Abstract:
The present invention discloses a contrast control method and circuitry for setting and compensating the contrast of a liquid crystal display (LCD). The present invention has circuitry for generating a contrast voltage normally applied to a control pin. Normally the actual contrast of the LCD is a sensitive function of the difference between the applied contrast voltage and the display power supply voltage. The present invention generates a reference voltage that is adjustable and made to vary inversely with temperature. The contrast control circuitry uses a feedback loop to make the difference voltage between the display power supply voltage and the contrast voltage equal to twice the reference voltage. A contrast setting made using the circuitry of the present invention now becomes independent of the display power supply voltage and compensated for variations in the temperature of the LCD. A high gain amplification method for reducing error voltages and providing wide dynamic range is also disclosed.

Description:
TECHNICAL FIELD 
     The present invention relates in general to display systems and to data processing systems which incorporate display systems, and in particular to liquid crystal displays (LCD) employing contrast control compensated for variations in power supply voltage and temperature. 
     BACKGROUND INFORMATION 
     Liquid crystal displays (LCDs) are used in many applications as the display of choice because of their small size, low power and low cost. As with any display system, users sometimes want to adjust the contrast between the displayed information and the background. LCDs typically have a contrast input voltage that is used to vary the contrast on a particular display. The contrast is a function of the power supply voltage used on the display and the voltage that is applied to the contrast control pin. The voltage that will generate a particular contrast depends on the display temperature and the actual supply voltage at the time an adjustment was made. If a user sets a contrast level, subsequent variations in the power supply voltage or temperature would require the user to re-adjust the contrast control to maintain the desired contrast. 
     Many approaches have been implemented in the prior art to deal with the problem of sensitivity of the contrast control setting to variations in power supply voltage and temperature. Some LCD systems try to compensate for only one of the variables while others use rather complex systems of microprocessors, analog to digital (A/D) converters, sensors and feedback systems to compensate for variations that occur when the LCD&#39;s power supply voltage or its ambient temperature vary. 
     In many LCD systems it is also desirable to have only one voltage to power the display and the circuitry within the display. Having only one power supply voltage can create additional problems in the dynamic range required for contrast control over possible variations in temperature and power supply voltage. Sometimes it is desirable to have a contrast control voltage that is near the level of the display power supply voltage. This dynamic range has led some display system designs to use multiple voltages for the LCD system. As a result, what is needed in the art is a simple and cost effective analog system for providing contrast control for a LCD system using only one supply voltage for the display as well as the contrast control circuitry. 
     Many modern data processing systems, including but not limited to personal computers, laptop or portable computers use LCDs s as output devices. These data processing systems are operated where it is desirable to have a LCD with an automatic contrast control adjustment. Therefore, the foregoing needs are particularly applicable to such data processing systems that employ a LCD as the primary or as one of the displays for system information. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the foregoing needs by providing an improved contrast control method and electronic circuitry to implement the contrast control method. More specifically the present invention provides a method where the difference between the display supply voltage and the contrast control voltage are made proportional to a reference voltage which itself is linearly and inversely proportional to temperature. One embodiment of the present invention also uses a novel circuit configuration to enable a high gain and a wide dynamic range for controlling the difference in the display voltage and the contrast control voltage. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a typical LCD display; 
     FIG. 2 illustrates a schematic of an embodiment of the present invention; 
     FIG. 3 illustrates a schematic of an electronic circuit implementation of the present invention; 
     FIG. 4 is a block diagram of a data processing system employing a LCD with the contrast control of the present invention; 
     FIG. 5A illustrates a zener diode reference circuit; and 
     FIG. 5B illustrates a three terminal bandgap reference circuit. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth such as specific voltages or resistor values, etc. to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning timing considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art. 
     Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     FIG. 1 is a simple block diagram showing a LCD system  106 , power supply voltage V BUS    100  and contrast voltage V CONTRAST    104 . V BUS    100  is a regulated display power supply voltage and V CONTRAST    104  is a control voltage applied to a control pin to change contrast. If no compensation were provided, LCD  106  would have a contrast level seen by a viewer which may vary. The difference between V BUS    100  and V CONRAST    104 , which effects the contrast of a LCD, varies if either V BUS    100  or V CONTRAST  undergo variations. V BUS    100  may vary from regulation and V CONTRAST  can vary due to drift, temperature or component aging in its generation circuitry. 
     FIG. 2 illustrates several features of the present invention. The displayed contrast on LCD  106  is dependent on the difference between the display power supply voltage V BUS    100  and the contrast control voltage V CONTRAST    104 . Differential amplifier  201  generates the difference between V BUS    100  and V CONTRAST    104 . Differential amplifier  202  generates the difference between the output of amplifier  201  and a generated reference voltage V REFTH    103 . The output of amplifier  202  is this difference voltage amplified by a gain G and the output of amplifier  202  becomes the contrast voltage V CONTRAST    104 . The voltage V CONTRAST    104  is used as a feedback to the negative input of differential amplifier  201 . V CONTRAST    104  can be shown to be the following: 
     
