JP2004004512A - Drive circuit of liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem of abnormality in signals in a level shifter caused when a TFT having a relatively slow operation rate is used to integrate a drive circuit in a liquid crystal display. <P>SOLUTION: Abnormality in signals is prevented by setting the size of a transistor in the level shifter 11. The output of the level shifter 11 is inputted into a gate circuit 11 which controls timing, and the subsequent circuits are driven by the output signals of the gate circuit 11. By constituting the above circuits, abnormality in signals caused in the level shifter 11 does not influence the signals in the subsequent circuits of the level shifter 11. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a driving circuit for a liquid crystal display device. In particular, it relates to a built-in drive circuit using a TFT.
[0002]
[Prior art]
2. Description of the Related Art In a liquid crystal display device that displays an image using electro-optical characteristics of a liquid crystal, a thin film element such as a TFT is formed on a transparent substrate as a switching element of each pixel, and an excellent display is achieved by controlling a voltage applied to the liquid crystal. Success in gaining quality. Further, a driving circuit built-in technology for integrally forming a driving circuit of a liquid crystal display with a TFT around a pixel portion on a substrate instead of an LSI has been widely used. This technology with a built-in drive circuit has made it possible to reduce the size and cost of the liquid crystal display device.
[0003]
The driving circuit of the liquid crystal display device includes a data driver and a scanning driver. While one scanning line in the horizontal direction is selected by the scanning driver, a video signal is written from the data driver to the pixel connected to the scanning line via the data line.
[0004]
The scan driver mainly includes a logic circuit unit having a shift register and the like and a buffer circuit unit. In general, the timing of the scanning signal is controlled by a shift register, and the scanning line is driven via a buffer so that the scanning line with a large load can be sufficiently driven at this timing.
[0005]
The data driver mainly includes a logic circuit unit including a shift register and the like and a sampling circuit unit. The sampling timing is controlled by the shift register, and the video signal is written to the data line by the sampling circuit according to the timing.
[0006]
At present, in a liquid crystal display device with a built-in drive circuit, there is an increasing need to lower the voltage of the drive circuit in order to reduce power consumption and reduce signal noise. However, the driving voltage of the liquid crystal is about ± 5 V, and the driving voltage of the driving circuit is relatively high at about 15 V. In order to satisfy these contradictory conditions, a driving circuit having a structure in which a logic circuit portion for controlling driving timing is driven at a low voltage and only a sampling circuit for sampling a video signal is driven at a high voltage is required. For this reason, two types of voltage levels must be used as power supply voltages inside the drive circuit, and a level conversion function for driving a sampling circuit by boosting a low-voltage timing signal output from the logic circuit unit is provided. A required circuit is required between the logic circuit section and the sampling circuit section. Due to such demands, the importance of a level shifter, which is a level conversion circuit in a built-in drive circuit, has been greatly increased.
[0007]
Based on the above background, FIG. 1 shows an example of a drive circuit (data driver) of a conventional liquid crystal display device including a level shifter (from page 609 of SID '92 DIGEST). FIG. 2 shows a timing chart of signals at points A to L inside the drive circuit.
[0008]
The shift register is driven by a first power supply voltage V1 having a positive value with respect to a reference potential Vss (usually equal to GND), and the sampling circuit is driven by a second power supply voltage V2 higher than V1. A level shifter (11) is provided between the shift register and the sampling circuit for boosting the first power supply voltage to the second power supply voltage. In general, the output of the level shifter is often connected to a sampling circuit through a buffer circuit (12).
[0009]
[Problems to be solved by the invention]
When the internal signal is actually examined by measurement in the configuration of the conventional driving circuit, an abnormality of the internal signal as shown in the timing chart of FIG. 3 is observed. It has been found that this signal abnormality caused by the level shifter results in a problem that the operation timing of the sampling circuit is shifted.
[0010]
In particular, among the level shifter output signals (signals at G, H, I, J, K, L,...) Of the timing chart shown in FIG. 3, negative logic input signals (signals at B, D, F,. It has been found by measurement that signal anomalies (31, 32, 33) occur at the fall of the positive logic output signal (signal at G, I, K,...) Output by the level shifter in response to the rise of .
[0011]
Originally, the role of a level shifter is to convert the level of an internal signal. However, when a level shifter is configured using a TFT whose operation speed is slower than that of a MOSFET, a signal delay inside the circuit or a signal abnormality becomes a problem. That is, the provision of the level shifter composed of the TFT inside the drive circuit causes a failure in the original signal transmission, and makes it impossible to output a correct logic at a correct timing. In particular, since the level shifter has a cascade connection (in FIG. 1, a connection method in which two PMOS transistors connect the other drain end to the gate), a dynamic operation state occurs, and a TFT having a low operation speed is used. And this lasts for a relatively long time, so that the problem peculiar to the level shifter becomes remarkable.
