Abstract:
To prevent inadvertent illumination of a light emitting diode (or set of light emitting diodes) by stray currents at extremely low levels, a quiescent current limiting resistive load is connected in parallel with the light emitting diode, sized to conduct a desired minimum current at the lowest forward voltage drop at which the light emitting diode is expected to properly illuminate. Rather than connecting the resistive load across the input/output ports of the driver circuit, in parallel with any biasing resistance and the light emitting diode, the load is connected directly in parallel with the light emitting diode. Additional current through the quiescent current limiting resistive load as the voltage across the input/output ports increase is thus effectively capped by the maximum forward voltage drop across the light emitting diodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims priority as a continuation-in-part of U.S. patent application Ser. No. 09/675,752 entitled ENHANCED TRIM RESOLUTION VOLTAGE-CONTROLLED DIMMING LED DRIVER and filed Sep. 29, 2000, now U.S. Pat. No. 6,323,598 and is also related to the subject matter of commonly assigned, co-pending U.S. patent application Ser. No. 09/949,139 entitled VOLTAGE DIMMABLE LED DISPLAY PRODUCING MULTIPLE COLORS and filed Sep. 7, 2001. The content of the above-identified applications are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to driver circuits for light emitting diode illumination sources and, more specifically, to voltage-controlled dimming driver circuits for light emitting diode illumination sources employed in place of incandescent lamps within aircraft crewstation instrumentation. 
     BACKGROUND OF THE INVENTION 
     Commercial and military aircraft instrumentation displays, like many other display systems, frequently employ illuminated indicators and controls. Traditionally, incandescent lamps operating at 5 VAC, 14 VDC or 28 VDC have been employed as illumination sources for illuminated pushbutton switches, indicators and annunciators within aircraft instrumentation. The illumination from such incandescent lamps is generally optically filtered to produce a wide range of human visible or night vision imaging system (NVIS) colors, and the small size of incandescent lamps allows multiple lamps to be used within the same display to illuminate different regions of the display in different colors. 
     The luminance required of incandescent displays varies from approximately 400 foot-lamberts at full rated voltage for sunlight-readability in daytime flying to 15 foot-lamberts for commercial/general aviation night flying, 1.0 foot-lamberts for military night flying, and 1.0 foot-lamberts for night flying utilizing NVIS night vision goggles. Because the luminance of incandescent lamps varies with applied voltage within a certain range, output luminance levels of displays are adjusted for night flying conditions by reducing the supplied voltage to approximately one-half or less of the normal full rated operating voltage (i.e. voltage-controlled dimming). 
     The inherent characteristics of incandescent lamps, however, lead to noticeable chromaticity shifts as the applied voltage is reduced. Moreover, incandescent lamps suffer other disadvantages when employed in aircraft instrumentation, including high power consumption, high inrush current, uncomfortably high touch temperatures, and unreliability in high vibration environments. As a result, considerable effort has been expended to incorporate more stable, efficient and reliable technologies, such as light emitting diodes (LEDs), into aircraft crewstation illuminated displays, and to retrofit existing displays. 
     The use of light emitting diodes as a retrofit in illuminated displays for aircraft crewstation instrumentation generally requires connection to aircraft wiring, circuitry and systems originally designed to operate with incandescent lamps. However, light emitting diodes—unlike incandescent lamps—can produce low but detectable levels of illumination with as little as a few microamperes (μA) of current. For a variety of reasons, currents at such levels exist in aircraft wiring and avionics boxes coupled to illuminated displays when the displays are not supposed to be illuminated, and may result in inadvertent or unintentional illumination when light emitting diodes are employed as an illumination source. Experimentation has revealed that indium gallium nitride light emitting diodes (blue, green, or yellow, depending on the indium concentration, or white if packaged with phosphor) are particularly vulnerable to such inadvertent low luminance levels. 