       
         
           V 
           CONTRAST 
           =[V 
           BUS 
           −V 
           CONTRAST 
           −V 
           REFTH 
           ]×G 
         
       
     
     If G is large (&gt;&gt; l ) then the difference between V BUS    100  and V CONTRAST    104  can be shown to approach the following: 
     
       
         
           V 
           BUS 
           −V 
           CONTRAST 
           =V 
           REFTH 
         
       
     
     V REFTH    103  is a reference voltage that is independent of V BUS    100 , optionally adjustable by varying resistor  206  and made to vary linearly with temperature. Since the viewed contrast level is a function of the difference between the supply voltage V BUS    100  and the contrast voltage V CONTRAST    104 , the compensation system shown in FIG. 2 generates a viewed contrast level that is independent of supply voltage V BUS    100 . Since a previously set contrast level would need to also be adjusted from the set value if the temperature changed, the reference generator  205  is designed to have the required variation in V REFTH    103  necessary to keep a set contrast at a viewer&#39;s desired value. 
     In many LCD systems, it is desirable to have a single power supply voltage for all elements in the system. In an embodiment of the present invention, a single power supply voltage, V BUS    100 , is used to generate V REFTH    103  and to power the amplifiers needed to generate the compensated contrast level control. If a single power supply is used, there are times when the desired voltage, V BUS    100  minus V CONTRAST    104 , becomes very nearly equal to the power supply voltage V BUS    100 . The reference voltage V REFTH    103  would need to be nearly equal to the voltage from which it is generated. In this case, the V REFTH  generator  205  would have to be more complex and more costly. The simple circuit of FIG. 2 is not used because of the high gain required in differential amplifier  202  and the desire to use a single supply voltage. If amplifier  202  was a closed loop amplifier with a necessary high gain G, then the circuit would be prone to potential stability problems. 
     FIG. 3 illustrates an embodiment of the present invention where all of the considerations discussed have been implemented. FIG. 3 illustrates the three main elements (differential amplifier  201 , differential amplifier  202  and V REFTH  generator  205 ) of FIG. 2 in dotted lines. The embodiment of the present invention shown in FIG. 3 uses the same power supply voltage, V BUS    100 , to power the LCD  106 , to generate the reference voltage V REFTH    103  and to power the amplifiers  304 ,  307 , and  310 . Since the difference voltage V BUS    100 −V CONTRAST    104  has a dynamic range that takes it near the supply voltage V REF    312 , resistors  301  and  303  are used to divide V BUS    100  by two and resistors  302  and  305  are used to divide V CONTRAST    104  by two V REFTH    103  can now be derived from V BUS    100  with a simple zener diode circuit or a simple three terminal bandgap reference circuit and a resistor divider. Amplifier  307  now has as its inputs (V BUS    100 −V CONTRAST    104 )/2 and V REFTH    103 . V REFTH    103  can now be less than V REF    312  and V BUS    100 −V CONTRAST    104  can be two times V REFTH    103 . In one embodiment of the present invention V BUS    100  is 5 volts and V REF    312  is 2.5 volts. 
     Most modem operational amplifiers used to make differential amplifiers can have their output voltage operate very near their supply voltages. Operational amplifiers are characterized by high input impedance and a very high but variable differential gain. To stabilize the gain of a particular amplifier, negative feedback is used to make the closed loop gain of a stage the ratio of two resistors. A very high closed loop gain in a stage may result in instability because of the large resistors necessary and parasitic capacitance. 
