Light emitting diode (LED) lighting device and controller and method for controlling same

Abstract

The invention relates to a light emitting diode (LED) lighting device and a controller and a method for controlling the same. A controller for controlling a plurality of series connected LED strings includes a reference voltage generator configured to generate a plurality of reference voltages. The reference voltage generator is further configured to vary the plurality of reference voltages based on the sampled voltage signal. The sampled voltage signal may include one or both of a peak voltage of a rectified input AC voltage and a peak voltage of an LED diode forward voltage (Vf). The controller further includes a plurality of linear regulators, each linear regulator coupled between a cathode of a corresponding LED string and a common current sense node, and each linear regulator configured to regulate a current through the corresponding LED string in accordance with a corresponding reference voltage.

Description

[0001] Information about divisional applications

[0002] This application is a divisional application of an invention patent application with an application date of December 20, 2018, application number 201811565530.5, and invention name “Light Emitting Diode (LED) Lighting Device and Controller and Method for Controlling the Same”. Technical Field

[0003] The present invention relates generally to light emitting diodes, and more particularly to constant current linear drivers with high power factor. Background Art

[0004] Light emitting diodes (LEDs) offer many advantages over conventional lighting devices, such as long life, high efficiency, and non-toxic materials. With the development of electronic technology, light emitting diodes are finding wider applications. For example, in consumer applications, LED bulbs are expected to be a replacement for conventional white light incandescent bulbs or fluorescent bulbs. In addition, more and more electronic devices use LCDs as displays, and LEDs are becoming more and more popular as backlight sources.

[0005] Constant current linear regulators are widely used as LED drivers because of their simplicity, low cost, stability, and suitability for PWM dimming. To provide a high power factor, conventional LED controllers use linear regulators to control LED strings. The controller can vary the number of LEDs to be turned on according to the magnitude of the input voltage. In other words, the controller turns on more LEDs in the string when the input voltage increases, and reduces the number of turned-on LEDs when the voltage decreases. Therefore, the total current flowing through the LED string changes in a step-wise manner to track the input voltage, thereby improving the power factor.

[0006] The disadvantage of these controllers is that the total LED current can vary with changes in supply voltage or operating temperature. For example, the total LED current is higher at higher input voltages, and the LED current is lower at lower input voltages. Therefore, the brightness of the LED string can vary with input voltage. These controllers can also be susceptible to variations in the LED forward voltage Vf due to device-to-device variations and temperature coefficient (TC). These conditions prevent the LED current from having a desired stable average LED current, resulting in variations in the brightness of the LED lighting device. Summary of the invention

[0007] The inventors have determined the cause of the LED brightness variation described above. To obtain a good power factor, in a PF (power factor) controller, the LED is turned on when the voltage across the LED is equal to or higher than the diode Vf voltage, and turned off when the voltage across the LED is lower than Vf. When the LED is on, the current is regulated by a linear feedback loop. Conventional controllers use a fixed reference voltage to control the linear regulator. Therefore, the LED current corresponding to the sequential on-step is activated at a fixed fraction of the input voltage. Therefore, any voltage fluctuations from the rectifier (AC amplitude changes) or Vf fluctuations from the LED can change the timing of the LED turning on and off. This LED on / off timing fluctuation will cause the total power entering the LED to change and then make the total lumen unstable (as perceived).

[0008] The present invention teaches an LED controller for controlling the number of LED diodes connected in series in a lighting device, the LED controller can provide high power factor and can maintain constant brightness under varying peak voltage conditions of at least one of an AC voltage supply and an LED diode forward voltage (which is temperature dependent). The high power factor is achieved by maintaining the LED current in phase with the rectified AC input voltage. The LED diodes can be turned on sequentially in stages according to variations in the rectified AC input voltage. Constant brightness is achieved by varying the transition point of each stage according to the peak voltage of at least one of the rectified AC input voltage and the LED diode forward voltage (e.g., varying the current setting voltage reference).

[0009] As an example, the present invention teaches an LED controller for controlling a plurality of LED strings connected in series. The LED string may have one or more LED diodes. The LED controller includes an input node for receiving a rectified input AC voltage from a rectifier circuit and a current sensing node for sensing the current flow in the LED string. The LED controller also includes a plurality of linear regulators. Each linear regulator is coupled between a corresponding LED string and a current sensing node. Each linear regulator is coupled to a reference voltage and controls the current flow through the corresponding LED string based on the reference voltage. A reference voltage generator provides a reference voltage so that a continuously higher reference voltage is coupled to the linear regulator, which is coupled to an LED string further down in the series. This arrangement allows the LED controller to turn on the LED string in sequence, and the LED current can be made in phase with the rectified AC input voltage to maintain a higher power factor. In addition, the reference voltage generator receives the rectified input AC voltage and provides a plurality of reference voltages based on the peak voltage of the rectified input AC voltage, so that each of the plurality of reference voltages is adjusted to be lower for a rectified input AC voltage having a higher peak voltage.

[0010] In an example, a reference voltage generator may include a bandgap voltage generator, a peak voltage detector, and an operational amplifier. The bandgap voltage generator provides a bandgap reference voltage. The peak voltage detector is coupled to the rectified AC input voltage and generates a peak voltage signal representing the peak voltage of the rectified AC input voltage. The operational amplifier has a positive input node coupled to the bandgap reference voltage, a negative input node coupled to the peak voltage signal, and an output node for providing a base reference voltage that varies inversely with the peak voltage of the rectified input AC voltage. The reference voltage generator may also include a voltage divider circuit for receiving the base reference voltage and deriving a plurality of reference voltages.

[0011] In another example, the reference voltage generator may have a peak voltage detector for receiving a sampled LED diode forward voltage signal Vf from an LED diode in an LED string and providing a base reference voltage that varies in proportion to the sampled LED diode forward voltage signal Vf.

[0012] In yet another example, the reference voltage generator may have a peak voltage detector for coupling to a cathode of one of the plurality of LED strings to receive a sampled voltage in order to track changes in the peak voltage of the rectified AC input voltage and the LED diode forward voltage.

[0013] Each of the linear regulators may have a transistor for controlling the current flow in the current path and an operational amplifier for providing a control signal for controlling the transistor. The operational amplifier receives a preset reference voltage at a first input terminal and receives a feedback signal from the current path at a second input terminal. The operational amplifier also outputs a control signal for controlling the current flow in the current path.

[0014] definition

[0015] Within the context of the present invention, the terms used in this disclosure generally have their ordinary meanings in the art. Certain terms are discussed below to provide additional guidance to practitioners regarding the description of the present invention. It will be appreciated that the same thing can be expressed in more than one way. Therefore, alternative language and synonyms may be used.

[0016] A regulator or voltage regulator is a device used to automatically maintain a constant voltage level.

[0017] A linear regulator is an electronic circuit used to maintain a stable voltage. A linear regulator can place the regulating device in parallel with the load (shunt regulator) or can place the regulating device between the source and the regulated load (series regulator). The regulating device is made to act as a variable resistor, continuously adjusting the voltage divider network to maintain a constant output voltage, and constantly dissipating power. In contrast, a switching regulator uses an acting device that is turned on or off to maintain an average output value.

[0018] A constant current regulator is a regulator that provides a constant output current. Constant current or constant voltage is understood as a current or voltage that maintains a constant value with a deviation range depending on design and manufacturing process variations or within limits according to specifications (for example, within ±10%, ±5% or ±1%).

[0019] A light emitting diode (LED) is a two-lead semiconductor light source. An LED is a pn junction diode that emits light when activated by an electric current. When a suitable voltage is applied to the leads, electrons are able to recombine with holes within the device, releasing energy in the form of photons.

[0020] The diode forward voltage is the voltage dropped across a forward biased diode that is conducting. For example, a silicon PN junction diode may have a forward voltage of approximately 0.7 volts depending on the doping concentrations of the P and N regions.

[0021] An LED string is two or more LED diodes connected in series.

[0022] An operational amplifier (op-amp or opamp) refers to a DC-coupled high-gain electronic voltage amplifier with differential inputs and usually single-ended output. An operational amplifier can be characterized by high input impedance and low output impedance and can be used to perform mathematical operations in analog circuits.

[0023] A voltage reference is an electronic device that ideally produces a fixed (constant) voltage regardless of the load on the device, power supply variations, temperature changes, and the passage of time.

[0024] A bandgap voltage generator (or bandgap voltage reference) is a temperature-independent voltage reference circuit used in integrated circuits. A bandgap voltage generator is configured to generate a fixed (constant) voltage regardless of power supply variations, temperature changes, and circuit loads from the device. A bandgap voltage generator typically has an output voltage of about 1.25V (close to the theoretical 1.22eV bandgap of silicon at 0K).

[0025] The reference voltage is a voltage value used as a target for a comparison operation.

[0026] An analog signal is a continuous signal with time-varying characteristics. An analog signal differs from a digital signal, which comprises a sequence of discrete values ​​that can assume only one of a finite number of values.

[0027] A feedback control loop refers to a feedback system arranged to regulate an output at a set point or reference value. Feedback occurs when the output of a system is routed back to an input as part of a chain forming a circuit or loop.

[0028] The power factor of an electric power system is defined as the ratio of the amount of flow to the load to the apparent power in the circuit. To deliver the same amount of useful power, a load with a low power factor draws more current than a load with a high power factor. In a distributed system, a load with a low power factor has higher energy losses.

[0029] When the term "same" is used to describe two quantities, this means that the values ​​of the two quantities are determined to be the same within the limits of measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a simplified block diagram illustrating an LED lighting device embodying certain aspects of the present invention;

[0031] Figure 2 is a simplified schematic diagram illustrating an LED lighting device embodying certain aspects of the present invention;

[0032] Figure 3 is a diagrammatic illustration of some aspects of the invention embodying Figure 2 A waveform diagram showing the relationship between the LED current and the rectified input voltage under the control of the control circuit;

[0033] Figure 4A is a waveform diagram illustrating the relationship between LED current and two rectified input voltages having different peak voltages embodying certain aspects of the present invention;

[0034] Figure 4B is a waveform diagram illustrating the relationship between LED current and two rectified input voltages having different peak voltages embodying certain aspects of the present invention;

[0035] Figure 5 is a simplified schematic diagram illustrating an LED control circuit embodying certain aspects of the present invention;

[0036] Fig. 6A is a waveform diagram illustrating the relationship between simulated LED current and rectified input voltage in an LED controller with an adjustable reference voltage embodying certain aspects of the present invention;

[0037] Figure 6Bis a graph illustrating the variation of LED current over a range of peak AC input voltages embodying certain aspects of the present invention;

[0038] Figure 7 is a simplified schematic diagram illustrating an LED control circuit 700 embodying certain aspects of the present invention;

[0039] Figure 8 is a waveform diagram illustrating the operation of another LED control circuit embodying certain aspects of the present invention;

[0040] Fig. 9 is a waveform diagram illustrating the relationship between LED current and rectified input voltage in an LED controller with an adjustable reference voltage embodying certain aspects of the present invention;

[0041] Fig.10 is a waveform diagram illustrating the relationship between LED current and rectified input voltage in another LED controller with an adjustable reference voltage embodying certain aspects of the present invention;

[0042] Fig.11 is a simplified schematic diagram illustrating an LED control circuit embodying certain aspects of the present invention;

[0043] Fig.12 is a simplified schematic diagram illustrating an LED control circuit 1200 embodying certain aspects of the present invention; and

[0044] Fig.13 is a flow chart illustrating a method for controlling a plurality of serially connected LED strings. DETAILED DESCRIPTION

[0045] The present invention teaches a linear regulator based high power factor controller that can provide a constant current by using a reference voltage that tracks changes in at least one of a peak supply voltage and a temperature. In a specific example, the reference voltage is modified using the peak value of the input voltage. In another example, the forward voltage of an LED or LED string is used as a representation of the temperature used to modify the reference voltage. In yet another example, both the input voltage and the LED forward voltage are monitored to modify the reference voltage. Thus, in addition to providing a high power factor, a cost-effective LED driver is described that can also provide a constant LED light brightness perceived by a user, which can be measured in lumens. Within a cycle of a rectified time-varying current, the brightness can be related to the LED current, which can be measured by the average value of the LED current or the RMS (root mean square) value of the LED current.

[0046] Figure 1 is a simplified block diagram illustrating an LED lighting device embodying certain aspects of the present invention. Figure 1As shown in FIG. 1 , the LED lighting device 100 includes a rectifier 110 for converting an AC input voltage VAC into a rectified input voltage Vin, which may be a time-varying DC voltage. The LED lighting device 100 also has an LED lamp 120 and a control circuit 130. The LED lamp 120 may have a plurality of LED strings connected in series. Figure 1 In the example of FIG. 1 , the LED lamp 120 has a plurality of LED strings 121, 122, 123, and 124 connected in series. Each of the four LED strings 121-124 may have one or more light emitting diodes (LEDs). The control circuit 130 may maintain constant LED lumens while providing a high power factor even under conditions where the AC input voltage fluctuates or the LED forward voltage varies.

[0047] Figure 2 is a simplified schematic diagram illustrating an LED lighting device embodying certain aspects of the present invention. Figure 2 As shown in FIG. 1 , the LED lighting device 200 includes a rectifier 210 for converting an AC input voltage VAC into a rectified input voltage Vin, which may be a time-varying DC voltage. The LED lighting device 200 also has an LED lamp 220 and a control circuit 230. The LED lamp 220 may have a plurality of LED strings connected in series. Figure 2 In the example of FIG. 2 , the LED lamp 220 has a plurality of LED strings 221, 222, 223, and 224 connected in series. Each of the four LED strings 221-224 may have one or more light emitting diodes (LEDs).

[0048] To simplify the illustration, in this example, LED lamp 220 shows only four LED strings. However, it should be understood that any number of LED strings can be used in the LED lamp. In addition, multiple LEDs can be grouped into different numbers of strings.

[0049] exist Figure 2 In the example of , the control circuit 230 is a linear LED driver and power factor (PF) controller IC, which can provide a 4-stage (or 4-channel) current control scheme. The control circuit 230 has four nodes or four current pins D1, D2, D3 and D4 for coupling to the cathodes of the four LEDs 221-224 respectively for providing four different current paths. The four nodes D1, D2, D3 and D4 are coupled to four linear regulators 231, 232, 233 and 234 respectively. The linear regulators 231, 232, 233 and 234 are coupled to a current sensing node CS236, which can be coupled to a current sensing resistor Rcs. The voltage at the current sensing node 236 is labeled as Vcs, which is the voltage across the current sensing resistor Rcs and represents the LED current flowing through the current path.

[0050] Each of the linear regulators 231-234 has transistors M1-M4 coupled to the LED current path and an amplifier AMP in a feedback control loop configuration with the transistors. Each of the amplifiers receives reference voltages REF1-REF4 and maintains the voltage at the source terminal of the transistor at the reference voltage. Thus, the linear regulators can maintain the current through the transistors at a constant determined by the reference voltage and the current sense resistor Rcs, for example, REF1 / Rcs, REF2 / Rcs, REF3 / Rcs, and REF4 / Rcs.

[0051] The operation of the linear regulators 231-234 can be illustrated using the first linear regulator 231 as an example. In the linear regulator 231, an operational amplifier (op-amp) AMP is used in the feedback loop to regulate the current in the LED load 221 to a target current value determined by the reference voltage REF1. The output of the operational amplifier AMP is coupled to a switching transistor M1, which is coupled to the LED load 221. The LED load current is controlled by the switching transistor M1 and sensed at the sense resistor Rcs. The voltage Vcs at the sense resistor Rcs is obtained at node 236 (CS) and used as the feedback voltage coupled to the negative input of the operational amplifier AMP. The operational amplifier AMP is used to keep Vcs equal to REF1, thereby maintaining the LED current at the target value REF1 / Rcs. In the case of PWM dimming, the PWM pulse is used as the enable signal (EN) for the operational amplifier. The operational amplifier AMP can also be enabled or disabled by the dimming signal, which will not be described in detail here.

[0052] The control circuit 230 also includes a reference voltage generator 240 that provides reference voltages REF1-REF4 such that REF1 < REF2 < REF3 < REF4. The controller 230 can provide an LED driver with a high power factor, as illustrated below with reference to Figure 3 Furthermore, the inventors have observed that the brightness of the lighting device 200 can vary due to changes in the LED current, which are caused by changes in the operating conditions (e.g., changes in the input voltage VIN or the LED diode forward voltage Vf). Therefore, the reference voltage generator 240 also receives the sampled voltage signal 241 Vin / Vf in order to modify the reference voltages REF1-REF4 to maintain a consistent brightness of the lighting device. The sampled voltage signal can represent at least one of the change in the input voltage VIN and the change in the LED diode forward voltage Vf, as further described below.

[0053] Figure 3 is an illustration based on the inventors' analysis of certain aspects of the present invention in Figure 2 The control circuit 230 regulates the LED current by sequentially drawing current into the four LED current pins D1-D4. Figure 2 and 3 For explanation. Within a rectified AC power sinusoidal cycle, only one of the four current paths conducts current at a time. For example, as soon as the voltage between VIN and D1 reaches a voltage sufficient to cause the first LED string between VIN and current path D1 to conduct current (i.e., a voltage higher than the forward diode voltage Vf), node D1 begins to draw current. The reference voltage REF1 in linear regulator 231 is selected to regulate the voltage Vcs at the CS pin to 25% of the maximum reference voltage to set the current in the D1 current path to be less than or equal to 25% of the LED peak current. As Figure 3 As shown in FIG. 3 , after VIN 310 reaches a voltage V1 (as set by reference voltage REF1 ) high enough to turn on D1 channel, LED current 320 is maintained at a first level I1 .

[0054] Similarly, the current path through node D2 is the second path that begins to draw current when the voltage between D1 and D2 reaches a value sufficient to cause the second LED string 222 between D1 and D2 to conduct current. With respect to D2, the reference voltage REF2 in the linear regulator 232 is selected to regulate the voltage at pin CS to 50% of the maximum reference voltage. While D2 is turned on, the linear regulator 231 turns off the current path D1 because after VIN 310 reaches a voltage V2 (as set by the reference voltage REF2) high enough to turn on the D2 channel, the voltage at the negative input of the amplifier is now higher than REF1, allowing the full current I2 to flow through the first and second LED strings. In a similar manner, the current path through D3 is the third stage that begins to draw current when the voltage between nodes D2 and D3 reaches a value sufficient to cause the third LED string 223 between D2 and D3 to conduct current. With respect to D3, the reference voltage REF3 in the linear regulator 223 is selected to regulate the voltage at pin CS to 75% of the maximum reference voltage. At this point, VIN 310 reaches voltage V3 (as set by REF3), and linear regulator 233 turns D2 off, allowing the entire current I3 to flow through the first, second, and third LED strings, and through node D3 to current sense node CS 236. This process continues until the last current path through D4 is activated, at which point the control circuit regulates the voltage of the CS pin to 100% of the maximum reference voltage, and the first three stages D1, D2, and D3 are all turned off, so that after VIN 310 reaches voltage V4 (as set by REF4), the entire current I4 now flows through all LED strings and current sense resistor Rcs.

