Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
  • H05B45/20—Controlling the colour of the light
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
  • H05B45/10—Controlling the intensity of the light
  • H05B45/14—Controlling the intensity of the light using electrical feedback from LEDs or from LED modules
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
  • H05B45/40—Details of LED load circuits
  • H05B45/44—Details of LED load circuits with an active control inside an LED matrix
  • H05B45/46—Details of LED load circuits with an active control inside an LED matrix having LEDs disposed in parallel lines

Definitions

• the present invention relates in general to the field of illumination.
• the present invention relates to an illumination system comprising a plurality of LEDs and being capable of generating a light output with a controllable color point.
• Illumination systems for generating light are commonly known, and the same applies to the use of LEDs as light source in such illumination systems. Therefore, a detailed explanation thereof will be omitted here.
• An obvious requirement is that the system can be switched ON and OFF.
• a second requirement is dimmability: it is desirable that the intensity of the light output can be varied.
• a third requirement is color variability: it is desirable that the color of the light output can be varied.
• colors as perceived by the human eye can be described in a two-dimensional color space.
• pure or monochromatic colors i.e. electromagnetic radiation having one frequency within the visible spectrum
• This curve together with a straight line connecting said end points, forms the well-known color triangle. Points within this triangle correspond to so-called mixed colors.
• An important feature of colors is that, when the human eye receives light originating from two light sources with different color points, the human eye does not distinguish two different colors but perceives a mixed color, wherein the color point of this mixed color is located on a straight line connecting the two color points of the two light sources, while the exact position on this line depends on the ratio between the respective light intensities.
• the overall intensity of the mixed color corresponds to the respective light intensities added together.
• three light sources it is possible to render any color point within the triangle defined by the three respective color points.
• a specific type of illumination system is a daylight lamp capable of generating white light and/or capable of simulating the change in light color of daylight from sunrise to sunset.
• Another specific type of illumination system is a replacement for an incandescent lamp, having the same "warm" light output.
• a light source particularly suitable in color systems is the LED, in view of its size and cost, and considering the fact that an LED produces monochromatic light.
• illumination systems have been developed comprising 3 or 4 (or even more) different LED types.
• the RGBW system is mentioned, comprising RED, GREEN, BLUE and WHITE LEDs.
• the LED receives current pulses of a certain duration at a certain repetition frequency, selected to be sufficiently high such as not to lead to perceivable flicker.
• an LED driver For driving an LED, an LED driver is used, capable of generating the required LED current at the corresponding drive voltage.
• An important object of the present invention is to provide an illumination system comprising 4 different LED groups driven by one common driver, in which dimmability and color variability are possible.
• the gist of the present invention is also applicable, however, in an illumination system comprising 2 or 3 different LED groups, or comprising 5 or more different LED groups.
• an LED driver is typically implemented as a current source.
• an LED like any other type of diode, has as a characteristic an almost constant voltage when in its forward conductive state, indicated as forward voltage.
• the driver output current is determined by the driver
• the driver output voltage is determined by the LED.
• an illumination system comprises a controllable current distribution means having one input receiving the driver current and having a plurality of outputs coupled to the respective LED groups for providing the respective LED currents.
• the driver actively sets its output voltage, which is used as a control signal for the current distribution means.
• the current distribution means sets a specific ratio of the respective LED currents.
• controllable current distribution means may comprise a processor provided with a memory containing information defining a relationship between input voltage and output current ratio.
• controllable current distribution means consists of a specific hardware configuration of the LED system.
• figure 1 shows a block diagram schematically illustrating a prior art design of an illumination system
• figure 2 is a graph schematically illustrating the electrical behaviour of a diode
• figure 3 is a block diagram schematically illustrating the design of an illumination system according to one embodiment of the present invention.
• figure 4A is a block diagram schematically illustrating a possible embodiment of the LED system according to the present invention.
• figure 4B is a graph showing the light output of the LED system of figure 4A as a function of the input voltage
• figure 4C is a block diagram schematically illustrating a possible embodiment of the LED system according to the present invention.
• figure 5A is a block diagram schematically illustrating a possible embodiment of the LED system according to the present invention.
• figure 5B is a graph showing the light output of the LED system of figure 5 A as a function of the input voltage
• figure 6A is a block diagram schematically illustrating a possible embodiment of the LED system according to the present invention.
