Continuous step driver

A light emitting diode (LED) lamp includes an LED cluster including LED groups arranged in series, a power source configured to provide an input power to the LED cluster, and a driving unit configured to adjust a number of the LED groups connected to a current path of the LED cluster in series based on the input power to the LED cluster.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

This disclosure is directed to a light-emitting diode (LED) lamp, and more particularly to an apparatus and method for more efficiently driving an LED lamp.

2. Related Art

An LED lamp is a type of solid state lighting (SSL) that uses one or more LEDs as a light source. LED lamps are usually constructed with one or more clusters of LEDs in a suitable housing.FIG. 1Ashows a configuration of a conventional LED lamp100. The LED lamp100includes a voltage source110, a rectifier120, a current source130and an LED cluster140. The LED cluster140typically includes a plurality of LEDs140A to140N connected in series to form an LED string coupled between the current source130and a ground150. The LED cluster140may include more than one LED string coupled in parallel between the current source130and the ground150. The voltage source110may be an AC voltage source. The AC voltage from the voltage source110is converted to a DC voltage by the rectifier120and provided as an input voltage VINPUTto the LED cluster140. The current source120may be configured to impose a maximum current IMAXof a current ILEDflowing through the LED cluster140.

FIG. 1Bis a graph showing changes in the current ILEDin response to a sinusoidal input voltage VINPUT. Initially at time t0, the input voltage VINPUTand the current ILEDis the lowest (i.e., zero) and the LED cluster140may stay turned off until the input voltage VINPUTrises and reaches a sufficient potential level (i.e., a threshold level VTH) at which time the LED cluster140is turned on and the current ILEDbegins to flow therethrough at time t1. As the input voltage VINPUTfurther increases, the current ILEDalso increases until it reaches the maximum current IMAXset by the current source130at time t2(The input voltage VINPUTat the time t2is referred to as a maximum voltage VMAX). Upon reaching the maximum current IMAX, the current ILEDstays substantially the same even though the input voltage VINPUTrises over the maximum voltage VMax. After reaching the peak of sinusoidal curve, the input voltage VINPUTfalls but the current ILEDstays at the maximum current IMAXuntil the input voltage VINPUTfurther falls below the maximum voltage VMAXat time t3. After passing the time t3, the current ILEDbegins to decrease as the input voltage VINPUTfurther decreases from the maximum voltage VMAX. The current ILEDis then discontinued when the input voltage VINPUTfalls below the threshold level VTHat time t4. This pattern is repeated in the subsequent input voltage cycles.

The LED lamp100shown inFIG. 1A, however, suffers various drawbacks, some of which may contribute to inefficient power consumption. For example, between the times t2and t3, the LED cluster140cannot convert the input voltage VINPUThigher than the maximum voltage VMAXto light and the excessive energy is instead converted to heat. Furthermore, the LED cluster140may be turned on only for the period between the times t1and t4, i.e., when the input voltage VINPUTis higher than the threshold level VTH. Thus, the LED lamp100suffers a relatively short duty cycle compared to the input voltage cycle. The duty cycle may be even further shortened when LED cluster140has a higher threshold level VTH.

Accordingly, there is a need for an improved LED lamp configuration and power scheme to increase the energy efficiency and improve the light-generating operation.

SUMMARY OF THE DISCLOSURE

According to an aspect of the disclosure, a light emitting diode (LED) lamp includes an LED cluster including LED groups arranged in series, a power source configured to provide an input power to the LED cluster, and a driving unit configured to adjust a number of the LED groups connected to a current path of the LED cluster in series based on the input power to the LED cluster.

Each LED group may include one or more LED strings arranged in parallel, and each LED string may include one or more LEDs arranged in series. The input power may have a sinusoidal waveform. The power source may include an AC voltage source configured to generate an AC input power, a rectifier configured to convert the AC input power to a DC input power, and a current source configured to limit a maximum input current for the LED cluster.

