Abstract:
A inductorless method and apparatus for driving electroluminescent (EL) panels enables fabrication of an integrated circuit that provides a complete EL panel driver solution in one package. The high voltage required to drive the EL panel is generated by a plurality of charge pump circuits. A multi-stage charge pump may be used to provide a high voltage power supply that does not require an external capacitor to store energy for the power supply.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to integrated circuits, and more specifically, to an integrated circuit for implementing a inductorless electroluminescent panel driver.  
           [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]    Traditionally, an integrated circuit controlled for implementing an EL panel driver circuit contains an oscillator, a high voltage power supply and a switching circuit coupled to the oscillator and the high voltage power supply for creating the alternating high voltage output for connection to the EL panel.  
           [0006]    The high voltage power supply in an integrated circuit EL panel driver typically includes connections for either an inductor or a transformer to convert a low voltage input power supply to a high voltage that is then coupled to one or more terminals of the integrated circuit, which include a flyback switch transistor terminal and a terminal for coupling the high voltage DC output to the switching circuit that alternates the voltage supplied to the EL panel.  
           [0007]    With the high density integration requirements present in devices such as cellular telephones and notebook computers, and in devices having very small packages such as wristwatches, it is desirable to produce a single integrated circuit solution with a minimum of external components and terminals. However, it is impractical to incorporate inductors or transformers within an integrated circuit package.  
           [0008]    Therefore, it would be desirable to provide a inductorless apparatus and method for driving EL panels that may be incorporated within a single integrated circuit package.  
         SUMMARY OF THE INVENTION  
         [0009]    The above objective of providing a inductorless apparatus and method for driving EL panels is accomplished in an integrated circuit including a high voltage power supply and a switching circuit for producing an alternating high voltage output from the high voltage power supply. The high voltage power supply includes a plurality of charge pump circuits for generating the high voltage power supply output. The high voltage power supply may also include a multi-stage charge pump to reduce voltage loss associated with driving charge pump circuits from a low voltage power supply, which results in a lesser number of overall charge pumps required to achieve a the voltage level required to drive the EL panel.  
           [0010]    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  
       [0011]    [0011]FIG. 1 is a schematic diagram depicting an integrated circuit in accordance with a preferred embodiment of the present invention.  
         [0012]    [0012]FIG. 2 is a schematic diagram depicting details of charge pump circuit  12  of FIG. 1.  
         [0013]    [0013]FIG. 3 is a schematic diagram depicting details of the integrated circuit of FIG. 1.  
         [0014]    [0014]FIG. 4 is a schematic diagram depicting an integrated circuit in accordance with an alternative preferred embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    Referring now to the figures and in particular to FIG. 1, an integrated circuit in accordance with a preferred embodiment of the invention is shown. An electroluminscent (EL) panel driver integrated circuit  10  is coupled to EL panel  11  via two terminal connections. The terminal connections have an alternating high voltage DC output (for example +/−80V) that causes the EL panel to luminesce. An input voltage supply  13  provides the operating power for integrated circuit  10  and through integrated circuit  10 , the power to operate EL panel  11 . Input voltage supply  13  is coupled to integrated circuit  10  through two terminal connections. Thus, integrated circuit  10  requires only four terminals to interface to all external circuitry and does not require a transformer or inductor as needed in prior art circuits.  
         [0016]    Integrated circuit  10  achieves inductorless operation through use of a charge pump circuit  12  that includes a plurality of charge pumps to generate a high voltage DC output provided to a switching circuit  14  that alternates the high voltage DC output of charge pump circuit  12  to produce a high voltage AC signal for driving EL panel  11 . Switching circuit  14  is coupled to an oscillator  18  that provides a switching signal to switching circuit  14 . An optional terminal coupled to oscillator  18  provides for frequency adjustment, which will affect the “hue” of EL panel  11 . A resistor Rosc may be connected to an internal RC oscillator within oscillator  18  or an external clock, such as a programmable clock from a notebook computer internal port pin may be supplied. It should be noted that this additional terminal connection is not required for operation of integrated circuit  10 , but is an optional feature.  
         [0017]    The high voltage DC output of charge pump circuit  12  is controlled by a feedback circuit  16  that disables charge pump circuit  12  when a predetermined voltage level is generated at the output of charge pump circuit  12 , thus regulating the voltage supplied to switching circuit  14  and controlling the amplitude of the AC high voltage drive signal supplied to EL panel  11 .  
         [0018]    Referring now to FIG. 2, details of charge pump circuit  12  are shown. A square wave oscillator  22  produces a switching signal within charge pump circuit  12 . The switching signal is split into a first phase by the inverter chain formed by inverters I 1  and I 2  and a second phase by inverter I 3 . Square wave oscillator  22  also receives an enable signal from feedback circuit  16  that disables oscillator  22  when the output of charge pump circuit  12  reaches a predetermined high voltage level. Oscillator  22  is disabled until the output of charge pump circuit  12  falls below the predetermined high voltage level, thus providing regulation of the output of charge pump circuit  12 .  