     Because incandescent lamps were essentially immune to inadvertent illumination while light emitting diodes are not, additional driver circuitry is required for light emitting diodes to prevent inadvertent illumination. Requiring a minimum current of 1.0 milliamperes (mA) to illuminate the light emitting diode(s) has been determined through experimentation to be sufficient to prevent inadvertent illumination, even when a few hundred microamperes (μA) of current are unintentionally generated across the light emitting diode driver inputs. 
     For example, a typical light emitting diode driver circuit for employing light emitting diodes as illumination sources in retrofitting aircraft instrumentation is shown in FIG.  3 . Driver  300  includes a biasing resistor R 2  and a light emitting diode L 1  connected in series between input and output ports (“+” and “−”) to which the input voltage is applied. For an input voltage of 28 VDC, a typical resistance value for resistor R 2  would be 1250 ohms (Ω), resulting in a forward voltage drop of approximately 3.0 VDC across light emitting diode L 1  and a current through resistor R 2  and light emitting diode L 1  of approximately 20 mA. For night flying conditions, the applied input voltage across the input and output ports is reduced to a level where the forward voltage drop across light emitting diode L 1  is approximately 2.37 VDC and the total circuit current is approximately 50 μA. This 50 μA circuit current is a level known to be vulnerable to inadvertent illumination, rendering the driver  300  unsuitable. 
     There is, therefore, a need in the art for quiescent current limiting in light emitting diode driver circuits employed for aircraft crewstation instrumentation, and particularly power efficient quiescent current limiting. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in voltage-controlled dimming light emitting diode driver, a quiescent current limiting mechanism to prevent inadvertent illumination of a light emitting diode (or set of light emitting diodes) by stray currents at extremely low levels, which is implemented in the present invention by a resistive load connected in parallel with the light emitting diode. The quiescent current limiting resistive load is sized to conduct a desired minimum current at the lowest forward voltage drop at which the light emitting diode is expected to properly illuminate. Rather than connecting the resistive load across the input/output ports of the driver circuit, in parallel with any biasing resistance and the light emitting diode, the load is connected directly in parallel with the light emitting diode. Additional current through the quiescent current limiting resistive load as the voltage across the input/output ports increase is thus effectively capped by the maximum forward voltage drop across the light emitting diodes. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
     FIG. 1 depicts a circuit diagram for a voltage-controlled dimming light emitting diode driver with quiescent current limiting according to one embodiment of the present invention; 
     FIG. 2 depicts is a circuit diagram for a voltage-controlled dimming light emitting diode driver with quiescent current limiting according to another embodiment of the present invention; 
     FIG. 3 is a circuit diagram for a light emitting diode driver without quiescent current limiting; and 
     FIG. 4 is a circuit diagram for a light emitting diode driver with quiescent current limiting in an inefficient power configuration. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged device. 
     One rather self-evident configuration for connection of a load resistance within the unsatisfactory driver  300  shown in FIG. 3 is depicted in FIG.  4 . In addition to biasing resistor R 2  and light emitting diode L 1  connected in series between input and output ports (“+” and “−”), driver  400  also includes a quiescent current resistor R 1  connected across the input and output ports in parallel with resistor R 2  and light emitting diode L 1 . A resistance value of 2600 ohms (Ω) will insure that driver  400  consumes 1.0 mA of total current when the applied input voltage is adjusted so that the current through the light emitting diode L 1  (and resistor R 2 ) is reduced to the night flying setting of 50 μA. Unfortunately, however, the addition of resistor R 1  as shown adds an additional 10.7 mA of current when the applied input voltage is 28 VDC, the full rated voltage for the exemplary embodiment. The increase of 53.5% in overall power consumption by the driver circuit  400  over the design of FIG. 3 renders this configuration unsatisfactory. 
     FIG. 1 depicts a circuit diagram for a voltage-controlled dimming light emitting diode driver with quiescent current limiting according to one embodiment of the present invention. In addition to biasing resistor R 2  and light emitting diode L 1  connected in series between input and output ports (“+” and “−”), driver  100  also includes a quiescent current resistor R 1  connected in parallel across light emitting diode L 1 , in series with resistor R 2  between the input and output ports. 
