Abstract:
A drive system for powering LED triads includes a controller for supplying power to one or more LED triad modules with integral encoding of the desired hue and intensity information. The LED triad modules each include an LED triad and decoding circuitry for activating the individual LED elements of the triad according to the encoded hue and intensity information. In the illustrated configuration, the controller supplies power to the LED triad modules over a pair of conductors, and the supplied power is modulated using a four-phase encoding sequence that is decoded by the decoding circuitry of each LED triad module so that each LED triad module produces light of the desired hue and intensity.

Description:
TECHNICAL FIELD 
     The present invention relates to the provision of color lighting with a triad of red, green and blue light emitting diodes (LEDs), and more particularly to a low-cost drive system for controlling the hue and intensity of the emitted light. 
     BACKGROUND OF THE INVENTION 
     LEDs have been utilized in many monochrome lighting applications, and various manufacturers are now co-packaging triads of red, blue and green LEDs for applications where color control is desired. With such an LED triad, the hue of the emitted light is changed by varying the proportion of drive current among the red, green and blue LEDs, and the intensity of the emitted light is changed by varying the overall drive current while maintaining the proportionality of the individual red, green and blue drive currents. 
     While color control is often deemed to be desirable, the cost of introducing color controllable LEDs in traditionally monochrome applications can be cost prohibitive due to the increase in the number of wires required to address the individual LED devices. Instead of the traditional two wires needed for a monochrome lamp (incandescent or LED), four wires are ordinarily needed for an LED triad. This can be a particular disincentive in applications that require many lighting locations, such as in automotive interior lighting. Accordingly, what is needed is a drive system that reduces the wiring complexity required to control LED triads so that color controllable LEDs can be used more cost-effectively in a variety of applications. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved drive system for powering LED triads, including a controller for supplying power to one or more LED triad modules with integral encoding of the desired hue and intensity information. The LED triad modules each include an LED triad and decoding circuitry for activating the individual LED elements of the triad according to the encoded hue and intensity information. In the illustrated embodiment, the controller supplies power to the LED triad modules over a pair of conductors, and the supplied power is modulated using a four-phase encoding sequence that is decoded by the decoding circuitry of each LED triad module so that each LED triad module produces light of the desired hue and intensity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an LED triad drive system according to the present invention, including a controller and a number of LED triad modules; 
         FIG. 2  is a circuit diagram of a bridge circuit of the controller and one of the LED triad modules; 
         FIGS. 3A and 3B  are exemplary timing diagrams for controlling the bridge circuit of  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, and particularly to  FIG. 1 , the reference numeral  10  generally designates an LED triad drive system including single controller  12  that supplies power to an unspecified number of parallel-connected LED triad modules  14  (M 1 , M 2  . . . Mn) via first and second conductors  16   a  and  16   b . The controller  12  includes a DC power supply  18 , a processor  20  that receives hue and intensity input signals (H, I) on lines  22  and  24 , and a switching circuit  26  for coupling the power supply  18  to the conductors  16   a  and  16   b . Each LED triad module  14  includes a set of three co-packaged red, green and blue LEDs and decoding circuitry for coupling the individual LEDs to the conductors  16   a  and  16   b.    
     In general, the processor  20  and switching circuit  26  of controller  12  constitute an encoder for modulating the power supplied to the LED triad modules  14  based on the hue and intensity inputs, and decoding circuitry in each LED triad module  14  decodes the hue and intensity information and correspondingly activates the individual LEDs. While a particularly cost-effective encoding arrangement is described herein, it should be understood that the present invention is not limited to the disclosed arrangement, and that other suitable encoding/decoding arrangements and circuits can be devised by those skilled in the art. For example, it is possible to encode the hue and intensity information so that one of the two conductors  16   a ,  16   b  can be referenced to same ground potential as controller  12 ; in that case, the ground conductor may be eliminated by referencing the controller  12  and each of the LED triad modules  14  to a common ground potential, such as a conductive frame on which the controller  12  and LED triad modules  14  are mounted. 
     FIGS.  2  and  3 A- 3 B depict circuitry for implementing a preferred encoding/decoding scheme for the LED triad drive system  10  of  FIG. 1 . Referring to  FIG. 2 , the switching circuit  26  is configured as a full H-bridge that is pulse-width modulated by processor  20  via the inputs POS_CTRL and NEG_CTRL to define a four-phase encoding sequence that is decoded by each LED triad module  14 . The specific four-phase encoding sequence in the illustrated embodiment comprises a variable negative pulse for each red LED, a first variable positive pulse for each blue LED, a second variable positive pulse for each green LED, and a variable off interval. The repetition frequency of the sequence is sufficiently high (preferably 120 Hz or higher) so that there is no noticeable flicker due to the pulse modulation. Of course, it will be understood that the color order and pulse polarities of the encoding sequence are arbitrary, and may be different than shown. 
     The H-bridge outputs at terminals  34  and  36 , designated as VPOS and VNEG, are respectively connected to the conductors  16   a  and  16   b  so that the POS_CNTL and NEG_CNTL inputs control their relative polarity. When POS_CNTL is active (high), conductor  16   a  is coupled to the V+ terminal of power supply  18  via the VPOS output terminal  34  of switching circuit  26 , and conductor  16   b  is coupled to the controller ground via the VNEG output terminal  36  of switching circuit  26 . When NEG_CNTL is active (high), conductor  16   b  is coupled to the V+ terminal of power supply  18  via the VNEG output terminal  36 , and conductor  16   a  is coupled to the controller ground via the VPOS output terminal  34 . 