• figure 6B is a graph showing the light output of the LED system of figure 6A as a function of the input voltage
• figure 6C is a block diagram schematically illustrating a possible embodiment of the LED system according to the present invention.
• figure 7A is a block diagram schematically illustrating a possible embodiment of the LED system according to the present invention.
• figure 7B is a graph showing the light output of the LED system of figure 7A as a function of the input voltage
• figure 8A is a block diagram schematically illustrating a possible embodiment of the LED system according to the present invention.
• figure 8B is a graph showing the light output of the LED system of figure 8A as a function of the input voltage
• figure 9 A is a graph schematically illustrating an output voltage of a driver as a function of time according to the present invention
• figure 9B is a graph schematically illustrating an output voltage of a driver as a function of time according to the present invention.
• Figure 1 shows a block diagram schematically illustrating a prior art design of an illumination system 1 comprising driver means 10 and an LED system 20, wherein in this example the LED system 20 comprises four LEDs 21, 22, 23, 24.
• the driver means 10 actually comprises individual drivers 11, 12, 13, 14 dedicated to driving a corresponding one of the LEDs 21, 22, 23, 24.
• the illumination system 1 comprises a control device 2 receiving a user input signal Sui and calculating individual driver control signals for the individual drivers 11 , 12, 13, 14. The figure clearly shows that eight wires are needed to connect the driver means 10 to the LED system 20.
• Figure 2 is a graph schematically illustrating the electrical behaviour of a diode, particularly an LED.
• the horizontal axis represents voltage (arbitrary units), the vertical axis represents current (arbitrary units).
• a diode has two terminals, one being indicated as anode and the other being indicated as cathode. Assuming that a DC voltage is applied across the diode terminals, with the anode being positive and the cathode being negative; this will be indicated as positive bias (righthand side of the graph).
• Vth the current may be considered to be zero and the diode is said to be non-conductive (it is noted that in reality a very small current may flow, but this is neglected here). If the voltage magnitude is above said threshold value Vth, the current rises very steeply as a function of voltage and the diode is said to be forwardly conductive.
• the threshold voltage Vth may be considered to be constant for a single diode specimen, although the value may be different for different types of diode. For instance, for a standard germanium diode, Vth is about 0.3 V, for a standard silicium diode, Vth is about 0.7 V, and for power LEDs, Vth may be in the range of 1 V to 3 V.
• a driver 11, 12, 13, 14 has the characteristics of a voltage source: the load determines the current, and by precisely controlling the voltage, it is possible to set the current.
• the load determines the current, and by precisely controlling the voltage, it is possible to set the current.
• slight variations in the voltage result in large variations in the LED current, while the LED output intensity may be considered to be substantially proportional to the LED current, so that visible intensity variations may result. Therefore, it is typically preferred that a driver has the characteristics of a current source. If this is the case, the load determines the output voltage of the driver. Thus, in both cases, the driver output power is determined by the load.
• FIG. 3 is a block diagram schematically illustrating the design of an illumination system 100 according to one embodiment of the present invention.
• this system has driver means 110 and an LED system 120 comprising four LEDs 21, 22, 23, 24.
• the driver means 110 comprises just one driver 130 having output terminals 131, 132, and the LED system 120 having input terminals 121, 122 comprises controllable current distribution means 140.
• the figure shows that the driver 130 is powered from the mains M, but it is noted that this, although typical, is not essential.
• a control device 2 may receive a user input signal Sui, and may control the driver 130. It is noted that this control device and driver may be integrated.
• the driver 130 has the characteristic of a current source.
• the driver 130 has the characteristic of a voltage source.
• the precise characteristic of the driver should not be interpreted as being a limiting factor. While an ideal voltage source has a vertical characteristic and an ideal current source has a horizontal characteristic, a realistic power source typically has a sloping characteristic intersecting both the current axis and the voltage axis. Nevertheless, in all cases, an LED driven by the driver may have the same working point (a point in the graph of figure 2 defined by the combination of actual voltage and actual current).
• the driver 130 has the characteristic of a voltage source, and that the control device 2 is capable of setting the driver output voltage. It is noted that LED drivers having a controllable output voltage are known per se, so that a detailed explanation thereof is not needed here. According to the principles proposed by the present invention, the output voltage of the driver 130, i.e. the input voltage received by the current distribution means 140, is considered to be a control parameter for the distribution of the current among the LEDs 21, 22, 23, 24.
