filter  424  will smooth the dimmer response to provide more consistent operation between the low/high and mid-range of the dimming curve. In one embodiment, the filter bandwidth is therefore configured to be relatively wide (e.g., a third predetermined bandwidth) during active operation to allow it to respond quickly to dimmer switch  10 . The bandwidth during active operation may be similar to the first predetermined bandwidth using during startup conditions, or may be slightly narrower than the first predetermined bandwidth, but still wider than the second predetermined bandwidth used during stable dimming conditions. 
     In another embodiment, active operation is divided into one of two sub-operations: normal active operation or rapid active operation. Normal active operation occurs when the dimmer is adjusted at a normal rate (e.g., above a first threshold rate of change and below a second threshold rate of change in dimmer position) and the bandwidth of mapping filter  424  is configured to be relatively wide (e.g., the third predetermined bandwidth). Rapid active operation occurs when the dimmer is adjusted at a rapid rate (e.g., above the threshold rate of change in dimmer position such as when the dimmer switch is rapidly switched up and down). Here, the bandwidth of the filter is configured to have an even wider bandwidth than in the normal active operation (e.g., a fourth predetermined bandwidth). For example, in one embodiment, the bandwidth under rapid active operation is configured to be as wide as allowable to minimize group delay. 
     In one embodiment, mapping filter  424  is implemented as a first order Infinite Impulse Response (IIR) low pass filter with the bandwidth and group delay data related as follows for different detected operation conditions: 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Detected Condition 
                 Bandwidth (Hz) 
                 DC group Delay (ms) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Startup 
                 2.5 
                 52.8 
               
               
                 Stable 
                 1 
                 132 
               
               
                 Normal Active 
                 2 
                 66 
               
               
                 Rapid Active 
                 Max 
                 Min 
               
               
                   
               
            
           
         
       
     
     In alternative embodiments, different bandwidths may be used in the different operating conditions. 
     The power supply control state machine  440  determines whether or not the LED controller is operating under a startup condition based on signals received from the dimmer detection unit  410  and/or the dimming processor  420 . The power supply control state machine transmits a startup condition signal  452  to bandwidth control unit  450  indicating a state of the LED lamp as being either a startup state or a normal operation state. Power supply control state machine  440  also send startup condition signals to dimmer detection unit  410 , dimmer processing unit  420  and boost and current regulator control unit  430  via connections  462 ,  464 . This information may be used by the various units  410 ,  420 ,  430  to more accurately operate under startup conditions. 
     When bandwidth control unit  450  detects startup condition signal  452 , the bandwidth control unit  450  sets the mapping filter  424  to have the appropriate bandwidth (e.g., the first predetermined bandwidth) for startup conditions via mapping filter control signal  454 . Otherwise, if no startup condition is detected, bandwidth control unit  450  monitors the pre-filtered dimming ratio signal  426  and the post-filtered dimming ratio signal dim_out. When the difference between the pre-filtered dimming ratio signal  426  and dim_out is small (e.g., below a first predefined threshold), bandwidth control unit  450  determines that LED controller  300  is in a stable operating condition. Otherwise, when the difference is large because the dimming ratio changed significantly between current and previous samples, bandwidth control unit  450  determines that LED controller  300  is in an active operating condition. If the dimming ratio changes very rapidly (e.g., above a second threshold rate of change that is higher than the first threshold), the bandwidth control unit  450  determines that the LED controller  300  is operating under rapid active operation conditions. The bandwidth control unit  450  then adjusts bandwidth of the mapping filter  454  (e.g., to the fourth predetermined bandwidth) based on the detected condition via control signal  454 . 
     A number of different methods can be used to adjust the bandwidth of the mapping filter  424 . In one embodiment, the bandwidth of the mapping filter is adjusted by controlling the filter sampling rate via mapping filter control signal  454 . The following table is an example of the sampling rate changes under the various operating conditions. In the table, below 1X means that the sampling rate is half of the AC line frequency. “Recharge” means that the filter is configured to have as wide of a bandwidth as possible (e.g., maximum bandwidth configuration). For example, in the maximum bandwidth configuration, the internal registers (e.g., memory cells) of the filter  424  may be set to follow the input value to the filter  424 . This effectively bypasses the filter, thus having the effect of a very wide bandwidth. 
     
       
         
           
               
               
               
            
               
                   
                   
               
               
                   
                 Active Operation 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Startup 
                 Normal 
                 Rapid 
                 Stable Operation 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Mapping Filter 
                 2.5X 
                 2X 
                 recharge 
                 1X 
               
               
                   
               
            
           
         
       
     
     In alternative embodiments, the bandwidth can instead be adjusted by changing filter coefficients, switching filter types, or a combination of techniques. 
     The LED controller illustrated in  FIG. 4  can be implemented, for example, using digital circuits, analog circuits, or a combination of digital and analog circuits. Furthermore, one or more components of the LED controller  310  can be implemented as a non-transitory computer-readable storage medium that stores instructions for execution by a processor (e.g., a microprocessor or microcontroller). 
       FIG. 6  illustrates an embodiment of a process for adjusting the bandwidth of the adaptive filter  424  based on operating condition of LED lamp  300 . LED controller  310  determines  602  a startup state of LED lamp  300  as being in either a startup condition or a normal operating condition. Responsive to detecting the startup condition, adaptive filter  424  is configured  604  to have a first predetermined bandwidth suitable for startup conditions (e.g., relatively wide for fast response time). Otherwise, LED controller  310  determines  606  a rate of change in the desired dimming amount. LED controller then compares  608  the rate of change to a first threshold. Responsive to the rate of change not exceeding a first threshold rate of change, LED controller  310  determines that LED lamp  300  is operating in a stable condition and configures  610  adaptive filter  424  to have a second predetermined bandwidth suitable for stable conditions. The second predetermined bandwidth is generally narrower than the first predetermined bandwidth (e.g., by a factor of 2-2.5) in order to provide improved noise rejection. Responsive to the rate of change exceeding the first threshold rate of change, LED controller  310  compares  612  the rate of change to a second threshold, which is higher than the first threshold (i.e., faster rate of change). Responsive to the rate of change not exceeding the second threshold rate of change, LED controller  310  determines that the LED lamp is operating in a normal active condition and configures  614  adaptive filter  424  to have a third predetermined bandwidth suitable for normal active conditions. The third bandwidth is generally wider bandwidth than the second predetermined bandwidth (e.g., by a factor of 2-2.5) and may be similar to or slightly narrow than the first predetermined bandwidth. Responsive to the rate of change exceeding the second threshold rate of change, LED controller  310  determines that the LED lamp is operating in a rapid active condition and configures  616  adaptive filter  424  to have a fourth predetermined bandwidth suitable for rapid active conditions. The fourth predetermined bandwidth is generally wider than the first, second, and third predetermined bandwidths and in on one embodiment, is as wide as allowable by the adaptive filter  424 . 
     Upon reading this disclosure, those of skill in the art will appreciate still additional alternative designs for controlling dimming of an LED lamp using an adaptive filter. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention.