HK1099399B - Control on lcd display by visual light emitting diodes - Google Patents

Description

Control of video LEDs to LCD screen

Technical Field

The present disclosure relates to controller circuits for Light Emitting Diodes (LEDs).

Background

LEDs are increasingly used in the lighting industry, particularly for backlights for Liquid Crystal Displays (LCDs). Lighting devices using LEDs have numerous advantages over fluorescent lighting devices, such as power savings, small size, no use of hazardous materials, etc. In addition, the power supply for the LED is usually operated at a lower voltage, avoiding problems that may be caused by the high voltage of the power supply for the fluorescent lamp. For example, a Cold Cathode Fluorescent Lamp (CCFL) may require more than 1000 volts to start and operate, whereas a single LED may only require about 1-4 volts to operate.

To provide sufficient brightness, the display system requires multiple LEDs to emit a brightness comparable to a single fluorescent lamp. The challenge of using LEDs for lighting systems is to balance the currents in the individual LEDs while optimizing the brightness perceived by the human eye. The brightness of the color and the perception of the color by the human eye are quite different. For example, the human eye perceives yellow as more intense than green. Thus, in applications such as traffic lights, to achieve approximately equal visual brightness, the power allocated to the yellow light is less than that of the green light.

There are currently a variety of configurations for multiple LEDs used in lighting systems. Multiple LEDs may be connected in series, in parallel, or in a series-parallel combination.

Fig. 1A and 1B are power supply circuits 10 and 20, respectively, for parallel LEDs. The parallel LEDs receive a common supply voltage line from the supply circuit. Typically, the current is controlled either by monitoring the total amount of current flowing through all LEDs or by monitoring the current flowing through a single LED. Due to the difference in LED voltage drops, the current carried by each LED may be different, and thus the emitted brightness may be different. Non-uniformity in brightness can affect the life of the LED. Fig. 1C shows an improved power supply circuit 30 in which each LED shares an output. The power supply circuit in this example is complex and expensive. Such a configuration can only be used for low power LED systems containing a few LEDs.

Fig. 2A shows a supply circuit 40 for series-connected LEDs. When the current is large enough, the voltage drop across each LED varies from 1.0 to 4.0 volts. It is the current flowing through it that determines the brightness of the LED. The voltage drop depends on the LED fabrication and may vary widely. Thus, the series configuration has the advantage of being able to adjust the current in the entire string of LEDs so that all LEDs are approximately the same brightness. For a single string of LEDs, it is much better to adjust the current of the LED string according to the supply circuit than to adjust the voltage across the LED string. The power supply for such applications requires the use of current mode control to convert the power supply to a regulated output. In such applications, the number of LEDs in each LED string is limited, which also determines the voltage across the entire LED string. Too high a voltage may limit the application of low cost semiconductor devices in power supply circuits. For example, a 12.1 "LCD display screen, if 40 LEDs are used for illumination, the output voltage of the inverter may reach 150 volts. To generate such high voltages, the cost of the semiconductor switches used would be prohibitive.

Fig. 2B is a power supply 50 for series-parallel LEDs. Many LEDs are divided into strings to reduce the cost of the converter circuit, allowing the use of affordable semiconductor switches. This configuration has the advantage of series connection, i.e. the current supplied to the LEDs is of uniform magnitude within the same LED string. However, as with the parallel LED configuration, the difficulty is how to balance the currents of the individual LED strings. To address this problem, multiple power supplies may be used, each supplying power to one LED string. For example, each LED string is operated by a separate DC/DC converter. However, using multiple power stages to power an LED string is bulky, cost-effective, and complex in construction. Typically, this configuration requires that all power supplies be synchronized to avoid beat noise in the system.

Disclosure of Invention

One embodiment of the present invention uses a controller for the LED array. A direct current/direct current (DC/DC) converter circuit capable of powering the LED array is included in the controller. The LED array comprises at least one first LED string and one second LED string connected together in parallel, each comprising at least 2 LEDs. The controller may also include feedback circuitry that receives a first feedback signal from the first series of LEDs and a second feedback signal from the second series of LEDs. The first feedback signal is proportional to the current in the first LED string and the second feedback signal is proportional to the current in the second LED string. The feedback circuit may also compare the first feedback signal to the second feedback signal, and based on the comparison, control a voltage drop to adjust the current in the first string of LEDs with reference to the current in the second string of LEDs.

In one embodiment of the invention, a scheme is proposed to power an LED array comprising at least a first LED string and a second LED string connected together in parallel, each comprising at least 2 LEDs. The scheme may also include comparing a first feedback signal from the first string of LEDs to a second feedback signal from the second string of LEDs, the first feedback signal being proportional to a current in the first string of LEDs, the second feedback signal being proportional to a current in the second string of LEDs. The scheme may further include controlling a voltage drop of one of the first LED strings to adjust its current with reference to the current in the second LED string based on the comparison.

In the embodiments discussed herein, at least one system embodiment can provide an LED array comprising at least a first LED string and a second LED string connected together in parallel, each comprising at least 2 LEDs. The system may also provide a controller for powering the LED array, the controller further receiving a first feedback signal from the first LED string and a second feedback signal from the second LED string, the first feedback signal being proportional to a current in the first LED string and the second feedback signal being proportional to a current in the second LED string. The controller may also compare the two feedback signals and control a voltage drop across the first LED string to adjust the current in the first LED string with reference to the current in the second LED string based on the comparison.

Drawings

Features and advantages of embodiments of the present invention will be apparent from the following detailed description, in which like numerals represent like parts.

FIGS. 1A-C are conventional LED system layouts;

FIGS. 2A-B are another conventional LED system layout;

FIG. 3 is an exemplary system embodiment of the present invention;

FIG. 4 is another exemplary system embodiment of the present invention;

FIG. 5 is another exemplary system embodiment of the present invention;

FIG. 6 is another exemplary system embodiment of the present invention.

While the following detailed description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the invention is to be construed broadly and its scope is to be limited only by the appended claims.

Detailed Description

Fig. 3 is an exemplary system embodiment 100 of the present invention. The system 100 generally includes an LED array 102 and an LED backlight controller circuit 110. The LED array may be part of an LED backlight for a liquid crystal display. The LED array may include a plurality of LED strings 104, 106, and 108. The LED strings 104, 106 and 108 may each comprise a plurality of LEDs connected in series, e.g., the first LED string 104 comprises a plurality of LEDs connected in series, i.e., LED _11, LED _ 12. Similarly, the second LED string 106 may include a plurality of series connected LEDs, i.e., LED _21, LED _ 22.. LED _2n, and the third LED string may include a plurality of series connected LEDs, i.e., LED _31, LED _ 32.. LED _3 n. The LED strings 104, 106 and 108 are connected in parallel to a power supply, shown as Vout. Thus, the voltage across each LED string is Vout. Each LED string may generate a respective feedback signal 112, 114, and 116 (labeled Isen1, Isen2, and Isen3, respectively). Feedback signals 112, 114 and 116 are proportional to the current in each LED string, respectively.

The LED backlight controller circuit 110 may include a DC/DC converter circuit 120 that may generate a DC power supply Vout using a DC input 122. Controller circuit 110 may be unique or share one or more integrated circuits. In all embodiments herein, "integrated circuit" refers to a semiconductor device and/or microelectronic device, such as a semiconductor integrated circuit chip. Exemplary DC/DC converter circuits 110 may include buck, boost, buck-boost, Sepic, Zeta, Cuk, and/or other known or later developed circuit configurations. The controller circuit 110 may also include a feedback circuit 130, and the feedback circuit 130 may balance the current of the LED strings. In one embodiment of the present invention, the feedback circuit 130 can compare the current in one LED string with the current in another LED string or strings, and adjust the current of one LED string by adjusting the voltage drop of another LED string according to the difference obtained by the comparison or other necessary factors. The typical operation of the feedback circuit 130 will be described in more detail below.

Feedback circuit 130 may include amplification circuits 132, 134, and 136 for LED strings 104, 106, and 108, respectively. The feedback circuit may also include switches 142, 144, and 146 configured to direct the respective feedback signals 112, 114, and 116. In this manner, switches 142, 144, and 146 may be controlled such that the voltage drop across each LED string produces a desired current state within the LED string, as will be described further below. In this embodiment, the switches 142, 144, and 146 may each include a Bipolar Junction Transistor (BJT) that directs the respective current feedback signals 112, 114, and 116 from the emitter through the collector, controlling the value of the signal transmitted by the switches by controlling the base. Offset resistors 152, 154 and 156 may be connected to the inputs of the respective amplifiers to reduce or eliminate offset errors that may be generated by the amplifiers. Sense resistors 162, 164, and 166 may be connected to current feedback signals 112, 114, and 116, respectively, and the input to each amplifier may be a voltage signal across sense resistors 162, 164, and 166. Sense resistors may be used to generate values proportional to the feedback signals 112, 114, and 116. In order to equalize the currents in the respective LED strings, the respective sense resistors must be of uniform size. However, as described in the following embodiments, the detection resistance sizes may be selected to obtain different current values from each other in the respective LED strings.

The current level in any one of the LED strings may be proportional to Vout minus the voltage drop across the corresponding switch. For example, the current in the LED string 104 may be proportional to the difference of Vout minus V (the voltage across the switch 142). In this way, controlling the voltage drop across the switch 142 controls the current in the LED string 104. In this embodiment, with reference to the current in the LED string 106, the current in the LED string 104 can be controlled by controlling the voltage drop across the switch 142.

In this embodiment, the amplifier 132 may be configured to receive the current feedback signal 112 (from the first LED string 104) via the switch 142 and the current feedback signal 114 (from the second LED string 106) via the switch 144. More specifically, the configurable amplifier 132 receives a voltage signal (across the sense resistor 162) proportional to the current feedback signal 112 for the non-inverting input and a voltage signal (across the sense resistor 164) proportional to the current feedback signal 114 for the inverting input. Amplifier 132 may compare the relative values of signals 112 and 114 and generate a control signal 133. The control signal 133 may obtain a value based on the difference between the signals 112 and 114, or other factors as necessary. In this example, the feedback signal 112 may be used for the non-inverting input of the amplifier 132 and the signal 114 may be used for the inverting input of the amplifier 132. The control signal 133 may control the on state of the switch 142 by controlling the base voltage of the switch 142, or the like. The individual switches may be configured such that when the currents in the individual LED strings are equalized, the output of the amplifier is at a low state to fully saturate the individual switches. Doing so may reduce the power dissipation of the transistor in this state.

Controlling the on state of switch 142 operates the voltage drop across switch 142. For example, if signal 112 is greater than 114, amplifier 132 may generate a higher control signal 133 (relative to the state where signal 112 is less than or equal to 114). The higher control signal 133 is applied to the switch 142 such that the base current is reduced and thus the voltage drop across the switch 142 is increased. The increase in the voltage drop across the switch 142 causes the current 112 through the LED string 104 to decrease. This process may continue until the current values of currents 112 and 114 are equal. This operation illustrates a lower voltage drop across the LEDs in string 104 than across the LEDs in string 106.

Similarly, if signal 112 is less than signal 114, amplifier 132 may generate a lower control signal 133 (relative to the state where signal 112 is greater than or equal to signal 114). The lower control signal 133 applied to the switch 142 causes the base current to increase and the voltage drop across the switch 142 to decrease. Reducing the voltage drop across the switch 142 increases the current 112 in the LED string 104. This process may continue until currents 112 and 114 are equal in magnitude.

The configurable amplifier 136 receives the current feedback signal 116 (from the third LED string 108) via the switch 146 and the current feedback signal 112 (from the first LED string 104) via the switch 142. Amplifier 136 may compare the relative values of signals 116 and 112 and generate a control signal 137. The control signal 137 may obtain a value based on the difference between the signals 116 and 112, or other factors as necessary. In this example, the current feedback signal 116 may be applied to the non-inverting input of the switch 136 through the sense resistor 166, and the signal 112 may be applied to the inverting input of the amplifier 136 through the sense resistor 162 including the offset resistor 156. The control signal 137 may control the conductive state of the switch 146, such as controlling the base voltage of the switch 146. Controlling the conductive state of switch 146 may manipulate the voltage drop across switch 146. For example, if signal 116 is greater than signal 112, amplifier 136 may generate a higher control signal 137 (relative to the state where signal 116 is equal to or less than signal 112). The higher control signal 137 applied to the switch 146 causes the base current to decrease and the voltage drop across the switch 146 to increase. The increase in voltage drop across the switch 146 causes the current 116 in the LED string 108 to decrease. This process may continue until currents 116 and 112 are equal in magnitude.

Similarly, if signal 116 is less than signal 112, amplifier 136 may generate a lower control signal 137 (relative to the state where signal 116 is equal to or greater than signal 112). The lower control signal 137 is applied to the switch 146 such that the voltage drop across the switch 146 is reduced. Reducing the voltage drop across the switch 146 causes the current 116 in the LED string 108 to increase. This process may continue until currents 116 and 112 are equal in magnitude.

In this embodiment, the feedback signals 112, 114, and/or 116 may be provided to the DC/DC converter circuit 120. Based on the magnitude of the feedback signals 112, 114, and 116, or other factors as necessary, the DC/DC converter circuit 120 may adjust Vout such that the current in one or more of the LED strings 104, 106, and 108 may reach a predetermined or desired state. Although not shown, the controller circuit 110 may also comprise user-controllable circuitry (which may include software and/or hardware) to preset a desired LCD display brightness level in this embodiment. In this case, the DC/DC converter can adjust the power supplied to the LED array based on user preset values and the value of the feedback signal 116.

Feedback circuit 130 may also include a pass-through circuit 170 that may provide one or more of feedback signals 112, 114, and/or 116 to DC/DC converter circuit 120. In this embodiment, the pass circuit may operate as an OR gate (OR gate) that allows one OR more signals flowing through sense resistors 162, 164, and/OR 166 to reach the converter circuit 120. This allows the circuit 120 to still receive feedback information when one or more of the LED strings 104, 106, and/or 108 become open.

Fig. 4 is another exemplary embodiment 200 of the present invention. In this embodiment, the LED array 102' may include a red LED string 204, at least one of which may emit red light, a blue LED string 206, at least one of which may emit blue light, and a green LED string 208, at least one of which may emit green light. The LED strings 204, 206, and 208 may be connected in parallel to a power supply, shown as Vout. Thus, the voltage across each string is Vout. Each LED string may generate signals 212, 214, and 216 (labeled Isen1, Isen2, and Isen3, respectively). Signals 212, 214, and 216 may be proportional to the current in the respective LED strings.

In this embodiment, it may be desirable to adjust the ratio between red light emitted in LED string 204, blue light emitted in LED string 206, and green light emitted in LED string 208. Thus, feedback circuit 130' may include sense resistors 262, 264, and 266. The resistance values of the sensing resistors 262, 264 and 266 may be varied as desired for the application. The current signals 212, 214, and 216 may be adjusted by adjusting the resistance values of the sensing resistors 262, 264, and 266, respectively. As described in detail above, the signal on sense resistor 262 may be an input to amplifier 132 and is proportional to signal 212. Thus, the amplifier 132 generates the control signal based on the ratio of the sensing resistors 262 and 264, or other factors as necessary, such that the current level in the red LED string 204 is a predetermined multiple/factor of the current level in the blue LED string. Similarly, the amplifier 134 generates the control signal based on the ratio of the sensing resistors 264 and 266, or other factors as necessary, such that the current level in the blue LED string 206 is a preset multiple/factor of the current level in the green LED string 208. Likewise, the amplifier 136 generates the control signal based on the ratio of the sense resistors 266 and 262, or other factors as necessary, such that the current in the green LED string 208 is a multiple/factor of the current in the red LED string 204. In addition, the feedback circuit 130' in this embodiment operates in a similar manner as the feedback circuit 130 of fig. 3.

Fig. 5 is another exemplary embodiment 300 of the present invention. In this embodiment, the feedback circuit 130 ″ may include a burst-mode dimming circuit (burst-mode dimming circuit) for controlling the brightness of one or more of the LED strings 204, 206, and/or 208. The burst mode dimming circuit may adjust the brightness of the LED strings 204, 206, and/or 208 by adjusting the flow of the feedback signals 212, 214, and/or 216, as will be described further below.

Feedback circuit 130 "may include multiplexer circuits 302, 304, and 306. Multiplexer 302 may include a first input configured to receive a first Pulse Width Modulated (PWM) signal 372 and a second input configured to receive control signal 133. The multiplexer 302 generates an output signal 382 based on the control signal 133 and the PWM signal 372. The PWM signal 372 may comprise a low frequency burst mode signal that may be used to control the red LED string 204 to emit a particular intensity. For example, the PWM signal 372 may comprise a rectangular waveform having a selected on-off duty cycle, i.e., the rectangular waveform oscillates from high to low according to a selected duty cycle. The frequency of the PWM signal 372 may be selected to avoid flickering of the LED, for example, a few hundred hertz.

In actual operation, if the PWM signal 372 is high, the multiplexer output signal 382 is the control signal 133. Thus, when the PWM signal 372 is high, the control signal 133 may control the switch 142 in the manner described above. If the PWM signal 372 is low, the output signal 382 may be driven high, causing the switch 142 to open. Of course, the output signal 382 can also be driven high simply by inverting logic within the multiplexer when the PWM signal is low. In this example, the LED string 204 may be an open circuit through which current cannot pass through the LEDs. Thus, the LED string 204 can be repeatedly turned on and off at a selected duty cycle to adjust the average current through the LED string 204 to perform dimming control, thereby obtaining a desired LED string brightness.

The multiplexer 304 may include a first input configured to receive the PWM signal 374 and a second input configured to receive the control signal 135. The multiplexer 304 may generate an output signal 384 based on the PWM signal 374 and the control signal 135. PWM signal 374 may comprise a low frequency burst mode signal that may be used to control blue LED string 206 to emit a particular intensity. For example, the PWM signal 374 may comprise a rectangular waveform having a selected on-off duty cycle, i.e., the waveform oscillates from high to low according to a selected duty cycle. The frequency of the PWM signal 374 may be selected to avoid flickering of the LEDs, e.g., a few hundred hertz.

In operation, if the PWM signal 374 is high, the multiplexer output signal 384 is the control signal 135. Thus, when the PWM signal 374 is high, the control signal 135 can control the switch 144 in the manner previously described. If the PWM signal 374 is low, the output signal 384 can be driven high, causing the switch 144 to open. Of course, the output signal 384 could also be driven high when the PWM signal is low, simply by inverting logic within the multiplexer. In this example, the LED string 206 may be an open circuit, with no current passing through the LEDs therein. Thus, the LED string 206 may be repeatedly turned on and off at a selected duty cycle to adjust the average current flowing therethrough to achieve a desired LED string brightness.

The multiplexer 306 may include a first input configured to receive the PWM signal 376 and a second input configured to receive the control signal 137. The multiplexer 306 may generate an output signal 386 based on the PWM signal 376 and the control signal 137. The PWM signal 376 may comprise a low frequency burst mode signal that may be used to control the green LED string 208 to emit a particular intensity. For example, the PWM signal 376 may comprise a rectangular waveform having a selected on-off duty cycle, i.e., the waveform oscillates from high to low according to a selected duty cycle. The frequency of the PWM signal 376 may be selected to avoid flickering of the LEDs, e.g., a few hundred hertz.

