Abstract:
A liquid crystal display (LCD) bias generator generates a plurality of bias voltages, e.g., four bias voltages, needed to drive a segmented LCD. The LCD bias generator has a voltage generator, e.g., charge pump, that may generate a most positive voltage, e.g., substantially equal to or more positive than V DD , on the integrated circuit that may also be used for maintaining proper reverse bias operation of well ties and analog switches of the integrated circuit. Other necessary LCD bias voltages, e.g., three voltages, may also be derived from the LCD bias generator to provide bias and contrast control voltages required by the LCD. Having a more positive bias voltage than the power supply voltage, V DD , allows V DD  to cover a wider range of voltages, e.g., powered from a battery, by eliminating the need for complex analog switch and pad designs for the integrated circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to Indian Patent Application No. 1596/DEL/2006, which was filed on Jul. 7, 2006, entitled “Liquid Crystal Display Bias Generator,” and is hereby incorporated by reference herein for all purposes. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to liquid crystal displays, and more particularly, to a liquid crystal display bias generator. 
       BACKGROUND 
       [0003]    Present technology liquid crystal display (LCD) bias generators fabricated on integrated circuits generally use charge pumps to generate the necessary bias voltages for operation of the LCD. However, the charge pump outputs used for the LCD bias voltages may be greater or lesser than power supply, V DD , to the integrated circuit. By not knowing the exact relationship between power supply V DD  and the LCD bias voltages, complex well ties, level-shifters and complex switching may be required for proper operation of the integrated circuit LCD bias generator. This may cause problems in the design of the integrated circuit pads, e.g., floating pads may be required, and special analog switch circuitry may be needed so that normally reverse biased junctions are not undesirably forward biased. 
       SUMMARY 
       [0004]    Therefore there is a need for an LCD bias generator that may be used in generating a plurality of bias voltages, e.g., four bias voltages, needed to drive a segmented LCD. The LCD bias generator has a voltage generator, e.g., charge pump, that may generate a most positive voltage, e.g., substantially equal to or more positive than V DD , on the integrated circuit and may also be used for maintaining proper reverse bias operation of well ties and analog switches of the integrated circuit. Other necessary LCD bias voltages, e.g., three voltages, may also be derived from the LCD bias generator and may be used to provide bias and contrast control voltages required by the LCD. This allows V DD  to cover a wider range of voltages, e.g., powered from a battery, by eliminating the need for complex analog switch and pad designs for the integrated circuit. 
         [0005]    According to a specific example embodiment of this disclosure, a liquid crystal displace (LCD) bias voltage generator may comprise: a first voltage generator having an input and an output; a first adjustable voltage divider coupled to the input of the first voltage generator, a power supply common and a voltage reference; a first capacitor; a pair of first switches coupling the first capacitor to the output of the first voltage generator and the power supply common when closed and decoupling the first capacitor therefrom when open; a second voltage generator having an input and an output a second adjustable voltage divider coupled to the input of the second voltage generator, the power supply common and the voltage reference; the second voltage generator output coupled to a first LCD bias voltage node and generating a first LCD bias voltage thereon; a pair of second switches coupling the first capacitor to the first LCD bias voltage node and a second LCD bias voltage node when closed and decoupling the first capacitor therefrom when open; a pair of third switches coupling the first capacitor to the second LCD bias voltage node and a third LCD bias voltage node when closed and decoupling the first capacitor therefrom when open; a pair of fourth switches coupling the first capacitor to the third LCD bias voltage node and a fourth LCD bias voltage node when closed and decoupling the first capacitor therefrom when open, wherein a second LCD bias voltage on the second LCD bias voltage node is a sum of the first LCD bias voltage and a first boost voltage on the first capacitor, a third LCD bias voltage on the third LCD bias voltage node is a sum of the first LCD bias voltage, the second LCD bias voltage and a second boost voltage on the first capacitor, and a fourth LCD bias voltage on the fourth LCD bias voltage node is a sum of the first LCD bias voltage, the second LCD bias voltage, the third LCD bias voltage and a third boost voltage on the first capacitor. 
