Abstract:
A low noise method and apparatus for driving electroluminescent (EL) panels provides reduction in EL panel noise, and in the preferred embodiment, voltage compensation as the EL panel ages. The output of a high voltage power supply is applied to the EL panel through a circuit having an optimized time-constant, producing a waveform that reduces EL panel vibration. Additionally, if the time constant is derived in part from the capacitance of the EL panel, the time constant may be chosen such that aging of the EL panel maintains the time constant in the proper range to maintain noise reduction over the life of the panel and compensate for brightness reduction due to aging of the EL panel.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to electroluminescent (EL) panels, and more specifically, to a circuit for driving an EL panel to achieve low noise operation.  
           [0003]    2. Background of the Invention  
           [0004]    Electroluminescent (EL) panels are in common use as backlights for keyboards and displays. Recent uses of EL panels include wrist watches, cellular telephone displays and keyboards, notebook computers and personal digital assistants (PDAs). In order to produce illumination from an EL panel, an alternating high voltage power supply is required. An EL panel driver includes a high voltage power supply, and a mechanism for switching the high voltage power supply output to produce a high voltage output of alternating polarity for connection to the EL panel.  
           [0005]    EL panels are fabricated as a laminate structure, with a phosphor-dielectric layer sandwiched between a pair of electrodes. Outer cover layers are then applied to the outer surfaces of the electrodes, producing a insulated and protected panel. When the EL panel is driven with the above-described alternating high voltage signal, vibration is produced via a piezoelectric effect between the electrodes. Since typical drive frequencies are in the audible range, the vibration is perceivable to the user of an EL panel-equipped device as audible noise.  
           [0006]    Techniques have been adopted to reduce the EL panel noise, such as current limiting the discharge of the EL lamp and approximating a sinusoidal drive waveform, but these techniques leave residual audible noise that may be perceived by an end-user. Additionally, the circuitry required for implementing the above-described techniques is complex and requires die area and additional power for operation.  
           [0007]    EL panels also degrade over their lifetime due to ionic migration within the phosphor-dielectric layer. Decreasing frequency of the drive signal extends the phosphor life, but an EL panel will still become degraded and lose brightness over time. Brightness is sometimes adjustable in devices that incorporate EL panels, but a manual adjustment required by the user is less desirable than an automatic compensation for aging of the panel. The brightness of EL panels is often not adjustable, so the brightness of an EL panel in a typical installation decreases over time.  
           [0008]    Therefore, it would be desirable to provide a apparatus and method for driving EL panels that will reduce panel noise. It would further be desirable to provide and apparatus and method for driving EL panels that will compensate for panel aging effects.  
         SUMMARY OF THE INVENTION  
         [0009]    The above objective of providing a low noise apparatus and method for driving EL panels is accomplished in a circuit for driving EL panels and method for driving EL panels. The circuit includes an output for connection to the EL panel that is coupled to an output circuit for alternating an output of a high voltage power supply. The output circuit is driven by a waveform generator to provide an output alternating frequency and a waveshaping for shaping the alternating voltage at the output in a first-order exponential shape. A ratio of a full period of the waveform generator to a time constant of the exponential shape is within the range of 3 to 8.  
           [0010]    The above objective of compensating for loss of EL panel brightness due to aging is achieved in a circuit for driving EL panels and method for driving EL panels that sets the above-described time constant in conformity with the capacitance of the EL panel, so that as capacitance of the EL panel decreases as the EL panel ages, the drive voltage supplied to the EL panel increases.  
           [0011]    The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic diagram depicting a prior art circuit for driving an EL panel.  
         [0013]    [0013]FIG. 2 is a schematic diagram depicting an EL panel driver circuit in accordance with a preferred embodiment of the invention.  
         [0014]    [0014]FIG. 3 is a schematic diagram depicting an EL panel driver circuit in accordance with an alternative embodiment of the invention.  
         [0015]    [0015]FIGS. 4A and 4B are a waveform diagrams depicting drive waveforms generated by the EL panel driver circuits of FIG. 2 and FIG. 3.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    Referring now to the figures and in particular to FIG. 1, a prior-art circuit for driving an electroluminescent (EL) panel is depicted. The driver circuit is coupled to an EL panel  14  and produces an alternating high voltage DC output (for example +/−200V) that causes the EL panel to luminesce. A high voltage supply  16  provides the power to operate EL panel  14 . A waveform generator  18  generates a square-wave signal to drive the H-bridge  12  output circuit that alternates the output of the high voltage power supply  16  onto the El panel  14  terminals.  
         [0017]    The above-described circuit provides essentially a square wave drive signal to EL panel  14 , with some rounding of the square-wave edges due to the parasitic resistance of the devices within H-bridge  12  and the capacitance of EL panel  14 . Other driver circuit implementations within the prior art include circuits that discharge EL panel  14  prior to inverting the polarity of the output terminal signals and circuits included within the driver circuit that control the current of the discharge of EL panel  14 .  
         [0018]    When EL panel  14  is driven with the square-wave high voltage signal generated by the above-described circuits, audible noise is produced within the layers of EL panel  14 , resulting in the user of devices that incorporate the EL panel hearing unnecessary sounds. Since the typical drive frequencies used with an EL panel range from approximately 50 Hz to 3 Khz, this may be perceived as a “buzz” or a high-pitched whine.  
         [0019]    Techniques in the prior art to limit this noise as described in U.S. Pat. No. 5,789,870 include charging the EL panel with a series of narrow pulses, using the time constant of the EL panel to partially smooth the pulses, resulting in a waveshape approximating a triangular waveform. Also, discharging the EL panel through a constant current source reduces the amount of noise due to a rapid change in polarity when the output alternates. However, the above pulse and discharge techniques require additional circuitry and do not produce the ideal waveform for driving an EL panel. Also, the pulses used may contribute to EL panel noise if the frequency is within the audible range.  