     The present invention solves this problem by operating amplifier  307  open loop to achieve the highest gain possible. The inputs to amplifier  307  are very nearly equal when the error is the smallest. As the controlled voltage, V BUS    100 −V CONTRAST    104 , moves above and below V REFTH    103  the high gain of amplifier  307  causes its output to switch from its most positive value (V BUS    100 ) and its most negative value (ground). The output of amplifier  307  is integrated or averaged with resistor  308  and capacitor  309 . Amplifier  310  is operated as a voltage follower and buffers or isolates the integrator so it is not loaded by the input impedance of differential amplifier circuit  201  when V CONTRAST    104  is fed back to resistor  302 . The average value on the output of amplifier  307  becomes the desired contrast voltage necessary to generate a desired set contrast level on the LCD. 
     The reference generator circuit  205  has a resistor divider circuit comprised of resistors  313 ,  314 ,  317 ,  318 , optional variable resistor  206 , capacitor  316 , and thermistor  319 . The reference voltage V REF    312  can be generated with a zener diode, a commercially available three terminal bandgap reference, or using an other suitable reference circuit. On LCDs that have an optional customer set contrast level, variable resistor  206  is used to vary the contrast level of the LCD. After a particular contrast is set the circuitry of the present invention will maintain the contrast with variations in display supply voltage and display temperature. Resistor  320  is added in parallel to thermistor  319  to change the slope of its temperature versus resistance curve. Inexpensive thermistors may not have the required temperature versus resistance curve needed for a particular LCD. 
     Resistors  313  and  314  allow a non-standard resistor value to be realized in one leg of the resistor divider with standard resistor values. Resistors  317  and  318  serve the same purpose in the other leg of the resistor divider. The resistors are sized to give the desired range of values for the reference voltage V REFTH    103 . 
     A representative hardware environment for practicing the present invention is depicted in FIG. 4 which illustrates a typical hardware configuration of workstation  413  in accordance with the subject invention having central processing unit (CPU)  410 , such as a conventional microprocessor, and a number of other units interconnected via system bus  412 . Workstation  413  includes random access memory (RAM)  414 , read only memory (ROM)  416 , and input/output (I/O) adapter  418  for connecting peripheral devices such as disk units  420  and tape drives  440  to bus  412 , user interface adapter  422  for connecting keyboard  424 , mouse  426 , and/or other user interface devices such as a touch screen device (not shown) to bus  412 , communication adapter  434  for connecting workstation  413  to a data processing network, and display adapter  436  for connecting bus  412  to LCD  438 . LCD  438  would employ the contrast control of the present invention. CPU  410  may include other circuitry not shown herein, which will include circuitry commonly found within a microprocessor, e.g., execution unit, bus interface unit, arithmetic logic unit, etc. CPU  410  may also reside on a single integrated circuit. 
     FIG. 5A illustrates a zener diode circuit using V BUS    100 , resistor  500 , and zener diode  501  for generating V REF    312 . FIG. 5B illustrates a three terminal bandgap reference  502  for generating V REF    312 . Either of these circuits or other reference circuits could be used to generate a reference voltage V REF    312  that is independent of variations in V BUS    100 . 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.