The LED groups may include the first LED group connected to the power source and the second LED group connected to the first LED group in series. The driving unit may include switches including the first switch coupled between an output of the first LED group and ground and the second switch coupled between an output of the second LED group and ground, and a controller configured to turn on one of the first and second switches individually based on the input power to the LED cluster. The LED groups and the switches may have the same number.

The controller may include the first input connected to the power source to detect the input power, the first output connected to the first switch to turn on or off the first switch, and the second output connected to the second switch to turn on or off the second switch. The controller may be further configured to compare the input power to the first threshold level for turning on the first LED group only and the second threshold level for turning on the first and second LED groups simultaneously. The controller may be further configured to turn on the first switch only when the input power is equal to or larger than the first threshold level and less than the second threshold level and turn off the first switch and turn on second switch when the input power is greater than the second threshold level.

The LED groups may further include the third LED group connected to the second LED group in series, the driving unit further may further include the third switch coupled between an output of the third LED group and the ground, and the controller further may further include the third output connected to the third switch to turn on or off the third switch. The driving unit may be further configured to compare the input power to the third threshold level for turning on the first, second and third LED groups simultaneously, and connect the first, second and third LED groups in series to the current path of the LED cluster when the input power is equal to or larger than the third threshold level.

The driving unit may be further configured to adjust a number of the LED groups connected in series to the current path of the LED cluster based on at least one of the input power to the LED cluster and an output current from the LED cluster. The controller may further include the second input terminal connected to the switches to detect the output current therefrom.

According to another aspect of the disclosure, a method of operating a light emitting diode (LED) cluster includes providing an input power to the LED cluster comprising LED groups connectable in series, detecting the input power, and adjusting a number of the LED groups connected in series to a current path of the LED cluster based on the detected input power.

The input power may have a sinusoidal waveform. The LED groups may include the first LED group receiving the input power and the second LED group connected to the first LED group in series. The adjusting a number of the LED groups may include comparing the input power to the first threshold level for turning on the first LED group only and the second threshold level for turning on the first and second LED groups connected in series, connecting only the first LED group to the current path of the LED cluster when the input power is equal to or larger than the first threshold level and less than the second threshold level, and connecting the first and second LED groups in series to the LED current path when the input power is greater than the second threshold level.

The plurality of LED groups may further include the third LED group connected to the second LED group in series. The adjusting a number of the LED groups may further include comparing the input power to the third threshold level for turning on the first, second and third LED groups connected in series, and connecting the first, second and third LED groups to the LED current path in series when the input power is equal to or larger than the third threshold level.

The method may further include adjusting a number of the LED groups connected in series to the LED current path based on at least one of the input power and an output current from the LED cluster. The adjusting a number of LED groups connected in series to the current path may include detecting the output current from the LED cluster, comparing the output current to one or more current levels, and adjusting a number of the LED groups connected to the LED current path in series based on comparison between the detected LED output and the one or more current levels.

Additional features, advantages, and embodiments of the disclosure may be set forth or apparent from consideration of the following detailed description, drawings, and claims. Moreover, it is to be understood that both the foregoing summary of the disclosure and the following detailed description are exemplary and intended to provide further explanation without limiting the scope of the disclosure as claimed.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 2Ashows a configuration of an LED lamp200, constructed according to the principles of the disclosure. The LED lamp200may include a power source210, an LED cluster220, a driving unit230and/or the like. The power source210may be configured to generate an input voltage VINPUTfor the LED cluster220. The input voltage VINPUTmay have a periodic sinusoidal waveform, such as an input voltage waveform VINPUTshown inFIG. 2B. Other types of waveform are also contemplated for the input voltage VINPUT, such as, e.g., a triangular waveform, a square waveform, a sawtooth waveform or the like. Further, The wavelength, phase, frequency and/or other attributes of the input voltage VINPUTmay vary depending on the construction and capability of the LED lamp200.

