Abstract:
A light emitting diode (LED) driver pad comprising a multiplying digital to analog converter (MDAC), which allows for differing LED characteristics to be matched digitally. Either a plurality of MDACs are integrated onto a single integrated circuit, one MDAC per color of LED, or a single MDAC may be multiplexed to drive a plurality of different color LEDs. The MDAC allows for LED operating current to be set digitally, while allowing an overall brightness or intensity control, thus achieving uniform color balance over a range of operating characteristics.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to drivers for light emitting diodes (LEDs), more particularly to drivers for LEDs of different colors. 
     2. Art Background 
     With the advent of red, green, and blue light emitting diodes (LEDs) their use in color displays has increased. Separate red, green, and blue LEDs can be combined to produce many colors and intensities of light, for example white light for backlighting displays. Ideally to obtain color balance and provide brightness control while maintaining that color balance, the individual red, green, and blue devices would have the same characteristics, such as efficiency, light output for a given drive voltage and current, and so on. This is unfortunately not the case. LEDs for the different primary colors have widely differing drive requirements, luminous outputs, and efficiencies. Additionally, process variations result in performance differences among LEDs of the same color. Consequently, means must be provided in the LED driver circuitry to allow these differing characteristics to be matched. 
     What is needed is an LED driver design for incorporating into an integrated circuit that allows varying LED characteristics to be easily accommodated. 
     SUMMARY OF THE INVENTION 
     A light emitting diode (LED) driver pad is disclosed which allows for varying LED characteristics to be accommodated digitally. One embodiment of the pad integrates a multiplying digital to analog converter into the driver. A second embodiment of the pad integrates a multiplying digital to analog converter with settable minimum output current. A third embodiment uses one multiplying digital to analog converter multiplexed to operate a plurality of LEDs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described with respect to particular exemplary embodiments thereof and reference is made to the drawings in which: 
     FIG. 1 shows an LED driver according to the prior art, 
     FIG. 2 shows a first embodiment of the present invention, 
     FIG. 3 shows a second embodiment of the present invention, and 
     FIG. 4 shows a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     When red, green, and blue LEDs are used in color displays, the drive current through them must be controlled to maintain color balance. Brightness differences in LEDs, both within a color due to manufacturing process variations, and differences in operating characteristics between different color LEDs, make this difficult. If brightness control is to be provided while maintaining color balance, drive current to the separate color LEDs must be individually set. 
     A prior art method of doing this, as used in the MCVVQ101 Backlight Driver integrated circuit from Motorola Inc. is shown in FIG.  1 . For clarity, only one of three drivers is shown. In this driver, the current flowing through LED  100  is controlled by current source  110 . The operating current is set by resistor  120 . Switch  130  represents a master on/off control, and control line  140  allows for brightness control. This design is replicated three times on a single integrated circuit to control red, green, and blue LEDs. To achieve color balance, individual resistors  120  for each of the red, green, and blue drivers must be carefully and individually selected. Achieving precise color balance in the presence of process variation in LED characteristics with this driver design requires careful trimming of resistors  120 . 
     An alternative approach to obtaining precise color balance is to carefully prescreen LEDs and select only those within a narrow operating range. A third alternative is to sacrifice color balance by going with nominal or ballpark values for the performance of LEDs  100  and resistors  120 . 
     None of these three alternatives is particularly palatable, incurring either extra cost in screening LEDs, incurring extra manufacturing cost and time in selecting or trimming resistors  120 , or sacrificing precise color balance. 
     FIG. 2 shows a first embodiment of the present invention. A single color is shown; this design is replicated on the integrated circuit for each of the colors used, typically three times, for red, green, and blue. LED  200  is driven by multiplying digital to analog converter (MDAC)  220 . Multiplying digital to analog converters are known to the art, described for example in chapter 9 of  The Art of Electronics, Second Edition,  by Horowitz and Hill, Cambridge University Press, 1989. Digital inputs  230  control LED current. Control line  240  allows for intensity control, and is common to each MDAC so that a single control line  240  controls the operation of all MDACs. Control line  250  latches the data in MDAC  220 ; depending on the design, this latch may not be part of the MDAC, but may be part of the overall control circuitry (not shown). With this design, current through LED  200  is set digitally, allowing the operating point of each LED to be set easily during the manufacturing process, without needing to trim or select components such as resistors or LEDs, allowing close color balance to be achieved. In practice, four to six bits of resolution are adequate for MDAC  220 ; additional bits provide more resolution at the expense of increased pad complexity and size. While a current output MDAC is preferred in the present invention, it is understood that a voltage output MDACs may be used, each followed by a voltage to current converter. 