In operation, if the PWM signal 376 is high, the multiplexer output signal 386 is the control signal 137. Thus, when the PWM signal 376 is high, the control signal 137 may control the switch 146 in the manner previously described. If the PWM signal 376 is low, the output signal 386 may be driven high, causing the switch 146 to open. Of course, the output signal 386 may also be driven high when the PWM signal is low simply by inverting logic within the multiplexer. In this example, the LED string 208 may be an open circuit, with no current passing through the LEDs therein. Thus, the LED string 208 may be repeatedly turned on and off at a selected duty cycle to adjust the average current flowing therethrough to achieve a desired LED string brightness.

In one embodiment of the present invention, the duty cycle of one or more PWM signals can be adjusted to accommodate other PWM signals to make the effect perceived by the human eye better. For example, the duty cycle ratio of the PWM signal 372 controlling the red LED string to the PWM signal 374 controlling the blue LED string and/or the PWM signal 376 controlling the green LED string may be 2: 1. When dimming requires that the red LED be adjusted to 60% on 40% off, the green and blue LED strings may both be 30% on 70% off to optimize the visual effect, making the overall effect closer to white light quality. Thus, the duty cycles of the PWM signals 372, 374 and 376 in the present invention are duty cycles that can be selected and/or programmed to accommodate each other.

Fig. 6 is another exemplary system embodiment 400 of the present invention. In this embodiment, the DC/DC converter circuit 120' may comprise a boost converter. The boost converter includes a first error amplifier 402 for comparing a current feedback signal from the LED array 102' with a regulation signal. The error amplifier 402 compares the current detection signal Isen with the reference signal ADJ. The result of the signal is then compared to the ramp compensated current sense signal in the boost converter switch. The current through the switch adds a sawtooth signal through the adder 406. The output of 406 is one of the inputs to comparator 404. The output of comparator 404 is a square wave that is provided to a drive, such as an oscillator, to drive the switches in the boost converter.

As already mentioned above, the current ratio in the individual LED strings may be adjusted by burst mode dimming and/or selecting the resistance values of the sense resistors 262, 264, and/or 266. In this embodiment, the feedback circuit 130' ″ may include amplifiers 432, 434, and 436 for adjusting the effective resistances of the associated sensing resistors 262, 264, and 266, respectively. In this example, programmable input signals 422, 424, and 426 are provided to amplifiers 432, 434, and 436, respectively. The programmable input signals 422, 424, and 426 may be proportional to the current level required in a given string of LEDs.

In actual operation, the value of the input signal 422 may be increased or decreased, and accordingly, the effective resistance of the sensing resistor 262 may be increased or decreased. As already mentioned above, this may enable the current magnitudes in the first LED string and the second LED string to be scaled. The value of the input signal 424 may be adjusted up or down and, accordingly, the effective resistance of the sense resistor 264 may be adjusted up or down. As already mentioned above, this may enable the current magnitudes in the second LED string and the third LED string to be scaled. Similarly, the value of input signal 426 may be adjusted up or down, and accordingly, the effective resistance of sensing resistor 266 may be adjusted up or down. As already mentioned above, this may enable the current magnitudes in the third LED string and the first LED string to be proportioned. These operations may produce a desired and/or programmable current in one or more LED strings.

Of course, all embodiments discussed herein may be extended to include n LED strings. From the above discussion, if n LED strings are used, a corresponding number of amplifier circuits and switches may need to be used. Similarly, depending on the number of LED strings used, a corresponding number of multiplexer circuits may be required.

The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such equivalents, of the features shown and described (or portions thereof), and it is recognized that various modifications are possible within the scope of the claims. Other possible modifications, variations and alternatives also exist. Accordingly, it is intended that the claims include all such equivalents.

