Drive circuit for a display apparatus

Abstract: In a drive circuit for a display apparatus into which a digital video signal is input and in which a plurality of parallel signal electrodes are provided one of signal voltages having different levels is output in accordance with the digital video signal, or two adjacent ones of said signal voltages are simultaneously output. Alternatively, one of the signal voltages is supplied to a signal electrode in one portion of one output period, and another one of the signal voltages is supplied to the signal electrode in another portion of the output period. The length ratio of the two portions of one output period is appropriately determined according to a digital video signal, whereby an arbitrary voltage corresponding to the video signal data can be applied to the pixel.

Description: 1. Field of the Invention: This invention relates to a drive circuit for a display apparatus, and more particularly to a drive circuit for a display apparatus which is capable of gray-scale display by an amplitude modulation drive. In this specification, a matrix type liquid crystal display apparatus will be described as a typical example of a display apparatus, but this invention can also be applied to drive circuits for other types of display apparatus such as electroluminescent (EL) display apparatus and plasma display apparatus. 2. Description of the Prior Art: When driving a liquid crystal display (LCD) apparatus, since the speed of response of a liquid crystal is very slow as compared with a luminescent material used in a cathode ray tube (CRT) display apparatus, a special drive circuit is used. That is, in a drive circuit for an LCD apparatus, video signals which are sequentially sent are not immediately supplied to respective pixels, but the video signals are sampled for each of the respective pixels in one horizontal period and held for the horizontal period. The held video signals are output at the same time at the beginning of the next horizontal period, or at an appropriate point of time in the next horizontal period. After the output of video signal voltages to the respective pixels is begun, the signal voltages are held for a period of time sufficiently over the speed of response of the liquid crystal. In order to hold the signal voltages, a prior art drive circuit utilizes capacitors. Figure 32 shows a signal voltage output circuit (a source driver) for supplying drive voltages to a plurality of pixels (in this case, 120 pixels) on one scanning line selected by a scanning signal. A portion for the nth pixel of the source driver is shown in Figure 33. The portion includes an analog switch SW₁, a sampling capacitor C SMP , an analog switch SW₂, a holding capacitor C H , and an output buffer amplifier A. The operation of the signal voltage output in the prior art will be described with reference to the signal timing chart of Figure 34. Analog video signals v s to be input to the analog switches SW₁ are sequentially sampled in accordance with sampling clock signals T SMP1 - T SMP120 which correspond to the respective 120 pixels on one scanning line selected by each horizontal synchronizing signal H syn . By this sampling, the sequential instantaneous voltages V SMP1 - V SMP120 of the video signals v s are applied to the corresponding sampling capacitors C SMP . The nth sampling capacitor C SMP is charged up to the value of the video signal voltage V SMPn corresponding to the nth pixel, and holds this value. The signal voltages V SMP1 - V SMP120 which are sequentially sampled and held in one horizontal period are transferred from the sampling capacitors C SMP to the holding capacitors C H for holding outputs, in response to an output pulse OE which is supplied to all of the analog switches SW₂ at the same time. Then the signal voltages V SMP1 - V SMP120 are output to source lines O₁ - O₁₂₀ connected to the respective pixels through the buffer amplifiers A. To the drive circuit described above, analog video signals are supplied. When video signals are supplied in the form of digital data, a drive circuit shown in Figure 35 is used. For the sake of simplicity, the video signal data is composed of 2 bits (D₀, D₁). That is, video signal data have four values 0 - 3, and a signal voltage applied to each pixel is any one of four levels V₀- V₃. Figure 36 shows a portion for the nth source line O n in the circuit. The portion of the circuit comprises a D-type flip-flop (sampling memory) M SMP at a first stage and a flip-flop (holding memory) M H at a second stage which are provided for the respective bits (D₀, D₁) of the video signal data, a decoder DEC, and analog switches ASW₀ - ASW₃ each of which is provided between corresponding one of four external voltage sources V₀ - V₃ and a source line O n . The digital source driver operates as follows. The video signal data (D₀, D₁) are sampled at the rising of a sampling pulse T SMPn corresponding to the nth pixel, by the sampling memory M SMP . At the time when the sampling for one horizontal period is completed, an output pulse OE is fed to the holding memories M H . All the video signal data (D₀, D₁) held in the holding memories M H are simultaneously output to the respective decoders DEC. Each of the decoders DEC decodes the 2-bit video signal data (D₀, D₁). In accordance with the values (0 to 3), one of the analog switches ASW₀ - ASW₃ is conductive, and the corresponding one of the four external voltages V₀ - V₃ is output to the source line O n . In the example shown in Figure 36, since the video signal data is 2 bits, 4 (= 2²) levels of external voltages (V₀- V₃) to be supplied to the source line O n are required. When a 4-bit video signal data is supplied, a signal voltage output circuit has a configuration shown in Figure 37 in which 2⁴ = 16 levels of external voltages (V₀- V₃) are required. That is, in a drive circuit for a digital video signal constructed in such a manner, it is necessary to provide 2 n levels of external voltages, for n-bit video signal data. As described above, when the number of levels of the voltages to be externally supplied increases, the following problems (1) and (2) arise. (1) With the increase in number of levels of voltages to be supplied, the size of a voltage supplying circuit is enlarged. Therefore, the production cost is increased. (2) Since the number of input terminals of an LSI constituting the drive circuit including the above signal voltage output circuit increases, such an LSI is difficult to mount. US-A-4 921 334 describes a matrix liquid crystal display in which an extended gray-scale is realized by dithering the brightness level of an addressed pixel between adjacent brightness values in successive display frames. Thereby, the human eye percieves an intermediate brightness level. SUMMARY OF