The power source210may include a voltage source212, a rectifier214, a current source216and/or the like. The construction, functions and/or operations of the voltage source212, the rectifier214, the current source216may be similar to those of the voltage source110, the rectifier120and the current source130shown inFIG. 1A, respectively. The LED cluster220may include a plurality of LED groups222, such as, e.g., a first LED group222A, a second LED group222B, . . . , and an Nth LED group222N and/or the like, connected in series. Each of the LED groups222may include one or more LED strings connected in parallel and each LED string may include on or more LEDs connected in series, as shown in, for example,FIG. 2C.

The driving unit230may include a plurality of switches240, a controller250and/or the like. The switches240may be any type of switching device, for example, a transistor and/or the like, such as, e.g., a bipolar junction transistor (BJT), a metal oxide silicon field effect transistor (MOSFET) and/or the like. The number of switches240may be the same as that of the LED groups222included in the LED cluster220. However, the switches240may be fewer than the LED groups222when, for example, two or more LED groups222operate together as a single group. The switches240may include a first switch240A, a second switch240B, . . . , and an Nth switch240N and/or the like. The first switch240A may have an input connected to an output node224A of the first LED group222A, an output connected to a ground232and a control input connected to the controller250. The second switch240B may have an input connected to an output node224B of the second LED group222B, an output connected to the ground232and a control input connected to the controller250. Similarly, the Nth switch240N may have an input connected to an output node224N of the Nth LED group222N, an output connected to the ground232and a control input connected to the controller250.

The controller250may be configured to selectively turn on or off the switches240depending on a level (i.e., magnitude) of the input voltage VINPUT. The controller250may be connected to the power source210to detect the input voltage VINPUT. For example, as shown inFIG. 2A, the controller250may include an input terminal252connected to an output node218of the rectifier214to receive input voltage VINPUT. The controller250may further include a plurality of output terminals254, such as, e.g., a first output terminal254A, a second output terminal254B, . . . , and an Nth output terminal254N and/or the like, which are connected to the control inputs of the switches240A,240B, . . . ,240N and/or the like, respectively. More specifically, the first output terminal254A may be connected to the control input of the first switch240A, and the second output terminal254B may be connected to the control terminal of the second switch240B. Similarly, the Nth output terminal254N may be connected to the control terminal of the Nth switch240N.

To selectively turn on or off the switches240, the controller250may be configured to selectively output one of enable signals EN, such as, e.g., a first enable signal EN1, a second enable signal EN2, . . . , and an Nth enable signal ENNand/or the like, to the control inputs of the switches240, respectively, via the output terminals254A,254B, . . . ,254N, respectively. The controller250may be configured with a microcontroller, discrete analog/digital components and/or the like. With this configuration, the driving unit230may adjust the number of the LED groups222connected in series to a current path of the LED cluster220depending on a level of the input voltage VINPUT. The current path of the LED cluster220may be coupled between the power source210and the ground232.

For example,FIG. 2Bshows a graph showing the input voltage VINPUTand an LED current ILEDversus time in the LED cluster220shown inFIG. 2A. As noted above, the input voltage VINPUTmay have a periodic sinusoidal waveform with a peak level VPEAKat time t7and a half-wavelength period starting at time t0and ending at time t14. Other waveforms are also contemplated. The input voltage VINPUTmay be the lowest (e.g., zero) at the period starting and ending times t0, t14and the highest (e.g., VPEAK) at time t7. A first threshold level VTH1may be a minimum voltage level to turn on the first LED group222A only. A second threshold level VTH2may be a minimum voltage level to turn on the first and second LED groups222A,222B connected in series. Similarly, an Nth threshold level VTHNmay be a minimum voltage level to turn on the first to Nth LED groups222A to222N connected in series. The controller250may include a data storage (not shown), such as, e.g., read only memory (ROM) and/or the like, to store the threshold levels VTH, and a logic circuit (not shown) configured to compare the input voltage VINPUTwith the threshold levels VTHand output one of the enable signals EN based on the comparison. Zener diodes, BJTs, MOSFETs and/or the like may be used to create the logic circuit of the controller250.

