Crystal variation compensation circuit for liquid crystal display

Abstract: A liquid crystal display (15) comprises a circuit (10) for compensating for variations in the display crystals thereof. The compensating circuit utilizes four crystals (21,22,23,24) arranged outside of the viewing area. Two of the crystals (21,23) have a capacitance (1.5C X ) different from that of the other two and serve as reference crystals. The other two crystals (22,24) serve as detection crystals and can have a capacitance (C X ) equal to that of the display crystals. The detection crystals are biased to have a capacitance equal to that of the reference crystals. Changes in the detection crystal capacitance results in changes in the voltage across the crystals. The voltage changes are detected in a detection circuit (12) and are applied to an adjustment circuit (39,47,51\0. The adjustment circuit produces an adjustment signal which is used to adjust the crystal drive voltage and to adjust the detection circuit.

Description: This invention relates generally to liquid crystal displays (LCD's) and particularly to a circuit for compensating for variations in the characteristics of the crystals of such displays. The use of LCD's in instances where a visual display is required is increasing rapidly. Because of their low power consumption and economy, LCD's are being used to replace cathode ray tubes (CRT's) for many uses requiring a visual display. Unlike CRT's, liquid crystals show a rapid change in light output over a narrow range of driving voltages just above a threshold voltage. However, keeping a good grey scale, in a monochrome display, or color uniformity, in a color display, requires accurate control of the LCD drive voltages in the region of the threshold voltage. Manufacturing liquid crystals with uniform characteristics is an achievable goal with existing technology. However, the turn-on voltage characteristics of liquid crystals vary substantially over time because of such factors as temperature changes, aging, thickness of the crystals, et al. Because of these variations in characteristics, even when the crystals within a given display are manufactured with substantially identical turn-on characteristics, matching the drive voltage circuitry to the turn-on voltage of the crystals is extremely difficult. The difficulty is increased when the changes are not permanent. For example, temperature changes can cause substantial changes in the crystal characteristics. However, the temperature changes are not permanent. Accordingly, the light output of a portable LCD can vary substantially as the display is moved from one environment to another. For these reasons, there is a need for an LCD with a circuit for compensating for crystal variations. EP-A-012479 discloses a circuit for compensating for variations in the display crystals of a liquid crystal display (LCD), including a drive voltage circuit comprising: at least one reference crystal having a first capacitance and at least one detection crystal having a second capacitance, different from said first capacitance, arranged outside the viewing area of said display; a four-sided circuit, each of said sides having a midpoint and being connected at junctions, said four-sided circuit including said reference crystals and said detection crystals, for generating a differential signal in response to characteristic changes in said detection crystals; and an adjustment circuit including a differential amplifier, for receiving said differential signal and producing an adjustment signal representative of said differential signal and applying said adjustment signal to said drive voltage circuit. According to the present invention, there is provided a circuit for compensating for variations in the display crystals of a liquid crystal display (LCD), including a drive voltage circuit comprising: at least one reference crystal having a first capacitance and at least one detection crystal having a second capacitance, different from said first capacitance, arranged outside the viewing area of said display; a four-sided circuit, each of said sides having a midpoint and being connected at junctions, said four-sided circuit including said reference crystals and said detection crystals, for generating a differential signal in response to characteristic changes in said detection crystals; an adjustment circuit including a differential amplifier, for receiving said differential signal and producing an adjustment signal representative of said differential signal and applying said adjustment signal to said drive voltage circuit; characterized in that there are: first and second reference crystals, series connected between midpoints of two opposite sides of said four-sided circuit to form a central junction; first and second detection crystals respectively connected between said central junction and the midpoints of the other two sides of said four-sided circuit; and means for biasing said detection crystals to a capacitance substantially equal to the capacitance of said reference crystals arranged between the respective midpoints and said central junction. In the drawings: FIGURE 1 shows a preferred embodiment of a circuit for compensating for crystal variations in a LCD. FIGURE 2 shows the LCD threshold and gamma correction achievable with the preferred embodiment. FIGURE 3 illustrates a simplified portion of a LCD showing the relationship of the compensation crystals and the display area crystals. FIGURE 4 shows voltage regulators, which can be used with the invention, in more detail. In FIGURE 1, a circuit 10 for compensating for the variations of crystals of a liquid crystal display 15 (FIGURE 3) includes a detection circuit 12. The detection circuit 12 is in the form of a bridge, each leg of which includes a positive polarity voltage sensitive transfer gate 13 and a negative polarity voltage responsive transfer gate 14. The transfer gates 13 and 14 in each of the legs are coupled at junctions 16, 17, 18 and 19. A plurality of compensation crystals 21, 22, 23 and 24 are connected at a grounded junction 26 and to the nodes 16, 17, 18 and 