[0012]
Next, the cause of this problem will be described. For example, consider the potential at point G in FIG. When the input signal at the point B rises, the NMOS transistor 2 (15) is turned on to try to lower the potential at the point G from V2 to Vss. At this time, since the PMOS transistor 2 (16) is still on, the potential at the point G is not determined and enters a dynamic operation state. Therefore, a state where the potential is not determined as shown in FIG. 3 occurs.
[0013]
In the state of the dynamic operation as described above, a circuit including the point G, Vss, V2, the PMOS transistor 2, and the NMOS transistor 2 is represented by an equivalent circuit as shown in FIG. In the dynamic operation state, V2 and Vss are connected via the on-resistances (41, 42) of the PMOS transistor 2 and the NMOS transistor 2. At this time, a current flows from V2 to Vss, and the potential at point G is determined by the ratio of the on-resistance (41) of the PMOS transistor 2 to the on-resistance (42) of the NMOS transistor 2 by resistance division. That is, if the on-resistance of the PMOS transistor 2 is smaller than the on-resistance of the NMOS transistor 2, the potential at the point G becomes closer to V2, which causes a signal abnormality.
[0014]
Further, it has been found that the closer the potential at point G is to V2 than Vss, the longer the duration of signal abnormality.
[0015]
The present invention provides a TFT element configuration for reducing the signal abnormality generated in the level shifter, or a circuit configuration in which even if the signal abnormality occurs, its influence does not affect circuits subsequent to the level shifter.
[0016]
[Means for Solving the Problems]
The drive circuit of the liquid crystal display device according to the present invention is configured such that the size of the transistor connected to the high-voltage power supply side of the level shifter and the size of the transistor connected to the low-voltage side have an appropriate size ratio in which a signal abnormality is unlikely to occur. A gate circuit for adjusting the timing of the signal by comparing it with the output signal of the next stage is provided.
[0017]
【Example】
Embodiments of the present invention will be described below with reference to the drawings.
[0018]
FIG. 5 shows an overall view of an active matrix circuit incorporating a drive circuit of the liquid crystal display device of the present invention. As shown, a data driver and a scanning driver are arranged around the active matrix circuit. The scanning driver includes a shift register, a level shifter, a gate circuit, and a buffer circuit, and is connected to a scanning line. One data driver includes a shift register, a level shifter, a gate circuit, and a buffer circuit, and is connected to a data line. The active matrix circuit includes an active matrix element, a storage capacitor, and a transparent electrode for applying a voltage to the liquid crystal for each pixel. The gate of the active matrix element is connected to the scanning line, the source is connected to the data line, and the drain is connected to the transparent electrode.
[0019]
A scan signal is output from the scan driver, and a video signal is written from the data line to a liquid crystal capacitor and a storage capacitor connected to the selected scan line. The video signal is sampled by the data driver, and its timing is also controlled by the data driver.
[0020]
The present invention particularly relates to the data driver and the scanning driver.
[0021]
FIG. 6 shows an embodiment of the present invention in which the TFT size is adjusted so that signal abnormality hardly occurs. Here, a case is shown where the first power supply voltage V1 and the second power supply voltage V2 are both positive and there is a relationship of V1 <V2.
[0022]
The on-resistance of a TFT is determined by the channel length L and channel width W of the TFT. Unless L is extremely small, the on-resistance of the TFT is proportional to the value of L / W. As described above, the signal abnormality in the level shifter occurs because the on-resistance of the N-channel TFT is smaller than the on-resistance of the P-channel TFT. Therefore, at least the on-resistance of the N-channel TFT becomes smaller than the on-resistance of the P-channel TFT. You need to do that. In other words, the value of L / W of the P-channel TFT must always be set at least larger than the value of L / W of the N-channel TFT. In the level shifter shown in FIG. 6, when each TFT is in an ON state, the gate voltage of the P-channel TFT1 (54) and the P-channel TFT2 (56) is -V2 (because the source potential is equal to V2 and the gate potential is equal to GND). ), The gate voltages of the N-channel TFT1 (53) and the N-channel TFT2 (55) are V1. Here, since V2> V1, if the TFT sizes of the P-channel TFTs 1 and 2 and the N-channel TFTs 1 and 2 are the same, the on-resistance of the P-channel TFTs 1 and 2 is smaller than that of the N-channel TFTs 1 and 2. Causes abnormalities. Therefore, the level shifter according to the embodiment of the present invention is characterized in that the TFT size is set such that the ON resistance of the N-channel TFT is not always too large as compared with the ON resistance of the P-channel TFT. If the gate lengths of the N-channel TFT and the P-channel TFT are the same, the gate width of the N-channel TFT is at least larger than that of the P-channel TFT, and if the gate widths of the P-channel TFT and the N-channel TFT are the same, the gate length of the N-channel TFT Is set to be at least smaller than that of the P-channel TFT. More preferably, the TFT size is set such that the ON resistance of the N-channel TFT is always smaller than the ON resistance of the P-channel TFT at each gate voltage of the P-channel TFT and the N-channel TFT. By setting the TFT size in this way, the potential at the point G becomes close to Vss, so that signal abnormality hardly occurs, and even if signal abnormality occurs, it can be minimized.