Based on the comparison between the input voltage VINPUTand the first to Nth threshold levels VTH, the controller250may output one of the enable signals EN1to ENNto turn on one of the switches240A to240N, which in turn may change the number of the LED groups222connected to the current path of the LED cluster220. Initially at time t0, the input voltage VINPUTand the LED current ILEDmay be zero. Since there is no power, the controller250may not output any enable signal EN in order to keep the switches240turned off. Thus, the entire LED cluster220may be turned off until the input voltage VINPUTrises and reaches the first threshold level VTH1. Upon detecting that the input voltage VINPUTreaches the first threshold level VTH1at time t1, the controller250may output the first enable signal EN1via the first output terminal254A to turn on the first switch240A and to keep the second to Nth switches240B turned off. Thus, only the first LED group222A may be connected to the current path of the LED cluster220, and the LED current may flow through only the first LED group222A. In turn, only the first LED group222A may be turned on to generate light at time t1. As the input voltage VINPUTfurther increases, the LED current ILEDfurther increases until it reaches a first maximum current level IMAX1of the first LED group222A at time t2. The LED current ILEDmay temporarily stay substantially the same until the second LED group222B is connected to the first LED group222A.

When the input voltage VINPUTfurther rises to reach the second threshold level VTH2at time t3, the controller250may output the enable signal EN2via the second output terminal254B, thereby turning on the second switch240B only. This may resulting in establishing the LED current path via the first and second LED groups222A,222B connected in series, thereby turning on the first and second LED groups222A,222B to generate light. As the input voltage VINPUTfurther increases, the current ILEDalso increases until it reaches a second maximum current level IMAX2of the first and second LED groups222A,222B in series at time t4. At this moment, the LED current ILEDflowing through the LED groups222A,222B may temporarily stay substantially the same until the input voltage VINPUTfurther rises and reaches a third threshold level (not shown).

The controller250may repeat the same process to keep increasing the number of the LED groups220connected in series as the input voltage VINPUTincreases until all of the first to Nth LED groups222A to222N are connected in series to the LED current path. For example, when the input voltage VINPUTreaches the Nth threshold level VTHNat time t5, the controller250may output the Nth enable signal ENNvia the Nth output terminal254N to turn on the Nth switch240N only to connect all of the first to Nth LED groups222A to222N in series. The LED current ILEDmay flow the first to Nth LED groups222A to222N, thereby generating light at the maximum capacity of the LED cluster220. The LED current ILEDmay further increase as the input voltage VINPUTincreases until it reaches the Nth maximum current IMAXNof the first to Nth LED groups222A to222N connected in series. The maximum current IMAX, such as, e.g., the first maximum current IMAX1, the second maximum current IMAX2, . . . , the Nth maximum current IMAXN, and/or the like, may be set by manipulating the maximum current IMAXof the current source216. When the Nth maximum input current IMAXN, the LED current ILEDmay stay substantially the same even though the input voltage VINPUTfurther rises and reaches the peak level VPEAKat time t7.

After passing the peak level VPEAKat time t7, the input voltage VINPUTmay start falling, and the LED current ILEDmay also fall from the maximum current IMAXwhen the at time t8. Then, the controller250may start decreasing the number of the LED groups222connected to the LED current path until none of the LED groups222is connected to the LED current path. More specifically, when the input voltage VINPUTfalls below the Nth threshold level VTHNat time t9, the controller250may stop outputting the Nth enable signal ENNand start outputting an (N−1)th enable signal (not shown) to turn on an (N−1)th switch (not shown). Thus, The first LED group222A to an (N−1)th LED group (now shown) may be connected in series to the LED current path.