THE INVENTION This invention, as defined by claim 1, provides a drive circuit for a display apparatus in which a drive voltage is output, to be applied to a display drive line, in accordance with a digital video signal, said drive circuit comprising: time control means for receiving a portion of said digital video signal, and for producing a time division signal defining at least a first period and a second period included in a signal output period in accordance with said portion of said digital video signal, said display drive line being connected to a pixel during said signal output period; and voltage selecting means connected to said time control means, for receiving the remaining portion of said digital video signal and said time division signal, and selectively outputting, to said display drive line, one of a plurality of signal voltages during said first period of said signal output period and another of said plurality of signal voltages during said second period of said signal output period, in accordance with said remaining portion of said digital video signal and said time division signal, said plurality of signal voltages being different from each other. The subclaims 2 to 8 are directed to embodiments of the invention. The invention disclosed herein makes possible the objectives of: (1) providing a drive circuit which can drive a display device with a gray-scale using a reduced number of signal voltages having different levels; (2) providing a drive circuit which can produce a plurality of signal voltages with more fine steps than voltages supplied from a voltage source; and (3) providing a drive circuit which can drive a display apparatus with a fine gray-scale display. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram illustrating an example which is useful for understanding the invention. Figure 2 shows the logical relationships between the inputs and outputs of a selection circuit used in the example of Figure 1. Figure 3 shows an equivalent circuit of analog switches and one source line in the case that two analog switches are simultaneously conductive in the example of Figure 1. Figure 4 shows the relationships between digital video signals and voltages applied to pixels. Figure 5 is a circuit diagram illustrating in more detail the selection circuit of the example of Figure 1. Figure 6 is a block diagram illustrating one embodiment of the invention. Figure 7 shows the logical relationships between the inputs and outputs of a selection circuit used in the embodiment of Figure 6. Figure 8 is a timing chart illustrating the operation of the embodiment of Figure 6. Figure 9 shows an equivalent circuit of one source line. Figure 10 shows the variation in the voltage level applied to pixels in the embodiment of Figure 6. Figure 11 shows the relationships between digital video signals and voltages applied to pixels. Figure 12 is a circuit diagram illustrating in more detail the selection circuit of the embodiment of Figure 6. Figure 13 is a block diagram illustrating another embodiment of the invention. Figure 14 shows the logical relationships between the inputs and outputs of a time control circuit used in the embodiment of Figure 13. Figure 15 shows the logical relationships between the inputs and outputs of a selection circuit used in the embodiment of Figure 13. Figure 16 shows an equivalent circuit of one source line. Figure 17 illustrates the relationship between the variation in the voltage level output on a source line and the voltage applied to a pixel. Figure 18 illustrates the variation in the voltage applied to a pixel for various voltages output on a source line. Figure 19 is a circuit diagram illustrating in more detail the time control circuit and selection circuit of the embodiment of Figure 13. Figure 20 illustrates inputs and outputs of the time control circuit of the embodiment of Figure 13. Figure 21 illustrates the relationships between digital video signals and output voltages in the embodiment of Figure 13. Figure 22 is a block diagram illustrating a further embodiment of the invention. Figure 23 shows the logical relationships between the inputs and outputs of a time selection circuit used in the embodiment of Figure 22. Figure 24 shows the logical relationships between the inputs and outputs of a voltage selection selection circuit used in the embodiment of Figure 22. Figure 25 is a timing chart illustrating the operation of the embodiment of Figure 22. Figure 26 illustrates an equivalent circuit of one source line. Figure 27 illustrates the relationship between the variation in the voltage level output on a source line and the voltage applied to a pixel. Figure 28 illustrates the relationships between digital video signals and output voltages in the embodiment of Figure 22. Figure 29 is a circuit diagram illustrating in more detail the time selection circuit and voltage selection circuit of the embodiment of Figure 22. Figure 30 is a timing chart illustrating inputs of the time selection circuit. Figure 31 is a timing chart illustrating the relationship between the inputs of the time selection circuit and voltages applied to a pixel. Figure 32 is a circuit diagram of a drive circuit for analog video signals. Figure 33 illustrates a portion of the drive circuit of Figure 32 for one source line. Figure 34 illustrates the operation of the drive circuit of Figure 32. Figure 35 illustrates a drive circuit in which a digital sampling unit is provided for each source line. Figure 36 illustrates a portion of the drive circuit of Figure 35 for one source line. Figure 37 illustrates a portion of a drive circuit for one source line which receives 4-bit digital video signals. Figure 38 illustrates a portion of a drive circuit for one source line which receives 3-bit digital video signals. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows an example which is useful for understanding the invention. In Figure 1, a digital video signal data of 3 bits is supplied to a signal voltage output circuit for the nth signal line (source line) of a drive circuit for an LCD apparatus. The circuit comprises a sampling flip-flop M SMP , a holding flip-flop M H , a selection circuit 20, and five analog switches ASW₀ - ASW₄. Each of the sampling flip-flop M SMP and the holding flip-flop M H comprises three D-type flip-flops corresponding to the respective bits D₀, D₁ and D₂ of a digital video signal data. The terminals of the five analog switches ASW₀ - ASW₄ are connected to external voltage sources with five levels V₀ - V₄ (V₀ < V₁ < V₂ < V₃ < V₄), respectively. The other terminals are commonly connected to the nth source line O n . The control terminals of the analog switches ASW₀ - ASW₄ receive the outputs S₀ - S₄ of the selection circuit 20, respectively. The bits of a digital video signal data (D₀, D₁, D₂) supplied to the sampling flip-flop M SMP are sequentially sampled in accordance with a sampling pulse T SMPn corresponding to the source line O n and supplied to the holding flip-flop M H . After completing the sampling of video signal data for all pixels on one scanning line, the data (D₀, D₁, D₂) held in the holding flip-flop M H is fed to the selection circuit 20 through the inputs A, B and C by an output pulse OE applied to the holding flip-flop M H . The table of Figure 2 shows the relationship among the values of the inputs A, B and C and the values of the outputs S₀ - S₄ of the selection circuit 20. For example, when the video signal data (D₀, D₁, D₂) = (A, B, C) is (0, 0, 0), only the output S₀ is 1 and all the other outputs S₁ - S₄ are 0. Thus, only the analog switch ASW₀ is conductive and the external power source voltage V₀ is supplied to the source line O n . When (A, B, C) = (0, 1, 0), only the output S₁ is 1. Thus, only the analog switch ASW₁ is conductive and the power source voltage V₁ is supplied to the source line O n . In the same manner, the power source voltage V₂ is supplied when (A, B, C) = (0, 0, 1), V₃ is supplied when (A, B, C) = (0, 1, 1), and V₄ is supplied when (A, B, C) = (1, 1, 1). In the above cases, an external voltage having one level is supplied to the source line O n . When the video signal data (D₀, D₁, D₂) = (A, B, C) is (1, 0, 0), each one of the outputs S₀ and S₁ of the selection circuit 20 is 1, and the other outputs S₂, S₃, S₄ are 0. Thus, the two analog switches ASW₀ and ASW₁ are conductive at the same time, and both the external voltage sources V₀ and V₁ are connected to the source line O n . The equivalent circuit in this case is shown in Figure 3. In this figure, R ON0 indicates the resistance of the analog switch ASW₀ and R ON1 indicates the resistance of the analog switch ASW₁ in the conductive state. In the case of Figure 3, the voltage V ON supplied to the source line O n in a steady state is expressed as follows: V ON = R ON1 ·V₀ + R ON0 ·V₁ R ON0 + R ON1 This expression shows the condition of V₀ < V ON < V₁, and namely the voltage V ON supplied to the source line O n has a level between the levels of external voltages V₀ and V₁. When the on-resistances R ON0 and R ON1 are set equal to each other (R ON0 = R ON1 ), the voltage V ON is expressed by the following expression: V ON = (V₀ + V₁)/2 Accordingly, it is possible to supply a voltage with a middle level of V₀ and V₁. In the same way, in the embodiment, the on-resistances of the analog switches ASW₀ - ASW₄ are equal to each other. Thus, the voltage with a level of (V₁ + V₂)/2 can be supplied to the source line O n when (D₀, D₁, D₂) = (A, B, C) = (1, 1, 0), and the voltage with a level of (V₂ + V₃)/2 can be supplied to the source line O n when (A, B, C) = (1, 0, 1). The above example has been described in the case where two levels of external voltages are supplied to the source line O n at the same time. Figure 4 summarizes the relationship among the values of digital video signal data D₀, D₁ and D₂ to be fed to the drive circuit for an LCD apparatus of the example and the values of the voltage supplied to the source line O n . As seen from the table of Figure 4, the drive circuit of the example can perform a display of eight gray-scale levels with using external voltages having only five levels. Figure 5 shows an example in which the selection circuit 20 shown in Figure 1 is a logic circuit including AND gates and OR gates. In this example, the following logic expressions are led from the logic table of Figure 2, and realized by the arrangement of AND gates and OR gates shown in Figure 5: S₀ = A · B S₁ = A · B ·C + A ·B S₂ = A ·B·C + A· B S₃ = A· B ·C + A·B· C S₄ = A·B·C In this example, the digital video signal data is 3 bits. As shown in Figure 38, when one of the external voltages is selected to be supplied to a pixel as it is, the required number of external voltage levels is 2³ = 8. However, in this example, since a middle level between two external voltage levels can be produced, it is sufficient only to provide external voltages with five levels. In the same way, for 4-bit video signal data, it is necessary to provide external voltages with 2⁴ = 16 levels in the above prior art configuration. This example necessitates external voltages with 2³ + 1 = 9 levels. As described above, for n-bit video signal data, this example requires external voltage levels of 2 (n-1) + 1. On the contrary, conventionally, since the voltage levels to be supplied to a pixel are obtained from external voltages, it is necessary to provide external voltage levels of 2 n . Therefore, in this example, the burden of the voltage supplying circuit can be reduced and the number of terminals of the drive circuit can be reduced. According to the example, the degree of reduction in the number of required external voltage levels is greater with the increase in the number of bits of a video signal data. According to the example, when signal voltages with different levels are supplied to a pixel based on digital video signal data so as to perform a gray-scale display, there are two cases where an external voltage with one level is supplied to a signal electrode as the signal voltage, and where external voltages with two levels are supplied at the same time. When two levels are supplied at the same time, a voltage with a middle level between these two levels can be supplied to a pixel. Therefore, a predetermined number of scale levels can be attained without using the same number of external voltages but with a reduced number of external voltages. Accordingly, the external voltage supplying circuit can be made smaller and the number of terminals of the driver circuit for a display circuit can be reduced. Furthermore, since the steps between adjacent voltages applied to the pixel can be more minute than the steps between adjacent levels of external voltage sources, minute gray-scale display can be attained when voltages with minute differences are difficult to produce in an external voltage producing circuit. Figure 6 shows one embodiment of the invention. In the embodiment of Figure 6, the selection circuit 20 has only two inputs terminals A and B, and an AND gate 22 is provided. The sampling flip-flop M SMP and the holding flip-flop M H have the same configuration as those used in the embodiment of Figure 1. The upper 2 bits D₂ and D₁ of the video signal data held in the holding flip-flop M H are fed to the selection circuit 20 through the terminals A and B as they are. The lowest significant bit D₀ of the video signal data is fed into the AND circuit 22 together with a control signal TM, and the resulting signal CTM is fed to the selection circuit 20. Figure 7 shows the relationship among the values of the inputs A, B