• the functions f typically being mutually different such that together they define, for the color point of the overall light output, a certain predefined path in the color space.
• the current (function f) is only nonzero within a certain range of input voltages, while this range overlaps with a range of input voltages where all other LEDs have zero current, so that in this overlap range the light output has the pure color of said one LED or group of LEDs.
• the driver 130 supplies the summation of all LED currents.
• the current distribution means 140 does not comprise active processor means but consists of the hardware configuration of the LED system 120. In the following, some exemplary embodiments will be discussed.
• FIG 4A is a block diagram schematically illustrating a possible embodiment of the LED system according to the present invention, indicated in general by the reference numeral 420.
• the input terminals are indicated by reference numerals 121, 122.
• the LED system 420 comprises two groups of LEDs 451, 452. These groups are connected in parallel to the input terminals 121, 122.
• An impedance 461 is connected in series with the first group 451 of LEDs.
• An impedance 462 is connected in series with the second group 452 of LEDs. In the following explanation, it will be assumed that this impedance is resistive, for instance a resistor.
• the first group 451 is shown by the symbol of a single LED, but this does not mean that there is only one LED in the first group.
• the group may actually comprise a plurality of LEDs arranged in series and/or in parallel with each other. These LEDs may be mutually identical, but the group may also comprise LEDs of mutually different colors. Apart from the LEDs, other electrical components may be connected in series and/or in parallel to the LEDs, for instance common diodes. While each individual LED or diode has its individual threshold voltage, as explained with reference to figure 2, the group 451 as a whole has a group threshold voltage VT1 which typically corresponds to the summation of the threshold voltages of LEDs arranged in series. Thus, if the group 451 consists of a series arrangement of three identical LEDs each having an individual threshold voltage Vth, the group threshold voltage VT1 of the group is equal to 3Vth.
• the second group 452 has a group threshold voltage VT2, hereinafter simply indicated as second threshold voltage, larger than the group threshold voltage VT1 of the first group 451, hereinafter simply indicated as first threshold voltage.
• the impedance value of the second impedance 462 in series with the second LED group 452 may differ from the impedance value of the first impedance 461 in series with the first LED group 451.
• the impedance value of the second impedance 462 may be smaller than the impedance value of the first impedance 461, and the second impedance 462 may even be omitted, in which case the function of second impendance will be performed by the series wiring of the second LED group 452.
• figure 4B is a graph showing the light output LI of the first group of LEDs 451 and the light output L2 of the second group of LEDs 452 as a function of the input voltage Vi received at the input terminals 121, 122 of the LED system 420.
• VR1 Vi - VT1 will be the voltage across the resistor 461, so that the current magnitude will be equal to (Vi - VT1)/R1, with Rl indicating the resistance of the resistor 461.
• This current is proportional (in reality: almost linearly proportional) to the input voltage Vi, and hence the first light output LI is proportional to the input voltage Vi.
• the light output of the LED system 420 as a whole has the first color point.
• the ratio between Rl and R2 determines the ratio between the proportionality of LI and L2 versus Vi, respectively.
• R2 is smaller than Rl, so that the current in the second group 452 rises faster as a function of Vi as compared to the current in the first group 451, and it will be
• the number of LEDs in the second group 452 is larger than the number of LEDs in the first group 451 , such that all in all the second light output L2 rises faster than the first light output LI, as illustrated.
• the color points of the LEDs do not play any role. All individual LEDs may even be mutally identical.
• the group color point of the light output of all LEDs of the second group combined differs from the group color point of the light output of all LEDs of the first group combined, hereinafter simply indicated as first color point.
• first color point When all LED groups are placed relatively closely together, a human observer will perceive the overall light output as a blend having one blend color point.
• this blend color point travels in a straight line from the first color point towards the second color point.
• increasing the input voltage causes a change from red light to warm white light, which corresponds to the dimming of an incandescent lamp.
• Figure 4C illustrates a second embodiment 430, in which the second group of LEDs 452 is connected to a node of a voltage divider 430 formed by two resistors 431, 432 connected in series between the input terminals 121, 122.