[0055] VIN is a rectified AC voltage that rises and falls during half of the AC cycle. Under regulation by the control circuit 230, the LED current flowing through the current sensing node 236 increases with the rising magnitude of the rectified AC voltage. Similarly, the LED current flowing through the current sensing node 236 decreases with the falling magnitude of the rectified AC voltage. In this way, the magnitude of the LED current varies with the rising magnitude and falling magnitude of the rectified AC voltage, as shown in FIG. Figure 3 Therefore, the control circuit 230 can provide a high power factor in the LED lighting device.

[0056] As described above, Figure 2 The control circuit in can provide high power factor in LED lighting device by sequentially drawing current into different LED current paths. However, the brightness of LED lighting device can vary with the transition timing of changing to different LED currents, which can be affected by the peak AC input voltage of LED and the forward voltage of diode.

[0057] Figure 4Ais a waveform diagram illustrating the relationship between LED current and two rectified input voltages with different peak voltages based on the inventor's analysis of certain aspects of the present invention. If the reference voltages REF1-REF4 of the linear regulator are fixed, for example, with voltage settings V1-V4 based on the rectified AC voltage 412, then a rectified AC voltage 411 with a higher peak voltage (e.g., 220VAC) can cause the LED string to turn on earlier than a rectified AC voltage 412 with a lower peak voltage (e.g., 115VAC). The system can generate a quasi-sine wave-shaped LED current waveform 421 that is wider at a higher AC amplitude (e.g., 220VAC) than at a lower AC amplitude 422 (e.g., 115VAC). Therefore, the average LED current (or the total LED current in the AC cycle) can vary with the peak AC amplitude, causing the brightness of the LED lamp to vary. For example, in the AC voltage cycle, the higher the peak AC amplitude, the earlier the first LED string is turned on through the D1 current path circuit, so that the total LED current or the average LED current is higher at higher AC amplitudes.

[0058] Similarly, an LED with a lower diode forward voltage can be turned on at a lower voltage (i.e., earlier in the AC voltage cycle). Figure 2 If the reference voltage in the controller is kept fixed for diodes with different forward voltages, changes in the diode forward voltage may also result in changes in the brightness of the LED lighting device.

[0059] The present invention teaches adjusting the reference voltage to compensate for at least one of a change in the peak voltage of the rectified input voltage and a change in the LED diode turn-on voltage. For example, the reference voltage can be reduced for a higher supply voltage VIN or a lower diode forward voltage Vf so that the average LED current can remain constant. Similarly, the reference voltage can be increased for a lower supply voltage (VIN) or a higher diode forward voltage (Vf) so that the average LED current can remain constant.

[0060] Figure 4B FIG. 1 is a waveform diagram illustrating the relationship between LED current and two rectified input voltages with different peak voltages embodying certain aspects of the present invention. In this example, the reference voltages REF1-REF4 of the linear regulator are adjusted so that the reference voltage is reduced for a higher input voltage Vin to reduce the LED current at each stage of the input voltage cycle. Figure 4BAs shown in FIG. 4 , a rectified AC voltage VIN 441 having a higher peak voltage (e.g., 220VAC) can cause the LED string to turn on earlier than a rectified AC voltage 442 having a lower peak voltage (e.g., 115VAC). At each stage of the cycle of VIN, the LED current waveform 451 is wider at a higher AC amplitude (e.g., 220VAC) than at a lower AC amplitude 452 (e.g., 115VAC). However, at each stage of the VIN cycle, the magnitude of the LED current waveform 451 for a VIN having a higher peak voltage is lower than the magnitude of the LED current waveform 452 having a lower peak voltage because the reference voltages REF1-REF4 are adjusted to lower values ​​for the VIN having a higher peak voltage. Therefore, the total LED current or (equivalently) the average LED current in the VIN cycle can be maintained. Therefore, the brightness of the LED lighting device, which can be measured in lumens, can be maintained.

[0061] As an example, the control function described above can be implemented by designing the system with a predefined AC amplitude (e.g., 220VAC) and a nominal LED Vf (e.g., 3V) to meet the user's required LED RMS current. During the AC cycle, the LED current is configured as a function of the LED on time (duration between on and off). For example, if the LED in a certain channel is on for a longer duration than in the nominal case, then this LED current is reduced to have an inversely proportional change to make the LED RMS current (or the sum or integral of the currents in the AC cycle) constant.

[0062] Figure 5 is a simplified schematic diagram illustrating an LED control circuit 500 embodying certain aspects of the present invention. The control circuit 500 may be Figure 2 An example of a control circuit used in an LED lighting device. Figure 2 2, the control circuit 500 has four terminals D1-D4 for providing current paths for the LED strings and four linear regulators 511-514 for controlling the current in each current path based on one of four reference voltages REF1-REF4. In addition, the control circuit 500 also includes a reference voltage generator 540 for generating the four reference voltages REF1-REF4. The operation of the control circuit 500 is similar to the operation of the control circuit 230 described above. The control circuit 500 illustrates an exemplary implementation of a reference voltage generator.

[0063] like Figure 5As shown, the reference voltage generator 540 includes a peak voltage detector 550 for detecting changes in the rectified input voltage VIN. The peak voltage detector 550 can be connected to the rectified input voltage VIN through a voltage divider formed by resistors 552 and 553. The peak voltage detector 550 may further include an amplifier 554, a diode 551, a power-on reset switch POR, and a capacitor 555 having a parasitic discharge path (not shown), such that the peak detector can be reset and detect new peaks and provide a signal VPEAK 556 representing a sampled version of VIN. The sampled signal VPEAK 556 is buffered at the amplifier 557 to provide a signal 558 representing a sampled version of VIN. The reference voltage generator 540 further includes a bandgap reference circuit 541 for providing a reference voltage 542. The amplifier 543, together with resistors 544 and 545, receives the bandgap reference signal 542 and the input voltage sampling signal 558 to provide a base reference voltage 546 that tracks changes in VIN. A plurality of reference voltages REF1-REF4 can be derived based on the base reference voltage 546. For example, the output node 566 is coupled to the base reference voltage 546 through a resistor 561. A voltage divider (which includes resistors 562, 563, 564, and 565) is coupled to the output node 566 and provides the reference voltages REF1-REF4 to the linear regulators 511-514. In this circuit, the condition REF1 < REF2 < REF3 < REF4 is satisfied. The resistance values of the resistors 561-565 can be selected based on the desired voltage values of REF1-REF4.