and CTM of the selection circuit 20 and the output (one of S₀ - S₄) selected by the selection circuit 20. When, for example, the video signal data (D₀, D₁, D₂) is (0, 0, 0), (A, B, CTM) are always (0, 0, 0) irrespective of the value of the control signal TM, and the selection circuit 20 selects the output S₀. As a result, only the analog switch ASW₀ is conductive and the power source voltage V₀ is supplied to the source line O n . When the video signal data (D₀, D₁, D₂) is (1, 0, 0), the output of the selection circuit 20 depends on the value of the control signal TM. Since the input CTM is 0 during the Low level period of the control signal TM, the selection circuit 20 selects the output S₀ and the voltage V₀ is supplied to the source line O n . When the control signal TM becomes High, the input CTM is 1 and the selection circuit 20 selects the output S₁, whereby the voltage V₁ is supplied to the source line O n . The conditions of the output pulse OE, the control signal TM and the change of the signal for the source line O n are shown in the timing chart of Figure 8. The equivalent circuit of the load for the source line O n is shown in Figure 9. In this figure, R s represents the whole resistance of the source line, C s represents the capacitance of the source line, v c (t) indicates a potential at a point A, and V COM indicates a voltage applied to an opposite electrode. Actually, the capacitance C LC of the pixel is formed in parallel with the capacitance C s , as shown by a broken line in Figure 9. However, the capacitance C LC is extremely smaller than the source line capacitance C s (e.g., C s = 160 pF, C LC = 0.2 pF), so that the capacitance C LC may be disregarded. Accordingly, the potential v c (t) at the point A can be regarded as the voltage between the pixel electrode and the opposite electrode. Figure 10 shows the change in the voltage of the source line O n in more detail. As shown in Figure 10, the control signal TM is Low during the first half t₁ of the period of the output pulse OE and High during the last half t₂. The voltage v(t) of the source line O n changes as follows: v(t) = V₀ (0 < t ≦ t₁) and v(t) = V₁ (t₁ < t ≦ t₁ + t₂) The voltage v c (t) of the pixel connected to the source line O n is obtained by solving the following simultaneous equations: V₀ = R s ·i(t) + (1/C s )∫i(t)dt v c (t) = (1/C s )∫i(t)dt + V B where i(t) is a current flowing through the source line O n , and V B is a potential at the point A when t = 0, i.e., a voltage of the source line in the previous horizontal scanning period. The solution of these simultaneous equations is effected as follows: v c (t) = V B + V₀·{1 - exp(-t/(C s ·R s ))} Accordingly, the voltage v c (t) comes closer to the voltage V₀ as shown by a broken line in Figure 10. In view of the rising characteristic of the voltage, when designing a liquid crystal panel, the values of the capacitance C s and the resistance R s are determined so that the voltage v c (t) comes sufficiently closer to the voltage V₀ in the period of the output pulse OE. Next, the change in the voltage v c (t) after t₁ is obtained by solving the following simultaneous equations: V₁ - V₀ = R s ·i(τ) + (1/C s )∫i(τ)dτ v c ′(τ) = (1/C s )∫i(τ)dτ In these equations, a coordinate transformation is performed as follows: t = τ + t₁ v = v′ + V₀ The solution of the above simultaneous equations is effected as follows: v c ′(τ) = (V₁ - V₀)·{1 - exp (-τ/(C s ·R s ))} The time period required for the voltage of the pixel to reach the middle voltage (V₀ - V₁)/2 (for the v-coordinate axis, (V₀ + V₁)/2) of V₁ and V₀ is obtained by the following equation: (V₁ - V₀)/2 = (V₁ - V₀)·{1 - exp(-τ/(C s ·R s ))} The solution is: τ = (ln2)·C s ·R s = 0.693 x C s ·R s As is apparent from the equation, the time period τ is independent of the external voltages V₀ , V₁ and V B . Accordingly, the time period τ is constant when any two adjacent voltage levels are selected from the external voltages V₀ - V₄ by the selection circuit 20. Therefore, when the time period t₂ for which the control signal TM is High conforms with the time period τ obtained by the above equation, the voltage of the pixel can take the middle value between V₀ and V₁. The table of Figure 11 shows the relationship among the values of D₀, D₁ and D₂ of a video signal data and the voltage applied to the pixel in this embodiment. When the lowest significant bit D₀ of the video signal data is 0, the output of the selection circuit 20 is constant irrespective of the change in the control signal TM, so that the voltage supplied to the source line O n takes a constant value V₀, V₁, V₂ or V₃ . When the lowest significant bit D₀ is 1, the output of the selection circuit 20 changes as the control signal TM changes in one period of the output pulse OE as shown in Figure 8, and the external voltages with adjacent two levels are sequentially supplied to the source line O n . Especially, since the time period τ for which the voltage is supplied in the last half is determined by the above equation, the middle values (V₀ + V₁)/2, (V₁ + V₂)/2, (V₂ + V₃)/2, (V₃ + V₄)/2 of the respective adjacent levels are applied to the liquid crystal. Accordingly, in this embodiment, only five levels of external voltages V₀ - V₄ are required when eight levels of voltages corresponding to the 3-bit video signal data are supplied to the pixel. An example in which the selection circuit 20 shown in Figure 6 is realized by AND gates, OR gates and an inverter is shown in Figure 12. In this example, the following logic expressions are led from the logic table of Figure 7. In the following expressions, CTM is represented by T: S₀ = B · A · T S₁ = B · A ·T + B ·A· T S₂ = B ·A·T + B· A · T S₃ = B· A ·T + B·A· T S₄ = B·A·T As seen from above, when video signal data is n bits, only 2 (n-1) + 1 levels are required also in this embodiment. Figure 13 shows another embodiment of the invention. This embodiment comprises a sampling memory M SMP , an output holding memory (holding memory) M H , a time control circuit TC, a selection circuit SEL, and four analog switches ASW₁ - ASW₄. The sampling memory M SMP , holding memory M H and selection circuit SEL are similar in construction as those used in the embodiment of Figure 6. Among the outputs of the holding memory M H , the least significant bit D₀ of the digital video signal data is fed to the time control circuit TC at an input terminal A, and the other bits (in this embodiment, the upper 2 bits D₁ and D₂) are fed to the selection circuit SEL through the inputs B and C. The time control circuit TC receives, in addition to the 1-bit data from the holding memory M H (the least significant bit D₀ of the video signal data), an external pulse signal (a time control pulse signal) TM. The output CTM of the time control circuit TC is fed to the