The controller250may repeat the same process until the input voltage VINPUTfalls below the first threshold level VTH1at time t13. For example, when the input voltage VINPUTfalls below the third threshold level VTH3(not shown) at time t10, the controller250may stop outputting the third enable signal EN3(not shown) and start outputting the second enable signal EN2to turn on the second switch240B only, and the first and second LED groups222A,222B may be to the LED current path. When the input voltage VINPUTfalls below the second threshold level VTH2at time t11, the controller250may stop outputting the second enable signal EN2and start outputting the first enable signal EN1to connect only the first LED group222A to the LED current path. The LED current ILEDmay temporally stay the same until the input voltage VINPUTfurther falls below the first maximum current value IMAX1at time t12. When the input voltage VINPUTfalls further below the first threshold level VTH1at time t13, the controller250may stop outputting the first enable signal EN1to disconnect the LED current path, thereby turning off the entire LED cluster220temporarily. The same pattern may be repeated in the subsequent input voltage cycle.

Accordingly, by dividing the LED cluster220into a plurality LED groups222and adjusting the number of the LED groups222connected in series to the LED current path proportional to the input voltage VINPUT, one or more LED groups222may be turned on even when the input voltage VINPUTis far less than the threshold level required to turn on the entire LED cluster222simultaneously (e.g., the Nth threshold level VTHN). For example, inFIG. 2B, the LED cluster220may be turned on as early as time t1and stay turned on until as late as the time t13. In the prior art LED lamp configuration100, the LED cluster140would be turned on at the time t5and turned off at the time t9. Thus, the LED lamp200may exhibit a higher duty cycle and power factor compared to the prior art.

Also, the LED cluster220may be designed such that the Nth threshold level VTHNmay be as close as possible to the peak level VPEAKof the input voltage VINPUT. This may substantially reduce the amount of energy converted into heat, thereby improving the energy efficiency. Furthermore, as shown inFIG. 2B, the LED cluster220may be configured such that the LED current ILEDflowing therethrough may mimic the input voltage curve. Particularly, by increasing the number of LED groups222in the LED cluster220, the input voltage curve may be more closely mimicked, thereby further increasing the energy efficiency, power factor and duty cycle. Additionally, phase control dimmers may operate better according to the disclosure.

FIG. 2Cshows a configuration of an LED lamp200′, constructed according to the principles of the disclosure. The LED lamp200′ may be a specific embodiment of the LED lamp200shown inFIG. 2A. Thus, the construction and operation of the LED lamp200′ may be substantially the same with those of the LED lamp200. More specifically, in the LED lamp200′ ofFIG. 2C, the LED cluster220may include three LED groups222, such as, e.g., a first LED group222A, a second LED group222B and a third LED group222C connected in series. The first LED group222A may include three LED strings2222A1,222A2,222A3coupled in parallel. The second LED group222B may include two LED strings222B1,222B2coupled in parallel. The third LED group222C may include a single LED string222C1. Further, the LED lamp200′ may include three switches240, such as, e.g., a first switch240A, a second switch240B and a third240C, of which the input terminals are connected to the nodes224A,224B,224C, respectively, of the LED cluster220. The controller250may include three output terminals254, such as, e.g., a first output terminal254A, a second output terminal254B and a third output terminal254C connected to control terminals of the switches240A,240B,240C, respectively. The output terminals of the switches240A,240B,240C may be connected to the ground232.

FIG. 2Dshows a graph showing the LED current ILEDversus the input voltage VINPUTin the LED lamp200′ shown inFIG. 2C. Initially, the controller250may not output any of the enable signals EN, when the input voltage VINPUTis zero at time t0. When the controller250detects that the input voltage VINPUTreaches the first threshold level VTH1at time t1, the controller250may output the first enable signal EN1via the first output terminal254A to turn on the first switch240A. Only the first LED group222A may be connected to the LED current path and be turned on to generate light at this time. While the collective amount of the current flowing through the first LED group222A may be the same as the maximum current IMAXdictated by the current source216, the current I1flowing through each of the LED strings222A1,222A2,222A3may be a third of the maximum current IMAX.