     In driver  100 , the resistance of resistor R 1  is approximately 2370 Ω so that current through the resistor R 1  is about 1 mA when the voltage drop across light emitting diode L 1  and resistor R 1  is 2.37 VDC, the forward voltage drop required to produce a current of 50 μA through light emitting diode L 1 . The resistance of biasing resistor R 1  is approximately 1176 Ω to compensate for the additional circuit load. 
     Since the voltage drop across quiescent current limiting resistor R 1  is effectively limited to the maximum forward voltage drop across the light emitting diode L 1 , power dissipation by resistor R 1  at high input voltages is effectively capped. When the forward voltage drop across light emitting diode L 1  increases to 3.0 VDC (with roughly 20 mA of current passing through light emitting diode L 1 ), the current through quiescent current limiting resistor R 1  increases only to 1.26 mA. Thus, at 28 VDC applied across the input and output ports of driver  100 , the total current through the circuit is 21.26 mA, which results in only a 6.3% increase in current over the design in FIG.  3 . 
     Accordingly, quiescent current limiting resistor R 1  is preferably connected directly in parallel with the light emitting diode (or diodes, if a set of series connected LEDs is employed) in a driver circuit for a light emitting diode illumination source. Any biasing resistance should be connected in series with the parallel combination of the light emitting diode(s) and quiescent current resistor, and preferably no significant resistance should appear between a first terminal (anode) of the light emitting diode(s) and a first terminal of the quiescent current limiting resistor or between a second terminal (cathode) of the light emitting diode(s) and a second terminal of the quiescent current limiting resistor. The quiescent current limiting resistor is sized to require a desired minimum total current through the driver at the minimum forward bias voltage for illumination of the light emitting diode, and the resistance of the biasing resistor R 2  is selected with consideration for the additional load represented by the quiescent current limiting resistor R 1 . 
     FIG. 2 is a circuit diagram for a voltage-controlled dimming light emitting diode driver with quiescent current limiting according to another embodiment of the present invention. Circuit  200  includes four white light emitting diodes L 1 -L 4  series-connected in pairs L 1 /L 2  and L 3 /L 4  within two LED groups  201   a  and  201   b . A switching circuit  202  is connected between LED groups  201   a  and  201   b  to switch LED groups  201   a  and  20   b  from series-connection between input and output ports  204   a  and  204   b  to parallel-connection, or vice-versa, as the voltage applied across input and output ports  204   a-   204   b  is varied across a threshold or “kickover” value. 
     Switching circuit  202  includes a switching diode D 1  connected in series between LED groups  201   a  and  201   b , a first resistor R 3  connected in parallel with both LED group  201   a  and switching diode D 1 , and a second resistor R 4  connected in parallel with both LED group  201   b  and switching diode D 1 . 
     The cathode of switching diode D 1  is connected to the anode of the last light emitting diode L 2  (in the direction of the forward voltage drop across the LEDs) within LED group  201   a  and to one end of resistor R 4 ; the anode of switching diode D 1  is connected to the cathode of the first light emitting diode L 3  with LED group  201   b  and to one end of resistor R 3 . An opposite end of resistor R 3  is connected to the cathode of the first light emitting diode L 1  within LED group  201   a , and an opposite end of resistor R 4  is connected to the anode of the last light emitting diode L 4  within LED group  201   b.    
     LED groups  201   a  and  201   b  (comprising light emitting diode pairs L 1 /L 2  and L 3 /L 4 ) are connected by switching circuit  202  either in series or in parallel between input and output ports  204   a - 204   b , depending on the voltage applied across the input and output ports  204   a - 204   b . Switching circuit  202  provides kickover from parallel-connection to series-connection, and vice-versa, of LED groups  201   a - 201   b . Switching diode D 1  and resistors R 3  and R 4  enable the switching mechanism. 