selection circuit SEL together with the upper bits of the video signal data (in this embodiment, 2 bits (D₁, D₂)). Based on these inputs, the selection circuit SEL makes one of four outputs S₁ - S₄ High or 1 in accordance with the logic described below. Thus, the corresponding one of the analog switches ASW₁ - ASW₄ is conductive, and one of the external voltages V₁ - V₄ (V₁ < V₂ < V₃ < V₄) is supplied to the source line O n The logic table of Figure 14 illustrates the relationship between the input A (D₀) and the output CTM of the time control circuit TC. When the value of the input data is 0 (i.e., D₀ = 0), the input pulse signal TM is output as it is. When D₀ = 1, the output CTM is always 1. The logic table of Figure 15 illustrates the relationship among the inputs B, C and CTM and the output of the selection circuit SEL. When the input CTM is 0, all the outputs S₁ - S₄ are 0, irrespective of the values of the other inputs B and C (i.e., the upper bits of the video signal data). When the input CTM is 1, only the output S X (X = Y + 1) according to the value Y indicated by the other inputs (B and C) (in this embodiment, a 2-bit binary digit value in which C is the high bit and B is the lower bit) is 1. The embodiment of Figure 13 operates as follows when the time control circuit TC and the selection circuit SEL perform the outputs in accordance with the above-mentioned logic. When the least significant bit D₀ of the video signal data is 1, the output CTM of the time control circuit TC is always 1, and the selection circuit SEL operates in the same manner as the decoder (the decoder DEC shown in Figures 35 and 36) which receives the upper 2 bits (D₁, D₂) of the video signal data. In other words, one of the outputs S₁ - S₄ is selected to be 1 in accordance with the values of the upper 2 bits of the video signal data, so that the corresponding analog switch ASW i is conductive and one of four levels of external voltages V₁ - V₄ is supplied to the source line O n . When the least significant bit D₀ of the video signal data is 0 and the time control pulse signal TM is High or 1, the selection circuit SEL operates in the same manner as described above. During the Low level (0) period of the pulse signal TM, all of the outputs S₁ - S₄ of the selection circuit SEL are 0 irrespective of the values of the upper 2 bits of the video signal data, and the source line O n is in a high impedance state. That is, when the least significant bit of the video signal data is 0, the output period of the external voltages to the source line O n can be controlled by the pulse signal TM. The equivalent circuit of the load for the source line O n is shown in Figure 16. In Figure 16, R s indicates the resistance of the source line, C s indicates the capacitance of a liquid crystal pixel connected to the source line O n , and V COM represents a common voltage applied to the opposite electrode to the liquid crystal. It is assumed that the time when the time control pulse signal TM changes from Low to High is t = 0. The voltage v(t) supplied from the source drive of Figure 13 to the source line O n is expressed as follows: v(t) = 0 (t < 0) and v(t) = V i (0 ≦ t) The voltage v c (t) across the capacitance C s of the pixel which is a load for the source line O n in accordance with the change in the supplied voltage can be obtained by solving the following simultaneous equations: V i = R s ·i(t) + (1/C s )∫i(t)dt v c (t) = (1/C s )∫i(t)dt + V COM where i(t) indicates a current flowing through the source line O n . The solution of the simultaneous equations is effected as follows: v c (t) = V COM + V i ·{1 - exp(-t/(C s ·R s ))} The voltage v c (t) comes closer to the voltage V i as shown in Figure 17. Accordingly, before the voltage v c (t) comes sufficiently closer to the voltage V i , at the time when the voltage v c (t) reaches a desired value, the time control pulse signal TM is dropped to Low to stop the output to the source line O n . Thus, the voltage applied to a pixel can be set to an arbitrary value. Figure 18 shows the curves of the voltage v c (t) applied to the pixel when the voltages V i supplied to the source line O n are V₁ , V₂ , V₃ and V₄, respectively. In the case of Figure 18, it is assumed that the differences between the adjacent two levels of external voltages V₁ , V₂ , V₃ and V₄ are equal to each other. The respective voltages V i (t) are expressed as follows: v₁(t) = V COM + V₁·{1 - exp(-t/(C s ·R s ))} v₂(t) = V COM + V₂·{1 - exp(-t/(C s ·R s ))} v₃(t) = V COM + V₃·{1 - exp(-t/(C s ·R s ))} v₄(t) = V COM + V₄·{1 - exp(-t/(C s ·R s ))} As seen from Figure 18, the voltage v₄ (t) equals the voltage V₃ at time t₃, and the voltage v₃ (t) equals the voltage V₂ at time t₂. These times t₂ and t₃ are obtained by solving the following expressions, respectively: V₂ = V COM + V₃·{1 - exp(-t₂/(C s ·R s ))} V₃ = V COM + V₄·{1 - exp(-t₃/(C s ·R s ))} When V₄ - V₃ = V₃ - V₂ = V₂ - V₁ = ΔV, the voltages V₂, V₃ and V₄ are expressed as follows: V₂ = V₁ + ΔV V₃ = V₁ + 2ΔV V₄ = V₁ + 3ΔV Therefore, the above expressions can be rewritten as follows: V₁ + ΔV = V COM + (V₁ + 2ΔV) {1 - exp(-t₂/(C s ·R s ))} V₁ + 2ΔV = V COM + (V₁ + 3ΔV) {1 - exp(-t₃/(C s ·R s ))} By solving these expressions, times t₂ and t₃ are obtained as follows: t₂ = C s ·R s ·ln{(V₁ + 2ΔV)/(V COM + ΔV)} t₃ = C s ·R s ·ln{(V₁ + 3ΔV)/(V COM + ΔV)} The difference between times t₂ and t₃ is expressed as follows: t₃ - t₂ = C s ·R s ·ln{V₁ + 3ΔV)/(V₁ + 2ΔV)} Since V₁ + 3ΔV > V₁ + 2ΔV, the following condition is always satisfied: t₃ - t₂ > 0, that is, t₃ > t₂ In the same manner, the difference between times t₁ and t₂ is expressed as follows: t₂ - t₁ = C s ·R s ·ln{V₁ + 2ΔV)/(V₁ + ΔV)} Since t₂ > t₁, the following relationship is established: t₃ > t₂ > t₁ Therefore, respective times t which satisfy the following conditions exist: V₃ < v₄(t) < V₄ V₂ < v₃(t) < V₃ V₁ < v₂(t) < V₂ V₁ - ΔV < v₂(t) < V₁ These respective times t satisfy the following inequalities, respectively: t₃ < t t₂ < t t₁ < t 0 < t That is, a point of time when the voltage applied to the pixel is an arbitrary voltage having a level between the respective two of the levels V₁ , V₂ V₃ and V₄ uniquely exists. In expressions (2) and (3), if V₁ >> ΔV, t₃ ≒ t₂ ≒ t₁. In this case, times t when the voltages v₄(t), v₃(t), v₂ (t) and v₁(t) become the middle levels of the adjacent levels of the external voltages V₁, V₂, V₃ and V₄, i.e., (V₄ + V₃)/2, (V₃ + V₂)/2, (V₂ + V₁)/2 and V₁ - ΔV/2, respectively, are approximately equal to each other. The value of the voltage V₁ can be determined arbitrarily. Therefore, if the voltage V₁ is determined so as to satisfy the relationship of V₁ >> ΔV, the time periods t w for which the intermediate values (V₄ + V₃)/2, (V₃ + V₂)/2, (V₂ + V₁)/2 and V₁ - ΔV/2 are supplied to the pixel can be