19, respectively. In FIGURE 3, the compensation crystals 21 to 24 are arranged outside the viewing area of the LCD, and they are used to compensate for capacitance changes which occur to the crystals within the LCD because of aging, temperature changes, variations in thickness due to manufacturing, etc. The crystals 22 and 24 have substantially the same capacitance C X as the crystals within the LCD and serve as detection pixels in the compensation circuit. The capacitance of the crystals 21 and 23 exceeds that of the detection crystals 22 and 24 by a preselected amount, such as 50%. The crystals 21 and 23 serve as reference crystals in the detection circuit. The crystal 22 is voltage biased through an RC filter including a resistor 27, a capacitor 28 and the capacitance C X of the crystal. Similarly, the detection crystal 24 is voltage biased through an RC filter including a resistor 29 and a capacitor 31 and the capacitance C X of the crystal. The capacitance of the capacitors 28 and 31 is much greater than the capacitance C X of the crystals 22 and 24. The voltage applied to the reference crystal 21 is applied through an RC filter including a resistor 32, a capacitor 33, and the capacitance of the crystal. A capacitor 34 and resistor 36 also form an RC filter for the reference crystal 23. The capacitance of the capacitors 33 and 34 is much greater than the capacitance of the crystals 21 and 23. A node 37 of the bridge circuit 12 is coupled by a line 38 to one input terminal of a differential amplifier 39. A node 41, directly opposite from the node 37, is coupled to another input terminal of the differential amplifier 39 by a line 42. The other two nodes 43 and 44 of the bridge circuit 12 are positively and negatively voltage biased, respectively. The output of the differential amplifier 39 is connected to the input terminal of a positive voltage regulator 47 by lines 46 and 48. The output of the differential amplifier 39 is also connected to the input terminal of a negative voltage regulator 51 by the line 46 and a line 49. An inverter 52 receives polarity control signals over an input line 53. The polarity control signal is applied as a control input to the positive voltage regulator 47 by a line 54. Also, the output of the inverter 52 is applied as a control input to the negative voltage regulator 51 by a line 56. A transfer gate 57 receives the output of the inverter 52 over a line 58. Another transfer gate 59 receives the polarity control signal over a line 61. The transfer gates 57 and 59 can be a transistor type similar to the gates 13 and 14, with the lines 58 and 61 being coupled to the control electrodes of the transistors. When this type of transfer gate is used, both the transfer gates 57 and 59 have the same polarity and, thus, are alternately on and off in response to the polarity control signal on the line 53. A junction 62, between the transfer gates 57 and 59, is coupled to the filter resistors 27 and 29 by a line 63. The output voltage of the negative regulator 51 is coupled by a line 64 to the transfer gate 57, and by a line 66 to an input of a driver circuit 67. The output voltage of the positive regulator 47 is coupled by a line 68 to the transfer gate 59, and by a line 69 to another input terminal of the driver circuit 67. An output line 71 of the driver circuit 67 is coupled to the crystals of the LCD in known manner. Analog data, which varies from -V to +V, for example, ±5 volts, is applied to the driver circuit 67 by an input line 72. The line 72 is coupled to the control electrodes of inverting transfer gate 73 and 74. The transfer gates 73 and 74 have opposite polarity. Accordingly, the gate 73 is conductive when negative analog data are coming in from line 72, and the gate 74 is conductive when positive analog data are applied to the line 72. The output line 71 is coupled to a junction 76 between the transfer gates 73 and 74 and thus varies between a plus and a minus voltage V, in accordance with the polarity of the signal on the input line 72. The line 71 is also coupled to a junction 77 between oppositely poled inverting transfer gates 78 and 79. The junctions 76 and 77 are electrically coupled. The gates 73 and 78 are similarly poled, as are the gates 74 and 79. The control electrodes of the transfer gates 78 and 79 are coupled to the input line 72. The gates 78 and 79 are coupled to the positive voltage regulator 47 and negative voltage regulator 51 by the input line 69 and 66, respectively. Accordingly, the voltage on the output line 71 is adjusted by the regulators 47 and 51 in accordance with the polarity of the signal on the input line 72. Briefly stated, the compensation crystals 21 to 24 develop a differential signal across the detection circuit 12 in response to changes in the detection crystals 22 and 24. The differential signal is applied to an adjustment circuit which includes the differential amplifier 39, the regulators 47 and 51, and the transfer gates 57 and 59. The adjustment circuit produces an adjustment signal which adjusts the output signal on the line 71 of the drive circuit 67, and which also adjusts the biasing voltage on the detection crystals 22 and 24. The adjustment signal rebalances the detection circuit 12, and crystal characteristic changes are compensated for . In operation, either all of the positive transfer gates 13 or all of the negative transfer gates 14 are simultaneously turned on in synchronization with the polarity of the polarity control signal on the line 53. The reference crystals 21 and 23 are biased to keep them in the minimum capacitance off state. The detection crystals 22 and 24 are alternately biased positive and negative by the voltage regulators 47 and 51, through the transfer gates 57 and 59 and the line 63. This biasing of the detection crystals 22 and 24 causes their total capacitance to equal that of the reference crystals 21 and 23. When the positive transfer gates 13 in the legs of the bridge circuit 12 are conductive, the compensation crystals 21 and 24 are connected to the positive node 43 and positively charged. Simultaneously, the compensation crystals 22 