[0023]
This method can be applied to both a data driver and a scanning driver as a driving circuit of a liquid crystal display device.
[0024]
As a relation among the first power supply voltage V1, the second power supply voltage V2, and the reference potential Vss, when V1 and V2 are both positive with respect to Vss, when V1 and V2 are both negative, V1 is positive with respect to Vss. When V2 is negative, V1 is negative and V1 is positive with respect to Vss. In each case, | V1 | <| V2 | and | V1 |> | V2 | . In all these cases, the same effect as in the above embodiment can be obtained by making the L / W of the TFT directly connected to V2 smaller than the L / W of the TFT connected to Vss.
[0025]
Next, FIG. 7 shows an embodiment of the present invention which provides a circuit configuration which does not affect the circuits subsequent to the level shifter even if the signal abnormality shown in FIG. 3 occurs in the level shifter. In the drive circuit of the present invention, the output of the level shifter is connected to the gate circuit. Here, a circuit configuration example of a data driver particularly when a NAND gate (61) is used as a gate circuit is shown. FIG. 8 is a timing chart of signals at respective points inside the driving circuit.
[0026]
The output terminals (A to F) of the shift register are connected to a level shifter. One of the two boosted outputs of one level shifter (G, I,...) Is connected to the other output (J, L,...) Of the adjacent level shifter as shown in FIG. Input to the gate. After the timing of the output signals (M, N, O...) Of the NAND gates is further adjusted by the buffer circuit, the sampling circuit is driven.
[0027]
When the above circuit configuration is adopted, the drive signals of the sampling circuit are M to O in FIG. 8 with respect to the output signals of the shift register (A to F in FIG. 8). When the signal M is taken as an example, the signal M is determined by the NAND logic of the signals G and J. Therefore, the rising edge (71) of the signal M is defined by the rising edge (73) of the signal G, and the falling edge (72) of the signal M is defined by the falling edge (74) of the signal J. That is, the signal M is determined irrespective of the fall of the signal G, which is likely to cause a signal abnormality. By providing the gate circuit immediately after the level shifter, a normal operation can be maintained without affecting the timing of the driving signal of the sampling circuit even if a signal abnormality occurs in the level shifter.
[0028]
Here, a gate circuit such as a NOR gate and an AND gate may be used in addition to the NAND gate depending on the output logic of the shift register.
[0029]
FIG. 9 shows an embodiment in which a scan driver is configured by the same circuit configuration method as described above.
[0030]
The output of the shift register is input to the level shifter (11), and the output of the level shifter is input to the NAND gate (61) in the same manner as in the embodiment of FIG. Thereafter, the output of the gate is connected to the scanning line through a buffer circuit. With this circuit configuration, the scanning driver can be driven normally without the signal abnormality in the level shifter affecting the scanning signal, similarly to the data driver.
[0031]
Note that the drive circuit of the liquid crystal display device of the present invention can be applied not only to a circuit formed on an insulating substrate using TFTs, but also to a circuit using MOSFETs using ordinary single crystal silicon.
[0032]
【The invention's effect】
As described above, by using the drive circuit of the present invention, signal abnormality in the level shifter can be reduced, and even if a signal abnormality occurs, the sampling circuit can be driven at normal timing. As a result, the sampling circuit can be accurately driven at high speed even if the driving circuit of the liquid crystal display device is integrally formed using a TFT having a relatively slow operation speed. As a result, the horizontal resolution is improved. Further, the timing of the driving circuit on the scanning side can be accurately controlled, and a large-sized liquid crystal display device having a large number of scanning lines and a large wiring capacity can be accurately driven.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating an example of a driving circuit (data driver) of a conventional liquid crystal display device including a level shifter.
FIG. 2 is a timing chart of the driving circuit of FIG.
FIG. 3 is a diagram showing signal waveforms of the drive circuit of FIG.