respectively determined uniquely. Accordingly, by setting the width of the time control pulse signal TM shown in Figure 18 to be the time period t w , voltages with the intermediate values can be supplied to the pixel, only using a small number of external voltages. Generally, since ΔV becomes small with the increase in number of gray-scale levels to be displayed, the above-mentioned condition of V₁ >> ΔV can be more easily satisfied as the number of the gray-scale levels increases. Figure 19 shows an example in which the time control circuit TC and the selection circuit SEL shown in Figure 13 are realized by AND gates and OR gates. The time control circuit TC may be an OR gate. The selection circuit SEL can consist of four AND gates. The timing relationship between the output pulse OE and the time control pulse signal TM fed to the time control circuit TC is shown in Figure 20. The time control pulse signal TM becomes High at the rising of the output pulse OE, so that the external voltage V i is begun to be supplied to the source line O n . After the time period t w when the voltage of the pixel reaches the middle value, the time control pulse signal TM drops to Low. At this time, the next output pulse OE rises and the next output of voltage is started from the source driver to the source line. However, by the function of scanning electrodes (not shown), the pixel which is charged by (V i + V i+1 )/2 is disconnected from the source driver, so that the pixel keeps the condition charged by (V i + V i+1 )/2. The relationship among the values of digital video signal data D₀ , D₁ and D₂ and the voltage applied to the liquid crystal in the source driver of this embodiment is shown in Figure 21. It will be understood that, by replacing the voltage levels realized in this embodiment with the following voltages, the source driver of this embodiment has the same function as that of the circuit requiring eight levels of external voltages as shown in Figure 38: In this embodiment, the digital video signal data is 3 bits. Even if the video signal data is 4 bits, in the signal voltage output circuit according to the invention, the number of external voltage levels may be 2³ = 8, and only one time control pulse signal TM is required. According to the embodiment, when signal voltages with different levels are applied to a pixel based on digital video signal data and a gray-scale display is performed, the voltages are not supplied in the first half of one output period, but one of the levels of the external voltages is supplied to a pixel only during a predetermined time period in the last half. By previously selecting the length of the time period based on the capacitance of the pixel, when one output period is terminated, the signal voltage can reach a desired voltage. Accordingly, voltages having the middle values between the respective adjacent external voltages can be supplied to the pixel. Figure 22 shows a further embodiment. This embodiment comprises a signal voltage output circuit (a source driver) for the nth signal line (source line) O n of a drive circuit for an LCD apparatus to which digital video signal data of 4 bits is supplied. The circuit comprises a sampling memory M SMP , two memories for holding outputs (holding memories) M H1 and M H2 , a time selection circuit TS, a voltage selection circuit VS, and five analog switches ASW₀ - ASW₄. The sampling memory M SMP comprises four D-type flip-flops corresponding to the respective bits D₀, D₁, D₂ and D₃ of a digital video signal data. The sampling memory M SMP latches video signal data in accordance with the rising of a sampling pulse T SMPn , and supplies them to the holding memories M H1 and M H2 . Each of the two holding memories M H1 and M H2 includes two D-type flip-flops which correspond to the lower 2 bits and the upper 2 bits of the sampling memory M SMP , respectively. The data to be supplied to the holding memories M H1 and M H2 are latched at the rising of an output pulse OE, and fed to the time selection circuit TS and the voltage selection circuit VS, respectively. At this time, the outputs of the holding memory M H1 corresponding to the lower 2 bits D₀ and D₁ of the digital video signal data are fed to the time selection circuit TS through the input terminals A and B. The outputs of the holding memory M H2 corresponding to the upper 2 bits D₂ and D₃ are fed to the voltage selection circuit VS through the input terminals A and B. In addition to the 2-bit data (the lower 2 bits D₀ and D₁ of the video signal data) from the holding memory M H1 , the time selection circuit TS receives three external pulse signals (time division pulse signals) TM₁, TM₂ and TM₃. The three time division pulse signals TM₁, TM₂ and TM₃ are different in pulse width from each other. An output CTM from the time selection circuit TS is fed to the voltage selection circuit VS in conjunction with the upper 2 bits D₂ and D₃ of the video signal data. Based on these inputs, the voltage selection circuit VS makes one of five outputs S₀ - S₄ High or 1 in accordance with a logic described below. Thus, the corresponding one of the analog switches ASW₀ - ASW₄ is conductive, and one of the external voltages V₀ - V₄ is supplied to the source line O n (V₀ < V₁ < V₂ < V₃ < V₄, and the potential differences between the respective adjacent levels are equal). The relationship among the input data (D₀, D₁) at the inputs A and B and the output CTM of the time selection circuit TS is shown in the logic table of Figure 23. When the value of the input data is 0 (i.e., (D₀, D₁) = (0, 0)), the output CTM is always 0. When the value of the input data is 1, 2 or 3 (i.e., (D₀, D₁) = (0, 1), (1, 0) or (1, 1)), the output CTM is 0 or 1 in synchronization with the respective time division pulse signals TM₁, TM₂ and TM₃. The relationship among the inputs A, B and CTM and the output of the voltage selection circuit VS is shown in the logic table of Figure 24. When the upper 2 bits (D₂, D₃) of the video signal data are (0, 0) and the input CTM is 0, the output S₀ becomes 1. When (D₂, D₃) = (0, 0) and CTM = 1, the output S₁ becomes 1. For (D₂, D₃) having another value ((0, 1), (1, 0) or (1, 1)), in the same manner, the output S i (i = 1, 2, 3) becomes 1 when CTM is 0, and the output S i+1 becomes 1 when CTM is 1. Therefore, for one unit of video signal data, the voltage supplied to the corresponding source line O n from the source driver of Figure 22 changes from the lower voltage level V i to the higher voltage level V i+1 , in accordance with the change (from 0 to 1) of the output CTM from the time selection circuit TS. The value of the output CTM of the time selection circuit TS changes in accordance with the time division pulse signal TM k (k = 1, 2, 3) when the lower 2 bits (D₀, D₁) of the video signal data are