and 23 are connected to the negative node 44 and negatively charged. When the negative transfer gates 14 in the legs of the bridge circuit 12 are made conductive, the reference crystal 21 and the detector crystal 22 are connected to the node 37. Also, the reference crystal 23 and the detection crystal 24 are connected to the node 41. When the capacitance of the detection crystals equals that of the reference crystals, the net voltage between each of the nodes 37 and 41 and grounded junction 26 is zero, and the differential amplifier 39 receives no differential voltage and produces no output voltage on the line 46. However, changes in the characteristics of the detection crystals 22 and 24, due to temperature changes, aging, etc., are substantially the same as those of the crystals within the viewing area of the LCD. When a crystal characteristic change occurs, the capacitance of the detection crystals 22 and 24 is different from that of the reference crystals 21 and 23. Under this condition, when the transfer gates 14 are rendered conductive and connect the crystals to the nodes 37 and 41, a differential voltage exists between the nodes 37 and 41 and ground. This voltage is applied to the differential amplifier 39 which produces an output on the line 46. The output voltage on the line 46 is applied to the voltage regulators 47 and 51 in accordance with the polarity of the polarity control signal on the line 53, to cause the voltage on either of the lines 66 or 69 to change and produce a change in the drive voltage on the output line 71 of the driver circuit 67. The regulators 47 and 51 also change the biasing on the detection crystals 22 and 24 because of the connection to the detector crystals through the transfer gates 57 and 59. Accordingly, the capacitance of the crystals is adjusted to rebalance the bridge 12 in accordance with the changed conditions, until rebalancing is required because of another change of crystal characteristics. FIGURE 2 shows how the voltage circuit compensates for capacitance variations of the crystals in the LCD. The abscissa is the voltage on the output line 71 of the driver circuit 67, and the ordinate is the input voltage applied to the input line 72. The curve 81 is the output on the line 71 for a negative analog input on the input line 72 with a nominal threshold level of -1.8 volts for the crystals of the LCD. The curve 82 is a curve with the nominal threshold of +1.8 volts for positive input on the line 72. Curve 83 shows how the inventive circuit shifts the curve 82 when the threshold level of the crystals of the LCD increases 0.6 volts, to +2.4 volts, due to a temperature change, or a change in some other crystal characteristic. With the invention, when the display is returned to the temperature which existed prior to the shift, the inventive circuit will shift the threshold voltage back to the nominal +1.8 volts. FIGURE 4 shows the regulator 51 in more detail. The regulator includes four transfer gates 84, 85, 86, 87. The polarity control signal is applied by the line 56 to the control electrode of the transfer gate 84. The output voltage of the differential amplifier 39 is applied by the line 49 to the control electrode of the transfer gate 86. When the polarity control signal on the line 56 is low, the regulator is enabled and the signal in the lines 64 and 66 is regulated. When the polarity control signal on the line 56 is high, the lines 64 and 66 are clamped to the most negative potential, e.g., -5 volts. The positive regulator 47 is similar to the negative regulator 51, but the polarity of the transfer gates is opposite to that of the negative regulator 51. The regulators 47 and 51 can be of the type described in U.S. Patent 4,061,962, issued December 6, 1977 to Stewart.

Claims: A circuit (10) for compensating for variations in the display crystals of a liquid crystal display (LCD) (15), including a drive voltage circuit (67) comprising: at least one reference crystal (21,23) having a first capacitance (1.5C x ) and at least one detection crystal (22,24) having a second capacitance (C x ), different from said first capacitance, arranged outside the viewing area of said display; a four-sided circuit (12), each of said sides having a midpoint and being connected at junctions, said four-sided circuit including said reference crystals and said detection crystals, for generating a differential signal in response to characteristic changes in said detection crystals; an adjustment circuit (39,47,51) including a differential amplifier (39), for receiving said differential signal and producing an adjustment signal representative of said differential signal and applying said adjustment signal to said drive voltage circuit; characterized in that there are: first and second reference crystals (21,23), series connected between midpoints of two opposite sides of said four-sided circuit to form a central junction (26); first and second detection crystals (22,24) respectively connected between said central junction and the midpoints of the other two sides (16,18) of said four-sided circuit; and means for biasing said detection crystals to a capacitance substantially equal to the capacitance of said reference crystals arranged between the respective midpoints and said central junction. The circuit of claim 1, characterized in that each side of the said four-sided circuit (13) includes a positive (13) and a negative (14) voltage responsive transfer gate arranged on opposite sides of said midpoints. The circuit of claim 2, characterized in that the capacitance (C x ) of said detection crystals (21,23) is substantially equal to the capacitance of the liquid crystals (15) in said display device, and the capacitance (1.5C x ) of said reference crystals (22,24) is equal to approximately 1.5 times said capacitance of said detection crystals. The circuit of claim 3, characterized in that said means for biasing said detection crystals includes RC filters (27-31).

Inventors: STEWART ROGER GREEN, STEWART, ROGER GREEN
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1987
Main classification: G09G3
IPC codes: G02F1, G01D7, G09G3