FIG. 4 is a diagram showing an equivalent circuit in a dynamic operation state.
FIG. 5 is a diagram of a driving circuit of the liquid crystal display device of the present invention formed integrally with an active matrix circuit.
FIG. 6 is a circuit diagram of a level shifter according to the present invention.
FIG. 7 is a circuit diagram of a driving circuit (data driver) of the liquid crystal display device of the present invention.
8 is a timing chart of the drive circuit in FIG.
FIG. 9 is a circuit diagram of a drive circuit (scan driver) of the liquid crystal display device of the present invention.
[Explanation of symbols]
11 Level shifter section 12 Buffer circuit section 13 NMOS transistor 1
14 PMOS transistor 1
15 NMOS transistor 2
16 PMOS transistor 2
31, 32, 33 A portion where a signal abnormality has occurred 41 An on-resistance of the PMOS transistor 2 42 An on-resistance of the NMOS transistor 43 Gate capacitance 61 NAND gate portion 71 A rising edge of the signal at the M point 72 A falling edge 73 G of the signal at the M point 73 G Rising point signal 74 Falling point J signal

Claims (16)

A drive circuit for a liquid crystal display device having a reference potential power supply, a shift register driven by a first power supply, a sampling circuit driven by a second power supply, and a level shifter for converting a level from the first power supply to a second power supply , The value of L / W of the transistor of the level shifter having the gate length L and the gate width W directly connected to the second power supply is changed to the L / W of the transistor of the level shifter directly connected to the reference potential power supply. A drive circuit for a liquid crystal display device, wherein the transistor size is set to be larger than the value of / W. 2. The driving circuit according to claim 1, wherein the transistor is a thin film transistor (TFT). 3. The driving circuit according to claim 1, wherein the driving circuit is formed integrally with an active matrix circuit formed on an insulating substrate. 4. The liquid crystal display device driving circuit according to claim 1, wherein the driving circuit is a data driver for applying a video signal to a data line. Drive circuit for a liquid crystal display device having a reference potential power supply, a shift register driven by a first power supply, a buffer circuit driven by a second power supply, and a level shifter for converting a level from the first power supply to a second power supply , The value of L / W of the transistor of the level shifter having the gate length L and the gate width W directly connected to the second power supply is changed to the L / W of the transistor of the level shifter directly connected to the reference potential power supply. A transistor circuit for a liquid crystal display device, wherein the transistor size is set to be larger than the value of / W. 6. The driving circuit according to claim 5, wherein the transistor is a thin film transistor (TFT). 7. The driving circuit for a liquid crystal display device according to claim 5, wherein the driving circuit is formed integrally with an active matrix type liquid crystal display device formed on an insulating substrate. 8. The driving circuit according to claim 5, wherein the driving circuit is a scanning driver that outputs a scanning signal to a scanning line. A drive circuit for a liquid crystal display device having a reference potential power supply, a shift register driven by a first power supply, a sampling circuit driven by a second power supply, and a level shifter for converting a level from the first power supply to a second power supply , A first output of the level shifter and a second output of the level shifter having a phase shifted from the first output by one sampling period are input to a gate circuit, and an output signal of the gate circuit outputs a signal of the sampling circuit. A drive circuit for a liquid crystal display device, wherein timing of a drive signal is defined. 10. The driving circuit for a liquid crystal display device according to claim 9, wherein the driving circuit for the liquid crystal display device is formed by a thin film transistor (TFT). 11. The driving circuit for a liquid crystal display device according to claim 9, wherein the driving circuit is formed integrally with an active matrix type liquid crystal display device formed on an insulating substrate. 12. The driving circuit according to claim 9, wherein the driving circuit is a data driver that applies a video signal to a data line. Drive circuit for a liquid crystal display device having a reference potential power supply, a shift register driven by a first power supply, a buffer circuit driven by a second power supply, and a level shifter for converting a level from the first power supply to a second power supply , A first output of the level shifter and a second output of the level shifter having a phase shifted by one scanning cycle from the first output are input to a gate circuit, and an output signal of the gate circuit outputs a signal of the buffer circuit. A drive circuit for a liquid crystal display device, wherein timing of a drive signal is defined. 14. The driving circuit for a liquid crystal display device according to claim 13, wherein the driving circuit for the liquid crystal display device is formed of a thin film transistor (TFT). 15. The driving circuit for a liquid crystal display device according to claim 13, wherein the driving circuit is formed integrally with an active matrix type liquid crystal display device formed on an insulating substrate. 16. The driving circuit of a liquid crystal display device according to claim 13, wherein the driving circuit is a scanning driver that outputs a scanning signal to a scanning line.

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