         [0019]    A stacked capacitor-diode chain is used to generate the high voltage output from charge pump circuit  12 . The capacitors and diodes form a plurality of charge pumps within charge pump circuit  12  and the number of charge pumps that are stacked is determined by the input voltage, the desired output voltage and the losses due to the diode drops and ESR of the capacitors.  
         [0020]    The charge pump circuit functions as follows: During the first oscillator phase transition, the leftmost terminals of the odd-numbered capacitors will be at a logic low voltage level and transition to a logic high voltage level. At the transition, capacitor C 1  will charge capacitor Cout through diode D 1  and capacitor C 2  will be discharged to within a voltage drop of the voltage at the anode of diode D 2 . When the output of square wave oscillator  22  transitions to a logic low voltage level, capacitor C 1  will be charged through diode D 2 . As the switching action generated by square wave oscillator  22  continues, Cout will be charged to a multiple of the input power supply voltage less a number of diode voltage drops. The voltage is determined by the numbed of stacked charge pump stages. In the figure, the stacked stages are illustrated by a first stage comprising capacitors C 1  and C 2  along with diodes D 1  and D 2 , a second stage comprising capacitors C 3  and C 4  along with diodes D 3  and D 4 , and a final stage comprising capacitors Cy and Cz along with diodes Dy, Dz and Dr. Any number of charge pump stages may be inserted between the second charge pump stage and the final charge pump stage as illustrated in the figure by the dashed connections.  
         [0021]    The resulting voltage across capacitor Cout after many switching cycles will be the input power supply voltage multiplied by the number of charge pump stages, less a number of voltage drops equal to two plus the number of charge pump stages (the total number of diodes in the stacked charge pump ladder). Thus, for an 80V supply generated from a 3V input, and assuming a 0.5V diode drop, at least  32  charge pump stages are required in the stack. As the number of stages are increased, the drive capabilities of inverters I 2  and I 3  must be correspondingly increased and the voltage ratings of the capacitors in the charge pumps need to be increased to handle the higher voltages present in the stack.  
         [0022]    Referring now to FIG. 3, details of integrated circuit  10  of FIG. 1 are shown. Switching circuit  14  incorporates a full bridge formed by MOSFET transistors N 1 , N 2 , P 1 , and P 2 . High voltage level translators  32  provide the drive voltages for the gates of transistors N 1 , N 2 , P 1 , and P 2  so that N 1  and P 2  are enabled for one phase of oscillator  18  and transistors N 2  and P 1  are enabled for the alternate phase, producing an alternating high voltage output across EL panel  11 . Additional circuitry may be incorporated within switching circuit  14  to eliminate switching overlap, or to provide discharge time between enabling the transistor pairs so that a doubled EL panel voltage does not appear across the transistor pairs in the full bridge circuit.  
         [0023]    Feedback circuit  16  includes a comparator K 1  that compares a reference voltage V 1  to a voltage generated from the high voltage DC output of charge pump circuit  12  by the resistor divider comprising resistors R 1  and R 2 . The output of comparator K 2  is provided to charge pump circuit  12  to disable oscillator  22 , regulating the output of charge pump circuit  12 .  
         [0024]    Referring now to FIG. 4, an integrated circuit in accordance with an alternative embodiment of the invention is depicted. In the alternative embodiment, performance of the charge pump circuit is improved by using two cascaded charge pump circuits. A 3V to 5V charge pump circuit  42  is coupled to input voltage source  41 , which supplies a 3V input voltage. Charge pump circuit  42  uses an external capacitor C 40 , which must be larger than the capacitors of FIG. 2 as it stores and transfers a larger charge. An external capacitor C 41  on the output of charge pump circuit  42  is also used to hold the output voltage of charge pump circuit  42  which is approximately  5 V. Transistors may be used rather than diodes in charge pump circuit  42 , providing a lower voltage drop which is more critical in the conversion from 3V to 5V than the diode drops in a higher voltage converter.  
         [0025]    A 5V to 80V charge pump circuit  43 , which may be constructed in a manner consistent with the charge pump circuit of FIG. 2, converts the 5V output from charge pump  42  to an 80V output. A feedback circuit  46  is coupled to charge pump  42  and charge pump  43  to halt their operation when the output voltage of charge pump  43  has reached the predetermined high voltage output level. Switching circuit  44  alternates the polarity of the output of charge pump  43  to produce the AC drive signal for EL panel  11  and oscillator  48  provides switching signals to control switching circuit  44 .  
         [0026]    The alternative embodiment of FIG. 4 is preferred for an efficient design at input voltages below 5V, but since it requires external capacitors and consequent additional terminals for external connection, it is not preferred from a packaging standpoint.  
         [0027]    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.