When the input voltage VINPUTrises above the first threshold level VTH1and reaches the second threshold level VTH2at time t2, the controller250may output the second enable signal EN2via the second output terminal254B to turn on the second switch240B, thereby connecting the first and second LED groups222A,222B in series to the LED current path. Thus, the first and second LED groups222A,222B may be turned on to generate light. The current I1flowing through each of the LED strings222A1,222A2,222A3of the first LED group222A may be a third of the maximum current IMAX. A current I2flowing through each of the LED strings222B1,222B2of the second LED group222B may be a half of the maximum current IMAX.

When the input voltage VINPUTfurther increases and reaches the third threshold voltage VTH3at time t3, the controller250may output the third enable signal EN3to turn off the first and second switches240A,240B and turn on the third switch260C. The entire first, second and third LED groups222A,222B,222C may be connected to the LED current path, thereby fully turning on the LED cluster240. The current I1flowing through each of the LED strings222A1,222A2,222A3may be a third of the maximum current IMAX. The current I2flowing through each of the LED strings222B1,222B2may be a half of the maximum current IMAX. A current I3flowing through the LED strings222C1may be the same as the maximum current IMAX.

When the input voltage VINPUTpasses the peak level VPEAKat time t4and falls below the third threshold voltage VTH3at time t5, the controller250may output the second enable signal EN2to turn off the first and third switches240A and240C and turn on the second switch240B. In turn, the first and second LED groups222A,222B may be turned on and the third LED group222C may be turned off. When the input voltage VINPUTfurther falls and reaches the second threshold voltage VTH2at time t6, the controller250may turn off the second and third switches240B,240C and turn on the first switch240A to turn on the first LED group222A only. When the input voltage VINPUTfalls below the first threshold voltage VTH1at time t7, the controller250may turn off the first, second and third switches240A,240B,240C, thereby turning off the first, second and third LED groups222A,222B,222C.

FIG. 2Eshows a flowchart of a method500of operating the LED lamp200′ shown inFIG. 2C, according to the principles of the disclosure. However, the method500may be easily modified to address more or less LED groups222and applied to the LED lamp200shown inFIG. 2Awith any number of the LED groups222. Upon starting the method (at502), the input voltage VINPUTmay be applied to the LED cluster220(at510). Then the controller250may detect the level of the input voltage VINPUT(at520) for comparison with the first, second and third threshold levels VTH1, VTH2, VTH3. When the input voltage VINPUTis less than (i.e., not equal to or greater than) the first threshold voltage VTH1(NO at530), the controller250may continue to detect the input voltage VINPUT(at520) and compare the input voltage VINPUTto the first threshold level VTH1(at530). However, when the input voltage VINPUTis equal to or greater than the first threshold level VTH1(YES at530), the controller250may compare the input voltage VINPUTto the second threshold level VTH2(at540).

When the input voltage VINPUTis less than (i.e., not equal to or greater than) the second threshold level VTH2(NO at540), the controller250may output the first enable signal EN1(at545) to turn on the first switch240A and connect the first LED group222A to the LED current path. In turn, the first LED group222A may be turned on. The controller250may continue to detect the input voltage VINPUT(at520). However, when the input voltage VINPUTis equal to or greater than the second threshold level VTH2(YES at540), the controller250may compare the input voltage VINPUTwith the third threshold level VTH3(at550). When the input voltage VINPUTis less than (e.g., not equal to or greater than) the third threshold level VTH3(NO at550), the controller250may output the second enable signal EN2(at555) to connect the first and second LED groups222A,222B to the LED current path. In turn, the first and second LED groups222A,222B may be turned on, and the controller250may continue to detect the input voltage VINPUT(at520).