not (0, 0). Accordingly, for example as shown in the timing chart of Figure 25, the voltage V₀ is supplied during the time period t a beginning from the rising of the output pulse OE, and thereafter the voltage V₁ is supplied during the time period t b . The sum of the time periods t a and t b equals the period of the output pulse OE which is constant. However, as described above, the time division pulse signals TM k are different from each other in the ratio of the time periods t a and t b . Figure 26 shows an equivalent circuit of the load for the source line O n . The resistance of the source line O n is represented by R s , and the capacitance of the liquid crystal pixel connected to the source line O n is represented by C s . A common voltage applied to the opposite electrode of the liquid crystal display panel is indicated by V COM . In the case of Figure 25, the voltage v(t) supplied to the source line O n is changed as follows: v(t) = V₀ (0 < t < t a ) and v(t) = V₁ (t a ≦ t ≦ t a + t b ) During the time period of 0 < t < t a , the voltage V₀ is supplied to the source line O n . Accordingly, the voltage v c (t) across the capacitance C s of the pixel which is a load for the source line O n can be obtained from the following simultaneous equations: V₀ = R s ·i(t) + (1/C s )∫i(t)dt v c (t) = (1/C s )∫i(t)dt + V COM where i(t) represents a current flowing through the source line O n . By solving the simultaneous equations, the voltage v c (t) is obtained as follows: v c (t) = V COM + V₀·{1 - exp(-t/(C s ·R s ))} The voltage v c (t) comes closer to the voltage V₀ as shown by a broken curve in Figure 27. In view of this rising characteristic of the voltage, when designing a liquid crystal panel, the values of the capacitance C s and the resistance R s of a liquid crystal are determined so that the voltage v c (t) comes sufficiently closer to the voltage V₀ in a shorter period of time than the period of the output pulse OE. In other words, the values of capacitance C s and resistance R s are appropriately determined, whereby the voltage can come sufficiently closer to the voltage V₀ at the time t q . The change in the voltage v c (t) after the time t q can be obtained from the following simultaneous equations: V₁ - V₀ = R s ·i(τ) + (1/C s )∫i(τ)dt v c ′(τ) = (1/C s )∫i(τ)dt In these equations, the coordinate transformation is performed as follows: t = τ + t q v = v′ + V₀ The solution of the above simultaneous equations is effected as follows: v c (τ) = (V₁ - V₀)·{1 - exp(-τ/(C s -R s ))} When the difference between the voltages V₀ and V₁ is divided into four equal parts, the obtained voltages are represented by V₀₋₁, V₀₋₂ and V₀₋₃. The time periods t₁, t₂ and t₃ (from time t q ) required for the voltage v c (τ) to reach the voltages V₀₋₁, V₀₋₂ and V₀₋₃ are respectively obtained by the following equations: (V₁- V₀)/4 = (V₁ - V₀)·{1 - exp(-τ/(C s ·R s ))} (V₁- V₀)/2 = (V₁ - V₀)·{1 - exp(-τ/(C s ·R s ))} 3 (V₁ - V₀)/4 = (V₁ - V₀)·{1 - exp(-τ/(C s ·R s ))} The solutions are effected as follows: t₁ = 0.288 C s ·R s t₂ = 0.693 C s ·R s t₃ = 1.39 C s ·R s As is apparent from these equations, the time periods t₁, t₂ and t₃ are independent of the external voltages V₀ and V₁. That is, the time periods t₁, t₂ and t₃ are always constant even when any two adjacent voltage levels are selected from the external voltage levels V₀ - V₄ by the voltage selection circuit VS. Therefore, when the time period t b for which the time division pulse signals TM₁, TM₂ and TM₃ to be supplied to the time selection circuit TS are High is set to the time period t₁, t₂ or t₃ obtained from the above equations, the voltage levels V₀₋₁, V₀₋₂ and V₀₋₃ (which are obtained by dividing the difference between the voltage levels V₀ - V₁) can be arbitrarily applied to the liquid crystal. In the same manner, voltage levels V₁₋₁, V₁₋₂ and V₁₋₃ which are obtained in such a manner that the difference between the voltages V₁ and V₂ is divided into four equal parts, and voltage levels which are obtained in such a manner that the difference between the voltages V₂ and V₃ is divided into four equal parts can be produced by these time division pulse signals TM₁, TM₂ and TM₃. Figure 28 shows the relationship among the values of the digital video signal data D₀, D₁, D₂ and D₃ and the voltage applied to the liquid crystal in the source driver of this embodiment which operates as described above. When the lower 2 bits (D₀, D₁) of video signal data are (0, 0), the output of the time selection circuit TS is always 0, irrespective of the change in the time division pulse signal TM k (k = 1, 2, 3), so that the voltage with external voltage level V₀, V₁, V₂ or V₃ is supplied to the source line O n . When the lower 2 bits (D₀, D₁) have values other than (0, 0), the output of the time selection circuit TS changes from 0 to 1 with the change of the time division pulse signal TM k in one period of the output pulse OE as shown in Figure 25. When the lower 2 bits (D₀, D₁) are (1, 0), the time division pulse signal TM₁ is selected in the time selection circuit TS. Therefore, the time period t b for which the higher voltage is output is the shortest time period t₁ (conversely, the time period t a for which the lower voltage is output is longest), and the voltages applied to the pixel have the levels V₀₋₁, V₁₋₁, V₂₋₁, and V₃₋₁ each of which is the lowest one among the three voltage levels obtained by dividing the difference between the adjacent levels into four equal parts. When the lower 2 bits (D₀, D₁) are (0, 1), the time division pulse signal TM₂ is selected in the time selection circuit TS. Therefore, the time period t b is the middle time period t₂, and the voltages applied to the pixel have the levels V₀₋₂, V₁₋₂, V₂₋₂ and V₃₋₂ which are the middle levels among the adjacent levels. When the lower 2 bits (D₀, D₁) are (1, 1), the time division pulse signal TM₃ is selected in the time selection circuit TS. Therefore, the time period t b is the longest time period t₃, and the voltages applied to the pixel have the levels V₀₋₃, V₁₋₃, V₂₋₃, and V₃₋₃ each of which is the highest one among the three voltage levels obtained by dividing the difference between the adjacent levels into four equal parts. As described above, in this embodiment, by only using five levels of external voltages V₀, V₁, V₂, V₃, and V₄, 16 voltage levels can be applied to the pixels. As shown in Figure 37, in a conventional digital source driver, in order to apply 16 voltage levels to pixels, the same number of external voltage levels (i.e., 16 levels) are required. In the drive circuit according to the invention, the number of external voltage sources can be greatly reduced, whereby the size and the production cost of the circuit can be reduced. Figure 29 shows an example in which the time selection circuit