     The positive leg of switching circuit  26  includes an n-channel control transistor  38  gated on and off by the POS_CNTL input, a pull-up resistor  40 , a p-channel transistor  42  coupling the output terminal  34  to V+ via resistor  44 , and an n-channel transistor  46  coupling the output terminal  34  to controller ground. When the POS_CNTL input is low, transistor  46  conducts to couple output terminal  34  (and conductor  16   a ) to controller ground; and when POS_CNTL input is high, transistors  38  and  42  conduct to couple output terminal  34  (and conductor  16   a ) to V+. 
     The negative leg of switching circuit  26  includes an n-channel control transistor  48  gated on and off by the NEG_CNTL input, a pull-up resistor  50 , a p-channel transistor  52  coupling the output terminal  36  to V+ via resistor  54 , and an n-channel transistor  56  coupling the output terminal  36  to controller ground. When the NEG_CNTL input is low, transistor  56  conducts to couple output terminal  36  (and conductor  16   b ) to controller ground; and when NEG_CNTL input is high, transistors  48  and  52  conduct to couple output terminal  36  (and conductor  16   b ) to V+. 
     The variable negative pulse for activating the red LEDs is triggered by a high interval of NEG_CNTL, the first variable positive pulse for activating the green LEDs is triggered by a first high interval of POS_CNTL, the second variable positive pulse for activating the blue LEDs is triggered by a second high interval of POS_CNTL, and the variable off interval is corresponds to an interval where both POS_CNTL and NEG_CNTL are low. Obviously, the POS_CNTL and NEG_CNTL inputs cannot be high at the same time, and in fact, dead time intervals (22 microseconds, for example) are imposed between the red, green and blue control pulses to ensure there is no overlap. 
     The above-described pulse sequence of POS_CNTL and NEG_CNTL for one cycle of the 120 Hz control pulse waveform is graphically illustrated in the timing diagrams of  FIGS. 3A and 3B . The four-phase sequence in any given cycle includes a blue activation interval signified by the first POS_CNTL pulse  60 , a green activation interval signified by the second POS_CNTL pulse  62 , a red activation interval signified by the NEG-CNTL pulse  64 , and an off interval during which both POS_CNTL and NEG_CNTL are low.  FIG. 3A  depicts a minimum intensity condition in which the activation and off intervals are set to a prescribed minimum time such as 22 microseconds.  FIG. 3B , on the other hand, depicts a maximum intensity condition in which the activation intervals are set to a prescribed maximum time equal to nearly one-third of the cycle period. In both examples, the emitted light is white because the blue, green and red activation intervals are equal; changing the color of the emitted light simply involves changing the proportionality of the blue, green and red intervals. For example, the emitted light will be green when the blue and red activation intervals are set to the prescribed minimum intensity, and so on. 
     Returning to  FIG. 2 , each of the LED modules  14  includes an LED triad and decoding circuitry for decoding the above-described four-phase pulse sequence. In other words, the LED modules  14  are configured so that blue, green and red LED  66 ,  68 ,  70  are respectively activated during the blue, green and red activation intervals. The red LED  70  is poled such that it will be forward biased when the NEG_CNTL input is high, while the blue and green LEDs  66  and  68  are oppositely poled, and therefore reverse biased when the NEG_CNTL input is high. When the POS_CNTL input is high, the red LED  70  is reverse biased, and a steering circuit including a pair of cross-coupled transistors  72 ,  74 , a pair of capacitors  76 ,  78  and a pair of diodes  80 ,  82  determine which of the blue and green LEDs  66 ,  68  will be forward biased. When the first POS_CNTL pulse of a given LED activation sequence occurs, the capacitor  76  suppresses the gate voltage of transistor  74  to ensure that transistor  72  turns on first. Once transistor  72  turns on, it holds the cross-coupled transistor  74  off. Meanwhile, capacitor  78  charges through diode  82 . Accordingly, the blue LED  66  is forward biased during first POS_CNTL pulse, but not the green LED  68 . In the dead time interval between the first and second POS_CNTL pulses, the gate of transistor  72  is discharged though diode  80  to turn off transistor  72 . The capacitor  78  is prevented from discharging due to diode  82 , and maintains a forward voltage across transistor  74 . When the second POS_CNTL pulse occurs, transistor  74  immediately turns on, and then holds the cross-coupled transistor  72  off. Accordingly, the green LED  68  is forward biased during second POS_CNTL pulse, but not the blue LED  66 . At the end of the second POS_CNTL pulse, the dead time and ensuing NEG_CNTL pulse reset the decoding circuitry so that the above-described operation will be repeated in the next cycle. 
     In summary, the drive system of the present invention provides a novel and cost-effective way of driving one or more LED triads with a single controller and reduced wiring complexity. When the drive system is used to drive a plurality of LED triad modules  14  as shown in  FIGS. 1-2 , module-to-module hue and intensity variability due to variation in photonic efficiency of the individual LEDs is minimized by performance-binning the LED elements and then accounting for the remaining efficiency variations by judiciously selecting the resistance values of the resistors  84 ,  86  and  88  connected in series with the blue, green and red LEDs  66 ,  68  and  70 . 
     While the present invention has been described with respect to the illustrated embodiment, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.