When the input voltage VINPUTis equal to or greater than the third threshold level VTH3(YES at550), the controller250may output the third enable signal EN3(at560) to connect the first, second and third LED groups222A,222B,222C in series to the current path of the LED cluster220, thereby fully turning on the LED cluster220. As noted above, by adjusting the number of the LED groups222connected in series to the LED current path proportional to the input voltage VINPUT, the input voltage VINPUTmay be used to power one or more LED groups222even before the input voltage VINPUTreaches the threshold level of the LED cluster220. The same operational principles may be applied to the LED lamp200shown inFIG. 2Aregardless of how many LED groups222are included in the LED cluster220.

The method500described herein and its variations and modifications may be carried out with dedicated hardware implementation, such as, e.g., semiconductors, application specific integrated circuits (ASIC), programmable logic arrays and/or other hardware devices constructed to implement the method500and the like. However, the various embodiments of the disclosure described herein, including the method500and the like, may be implemented for operation as software program running on a computer processor. Furthermore, alternative software implementations, such as, e.g., distributed processing (e.g., component/object distributed processing or the like), parallel processing, virtual machine processing, any further enhancement, or any future protocol may also be used to implement the methods described herein.

FIG. 3shows a configuration of another LED lamp300, constructed according to the principles of the disclosure. The LED lamp300may be configured similar to the LED lamp200shown inFIG. 2A. For example, the LED lamp300may include a power source310, an LED cluster320, a driving unit330and/or the like. The power source310may include a voltage source312, a rectifier314and/or the like. The LED cluster320may include a plurality of LED groups322, such as, a first LED group322A, a second LED group322B, . . . , and an Nth LED group322N and/or the like, connected in series. The driving unit330may include a plurality of switches340, a controller350and/or the like. The plurality of switches340may be connected to the outputs of the LED groups322, respectively. The controller may have a plurality of outputs354connected to the switches340. Similar to the controller250, the controller350may be configured to output enable signals EN to the switches340to adjust a number of the LED groups322connected to a current path of the LED cluster320.

However, unlike the LED lamp200shown inFIG. 2A, the LED lamp300may adjust the number of the LED groups322connected to the current path based on at least one of an input voltage VINPUTand an output current IOUTPUTfrom the LED cluster320. Thus, the controller350may include at least one of a voltage input terminal352to detect an input voltage VINPUTand a current input terminal356to detect an output current IOUTfrom the LED cluster320. The voltage input terminal352may be connected to the power source310, for example, a node322connected to the power source310, for example, to an output node322of a rectifier314or the like, to receive the input voltage VINPUTprovided to the LED cluster320. An output current IOUTmay flow from the outputs of switches340to a ground332. Thus, the current input terminal356may be connected to a node334coupled between the switches340and the ground332. A resistor336may be coupled between a ground332and the node334to slow down the output current IOUTdrained to the ground332.

The controller350may be configured to operate based solely on the output current IOUTdetected via the current input terminal356. For example, the controller350may adjust the number of the LED groups322connected to the current path based on the output current IOUT. The controller350may store a plurality of threshold current values, compare the output current IOUTwith the threshold current values, and turn on one of the switches360A,360B to360N to adjust the number of the LED groups322connected in series to the LED current path of the LED cluster320. Thus, it may not necessary to impose a maximum value for the input current in this embodiment, and a current source may be omitted from the power source310. However, when the output current IOUTis too small to detect and/or is not directly related to the LED current ILEDflowing through the LED cluster340, the controller350may use both the input voltage VINPUTand the output current IOUT.

While the disclosure has been described in terms of exemplary embodiments, those skilled in the art will recognize that the disclosure can be practiced with modifications in the spirit and scope of the appended claims. These examples given above are merely illustrative and are not meant to be an exhaustive list of all possible designs, embodiments, applications or modifications of the disclosure.