Light emitting diode driving apparatus with high power and wide dimming range

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.

FIELD OF THE INVENTION

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

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.

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.

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.

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

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.

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.

DETAILED DESCRIPTION OF THE INVENTION

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.

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.

Referring to the drawings,FIG. 1is a schematic diagram showing an exemplary embodiment of a driver circuit10in accordance with the present invention. The driver circuit10may be coupled to a light source, such as one or more light-emitting diodes14(LEDs), of a display24having, among other components, a brightness or luminance control device20. Although the driver circuit10is described with respect to the LED light source14, other light-emitting elements may be used with the display24and driver circuit10.

In this exemplary embodiment, the driver circuit10includes, but is not limited to, a variable voltage regulator12, a logarithmic photodetector unit16, a comparator18, the luminance control device20, and a voltage-to-frequency converter22. The variable voltage regulator12includes an output that is coupled with the LEDs14, a first input coupled to the luminance control device20, and a second input coupled to the voltage-to-frequency converter22. The logarithmic photodetector unit16is configured to determine the relative luminance output from the LEDs14. The comparator18includes a first input coupled to the output of the photodetector unit16, a second input coupled to the luminance control device20, and an output. The exponential voltage-to-frequency converter22includes an input coupled to the output of the comparator18and an output coupled to the second input of the variable voltage regulator12. The luminance control device20includes an output coupled to the first input of the variable voltage regulator12and to the second input of the comparator18. In response to a luminance command from the luminance control device20and photopic feedback (e.g., light output) from the LEDs14, the driver circuit10regulates the power supplied to the LEDs14.

The voltage regulator12generates an output voltage signal having a frequency and a pulse width to power and control the luminance output of the LEDs14. In response to the luminance command from the luminance control device20, the voltage regulator12adjusts the pulse width of the output voltage signal, and in response to a variable frequency signal from the converter22, the voltage regulator12adjusts the frequency of the output voltage signal. For lower commanded brightness levels, the voltage regulator12generates an output voltage signal having a shorter pulse width and relatively lower frequency, and at higher commanded brightness levels, the voltage regulator12generates an output voltage signal having a relatively longer pulse width and a relatively higher frequency.

Photopic feedback from the LEDs14is provided by the photodetector unit16to the comparator18. The amount of light generated by the LEDs14is measured by the photodetector unit16and is logarithmically compressed to produce a feedback signal. In this exemplary embodiment, the luminance command is also a logarithmic function. The comparator18compares the feedback signal from the photodetector unit16with the luminance command from the luminance control device20and generates an error signal that drives the voltage-to-frequency converter22. The voltage-to-frequency converter22exponentially converts the error signal from the comparator18to a frequency signal thereby allowing greater control at lower frequencies (e.g., lower brightness levels) than a linear voltage-to-frequency converter.

FIG. 2is a schematic diagram illustrating a more detailed exemplary embodiment of the driver circuit10shown inFIG. 1. In this exemplary embodiment, the driver circuit30includes, but is not limited to, a boost converter32having an output for coupling with one or more strings of LEDs34and having a first input for receiving a supply voltage and a second input, a photodiode36having first and second terminals and configured to detect the light output from the LEDs34, a logarithmic amplifier38having an input coupled across the terminals of the photodiode36, a calibration circuit40having an input coupled to an output of the logarithmic amplifier38, an inversion circuit44having an input coupled to an output of the calibration circuit40, an error amplifier46having a first input coupled to a reference potential (e.g., a ground) and a second input coupled to the output of the inversion circuit44and the luminance command, a compensation circuit48coupled between an output and the second input of the error amplifier46, an exponential voltage-to-frequency converter50having an input coupled to the output of the error amplifier46, a single pulse generator52having a first input coupled to an output of the voltage-to-frequency converter50and having a second input and an output, a field-effect transistor (FET) driver56having an input coupled to the output of the single pulse generator52, a metal-oxide semiconductor FET (MOSFET) switch58having a gate electrode coupled to an output of the FET driver56and 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 converter32, and a pulse width control unit54having a first input coupled to the source electrode of the MOSFET switch58, a second input for receiving the luminance command, and an output coupled to the single pulse generator52.

Additionally, the driver circuit30includes a first capacitor60having a first terminal coupled to the output of the boost converter32and a second terminal coupled to a reference potential (e.g., a ground), a second capacitor62having a first terminal coupled to the output of the boost converter32and having a second terminal, and a switch64having an input for receiving the luminance command and for selectively coupling the second terminal of the second capacitor62with a reference potential (e.g., a ground). The first capacitor60has a relatively smaller capacitance than the second capacitor62and provides a base filtering of the output voltage signal from the boost converter32to 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 switch64couples the second capacitor62to Voutto 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 switch64decouples the second capacitor62from Vout.

The boost converter32converts a low supply voltage (e.g., 16 to 32V) to a high output voltage at Voutand supplies the LEDs34with 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 converter50and has a pulse width that is adjusted by a combination of the single pulse generator52and the pulse width control unit54. For each pulse transmitted by the single pulse generator52, the single pulse generator52initiates a rising edge of the pulse, and the pulse width control unit54initiates a falling edge of the pulse. Each pulse is initiated in response to a frequency signal from the voltage-to-frequency converter50, as described in greater detail hereinafter.

The photodiode36generates a current that is representative of and varies in response to the amount of light output from the LEDs34. As the device name implies, the logarithmic amplifier38compresses the current value generated by the photodiode to a voltage value based on a logarithmic scale. The logarithmic amplifier38is capable of compressing a substantially wide range of current values (e.g., over five decades). This voltage value may be adjusted by the calibration circuit40with respect to a maximum brightness level, and the inversion circuit44inverts the voltage value for summing at the error amplifier46. In one exemplary embodiment, the calibration circuit40is 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).

The error amplifier46compares 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 circuit48may be used to adjust the error signal as is well known to those of skill in the art.

The voltage-to-frequency converter50exponentially converts the logarithmic error signal from the error amplifier46to 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 converter50, the driver circuit30provides a logarithmically scaled control over the frequency of the output voltage signal and provides a wide frequency range for variation.

The single pulse generator52is triggered by the frequency signal from the voltage-to-frequency converter50to initiate pulses at the frequency of the frequency signal. The FET driver56biases the gate of the MOSFET58to switch the boost converter32on and off in response to each pulse from the single pulse generator52and in synchronization with the frequency signal from the voltage-to-frequency converter50. For each pulse that the boost converter32is switched on/off, the duration that the boost converter is switched on/off is determined by the pulse width. The pulse width control unit54adjusts the width of the pulse generated by the single pulse generator52in 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.

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.