         [0020]    The present invention provides low noise EL panel operation without requiring complex circuitry that is optimized to provide a minimum amount of noise. Additionally, one embodiment of the present invention provides brightness compensation over the lifetime of the panel. As phosphors within an EL panel degrade due to ionic migration, the luminescent intensity of the panel decreases for a given drive voltage, decreasing the useable life of the EL panel. Compensation for EL panel aging extends lamp life in applications where the brightness of the lamp is not adjustable. Since the life of an EL panel is typically on the order of thousands of hours, compensation for aging is especially valuable, in that the expected useful life of the remainder of a device in which an EL panel is incorporated may be 10,000 hours or more.  
         [0021]    Referring now to FIG. 2, an EL panel driver circuit in accordance with a preferred embodiment of the invention is depicted. The driver circuit is coupled to an EL panel  24  and produces an alternating high voltage DC output (for example +/−200V) that causes the EL panel to luminesce. A high voltage supply  26  provides the power to operate EL panel  24 . A waveform generator  28  generates a square-wave signal to drive the H-bridge  22  output circuit that alternates the output of the high voltage power supply. The alternating high voltage output of the H-bridge is filtered by a first-order waveshaping circuit formed by resistor R 1  and the inherent capacitance of EL panel  24 . The capacitance of EL panel  24  will generally be large, as EL panels generally exhibit a capacitance in the range of 250 pf to 1000 pf per square centimeter.  
         [0022]    The value of resistor R 1  is selected by determining the time-constant desired for the waveshaping circuit. Experimental determinations have been made resulting in an optimum time constant for a given drive frequency. If the ratio of a full period of the drive waveform to the time constant of the first-order circuit is within the range of 3 to 8, noise will be minimized, with an optimum noise reduction occurring using a ratio of approximately 4.  
         [0023]    Additionally, the range of time constants available for noise reduction can be used to advantage in compensating for EL panel aging. As the phosphor in the EL panel degrades, causing loss of brightness, the capacitance of the EL panel correspondingly decreases. By choosing a time constant that is initially higher (corresponding to setting the above-described ratio of 3), the amplitude of the alternating high voltage signal driving the EL panel is reduced. As the EL panel ages, the time constant will decrease, resulting in an increased drive voltage at the EL panel. A higher drive voltage produces a brighter luminescence from the EL panel. Thus, reduction in brightness due to aging may be compensated using the present embodiment of the invention.  
         [0024]    Referring now to FIG. 3, an EL panel drive circuit in accordance with an alternative embodiment of the invention is depicted. The driver circuit is coupled to an EL panel  34  and produces an alternating high voltage DC output (for example +/−100V) that causes the EL panel to luminesce. A high voltage supply  36  provides the power to operate EL panel  34 . A waveform generator  38  generates a square-wave signal to drive the H-bridge  32  output circuit that alternates the output of the high voltage power supply. But in contrast to the embodiment of FIG. 2, in the present embodiment the output of waveform generator  38  is shaped by a first-order waveshaping circuit  31 . Waveshaping circuit  31  in the exemplary alternative embodiment comprises a resistor R 2  and a capacitor C 1 , but alternative first-order circuits or other waveshaping circuits that approximate the desired response may be used.  
         [0025]    By shaping the drive signal ahead of H-bridge  32 , H-bridge will operate linearly during at least some portion of the drive signal waveform so that the first-order waveshape is maintained. H-bridge  32  may be designed so that only the transistors coupled to high voltage power supply  36  are operated linearly, only the transistors coupled to ground are operated linearly, or so that all transistors are operated linearly. In the illustrative embodiment, amplifier A 1  provides an offset non-inverted signal to one half of H-bridge  32  and amplifier A 2  provides an inverted signal to the other half of H-bridge  32 , providing a split-phase drive signal to operate H-bridge as a symmetrical bridge amplifier, but other configurations within the scope of the present invention are possible.  
         [0026]    Referring now to FIG. 4A, the drive signals generated by the driver circuits of FIG. 1 and FIG. 2 are illustrated. In the illustrative example, the high voltage power supply output is set to 200V peak-to-peak, but may range from approximately 80V to 300V depending on the particular EL panel and brightness requirements of a particular device. Square wave signal  44  is a square wave signal having a full period τ. Square wave signal  44  will appear at the output of H-bridge  22  of FIG. 2 but is not actually generated high voltage drive level in the embodiment of FIG. 3, as waveshaping ciruit  31  generally will operate at a lower amplitude and the high voltage signals will be controlled by the shape of the output of waveshaping circuit  31 . Output drive signal  42  is depicted as the optimum shaped signal, where τ/RC=4. As long as τ/RC is in the range of 3 to 8, audible noise will be adequately reduced.  
         [0027]    Referring now to FIG. 4B, the range of first-order waveshapes applicable to the present invention are depicted. Drive waveform  46  represents a waveshape where τ/RC=8 and drive waveform  48  represents a waveshape where τ/RC=3. To compensate for EL panel aging, the output of a drive circuit in accordance with the preferred embodiment of the present invention may be initially set to produce a waveform in accordance with drive waveform  48 . In embodiments wherein the capacitance is provided by the EL panel inherent capacitance, as the EL panel ages, the resulting reduction in capacitance will first yield a waveform in accordance with drive waveform  42  (the optimum noise reduction waveform). As the panel ages further, the further reduction in capacitance may yield a waveform in accordance with drive waveform  46 . Since the amplitude of waveforms  48 ,  42  and  46  are increasing in order, at least partial compensation of the loss of brightness due to EL panel aging may be achieved, while maintaining low noise operation.  
         [0028]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.