Claims (28)

1. A controller for a Light Emitting Diode (LED) array, comprising: a DC/DC converter circuit for powering an LED array comprising at least a first LED string and a second LED string connected together in parallel, each containing at least two LEDs;

characterized in that, this controller still includes:

the feedback circuit is used for receiving a first feedback signal from the first LED string and a second feedback signal from the second LED string, wherein the first feedback signal is in direct proportion to the current in the first LED string, the second feedback signal is in direct proportion to the current in the second LED string, the feedback circuit also compares the first feedback signal and the second feedback signal, and based on the comparison, the feedback circuit further controls the voltage drop to adjust the current in the first LED string by referring to the current in the second LED string.

2. The controller of claim 1, wherein:

the DC/DC converter circuit may be selected from buck, boost, buck-boost, Sepic, Zeta, Cuk DC/DC converter topologies.

3. The controller of claim 1, wherein:

the feedback circuit comprises at least one amplifier circuit for comparing a first feedback signal with a second feedback signal and generating a control signal, and a switch connected in series with the first feedback signal, wherein the control signal controls the voltage drop across the switch by controlling the state of the switch.

4. The controller of claim 3, wherein:

the feedback circuit further comprises a first detection resistor connected to the first feedback signal and an input terminal of the amplifier circuit, and a second detection resistor connected to the second feedback signal and a second input terminal of the amplifier circuit, wherein the first detection resistor and the second detection resistor have the same resistance value.

5. The controller of claim 3, wherein:

the feedback circuit further comprises a first detection resistor connected to the first feedback signal and an input terminal of the amplifier circuit, and a second detection resistor connected to the second feedback signal and a second input terminal of the amplifier circuit, wherein the first detection resistor and the second detection resistor have different resistances.

6. The controller of claim 3, wherein:

if the first feedback signal is greater than the second feedback signal, the amplifier circuit controls the switch to increase a voltage drop across the switch with reference to a current in the second LED string, thereby reducing the current in the first LED string.

7. The controller of claim 3, wherein:

if the first feedback signal is less than the second feedback signal, the amplifier circuit controls the switch to reduce the voltage drop across the switch with reference to the current in the second LED string, thereby increasing the current in the first LED string.

8. The controller of claim 1, wherein:

the DC/DC converter is capable of receiving at least a first signal proportional to the first feedback signal or a second signal proportional to the second feedback signal, the DC/DC converter adjusting the power supply to the LED array accordingly.

9. The controller of claim 3, wherein:

the feedback circuit further comprises a burst mode dimming circuit connected to at least one of the first LED string or the second LED string, wherein the burst mode feedback circuit can adjust the brightness of the first LED string or the second LED string by adjusting the flow of the first feedback signal or the second feedback signal.

10. The controller of claim 9, wherein:

the burst mode dimming circuit comprises a multiplexer circuit having a first input connected to a Pulse Width Modulation (PWM) signal, a second input connected to the control signal, and an output connected to the switch, the on-state of the switch being controlled by the control signal and the PWM signal.

11. An illumination system using LEDs, comprising: an LED array including at least a first LED string and a second LED string connected together in parallel, each containing at least two LEDs;

characterized in that, the system also includes:

a controller capable of powering the LED array, the controller further receiving a first feedback signal from the first LED string and a second feedback signal from the second LED string, the first feedback signal being proportional to a current in the first LED string and the second feedback signal being proportional to a current in the second LED string, the controller further comparing the first feedback signal to the second feedback signal, the controller further controlling a voltage drop to adjust the current in the first LED string with reference to the current in the second LED string based on the comparison.

12. The system of claim 11, wherein:

the controller includes a DC/DC converter circuit for providing DC power to the LED array, the DC/DC converter circuit being selectable from a buck, boost, buck-boost, Sepic, Zeta, Cuk DC/DC converter topology.