• this node provides a voltage derived from the input voltage Vi.
• the second group threshold voltage VT2 is lower than the first group threshold voltage, the second group 452 can only start to conduct if the input voltage Vi is equal to or higher than (R432 + R431)/R432 times VT2.
• Figure 5A illustrates a third embodiment 470.
• Figure 5B is a graph comparable to figure 4B, illustrating the behaviour of this third embodiment 470.
• the second resistor 462 is replaced by a resistor 471 in series with the parallel arrangement of first group 451 and second group 452.
• the operation is the same as the operation of the first embodiment 420, with this difference that the current magnitude will be equal to (Vi - VT1)/(R1+R3), with R3 indicating the resistance of the common series resistor 471.
• the driver output voltage will result in the LED system 420; 470 as a whole generating a blend light output of which the color point travels in a straight line from the first color point towards the second color point.
• the first color point is substantially red and the second color point is substantially white.
• the first group 451 consists of precisely one red LED and the second group 452 consists of precisely two white LEDs arranged in series.
• the blend color point will not quite reach the second color point, because the first group 451 is on at all times when the second group 452 is on.
• the light colors may even be mutually equal.
• embodiments are possible where the individual LED groups are placed at a substantial distance from each other, so that for the human observer the light generated by the first group of LEDs originates from a different location than the light generated by the second group of LEDs. This can be used for generating special light effects, such as for instance running lights, a light tube, etc. Also in such embodiment, it would be desirable to be able to switch off the first group while the second group is on.
• Figure 6A illustrates a fourth embodiment 620 of the LED system, comparable to the first embodiment 420 of figure 4A, where a current measuring sensor 672 is arranged between the cathode terminal of the second group 452 and the second input terminal 122, and where an NPN transistor 673 is arranged having its base terminal connected to the node between the current measuring sensor 672 and the second group of LEDs 452, having its emitter terminal connected to the second input terminal 122, and having its collector terminal connected to the node between the first resistor 461 and the first group of LEDs 451.
• NPN transistor instead of an NPN transistor, another type of controllable switch can be used, for instance a FET. The operation is as follows.
• the current through the first resistor 461 becomes equal to Vi/Rl, which may be relatively high if Rl is relatively low.
• the collector- emitter path of a second NPN transistor 674 is arranged between the first input terminal 121 and the first resistor 461.
• a bias resistor 675 is connected between the first input terminal 121 and the base terminal of said second NPN transistor 674.
• the collector terminal of the first NPN transistor 673 is connected to the node between the bias resistor 675 and the base terminal of said second NPN transistor 674.
• the operation is basically similar to the operation of LED system 620: when the input voltage rises above VT2, the increasing current in the second group of LEDs 452 will cause the base terminal of the second transistor 674 to be drawn to the level of the second input terminal 122, thus reducing and eventually cutting off the current in the first group of LEDs 451. Now the wasted current is limited by the bias resistor 675, which may have a much higher resistance than the first resistor 461.
• the light production response as a function of the input voltage Vi is mutually different for the individual groups of LEDs. This is caused by the groups having mutually different threshold voltages or receiving mutually different supply voltages derived from the input voltage, or both. Further, the ratio between the individual light outputs of the individual groups of LEDs is not constant. This even applies if the voltage-dependencies of the individual groups (dL/dVi) are mutually equal, which can be seen in figure 4B by giving the two sloping curves the same angle. In some of the embodiments, a coupling between one group and another group results in a decrease of one light output while the other light input increases as a function of the input voltage. All in all, in all embodiments, the overall color point of the combined light output is not constant but travels a path in color space as a function of input voltage Vi (unless of course the LEDs all emit the same color).
• the path traveled in color space is a straight line between the two color points corresponding to the two groups of LEDs.
• the inventive concept can be expanded in a modular fashion. So, it is possible to have a third group of LEDs, a fourth group of LEDs, etc, connected between the input terminals 121, 122, always with mutually different color point and mutually different threshold voltage. Broadly speaking, it is possible to have N groups of LEDs, each group being indicated as G(i), with i being an index ranging from 1 to N, N being a positive integer larger than 1. Each group G(i) has a group threshold voltage VG(i) and a color point CP(i).
• CP(j) ⁇ CP(i) may apply, and preferably VG(j)>VG(i) applies.