TS and the voltage selection circuit VS in Figure 22 consists of logic circuits comprising AND gates, OR gates and an inverter. The time selection circuit TS consists of AND gates and OR gates arranged in accordance with the logic table of Figure 23. The voltage selection circuit VS consists of AND gates, OR gates and an inverter according to the logic table of Figure 24. An example of a specific relationship among the output pulse OE and the time division pulse signals TM₁, TM₂, and TM₃ to be supplied to the time selection circuit TS is shown in the timing chart of Figure 30. As shown in Figure 30, falling edges of the time division pulse signals TM₁, TM₂, and TM₃ (High) conform with the rising edge of the output pulse OE. Figure 31 shows the relationship among the output pulse OE, the time division pulse signal TM₁, the output V OUT to the source line O n and the voltage V cs applied to the pixel, in the source driver of Figure 29 (or Figure 22) to which video signal data (D₀, D₁, D₂, D₃) of (0, 0, 0, 1) are supplied. Since the duration t₀ of the voltage V₀ supplied to the source line O n for the first time is sufficiently long, the voltage of the pixel comes sufficiently closer to the voltage V₀ during this duration. After the time period t₀, the voltage V₁ of next higher level is started to be supplied to the source line O n . As calculated by the above equations, the voltage V cs applied to the pixel becomes V₀₋₁ (= V₀ + (V₁ - V₀)/4) after the time period t₁. When the voltage of the pixel reaches the voltage V₀₋₁, the next output pulse OE rises and the next output of voltage from the source driver to the source line is started. At this time, by the function of scanning electrodes (not shown), the pixel which has been charged by the voltage V₀₋₁ is disconnected from the source driver, so that the pixel keeps the condition of being charged by the voltage V₀₋₁. In the above-described embodiment, the digital video signal data is 4 bits. For example, when the video signal data is 6 bits to comply with an increased gray-scale, the required number of levels of external voltage sources is 2⁶ = 64 in the conventional digital drive circuit shown in Figure 37. By contrast, in the drive circuit according to the invention, for example, 4 bits are assigned to the voltage selection circuit VS and 2 bits to the time selection circuit TS. Therefore, by providing external voltage sources with 2⁴ + 1 = 17 levels and three time division pulse signals TM₁ - TM₃, a display with 64 gray-scale levels can be attained. According to the invention, when a gray-scale display is performed by supplying signal voltages with different levels based on digital video signal data, one output period of the signal voltages is divided into two periods. In the first and last half of the periods, the external voltages with different levels are supplied to a pixel. By appropriately determining the ratio of time periods for supplying the voltage according to the capacitance of the pixel, a voltage with an arbitrary level between adjacent external voltage levels can be supplied to the pixel. Accordingly, it is unnecessary to prepare external voltages the number of which is the same as that of a predetermined gray-scale levels. Therefore, the external voltage supplying circuit can be made smaller and the number of terminals of the drive circuit for a display circuit can be reduced. Furthermore, since the steps between adjacent voltages applied to the pixel can be more minute than the steps between adjacent levels of external voltage sources, minute gray-scale display can be attained even when voltages with minute differences are difficult to produce in an external voltage producing circuit.

Claims: A drive circuit for a display apparatus in which a drive voltage is output, to be applied to a display drive line (O n ), in accordance with a digital video signal, said drive circuit comprising: time control means (22;TC;TS) for receiving a portion of said digital video signal, and for producing a time division signal (CTM) defining at least a first period and a second period included in a signal output period in accordance with said portion of said digital video signal, said display drive line being connected to a pixel during said signal output period; and voltage selecting means (ASW₀ - ASW₄,20;ASW₁ - ASW₄,SEL;ASW₀ - ASW₄,VS), connected to said time control means, for receiving the remaining portion of said digital video signal and said time division signal, and selectively outputting, to said display drive line, one of a plurality of signal voltages during said first period of said signal output period and another of said plurality of signal voltages during said second period of said signal output period, in accordance with said remaining portion of said digital video signal and said time division signal, said plurality of signal voltages being different from each other. A drive circuit according to claim 1, wherein said voltage selecting means (ASW₁ - ASW₄,SEL) further outputs, to said display drive line, none of said plurality of signal voltages during one of said first and second periods of said signal output period. A drive circuit according to claim 1, wherein said voltage selecting means (ASW₀ - ASW₄,20) further outputs, to said display drive line, one of said plurality of signal voltages during both of said first and second periods of said signal output period. A drive circuit according to claim 1, wherein said voltage selecting means is provided for each said display drive line of said display apparatus. A drive circuit according to claim 1, wherein said voltage selecting means comprises: a plurality of switch means (ASW₀ - ASW₄;ASW₁ - ASW₄), the number of said plurality of switch means being equal to the number of said plurality of signal voltages; and selection means (20;SEL;VS) for producing selection signals which respectively control said plurality of switch means. A drive circuit according to claim 1, wherein said time control means (20;TC;TS) receives a control signal (TM;TM₁-TM₃) and selectively outputs either said control signal or a constant value to said voltage selecting means in accordance with said portion of said digital video signal. A drive circuit according to claim 1, wherein the length of said first period and the length of said second period are fixed. A drive circuit according to claim 1, wherein the length of said first period and the length of said second period are determined by said time control means (TS) in accordance with said portion of said digital video signal.

Inventors: MIKI KATUMI, OKADA HISAO, TANAKA KUNIAKI, UEHIRA SHIGEYUKI, YANAGI TOSHIHIRO, MIKI, KATUMI, OKADA, HISAO, TANAKA, KUNIAKI, UEHIRA, SHIGEYUKI, YANAGI, TOSHIHIRO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: G09G3
IPC codes: G09G3, H04N3