13. The system of claim 11, wherein:

the controller comprises a feedback circuit, the feedback circuit comprises at least one amplifier circuit used for comparing a first feedback signal with a second feedback signal and generating a control signal, the feedback circuit also comprises a switch which is connected with the first feedback signal in series, and the control signal controls the conducting state of the switch so as to control the voltage drop between the two ends of the switch.

14. The system of claim 13, wherein:

the feedback circuit further includes a first sensing resistor coupled to the first feedback signal and an input of the amplifier circuit, and a second sensing resistor coupled to the second feedback signal and a second input of the amplifier circuit, wherein the first sensing resistor and the second sensing resistor are the same size.

15. The system of claim 13, wherein:

the feedback circuit further includes a first sensing resistor coupled to the first feedback signal and an input of the amplifier circuit, and a second sensing resistor coupled to the second feedback signal and a second input of the amplifier circuit, wherein the first sensing resistor and the second sensing resistor are different in magnitude.

16. The system of claim 13, wherein:

if the first feedback signal is larger than the second feedback signal, the control signal refers to the current in the second LED string, and the control switch increases the voltage drop between the two ends of the control signal so as to reduce the current in the first LED string.

17. The system of claim 13, wherein:

if the first feedback signal is smaller than the second feedback signal, the control signal refers to the current in the second LED string, and the control switch reduces the voltage drop across the control signal to increase the current in the first LED string.

18. The system of claim 12, wherein:

the DC/DC converter is capable of receiving at least a first signal proportional to the first feedback signal or a second signal proportional to the second feedback signal, the DC/DC converter adjusting the power supply to the LED array accordingly.

19. The system of claim 11, wherein:

the first LED string includes a plurality of LEDs of a same color selected from a red LED, a blue LED, and a green LED;

the second LED string includes a plurality of LEDs of the same color selected from red LEDs, blue LEDs, and green LEDs.

20. The system of claim 13, wherein:

the feedback circuit further includes a burst mode dimming circuit coupled to at least one of the first and second LED strings and capable of adjusting the brightness of the first or second LED string by adjusting the flow of the first or second feedback signal.

21. The system of claim 20, wherein:

the burst mode dimming circuit comprises a multiplexer circuit having a first input connected to a PWM signal, a second input connected to the control signal, and an output connected to the switch, the on-state of the switch being controlled by the control signal and the PWM signal.

22. A method of lighting using LEDs, comprising: supplying power to an LED array comprising at least a first LED string and a second LED string connected together in parallel, each comprising at least 2 LEDs;

characterized in that the method further comprises:

comparing a first feedback signal from the first LED string having a magnitude proportional to a current in the first LED string with a second feedback signal from the second LED string having a magnitude proportional to a current in the second LED string;

at least one voltage drop of the first LED string is controlled based on a comparison of the first feedback signal and the second feedback signal, and the current in the first LED string is regulated with reference to the current in the second LED string.

23. The method of claim 22, further comprising:

generating a control signal indicating a difference between the first feedback signal and the second feedback signal based on a comparison of the first feedback signal and the second feedback signal;

the conducting state of a switch connected in series with the first or second feedback signal is controlled to control the voltage drop across the switch.

24. The method of claim 23, wherein:

if the first feedback signal is larger than the second feedback signal, the control signal controls the switch to increase the voltage drop between the two ends of the switch, and the current in the first LED string is reduced by referring to the current in the second LED string.

25. The method of claim 23, wherein:

if the first feedback signal is smaller than the second feedback signal, the control signal controls the switch to reduce the voltage drop between the two ends of the switch, and the current in the first LED string is increased by referring to the current in the second LED string.

26. The method of claim 22, further comprising:

adjusting power supplied to the LED array based on one or both of the first feedback signal and the second feedback signal.

27. The method of claim 22, wherein:

adjusting the brightness of the first or second LED string by adjusting the flow of the first or second feedback signal.

28. The method of claim 27, wherein:

the flow of the first or second feedback signal is adjusted according to a PWM signal.