• Each group G(i) is connected in series with at least one impedance.
• Two or more groups may be coupled such as to have one group influence the other group's response.
• two or more groups may have a common series impedance.
• a current reduction circuit for one group may be controlled by the current in another group. It is even possible to have an increasing current in group G(j) that reduces all the current in all groups G(i) with i ⁇ j; figure 6D schematically illustrates the modular layout of such a device.
• LED groups of 3 mutually different color points which may suitably be R, G, B, or there are at least 4 LED groups of 4 mutually different color points, which may suitably be R, G, B, W.
• Figure 7A illustrates an embodiment of an LED system 720 for a situation where the driver 130 is capable of providing a positive and a negative voltage.
• the LED system 720 comprises two systems 620 of figure 6A, individually distinguished as 620A and 620B, connected antiparallel between the input terminals 121, 122.
• 620A and 620B connected antiparallel between the input terminals 121, 122.
• the voltage at the first input terminal 121 is positive with respect to the second input terminal 122
• only the first system 620A is operative, and its operation is identical to the operation of LED system 620 as illustrated in figure 6B.
• the voltage at the first input terminal 121 is negative with respect to the second input terminal 122
• only the second system 620B is operative, and its operation again is identical to the operation of LED system 620 as illustrated in figure 6B.
• Figure 7B illustrates the overal light output as a function of Vi.
• LI indicates the light output of group 451 A.
• L2 indicates the light output of group 452A.
• L3 indicates the light output of group 45 IB.
• L4 indicates the light output of group 452B. It can be seen that
• the light output is pure L 1 ;
• the light output is pure L3;
• the light output is pure L4;
• this LED system 720 is capable of selectively providing light having the color points R or G or B or W by a suitable selection of the driver output voltage.
• Figure 8A illustrates an embodiment of an LED driver 820 that can be seen as a further elaboration of the embodiment 470 of figure 5 A.
• the node between the first group of LEDs 451 and the first resistor 461 will be indicated as first node A, while the node between the first group of LEDs 451 and the common series resistor 471 will be indicated as second node B.
• the second group of LEDs 452 is connected between the first input terminal 121 and the second node B
• this embodiment 820 comprises a third group of LEDs 453 connected between the first node A and the second input terminal 122.
• this embodiment comprises a fourth group of LEDs 454 connected antiparallel with respect to the first group 451 between the first and the second node A and B, respectively.
• the third group 453 may have a third threshold voltage VT3 equal to or larger than the second threshold voltage VT2.
• the fourth group 454 has a fourth threshold voltage VT4.
• the third group has a third color point and the fourth group has a fourth color point.
• VT2 VT3
• the operation is as follows. Five different voltage ranges I, II, III, IV and V can be distinguished. In a first voltage range I, Vi is smaller than VTl and no current will flow.
• Vi is larger than VTl, and current only flows in the path formed by the series arrangement of resistor 461, first LEDs 451, and resistor 471.
• a voltage drop equal to VTl will develop across the first LEDs 451.
• the voltage drop V461 across resistor 461 will be equal to
• V461 R461 x (Vi - VTl) / (R461 + R471)
• V471 R471 x (Vi - VTl) / (R461 + R471)
• R461 and R471 indicating the resistance of the resistors 461 and 471, respectively.
• R461 R471.
• a fourth voltage range IV current only flows in a second and a third current path formed by the series arrangements of the second group 452 and resistor 471 and the series arrangements of the third group 453 and resistor 461, respectively.
• the voltage VA at the first node A will be equal to VT3, and the voltage VB at the second node B will be equal to Vi - VT2.
• the current in the second group 452 will be equal to (Vi - VT2)/R471
• the current in the third group 453 will be equal to (Vi - VT3)/R461.
• a third voltage range III between the second and fourth ranges, current flows in all of said paths, and first group 451, second group 452 and third group 453 are on.
• the precise current distribution between these paths will vary with Vi and will depend on the precise values of VTl, VT2, VT3, R461, R471.
• the lower boundary of the third voltage range III is determined by an input voltage level at which current flow becomes possible in the second or third path. As long as the voltage drop between first input terminal 121 and second node B, which can be expressed as V461 + VTl or as Vi - V471, is smaller than VT2, no current will flow in the second path. Current will start flowing in the second path as soon as Vi becomes higher than VX2, with
• VX2 VTl + (VT2 - VTl)x(R461 + R471)/R461.