         [0006]    According to another specific example embodiment of this disclosure, a liquid crystal display (LCD) bias voltage generator may comprise: a voltage generator having an input and an output; an adjustable voltage divider coupled to the input of the voltage generator, a power supply common and a voltage reference; a voltage boost capacitor; a pair of voltage boost switches coupling the voltage boost capacitor to the output of the voltage generator and the power supply common when closed and decoupling the voltage boost capacitor therefrom when open; a plurality of LCD bias voltage nodes; a plurality of LCD bias voltage capacitors, each of the plurality of LCD bias voltage capacitors coupled to a respective one of the plurality of LCD bias voltage nodes and to the power supply common; a plurality of LCD bias voltage node switch pairs coupling the voltage boost capacitor between adjacent ones of the plurality of LCD bias voltage nodes when closed and decoupling the voltage boost capacitor therefrom when open, wherein a first one of the plurality of LCD bias voltage capacitors is charged to a first voltage from the voltage boost capacitor such that the respective one of the plurality of LCD bias voltage nodes is at the first voltage, a subsequent one of the plurality of LCD bias voltage capacitors is charged to a sum of the voltage on the voltage boost capacitor and the voltage on the previous one of the plurality of LCD bias voltage capacitors such that each subsequent one of the plurality of LCD bias voltage nodes has a more positive bias voltage value than the previous one of the plurality of LCD bias voltage nodes. 
         [0007]    According to yet another specific example embodiment of this disclosure, a digital system having liquid crystal display (LCD) bias voltage generator may comprise: a digital device fabricated on an integrated circuit; a liquid crystal display (LCD) bias voltage generator fabricated on the integrated circuit, said LCD bias voltage generator comprising: a voltage generator having an input and an output; an adjustable voltage divider coupled to the input of the voltage generator, a power supply common and a voltage reference; a voltage boost capacitor; a pair of voltage boost switches coupling the voltage boost capacitor to the output of the voltage generator and the power supply common when closed and decoupling the voltage boost capacitor therefrom when open; a plurality of LCD bias voltage nodes; a plurality of LCD bias voltage capacitors, each of the plurality of LCD bias voltage capacitors coupled to a respective one of the plurality of LCD bias voltage nodes and to the power supply common; a plurality of LCD bias voltage node switch pairs coupling the voltage boost capacitor between adjacent ones of the plurality of LCD bias voltage nodes when closed and decoupling the voltage boost capacitor therefrom when open, wherein a first one of the plurality of LCD bias voltage capacitors is charged to a first voltage from the voltage boost capacitor such that the respective one of the plurality of LCD bias voltage nodes is at the first voltage, a subsequent one of the plurality of LCD bias voltage capacitors is charged to a sum of the voltage on the voltage boost capacitor and the voltage on the previous one of the plurality of LCD bias voltage capacitors such that each subsequent one of the plurality of LCD bias voltage nodes has a more positive bias voltage value than the previous one of the plurality of LCD bias voltage nodes. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    A more complete understanding of the present disclosure thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
           [0009]      FIG. 1  is a schematic block diagram of a digital device having a liquid crystal display (LCD) driver and an LCD coupled thereto; 
           [0010]      FIG. 2  is a more detailed schematic diagram of a bias generator for the LCD driver, according to a specific example embodiment of this disclosure; and 
           [0011]      FIG. 3  is a more detailed schematic diagram of a bias generator for the LCD driver, according to another specific example embodiment of this disclosure. 
       
    
    
       [0012]    While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0013]    Referring now to the drawings, the details of example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
         [0014]    Referring to  FIG. 1 , depicted is a schematic block diagram of a digital device having a liquid crystal display (LCD) driver and an LCD coupled thereto. A digital device  102 , e.g., microprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), programmable logic array (PLA), etc., may have a LCD driver integral with the digital device and fabricated on an integrated circuit die (not shown). The LCD driver of the digital device  102  may generate LCD bias voltages  116 ,  118 ,  120  and  122 . These LCD bias voltages  116 ,  118 ,  120  and  122  may be used to drive display segments and control contrast of the LCD  130 . The digital device  102  may be powered from a power supply voltage  112 , V DD , and a power supply common  114 , V SS . At least one of the LCD bias voltages, e.g., LCD bias voltage  122 , may be more positive than the power supply voltage  112 , V DD . Using a LCD bias voltage that is more positive than the power supply voltage  112 , V DD , may be used for maintaining proper reverse bias operation of well ties and analog switches of the integrated circuit, thus allowing V DD  to cover a wider range of voltages, e.g., powered from a battery, by eliminating the need for complex analog switch and pad designs for the integrated circuit. 