[0064] In Figure 5 , the amplifier 543 can be an operational amplifier and has a positive terminal for receiving the reference voltage 542 from the bandgap reference circuit 541. The amplifier 543 also has a negative terminal for receiving the input voltage sampling signal 558. Thus, the base reference voltage 546 at the output of the amplifier 543 is configured to change in a direction opposite to the input voltage sampling signal 558. In other words, when the input voltage sampling signal 558 increases, the base reference voltage 546 is decreased. When the input voltage sampling signal 558 decreases, the base reference voltage 546 is increased. Based on the base reference voltage 546, a voltage divider (which includes resistors 562, 563, 564, and 565) provides the reference voltages REF1-REF4 to the linear regulators 511-514.

[0065] Fig. 6A is a waveform diagram showing the relationship between the simulated LED current and the rectified input voltage in an LED controller with an adjustable reference voltage, which illustrates certain aspects of the present invention. The LED current waveform is for a Figure 5The LED lighting device of the LED control circuit 500 is simulated. The rectified input voltage 611 has a peak voltage of 160V, the rectified input voltage 612 has a peak voltage of 235V, and the rectified input voltage 613 has a peak voltage of 311V. The reference voltages REF1-REF4 are adjusted so that the rectified input voltage 613 with the highest peak voltage of 311V has the lowest reference voltage. And the rectified input voltage 611 with the lowest peak voltage of 160V has the highest reference voltage. Fig. 6A , the LED current 621 for the rectified input voltage 611 has a minimum peak voltage of 160V, the LED current 622 for the rectified input voltage 612 has a peak voltage of 235V, and the LED current 623 for the rectified input voltage 613 has a maximum peak voltage of 311V. It can be seen that the LED currents 621, 622, and 623 have the same total current in half of the AC cycle, as represented by the area under each current curve. In other words, under the condition of rectified voltages with different peak voltages, the average LED current is the same. Therefore, the brightness of the LED lighting device can be maintained.

[0066] Figure 6B is a graph illustrating the variation of LED current over a range of peak AC input voltages embodying certain aspects of the present invention. Figure 6B The data presented are based on data from Figure 5 FIG60 is a diagram showing an LED current waveform simulated by an LED lighting device of an LED control circuit of a control circuit 500. The horizontal axis shows the peak voltage of the rectified input AC voltage in the range from about 200V to 240V in volts (V), representing a ±10% variation of the input AC peak voltage centered on 220V. The vertical axis shows the range of LED current in milliamperes (mA). Curve 631 illustrates that the variation of the LED current averaged over the AC cycle varies by about ±0.5% over the input peak voltage range of 200V to 240V. Curve 632 illustrates that the variation of the RMS (root mean square) LED current over the AC cycle varies by about ±1.2% over the input peak voltage range of 200V to 240V. The RMS value of the current can represent the average power over the AC cycle. Therefore, whether measured as an average current or as an RMS current, the LED current can be substantially constant under rectified voltages with different peak voltages. Figure 6BIn the example illustrated in FIG, the LED current in the AC cycle varies by about ±1% within a ±10% variation of the input AC peak voltage. Therefore, the brightness of the LED lighting device can be maintained, which is proportional to the LED current. In contrast, in a conventional power factor LED controller, the LED current in the AC cycle can vary by ±10% within a ±10% variation of the input AC peak voltage, resulting in a considerable variation in the brightness of the LED lighting device.

[0067] exist Figure 6B In the example illustrated in FIG, the LED current within the AC cycle varies by approximately ±1% within a ±10% variation of the input AC peak voltage. In other embodiments, depending on the range of voltage variation, the LED control circuit described herein can maintain the average LED current within a specific percentage (for example, 10%, 5%, or 2%) of the target average LED current.

[0068] Figure 7 is a simplified schematic diagram illustrating an LED control circuit 700 embodying certain aspects of the present invention. The control circuit 700 may be Figure 2 Another example of a control circuit used in an LED lighting device. Figure 5 , the control circuit 700 has four terminals D1-D4 for providing a current path for the LED string and four linear regulators 511-514 for controlling the current in each current path based on one of four reference voltages REF1-REF4. In addition, the control circuit 700 also includes a reference voltage generator 740 for generating four reference voltages REF1-REF4. The operation of the control circuit 700 is similar to the operation of the control circuit 500 described above. Unlike the control circuit 500, the control circuit 700 illustrates an exemplary implementation of the reference voltage generator 740, which provides the reference voltages REF1-REF4 based on the change of the LED diode forward voltage Vf.

[0069] The reference voltage generator 740 is similar to Figure 5 , there is one significant difference. Figure 5 The reference voltage generator 540 in generates a signal representing the input voltage VIN. In contrast, Figure 7 The reference voltage generator 740 generates a signal representing the forward voltage Vf of the LED diode. Figure 7 As shown in FIG. 1 , the reference voltage generator 740 includes a voltage across the LED 4A diode Vf sampling circuit or a diode forward voltage sampling circuit 573 where a diode is coupled to two nodes D3 and D4. The coupling points are labeled 571 and 572. The diode Vf sampling circuit 573 may include circuit components (e.g., a level shifter) for generating a voltage signal 574 that represents the diode forward voltage Vf and is appropriately scaled for the peak voltage detection circuit 550. Similar to Figure 5 the reference voltage generator 540 in

[0070] Figure 8 is a waveform diagram showing the relationship between the simulated LED current and the LED diode forward voltage in an LED controller that embodies certain aspects of the present invention. Figure 8 Depicts the simulated LED current for an LED lighting device with a Figure 7 control circuit 700. The control circuit 700 can maintain a constant average LED current under conditions of different diode forward voltages Vf by adjusting the reference voltages REF1 - REF4 based on the LED diode forward voltage Vf. For the simulated current waveforms, three different operating temperatures 0 °C, 62.5 °C, and 125 °C are used to obtain different diode forward voltages Vf. The rectified input line voltage VIN is kept constant, as shown by the three VIN waveforms 811, 812, and 813. In Figure 8 the waveform 821 represents the LED current waveform at 0 °C, the waveform 822 represents the LED current waveform at 62.5 °C, and the waveform 823 represents the LED current waveform at 125 °C. It can be seen that the LED currents 821, 822, and 823 have the same total current in half of the AC cycle, as represented by the area under each current curve. In other words, the average LED current is the same under conditions of different LED diode forward voltages Vf or different temperatures.

