Abstract:
Apparatus are provided for driving an LED light source and controlling a light output of the light source over a wide luminance range in response to a luminance input. The apparatus includes: a voltage source having an output configured to couple with the light source, a first input configured to receive the luminance input, and a second input; a photodetector unit configured to logarithmically compress the luminance range and determine a voltage based on a luminance of the light source in the compressed range; a comparator having an input coupled to the photodetector unit and having an output; and, a signal converter having an input coupled to the output of the comparator and having an output coupled to the second input of the voltage source. The voltage source is configured to generate an output signal at the output of the voltage source. The output signal has a frequency and a pulse width based on the luminance input. The comparator is configured to determine an error signal based on a comparison of the luminance input with the voltage. The converter is configured to exponentially convert the error signal to the frequency.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to a display device, and more particularly, to method and apparatus for lighting and controlling lighting in the display devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     A display used in avionics environments (e.g., on board aircraft) may be operated in a variety of ambient lighting conditions. For example, a cockpit is generally brighter during a daytime flight than a nighttime flight, and the display may be required to generate a brighter image during the daytime flight than the nighttime flight. For each lighting condition, the display provides a sufficiently bright image for viewing by aircraft personnel.  
         [0003]     Fluorescent lamps have been used as a light source for backlit displays. When increasing brightness of the backlit display, the fluorescent lamp phosphor tends to generate more heat, and operation of the fluorescent lamp at higher brightness may exceed the capability of the phosphor. To maintain efficiency, one common practice is to cool a portion of the fluorescent lamp to maintain a “cold-spot” which results in a lower overall temperature of the fluorescent lamp. With greater brightness demands, such as common with avionics displays, maintaining the cold-spot of the fluorescent lamp becomes increasingly more difficult.  
         [0004]     Light-emitting diodes (LEDs) have been used as a light source for backlit displays and also generate heat when operating to provide increased brightness. In comparison with the fluorescent lamp, removing heat from an LED is more readily accomplished. The overall temperature of the LED is generally lower than the fluorescent lamp when operating under greater brightness demands.  
         [0005]     Accordingly, it is desirable to provide an apparatus for powering an LED based light source and controlling an output thereof. In addition, it is desirable to provide an avionics light source having a broad dimming range and high brightness. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     Apparatus and method are provided for driving a light source and controlling a light output of the light source over a wide luminance range. In one exemplary embodiment, an apparatus for driving a light source over a luminance range in response to a luminance input is provided including, but not limited to, a voltage source having an output configured to couple with the light source, a photodetector unit configured to logarithmically compress the luminance range within a voltage range and determine a luminance of the light source in the voltage range, a comparator having an input coupled to the photodetector unit and an output, and a signal converter having an input coupled to the output of said comparator and having an output coupled to the first input of the voltage source. The voltage source further includes a first input configured to receive the luminance input and a second input. The comparator is configured to determine an error signal based on a comparison of the luminance input and the luminance. The converter is configured to exponentially convert the error signal to a frequency. The voltage source is configured to pulse an output voltage at the frequency at the output of the voltage source.  
         [0007]     In another exemplary embodiment, a lighting apparatus for a display having a luminance input is provided including, but not limited to a light source configured to generate a luminance based on the luminance input, a voltage source having an output configured to couple with the light source having first and second inputs, a photodetector unit configured to generate a first signal in response to the luminance and logarithmically convert the first signal to a second signal, a comparator having an input coupled to the photodetector unit and having an output, and a signal converter having an input coupled to the output of the comparator and having an output coupled to the second input of the voltage source. The first input of the voltage source is configured to receive the luminance input. The voltage source is configured to generate an output signal for powering the light source. The output signal has a voltage value, a frequency, and a pulse width. The pulse width is based on the luminance input. The comparator is configured to determine an error signal based on a comparison of the luminance input with the second signal. The converter is configured to exponentially convert the error signal to the frequency. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0009]      FIG. 1  is a schematic diagram showing an exemplary embodiment of a driver circuit in accordance with the present invention; and  
         [0010]      FIG. 2  is a schematic diagram illustrating a more detailed exemplary embodiment of the driver circuit shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.  
         [0012]     The present invention is described in terms of functional block diagrams. Those of skill in the art will appreciate that such functional blocks may be realized in many different forms of hardware, firmware, and/or software components configured to perform the various functions. For example, the present invention employs various integrated circuit components, e.g., memory elements, digital signal processing elements, look-up tables, and the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. Such general techniques are known to those skilled in the art and are not described in detail herein.  