         [0015]    Referring now to  FIG. 2 , depicted is a more detailed schematic diagram of a bias generator for the LCD driver, according to a specific example embodiment of this disclosure. A first voltage generator  214 , e.g., first charge pump, may be used to generate voltages across capacitor  104 . Whenever switches  230   a  and  230   b  are closed, the first voltage generator  214  may charge the capacitor  104  to a voltage determined by a programmable adjustable voltage divider  216 . The first voltage generator  214  may have gain that may multiply the input voltage from the adjustable voltage divider  216  to a greater amplitude output voltage, e.g., what is applied to the capacitor  104 . A reference voltage  224 , e.g., from a bandgap voltage reference (not shown), may be applied to the adjustable voltage divider  216 . The adjustable voltage divider  216  may be digitally programmed over a first voltage adjustment reference bus  222 . Thus, a plurality of different voltages may be applied to the capacitor  104  depending on LCD bias voltage requirements as more fully discussed herein. 
         [0016]    A second voltage generator  212 , e.g., second charge pump, may be used to generate certain voltages desired for the LCD bias voltage  116 . The capacitor  210  may be used for voltage waveform smoothing and transient reduction. The second voltage generator  212  may have an output voltage determined by a programmable adjustable voltage divider  218 . The second voltage generator  212  may have gain that may multiply the input voltage from the adjustable voltage divider  218  to a greater amplitude output voltage, e.g., what is applied as the LCD bias voltage  116 . The reference voltage  224 , e.g., from the bandgap voltage reference (not shown), may be applied to the adjustable voltage divider  218 . The adjustable voltage divider  218  may be digitally programmed over a second voltage adjustment reference bus  220 . Thus, a plurality of different voltages may be generated depending upon the LCD bias voltage  116  requirements as more fully discussed herein. 
         [0017]    The LCD bias voltage  116  may be the lowest voltage of the LCD bias voltages. The LCD bias voltage  116  may be any voltage programmed on the second voltage adjustment reference bus  220 , however, as an example but without limitation from this example, the LCD bias voltage  116  may be about 0.54 volts and the voltage across the capacitor  104  may be about 1.02 volts, hereinafter referred to as the “LCD boost voltage.” The reference voltage  224  may be about 1.2 volts, and with that reference voltage  224  the LCD bias voltage  116  may be set at any value from about zero volts to about 1 volt. 
         [0018]    Once the LCD bias voltage  116  has been generated from the second voltage generator  212 , as programmed over the second voltage adjustment reference bus  220 , and a voltage generated on the capacitor  104 , i.e., switches  230   a  and  230   b  are closed, the LCD bias voltage  118  may be generated by opening switches  230   a  and  230   b , and closing switches  232   a  and  232   b . From the example voltages above, when the LCD bias voltage  116  is about 0.54 volts and the LCD boost voltage (voltage across capacitor  104 ) is about 1.02 volts, the LCD bias voltage  118  will be the sum thereof, i.e., about 1.56 volts. Capacitor  110  will then charge to and hold the LCD bias voltage  118  at about 1.56 volts. Switches  232   a  and  232   b  may be opened and the LCD bias voltage  118  will remain at about 1.56 volts across the capacitor  110 . 
         [0019]    In a similar fashion to generate the LCD bias voltage  120 , the LCD boost voltage may be regenerated across the capacitor  104  by closing switches  230   a  and  230   b  and applying a voltage from the first voltage generator  214  as programmed over the first voltage adjustment reference bus  222 . In the present example, the LCD boost voltage may be about 1.02 volts and the LCD bias voltage  118  may be about 1.56 volts across the capacitor  110 . When switches  230   a  and  230   b  open, and switches  234   a  and  234   b  close, the LCD bias voltage  120  will be the sum of the LCD bias voltage  118  at about 1.56 volts and the LCD boost voltage (voltage across capacitor  104 ) at about 1.02 volts. Capacitor  108  will then charge to and hold the LCD bias voltage  120  at about 2.58 volts. Switches  234   a  and  234   b  may be opened and the LCD bias voltage  120  will remain at about 2.58 volts across the capacitor  108 . 
         [0020]    The LCD bias voltage  122  (the most positive voltage of the digital device  102 ), may be generated with the LCD boost voltage being regenerated across the capacitor  104  by closing switches  230   a  and  230   b  and applying a voltage from the first voltage generator  214  as programmed over the first voltage adjustment reference bus  222 . In the present example, the LCD boost voltage may be about 1.02 volts and the LCD bias voltage  120  may be about 2.58 volts across the capacitor  108 . When switches  230   a  and  230   b  open, and switches  236   a  and  236   b  close, the LCD bias voltage  120  will be the sum of the LCD bias voltage  120  at about 2.58 volts and the LCD boost voltage (voltage across capacitor  104 ) at about 1.02 volts. Capacitor  106  will then charge to and hold the LCD bias voltage  122  at about 3.60 volts. Switches  236   a  and  2326   b  may be opened and the LCD bias voltage  122  will remain at about 3.60 volts across the capacitor  106 . 