[0071] Fig. 9 is a waveform diagram showing the relationship between the LED current and the rectified input voltage in an LED controller with an adjustable reference voltage that embodies certain aspects of the present invention. The LED current waveform is simulated for an LED lighting device with an LED control circuit 500 similar to Figure 5 In Figure 5 the LED control circuit 500 has a peak detector VPEAK for measuring the AC peak for control. The peak voltage value is DC, such that from Figure 5The LED current generated (such as Fig. 6A ) has a rectangular waveform in each channel. In contrast, Fig. 9 The waveform is obtained by adding a non-inverting op amp to Figure 5 The non-inverting op amp output is then added to the VIN voltage through an adder or multiplier. Figure 5 The LED current shown in sets the reference voltage. Since the sampled AC line voltage is sinusoidal, the LED current is sinusoidal in each channel.

[0072] exist Fig. 9 , the rectified input voltage 911 has a peak voltage of 160V, the rectified input voltage 912 has a peak voltage of 235V, and the rectified input voltage 913 has a peak voltage of 311V. The reference voltages REF1-REF4 are adjusted so that the rectified input voltage 913 having the highest peak voltage of 311V has the lowest reference voltage, and the rectified input voltage 911 having the lowest peak voltage of 160V has the highest reference voltage. Fig. 9 , LED current 921 is for a rectified input voltage 911 with a lowest peak voltage of 160V, LED current 922 is for a rectified input voltage 912 with a peak voltage of 235V, and LED current 923 is for a rectified input voltage 913 with a highest peak voltage of 311V. It can be seen that LED currents 921, 922, and 923 have the same total current in half of the AC cycle, as represented by the area under each current curve. In other words, under the condition of rectified voltages with different peak voltages, the average LED current is the same. Therefore, the brightness of the LED lighting device can be maintained.

[0073] It would be desirable for an ideal AC offline linear LED lamp to have both high power factor and high efficiency. Unfortunately, power factor and efficiency are not related, and a compromise usually needs to be made. Figure 3 The waveforms shown in are examples of good power factor and good efficiency when the LED string is carefully selected. Fig. 9 The waveform shown in is an example with higher power factor (LED current profile is more in phase with AC input voltage) but with reduced efficiency (current peak is consistent with AC input voltage peak, so that IC has to dissipate maximum power). In another example, Fig.10 The waveforms shown in are examples with reduced power factor (the LED current profile is less in phase with the AC input voltage) but with higher efficiency (the current peak is at a lower AC input voltage, causing the IC to dissipate less power).

[0074] Fig.10FIG. 1 is a waveform diagram illustrating the relationship between LED current and rectified input voltage in another LED controller having an adjustable reference voltage embodying certain aspects of the present invention. Fig.10 , the rectified input voltage 1011 has a peak voltage of 160V, the rectified input voltage 1012 has a peak voltage of 235V, and the rectified input voltage 1013 has a peak voltage of 311V. The reference voltages REF1-REF4 are adjusted so that the rectified input voltage 1013 having the highest peak voltage of 311V has the lowest reference voltage, and the rectified input voltage 1011 having the lowest peak voltage of 160V has the highest reference voltage. Fig.10 , the LED current 1021 is for the rectified input voltage 1011 with the lowest peak voltage of 160V, the LED current 1022 is for the rectified input voltage 1012 with the peak voltage of 235V, and the LED current 1023 is for the rectified input voltage 1013 with the highest peak voltage of 311V. It can be seen that Fig.10 is an example of having reduced power factor (LED current profile is less in phase with AC input voltage) but higher efficiency (current peaks are at lower AC input voltage, causing the IC to dissipate less power). Fig.10 The illustration is based on a Figure 5 The LED control circuit 500 is a waveform diagram of the LED lighting device of the LED control circuit. Figure 5 In FIG. 5 , the LED control circuit 500 has a peak detector VPEAK for measuring the AC peak value for control. In contrast, Fig.10 The waveforms in are based on a control circuit in which a peak voltage detector is not used. Instead, the AC line voltage at VIN is measured through a voltage divider network for control. Inverting amplifier 543 produces a higher LED current at a lower AC input voltage with an inverse sine distribution curve.

[0075] Fig.11 is a simplified schematic diagram illustrating an LED control circuit 1100 embodying certain aspects of the present invention. The control circuit 1100 may be Figure 2 Another example of a control circuit used in an LED lighting device. Figure 51 , the control circuit 1100 has four terminals D1-D4 for providing a current path for the LED string and four linear regulators 511-514 for controlling the current in each current path based on one of four reference voltages REF1-REF4. In addition, the control circuit 1100 also includes a reference voltage generator 1140 for generating the four reference voltages REF1-REF4. The operation of the control circuit 1100 is similar to the operation of the control circuit 500 described above. Unlike the control circuit 500, the control circuit 1100 illustrates an exemplary implementation of the reference voltage generator 1140, which provides the reference voltages REF1-REF4 based on changes in both the input voltage VIN and the LED diode forward voltage Vf.

[0076] The reference voltage generator 1140 is similar to Figure 5 , there is one significant difference. Figure 5 The reference voltage generator 540 in generates a signal representing the input voltage VIN. In contrast, Fig.11 The reference voltage generator 1140 in the embodiment generates a signal representing the combined effect of the input line voltage VIN and the LED diode forward voltage Vf.

[0077] The reference voltage generator 1140 includes a first peak voltage detector 550 for detecting a change in the rectified input voltage VIN. The first peak voltage detector 550 is similar to Figure 5 550. The peak voltage detector 550 may be connected to the rectified input voltage VIN through a voltage divider formed by resistors 552 and 553. The peak voltage detector 550 may also include an amplifier 554, a diode 551, a power-on reset switch POR, and a capacitor 555 with a parasitic discharge path (not shown) so that the peak detector can be reset and detect a new peak value and provide a signal VPEAK 556 representing a sampled version of VIN. The sampled signal VPEAK 556 is buffered at an amplifier 557 to provide a signal 558 representing a sampled version of VIN.

[0078] The reference voltage generator 1140 has a first operational amplifier 543 having a positive terminal for receiving a reference voltage 542 from a bandgap reference circuit 541. The amplifier 543 also has a negative terminal for receiving an input voltage sampling signal 558. The amplifier 543, together with resistors 544 and 545, provides a first base reference voltage 546 that tracks changes in VIN.