         [0013]     The present invention is a driver circuit for powering and controlling an LED based light source (e.g., backlight) for displays that is ideally suited for avionics environments. The driver circuit efficiently delivers power to the light source over a substantially wide dimming range (e.g., greater than about 50,000 to 1) and to fulfill high brightness demands typically associated with avionics displays. In heads-up displays (HUDs), such as used in the avionics environment, the invented driver circuit efficiently powers the LED light source over a wide dimming range and provides sustainable brightness levels to meet the varying operating condition of the HUDs.  
         [0014]     Referring to the drawings,  FIG. 1  is a schematic diagram showing an exemplary embodiment of a driver circuit  10  in accordance with the present invention. The driver circuit  10  may be coupled to a light source, such as one or more light-emitting diodes  14  (LEDs), of a display  24  having, among other components, a brightness or luminance control device  20 . Although the driver circuit  10  is described with respect to the LED light source  14 , other light-emitting elements may be used with the display  24  and driver circuit  10 .  
         [0015]     In this exemplary embodiment, the driver circuit  10  includes, but is not limited to, a variable voltage regulator  12 , a logarithmic photodetector unit  16 , a comparator  18 , the luminance control device  20 , and a voltage-to-frequency converter  22 . The variable voltage regulator  12  includes an output that is coupled with the LEDs  14 , a first input coupled to the luminance control device  20 , and a second input coupled to the voltage-to-frequency converter  22 . The logarithmic photodetector unit  16  is configured to determine the relative luminance output from the LEDs  14 . The comparator  18  includes a first input coupled to the output of the photodetector unit  16 , a second input coupled to the luminance control device  20 , and an output. The exponential voltage-to-frequency converter  22  includes an input coupled to the output of the comparator  18  and an output coupled to the second input of the variable voltage regulator  12 . The luminance control device  20  includes an output coupled to the first input of the variable voltage regulator  12  and to the second input of the comparator  18 . In response to a luminance command from the luminance control device  20  and photopic feedback (e.g., light output) from the LEDs  14 , the driver circuit  10  regulates the power supplied to the LEDs  14 .  
         [0016]     The voltage regulator  12  generates an output voltage signal having a frequency and a pulse width to power and control the luminance output of the LEDs  14 . In response to the luminance command from the luminance control device  20 , the voltage regulator  12  adjusts the pulse width of the output voltage signal, and in response to a variable frequency signal from the converter  22 , the voltage regulator  12  adjusts the frequency of the output voltage signal. For lower commanded brightness levels, the voltage regulator  12  generates an output voltage signal having a shorter pulse width and relatively lower frequency, and at higher commanded brightness levels, the voltage regulator  12  generates an output voltage signal having a relatively longer pulse width and a relatively higher frequency.  
         [0017]     Photopic feedback from the LEDs  14  is provided by the photodetector unit  16  to the comparator  18 . The amount of light generated by the LEDs  14  is measured by the photodetector unit  16  and is logarithmically compressed to produce a feedback signal. In this exemplary embodiment, the luminance command is also a logarithmic function. The comparator  18  compares the feedback signal from the photodetector unit  16  with the luminance command from the luminance control device  20  and generates an error signal that drives the voltage-to-frequency converter  22 . The voltage-to-frequency converter  22  exponentially converts the error signal from the comparator  18  to a frequency signal thereby allowing greater control at lower frequencies (e.g., lower brightness levels) than a linear voltage-to-frequency converter.  
         [0018]      FIG. 2  is a schematic diagram illustrating a more detailed exemplary embodiment of the driver circuit  10  shown in  FIG. 1 . In this exemplary embodiment, the driver circuit  30  includes, but is not limited to, a boost converter  32  having an output for coupling with one or more strings of LEDs  34  and having a first input for receiving a supply voltage and a second input, a photodiode  36  having first and second terminals and configured to detect the light output from the LEDs  34 , a logarithmic amplifier  38  having an input coupled across the terminals of the photodiode  36 , a calibration circuit  40  having an input coupled to an output of the logarithmic amplifier  38 , an inversion circuit  44  having an input coupled to an output of the calibration circuit  40 , an error amplifier  46  having a first input coupled to a reference potential (e.g., a ground) and a second input coupled to the output of the inversion circuit  44  and the luminance command, a compensation circuit  48  coupled between an output and the second input of the error amplifier  46 , an exponential voltage-to-frequency converter  50  having an input coupled to the output of the error amplifier  46 , a single pulse generator  52  having a first input coupled to an output of the voltage-to-frequency converter  50  and having a second input and an output, a field-effect transistor (FET) driver  56  having an input coupled to the output of the single pulse generator  52 , a metal-oxide semiconductor FET (MOSFET) switch  58  having a gate electrode coupled to an output of the FET driver  56  and having a source electrode and a body electrode coupled to the source electrode and a drain electrode coupled with the second input of the boost converter  32 , and a pulse width control unit  54  having a first input coupled to the source electrode of the MOSFET switch  58 , a second input for receiving the luminance command, and an output coupled to the single pulse generator  52 .  