         [0021]    It is contemplated and within the scope of the present disclosure that the LCD boost voltage (voltage across the capacitor  104  may be any voltage that may be programmed over the first voltage adjustment reference bus  222 . The LCD bias voltage  116  may be any voltage that may be programmed over the second voltage adjustment reference bus  220 . The LCD bias voltage  120  may be the sum of the LCD bias voltage  118  and the LCD boost voltage. The LCD bias voltage  122  may be the sum of the LCD bias voltage  120  and the LCD boost voltage. The LCD bias voltage  122  (most positive) may be the sum of the LCD bias voltage  120  and the LCD boost voltage. 
         [0022]    Referring now to  FIG. 3 , depicted is a more detailed schematic diagram of another bias generator for the LCD driver, according to a specific example embodiment of this disclosure. An LCD bias generator  214 , e.g., charge pump, may be used to generate voltages across capacitor  104 . Whenever switches  230   a  and  230   b  are closed, the LCD bias generator  214  may charge the capacitor  104  to a voltage determined by a programmable adjustable voltage divider  216 . The LCD bias generator  214  may have gain that may multiply the input voltage from the adjustable voltage divider  216  to a greater amplitude output voltage, e.g., what is applied to the capacitor  104 . A reference voltage  224 , e.g., from a bandgap voltage reference (not shown), may be applied to the adjustable voltage divider  216 . The adjustable voltage divider  216  may be digitally programmed over a voltage adjustment reference bus  222 . Thus, a plurality of different voltages may be applied to the capacitor  104  depending on LCD bias voltage requirements as more fully discussed herein. 
         [0023]    To generate the LCD bias voltage  116 , a first voltage may be generated across the capacitor  104  by closing switches  230   a  and  230   b . The first voltage may be defined by what voltage is programmed into the adjustable voltage divider  216  through the voltage adjustment reference bus  222 . When switches  230   a  and  230   b  open, and switches  328   a  and  328   b  close, the LCD bias voltage  116  will be at the first voltage. Capacitor  310  will then charge to and hold the LCD bias voltage  116  at about the first voltage. Switches  328   a  and  328   b  may be opened and the LCD bias voltage  116  will remain at about the first voltage across the capacitor  310 . 
         [0024]    To generate the LCD bias voltage  118 , a second voltage may be generated across the capacitor  104  by closing switches  230   a  and  230   b . The second voltage may be defined by what voltage is programmed into the adjustable voltage divider  216  through the voltage adjustment reference bus  222 . When switches  230   a  and  230   b  open, and switches  232   a  and  232   b  close, the LCD bias voltage  118  will be the sum of the second voltage and the first voltage (LCD bias voltage  116 ). Capacitor  110  will then charge to and hold the LCD bias voltage  118  at about the sum of the first and second voltages. Switches  232   a  and  232   b  may be opened and the LCD bias voltage  118  will remain at about the sum of the first and second voltages across the capacitor  110 . 
         [0025]    To generate the LCD bias voltage  120 , a third voltage may be generated across the capacitor  104  by closing switches  230   a  and  230   b . The third voltage may be defined by what voltage is programmed into the adjustable voltage divider  216  through the voltage adjustment reference bus  222 . When switches  230   a  and  230   b  open, and switches  234   a  and  234   b  close, the LCD bias voltage  120  will be the sum of the third, second and first voltages. Capacitor  108  will then charge to and hold the LCD bias voltage  118  at about the sum of the first, second and third voltages. Switches  234   a  and  234   b  may be opened and the LCD bias voltage  120  will remain at about the sum of the first, second and third voltages across the capacitor  108 . 
         [0026]    To generate the LCD bias voltage  122 , a fourth voltage may be generated across the capacitor  104  by closing switches  230   a  and  230   b . The fourth voltage may be defined by what voltage is programmed into the adjustable voltage divider  216  through the voltage adjustment reference bus  222 . When switches  230   a  and  230   b  open, and switches  236   a  and  236   b  close, the LCD bias voltage  122  will be the sum of the fourth, third, second and first voltages. Capacitor  106  will then charge to and hold the LCD bias voltage  120  at about the sum of the first, second, third and fourth voltages. Switches  236   a  and  236   b  may be opened and the LCD bias voltage  122  will remain at about the sum of the first, second, third and fourth voltages across the capacitor  106 . 
         [0027]    While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.