[0079] In addition, the reference voltage generator 1140 also generates a signal that can represent the forward voltage Vf of the LED diode. Figure 7The reference voltage generator 740 therein, and the reference voltage generator 1140 includes a diode Vf sampling circuit 573 with diodes coupled across the LED to two nodes D3 and D4. The coupling points are labeled 571 and 572. The diode Vf sampling circuit 573 may include circuit components (e.g., a level shifter) for generating a voltage signal 574 that represents the diode forward voltage Vf and is appropriately scaled for a second peak voltage detection circuit 650, which may be similar to the peak voltage detection circuit 550. The peak voltage detector 650 may include an amplifier 654, a diode 651, a power-on reset switch POR, and a capacitor 655 having a parasitic discharge path (not shown) such that the peak detector can be reset and detect a new peak and provide a signal VPEAK 656 representing a sampled version of Vf. The sampled signal VPEAK 656 is buffered at an amplifier 657 to provide a signal 658 representing a sampled version of the LED diode forward voltage Vf. 4 In, the second operational amplifier 643 has a positive terminal for receiving the LED diode forward voltage Vf sampling signal 658. The amplifier 643 also has a negative terminal for receiving a reference voltage 542 from a bandgap reference circuit 541. The amplifier 643, together with resistors 644 and 645, provides a second base reference voltage 646 that tracks changes in the LED diode forward voltage Vf.

[0080] In Fig.11 The reference voltage generator 1140 includes an output node 1166 that is coupled to the first base reference voltage 546 through a resistor 561 to track changes in VIN. The output node 1166 is also coupled to the second base reference voltage 646 that tracks changes in the LED diode forward voltage Vf. The reference voltage generator is configured to decrease each of a plurality of reference voltages when the sampled voltage signal is increased and increase each of the plurality of reference voltages when the sampled voltage signal is decreased in response to a rectified input AC voltage. The reference voltage generator is configured to increase each of a plurality of reference voltages when the sampled voltage signal is increased and decrease each of the plurality of reference voltages when the sampled voltage signal is decreased in response to the LED diode forward voltage.

[0081] A voltage divider (which includes resistors 562, 563, 564, and 565) is coupled to the output node 1166 and provides reference voltages REF1 - REF4 to linear regulators 511 - 514. In this circuit, the condition REF1 < REF2 < REF3 < REF4 is satisfied. The resistance values of the resistors 561 - 565 and 661 can be selected based on the desired voltage values of REF1 - REF4.

[0082] ​

[0083] Fig.12 is a simplified schematic diagram illustrating an LED control circuit 1200 that embodies certain aspects of the present invention. The control circuit 1200 is another example of a control circuit that can be used in an LED lighting device in Figure 2 . Similar to the control circuit 500 in Figure 5 , the control circuit 1200 has four terminals D1 - D4 for providing a current path for an LED string and four linear regulators 511 - 514 for controlling the current in each current path based on one of four reference voltages REF1 - REF4. In addition, the control circuit 1200 also includes a reference voltage generator 1240 for generating the four reference voltages REF1 - REF4. The operation of the control circuit 1200 is similar to the operation of the control circuit 500 described above. Different from the control circuit 500, the control circuit 1200 illustrates an exemplary embodiment of the reference voltage generator 1240, which provides the reference voltages REF1 - REF4 based on the variations of both the input voltage VIN and the LED diode forward voltage Vf.

[0084] The reference voltage generator 1240 is similar to the reference voltage generator 540 in Figure 5 , with one significant difference. Figure 5 The reference voltage generator 540 in Fig.12 generates a signal representing the input voltage VIN. In contrast, the reference voltage generator 1240 in Fig.12 generates a signal representing the combined effect of the input line voltage VIN and the LED diode forward voltage Vf. As shown in 4 , the reference voltage generator 1240 includes a sampling node 1271 coupled to D4 at one end of the LED

[0085] diode. The voltage signal 1274 at the sampling node 1271 can vary and represents a sample of the input line voltage VIN and the diode forward voltage Vf. Fig.12 As shown in Figure 5 , the voltage signal 1274 is scaled by a voltage divider (resistors 1272 and 1273) for the peak voltage detection circuit 550. Similar to the reference voltage generator 540 in

[0086] Fig.1313 is a flow chart illustrating a method for controlling a plurality of serially connected LED strings. The method includes: receiving a rectified input AC (alternating current) voltage (step 1310), and generating a plurality of reference voltages (step 1320). The method also includes: varying the plurality of reference voltages based on a sampled voltage signal (step 1330). The sampled voltage signal may represent one or both of a peak voltage of the rectified input AC voltage and a peak voltage of an LED diode forward voltage Vf. The method further includes: controlling a current flow in each of a plurality of LED strings to be in phase with the rectified input AC voltage based on a corresponding reference voltage using one of a plurality of linear regulators (step 1340). At step 1350, the method includes: maintaining a target average LED current during a change in at least one of the rectified input AC voltage and the LED diode forward voltage.

[0087] The method 1300 described above may be used Figure 2 System 200, Figure 5 System 500, Figure 7 System 700, Fig.11 System 1100 or Fig.12 The system 1200 may be implemented to regulate the LED current to be in phase with the rectified AC voltage, thereby providing a high power factor. The method 1300 may be used to maintain a constant average LED current and therefore constant brightness during changes in operating conditions. In addition, the method 1300 may be implemented to maintain a constant average LED current during changes in the rectified AC voltage, such as during Figure 5 The method 1300 may also be implemented to maintain a constant average LED current during changes in the LED diode forward voltage Vf, as in Figure 7 In addition, method 1300 can also be implemented to maintain a constant average LED current during changes in the rectified AC voltage and during changes in the LED diode forward voltage Vf, as in Fig.11 System 1100 and Fig.12 The system 1200 is described in detail.

Claims

1. A controller for controlling a light emitting diode (LED) string, wherein the LED string comprises one or more light emitting diodes connected in series, the controller comprising: an input node for receiving a rectified input AC (alternating current) voltage; a common current sensing node for sensing the current flow in the LED string; a reference voltage generator configured to generate a reference voltage, the reference voltage generator further configured to vary the reference voltage based on a sampled voltage signal, the sampled voltage signal comprising one or both of a peak voltage of the rectified input AC voltage and a peak voltage of an LED diode forward voltage, wherein the reference voltage generator is configured to: responding to one or both of a peak voltage of the rectified input AC voltage and a peak voltage of an LED diode forward voltage; In response to the rectified input AC voltage, decreasing the reference voltage when the sampled voltage signal is increasing, and increasing the reference voltage when the sampled voltage signal is decreasing; and In response to the LED diode forward voltage, increasing the reference voltage when the sampled voltage signal is increased, and decreasing the reference voltage when the sampled voltage signal is decreased; and a linear regulator coupled between cathodes of the LED string and a common current sensing node, the linear regulator configured to control a current through the LED string based on the reference voltage; Wherein the controller is configured to regulate current through the LED string to be in phase with the rectified input AC voltage and maintain a target average LED current during changes in one or both of the rectified input AC voltage and the LED diode forward voltage. 2 . The controller of claim 1 , wherein the reference voltage generator is coupled to the rectified AC input voltage to receive the sampled voltage signal.