• VX3 VTl + (VT3 - VTl)x(R461 + R471)/R471.
• the upper boundary of the third voltage range III is determined by an input voltage level at which current flow becomes impossible in the first path.
• the voltage difference between the two nodes A and B can be expressed as VT2 + VT3 - Vi. If this voltage difference is less than VT1, the first group 451 cannot conduct current.
• the upper boundary of the third voltage range III is equal to VT3 + VT2 - VT1.
• node A While initially node A is positive with respect to node B, it follows from the above that node A is negative with respect to node B if Vi > VT2 + VT3. If the negative voltage difference between nodes B and A becomes larger than VT4, the fourth group of LEDs 454 can conduct current. This occurs in a fifth range V where Vi > VT1 + VT2 + VT3.
• the four color points may be mutually different.
• the third group 453 has the same threshold voltage as the second group 452 and also has the same color point, while also the two resistors 461 and 471 have the same resistance value.
• the second and third groups are driven in a synchronous manner and produce the same light output color.
• the first group 451 has a red color point
• the second and third groups 452 and 453 have a white color point
• the fourth group 454 has a blue color point.
• Such an embodiment is particularly useful as a daylight lamp.
• the driver 130 is capable of providing a negative voltage, there will be a sixth operative range where current only flows in a fourth path defined by the series arrangement of second resistor 471, fourth group of LEDs 454, and first resistor 461.
• the description can be the same as for the second range II, with the first and fourth groups 451 and 454 having switched places.
• the device is capable of rendering three pure colors by suitably setting the input voltage for the LED system.
• the LED system 820 can be made completely symmetrical by adding a fifth group of LEDs 455 (curve L5 in figure 8B) antiparallel to the second group of LEDs 452 and a sixth group of LEDs 456 (curve L6 in figure 8B) antiparallel to the third group of LEDs, as illustrated in figure 8 A in dotted lines.
• the color points of these fifth and sixth groups may be mutually equal. Further, the color points of these fifth and sixth groups may be equal to the color points of the second and third groups, but they may also be different to define a fourth color: in that case, there will be a seventh operative range where the output light only contains this fourth color, and the device is capable of rendering four pure colors by suitably setting the input voltage for the LED system.
• Figure 9 is a graph schematically illustrating the output voltage of the driver 130 (hence input voltage Vi) as a function of time.
• the control device 2 controls the driver 130 so that the output voltage Vi is within the second operative range II from time tl to time t2, so the generated light output will have the first color point. From time t2 to time t3, the control device 2 controls the driver 130 so that the output voltage Vi is within the fourth operative range IV, so the generated light output will have the second/third color point.
• the control device 2 controls the driver 130 so that the output voltage Vi is within the sixth operative range VI, so the generated light output will have the color point of the fourth LEDs 454.
• the control device 2 controls the driver 130 so that the output voltage Vi is within the seventh operative range VII, so the generated light output will have the fourth color point of the fifth/sixth LEDs 455, 456.
• the time interval from tl to t5 will be indicated as color period T.
• this color period T When this color period T is short enough, the human eye will not perceive a sequence of four different colors but rather a blend color; the precise color point of this blend color will depend on the precise durations of the four time intervals and on the precise voltage values within the four time intervals, as should be clear to a person skilled in the art.
• Figure 9A illustrates that the driver's output voltage Vi is maintained constant during said time intervals, but that is not necessary. It is even not necessary that the output voltage Vi is controlled stepwise: it is for instance possible that the output voltage Vi is controlled to have a wave shape such as a sawtooth or a sine.
• figure 9B shows a variation, wherein in each of the time intervals the voltage has the value discussed above for a first amount of time, and is zero for the remaining amount of time. By varying the duty cycle of the voltage in this time interval, the average intensity of the corresponding light output can be controlled between zero and a maximum.
• the present invention succeeds in providing an illumination system comprising an LED system and a single driver for driving this LED system, with a two- wire connection between driver and LED system, which illumination system is capable of rendering all colors within the color triangle RGB, or any other color triangle.
• the driver's output current can be used as a control parameter leading to a certain predetermined current distribution and hence output color.

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