     In operation, circuit  200  operates in two modes: high luminance mode above the kickover point, where the applied input voltage across ports  204   a - 204   b  is greater than the combined forward voltage drops (turn-on voltages) of light emitting diodes L 1 -L 4  and switching diode D 1 ; and low luminance mode below the kickover point, where the applied input voltage across ports  204   a - 204   b  is less than the combined forward voltage drops of light emitting diodes L 1 -L 4  and switching diode D 1  (but greater than the combined forward voltage drops of either of light emitting diode pairs l 1 /L 2  or L 3 /L 4 ). 
     In high luminance mode, switching diode D 1  conducts, and most of the current between ports  204   a - 204   b  passes through the series connected path of light emitting diode pair L 1 /L 2 , switching diode D 1 , and light emitting diode L 3 /L 4 . The primary current path for high luminance control is established by the high luminance resistor R 2 . 
     In low luminance mode, switching diode D 1  stops conducting and the current passes through the two parallel paths comprising: light emitting diode pair L 1 /L 2  and resistor R 4 ; and resistor R 3  and light emitting diode pair L 3 /L 4 . Low luminance mode therefore results when the applied input voltage is insufficient to allow forward current to flow through switching diode D 1 . The primary current path for low luminance control is established by low luminance resistors R 3 -R 4 . 
     Zener diodes Z 1  and Z 2 , in conjunction with high luminance resistor R 2 , provide circuit protection against transients, conducted electromagnetic susceptibility, or an electrostatic discharge event. Zener diodes Z 1  and Z 2  also prevent failure of the entire set of light emitting diodes L 1 -L 4  should a single light emitting diode L 1 -L 4  fail in an electrically open state, providing an alternate current path to maintain circuit integrity with two light emitting diodes still illuminating under such a catastrophic failure condition. 
     In addition to setting the kickover point as a function of input voltage applied across ports  204   a - 204   b , resistor R 2  serves to limit the current of a transient or overvoltage event and also serves to limit the operating current to safe levels in order to prevent a catastrophic failure of the display circuitry. 
     Exemplary values for the relevant components depicted in FIG. 2 are: resistor R 1 =4.32 kiloohms (KΩ); resistor R 2 =1.5 KΩ; resistors R 3  and R 4 =20 KΩ; and light emitting diodes L 1 -l 4  each having forward voltage drops in the range 2.5-3.3 VDC. 
     Resistor R 1  provides a quiescent current path to prevent false or unintentional illumination at low current levels, which otherwise may produce detectable illumination at levels of as low as a few microamperes (μA). Resistor R 1  is located to allow the rise in current across the resistor with applied voltage to halt at the combined forward voltage drops of light emitting diodes L 1 -L 4  and switching diode D 1 , reducing unnecessary power dissipation at higher input voltages. 
     As described above, quiescent current limiting resistor R 1  is connected directly in parallel with light emitting diodes L 1 -L 4 . No significant resistances appears in series between either terminal of resistor R 1  and the corresponding connected terminal of light emitting diode series L 1 -L 4 . The presence of additional resistances R 3  and R 4  also connected in parallel with light emitting diode pairs L 1 /L 2  and L 3 /L 4  does not significantly detract from the power efficiency. improvements of connecting resistor R 1  as shown rather than directly across the input and output ports  204   a  and  204   b.    
     In the configuration shown, the additional current draw over a design lacking quiescent current limiting resistor R 1  is the combined forward voltage drops of light emitting diodes L 1 -L 4  and switching diode D 1  divided by the resistance of resistor R 1 . Power dissipation by resistor R 1  therefore does not scale with increases in voltage across the input and output ports, but is instead effectively capped by the maximum forward voltage drop across the light emitting diode(s) employed to provide illumination. 
     Although the present invention has been described in detail, those skilled in the art will understand that various changes, substitutions, variations, enhancements, nuances, gradations, lesser forms, alterations, revisions, improvements and knock-offs of the invention disclosed herein may be made without departing from the spirit and scope of the invention it its broadest form.