3. The controller according to claim 2, wherein the reference voltage generator comprises: A bandgap voltage generator, which is used to provide a bandgap reference voltage; a peak voltage detector, coupled to the sampled voltage signal and configured to provide a peak voltage signal of the sampled voltage signal; An operational amplifier having: a positive input node coupled to the bandgap reference voltage; a negative input node coupled to the sampled voltage signal; and An output node provides a base reference voltage that varies inversely with the sampled voltage signal.

4. The controller of claim 1, wherein the reference voltage generator is coupled to a diode forward voltage sampling circuit to receive a sampled voltage signal representative of an LED diode forward voltage signal from an LED diode in an LED string.

5. The controller according to claim 4, wherein the reference voltage generator comprises: A bandgap voltage generator, which is used to provide a bandgap reference voltage; a peak voltage detector for coupling to the sampled voltage signal and providing a peak voltage signal of the sampled voltage signal; An operational amplifier having: a negative input node coupled to the bandgap reference voltage; a positive input node coupled to the sampled voltage signal; and An output node provides a base reference voltage that varies in proportion to the sampled voltage signal.

6. The controller of claim 1 , wherein the reference voltage generator is coupled to the rectified AC input voltage to receive a first sampled voltage signal, and the reference voltage generator is coupled to a diode forward voltage sampling circuit to receive a second sampled voltage signal representing an LED diode forward voltage signal from an LED diode in an LED string.

7. The controller according to claim 6, wherein the reference voltage generator comprises: A bandgap voltage generator, which is used to provide a bandgap reference voltage; a first peak voltage detector for coupling to a first sampled voltage signal and providing a first peak voltage signal of the first sampled voltage signal; A first operational amplifier having: a positive input node coupled to the bandgap reference voltage; a negative input node coupled to the first peak voltage signal; and a second peak voltage detector for coupling to the second sampled voltage signal and providing a second peak voltage signal of the sampled voltage signal; A second operational amplifier having: a negative input node coupled to the bandgap reference voltage; a positive input node coupled to the second peak voltage signal; and An output node is coupled to both the first operational amplifier and the second operational amplifier to provide a basic reference voltage that varies with the first sampled voltage signal and the second sampled voltage signal.

8. The controller of claim 1, wherein the reference voltage generator is coupled to a cathode of an LED diode to receive the sampled voltage signal representing the rectified AC input voltage and an LED diode forward voltage signal.

9. The controller of claim 1, wherein the reference voltage generator further comprises a voltage divider circuit for receiving a base reference voltage and generating the reference voltage.

10. The controller of claim 1, wherein the linear regulator comprises: transistors, which are used to control the flow of current in a current path; An operational amplifier having a first input terminal and a second input terminal, the operational amplifier being configured to: receiving a preset reference voltage at the first input terminal; receiving a feedback signal from the current path at the second input terminal; and A control signal for controlling the current flow in the current path is output.

11. The controller of claim 1 , wherein maintaining a target average LED current comprises: Maintain the average LED current within 1% to 10% of the target average LED current.

12. The controller of claim 1, wherein the LED string comprises one or more light emitting diodes.

13. An LED (light emitting diode) lighting device, comprising: a rectifier circuit for rectifying an input AC (alternating current) voltage to provide a rectified input AC voltage; A plurality of LED strings connected in series; a reference voltage generator configured to generate a plurality of reference voltages, the reference voltage generator further configured to vary the plurality of reference voltages based on a sampled voltage signal, the sampled voltage signal including both a peak voltage of the rectified input AC voltage and a peak voltage of an LED diode forward voltage; and A plurality of linear regulators are provided, each linear regulator being coupled between a cathode of a corresponding LED string and a common current sensing node, wherein each linear regulator is configured to regulate a current through the corresponding LED string according to a corresponding reference voltage.

14. The device of claim 13, wherein the reference voltage generator is configured to: In response to the rectified input AC voltage, decreasing each of the plurality of reference voltages when the sampled voltage signal is increasing, and increasing each of the plurality of reference voltages when the sampled voltage signal is decreasing; and In response to the LED diode forward voltage, each of the plurality of reference voltages is increased when the sampled voltage signal is increased, and each of the plurality of reference voltages is decreased when the sampled voltage signal is decreased.

15. The apparatus of claim 13, wherein the reference voltage generator comprises: A bandgap voltage generator, which is used to provide a bandgap reference voltage; a first peak voltage detector for coupling to a first sampled voltage signal and providing a first peak voltage signal of the first sampled voltage signal; A first operational amplifier having: a positive input node coupled to the bandgap reference voltage; a negative input node coupled to the first peak voltage signal; and a second peak voltage detector for coupling to the second sampled voltage signal and providing a second peak voltage signal of the sampled voltage signal; A second operational amplifier having: a negative input node coupled to the bandgap reference voltage; a positive input node coupled to the second peak voltage signal; as well as An output node is coupled to both the first operational amplifier and the second operational amplifier to provide a reference voltage that varies with the first sampled voltage signal and the second sampled voltage signal.

16. The device of claim 13, wherein the plurality of serially connected LED strings comprises three serially connected LED strings, each LED string comprising one or more light emitting diodes.

17. The device of claim 13, wherein the reference voltage generator is coupled to a diode forward voltage sampling circuit to receive the sampled voltage signal representative of an LED diode forward voltage signal from an LED diode in an LED string.

18. A method for controlling a plurality of LEDs (light emitting diodes) connected in series, the method comprising: Receiving rectified input AC (alternating current) voltage; Generate a reference voltage; varying the reference voltage based on a sampled voltage signal, the sampled voltage signal comprising a peak voltage of the rectified input AC voltage and a peak voltage of a LED diode forward voltage; controlling the flow of current in the plurality of LEDs to be in phase with the rectified input AC voltage based on the reference voltage using a plurality of linear regulators; and A target average LED current is maintained during changes in the rectified input AC voltage and the LED diode forward voltage.

19. The method of claim 18, wherein varying the reference voltage based on the sampled voltage signal comprises: In response to the rectified input AC voltage, decreasing the reference voltage when the sampled voltage signal is increasing, and increasing the reference voltage when the sampled voltage signal is decreasing; and In response to the LED diode forward voltage, the reference voltage is increased when the sampled voltage signal is increased, and the reference voltage is decreased when the sampled voltage signal is decreased.

20. The method of claim 18, further comprising: The sampled voltage signal representative of the rectified AC input voltage and an LED diode forward voltage signal is received.

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