     FIG. 3 shows a second embodiment of the present invention using a single MDAC multiplexed to drive three LEDs. LEDs  300 ,  302 , and  304  connect to MDAC  320  through switches  310 ,  312 , and  314  respectively. Digital lines  330  control the current, with line  340  providing intensity control and line  350  latching the data. As before, this latch may be part of MDAC  320  or may be part of the control circuitry. Where a design based on FIG. 2 uses one MDAC for each LED, FIG. 3 multiplexes a single MDAC. This requires external control circuitry (not shown) to scan across the LEDs, closing switches  310 ,  312 , and  314  while providing the correct digital inputs for the corresponding LED at digital inputs  330  and latch control  350 . 
     FIG.  4 . shows an embodiment of the present MDAC invention as implemented using complimentary metal oxide semiconductor (CMOS) technology. This structure is replicated for each of the different color LEDs driven. While a 4 bit device is shown, this may be extended as is known to the art. Data latching previously described is not shown. The MDAC may also be implemented using bipolar technology, or other MOS structures known to the art. LED  400  connects between positive supply terminal  402  and switching terminal  410 . Switches  420 ,  422 ,  424 ,  426  are controlled by their corresponding gates  430 ,  432 ,  434 ,  436 . Current sources  440 ,  442 ,  444 ,  446  form a binary ladder, with each current source supplying twice the current of the previous. Thus current source  440  causes 1× the design current to flow through LED  400  and switch  420 , current source  442  causes 2× the design current to flow, current source  444  causes 4× the design current to flow, and so on. This binary weighting allows the current flowing through LED  400  to be easily adjusted by turning on the appropriate switches  420 ,  422 ,  424 ,  426 . 
     As shown in FIG.  4 . the gates of current sources  440 ,  442 ,  444 ,  446  are tied together and fed from a common source comprised of transistors  450 ,  452 ,  454 , and  456 . By adjusting the current flowing into node  480 , the voltage on gates of current sources  440 ,  442 ,  444 ,  446  is varied, thereby changing the current flowing through the current sources. In this manner the level of the signal presented at node  460  is effectively multiplied by the binary weighting of the current sources ( 440 ,  442 ,  444 ,  446 ) which are activated by their corresponding gates  430 ,  432 ,  434 ,  436 . Gate  470  of transistor  454  provides the ability to effectively shut down the converter. When gate  470  is high, transistor  454  conducts, turning off transistors  440 ,  442 ,  444 ,  446 , and  452 . Transistor  456  provides isolation. As in FIG. 2, node  480  for each of the MDACs present are tied together, providing common control of all MDACs. Transistor  456  provides isolation between sections of each MDAC. 
     The MDAC of FIG. 4 may also be combined with the multiplexing arrangement shown in FIG. 3 for scanned LEDs. 
     In some applications it may be advantageous to keep a default amount current flowing through LED  400 . Having this default amount of current flowing in the LED reduces the number of bits that must be controlled. With the default current, it may be possible to reduce the number of bits in the MDAC to two or three. This may be accomplished by keeping one bit of the MDAC turned on. In the implementations shown in FIGS. 2 and 3, this is accomplished by tying one bit of the MDAC high. This bit does not need to be the least significant bit. In an implementation such as that shown in FIG. 4, this is done by tying the gate  430  of the appropriate switch  420  high, causing current to flow continuously through current source  440  and LED  400 . In another implementation, a separate switch and current source may be used, with the gate of that switch tied high. 
     The foregoing detailed description of the present invention is provided for the purpose of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Accordingly the scope of the present invention is defined by the appended claims.