         [0019]     Additionally, the driver circuit  30  includes a first capacitor  60  having a first terminal coupled to the output of the boost converter  32  and a second terminal coupled to a reference potential (e.g., a ground), a second capacitor  62  having a first terminal coupled to the output of the boost converter  32  and having a second terminal, and a switch  64  having an input for receiving the luminance command and for selectively coupling the second terminal of the second capacitor  62  with a reference potential (e.g., a ground). The first capacitor  60  has a relatively smaller capacitance than the second capacitor  62  and provides a base filtering of the output voltage signal from the boost converter  32  to minimize voltage spikes that may contribute to electromagnetic interference (EMI). At higher brightness levels, the output voltage signal has a higher frequency that approaches the characteristics of a direct current (DC) voltage. Upon receiving a luminance command corresponding to the higher brightness levels, the switch  64  couples the second capacitor  62  to V out  to assist in filtering the output voltage signal. At lower brightness levels (e.g., upon receiving a luminance command corresponding to the lower brightness levels), the switch  64  decouples the second capacitor  62  from V out .  
         [0020]     The boost converter  32  converts a low supply voltage (e.g., 16 to 32V) to a high output voltage at V out  and supplies the LEDs  34  with the output voltage signal. The particular supply voltage value may be selected based on conventional input voltage values used for displays. The output voltage signal has a frequency that is adjusted by the voltage-to-frequency converter  50  and has a pulse width that is adjusted by a combination of the single pulse generator  52  and the pulse width control unit  54 . For each pulse transmitted by the single pulse generator  52 , the single pulse generator  52  initiates a rising edge of the pulse, and the pulse width control unit  54  initiates a falling edge of the pulse. Each pulse is initiated in response to a frequency signal from the voltage-to-frequency converter  50 , as described in greater detail hereinafter.  
         [0021]     The photodiode  36  generates a current that is representative of and varies in response to the amount of light output from the LEDs  34 . As the device name implies, the logarithmic amplifier  38  compresses the current value generated by the photodiode to a voltage value based on a logarithmic scale. The logarithmic amplifier  38  is capable of compressing a substantially wide range of current values (e.g., over five decades). This voltage value may be adjusted by the calibration circuit  40  with respect to a maximum brightness level, and the inversion circuit  44  inverts the voltage value for summing at the error amplifier  46 . In one exemplary embodiment, the calibration circuit  40  is coupled to a variable resistor (e.g., a potentiometer) having one terminal coupled to a first reference voltage (e.g., Vref) and another terminal coupled to a second reference voltage (e.g., ground).  
         [0022]     The error amplifier  46  compares the luminance command value with the inverted voltage value, both of which are logarithmic functions, and outputs a logarithmic error signal based on any difference between the luminance command value and the inverted voltage value. The compensation circuit  48  may be used to adjust the error signal as is well known to those of skill in the art.  
         [0023]     The voltage-to-frequency converter  50  exponentially converts the logarithmic error signal from the error amplifier  46  to a frequency signal. The net effect of exponentially converting the logarithmic error signal to a frequency is that a small amount of change in brightness level at higher brightness levels generally amounts to a similar change at lower brightness levels. Using the logarithmic based error signal in combination with the exponential voltage-to-frequency converter  50 , the driver circuit  30  provides a logarithmically scaled control over the frequency of the output voltage signal and provides a wide frequency range for variation.  
         [0024]     The single pulse generator  52  is triggered by the frequency signal from the voltage-to-frequency converter  50  to initiate pulses at the frequency of the frequency signal. The FET driver  56  biases the gate of the MOSFET  58  to switch the boost converter  32  on and off in response to each pulse from the single pulse generator  52  and in synchronization with the frequency signal from the voltage-to-frequency converter  50 . For each pulse that the boost converter  32  is switched on/off, the duration that the boost converter is switched on/off is determined by the pulse width. The pulse width control unit  54  adjusts the width of the pulse generated by the single pulse generator  52  in response to the luminance command. In one exemplary embodiment, the width is adjusted on a scale of about 5:1 power variation although other ratios of power variation may be used.  
         [0025]     Although the invented driver circuit is described in terms of powering conventional LED based light sources, the driver circuit may also be applied to a variety of light sources having a wide dimming range. For example, the driver circuit is suited for a full-color display implementation having colored lighting elements to power and control each of such elements over a wide dimming range.  
         [0026]     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.