JPH022511A - Driving device for active matrix type liquid crystal display device - Google Patents

Abstract

PURPOSE:To prevent a liquid crystal cell from deteriorating in contrast by applying a reverse bias to the gate of a thin film transistor TFT according to the operation time of the liquid crystal display device when the TFT is not in operation and returning the drain current-gate voltage characteristics of the TFT to the initial position with the reverse bias in the nonoperation state by a negative-directional shift quantity. CONSTITUTION:An operation time counting means 1 provided to the driving device 10 counts the operation time of the liquid crystal display device and a reverse bias circuit 2 applies the specific bias voltage opposite in polarity from a bias voltage applied to the gate of the TFT when the liquid crystal display device is in operation to be gate of the TFT of the counting means 1 when the liquid crystal display device is not in operation. At such a time, an application time control means 3 provided to the driving device 10 controls the application time of the reverse bias circuit 2 according to the counted time 1 of the means 1 and the applied voltage of the reverse bias circuit 2. The characteristics of the TFT which shift in the operation of the liquid crystal display device are canceled when the device is not in operation. Consequently, the contrast of the liquid crystal cell is prevented from deteriorating.

Description

[Detailed Description of the Invention] [Summary] Regarding a driving device for an active matrix type liquid crystal display device that writes data given to a data electrode into a liquid crystal cell via a TFT, the active matrix type liquid crystal display device can prevent deterioration of the TFT. In order to stabilize the display quality of the liquid crystal display device, the present invention includes an operating time counting means for counting the operating time of the active matrix type liquid crystal display device, and a means for counting the operating time of the active matrix type liquid crystal display device; a reverse bias circuit that applies a predetermined bias voltage of opposite polarity to the bias voltage applied to the gate to the gate of the TFT; and an application time control means for controlling the application time of the reverse bias circuit.

[Industrial application field]

The present invention relates to a driving device for an active matrix liquid crystal display device, and more particularly to a driving device for an active matrix liquid crystal display device configured to prevent deterioration in display quality of the liquid crystal display device.

In recent years, active matrix drive type flat displays have been actively researched and developed. Since this active matrix drive type flat display can have a smaller depth than a cathode ray tube, it has also been commercialized as a display device for pocket-type televisions, laptop-knob type computers, and the like.

However, in the active matrix driving method, the display quality of the liquid crystal display device may deteriorate due to deterioration of the characteristics of TFTs (F-4 film transistors) used as switching elements, especially a-3i TFTs (amorphous silicon thin film transistors). , this improvement is desired.

[Conventional technology]

Figure 5 shows a conventional active matrix liquid crystal display device 1.
This shows the configuration of 00. In general, a conventional active matrix display device is constructed by sandwiching a display layer on which liquid crystal cells are arranged between two glass substrates. On one glass substrate, a scanning electrode of a TFT, which is a switching element, and a liquid crystal cell are placed on one glass substrate. One electrode (hereinafter referred to as a counter electrode) is formed on the other glass substrate, and a data electrode is formed on the other glass substrate as a common electrode of the liquid crystal cell. Each data electrode 4 is driven by a data electrode driver 40, and each scan electrode 5 is driven by a scan electrode driver 50, but here, the connection with the TFT 8 of one liquid crystal cell 3 in the display layer and the connection with each electrode Only the connection will be explained.

As shown in FIG. 5, the active matrix liquid crystal display device 100 includes a data electrode 4, a scanning electrode 5, and a counter electrode 7.
There are three electrodes, and one liquid crystal cell ([,C) 3 and a T'FT 8 for controlling this liquid crystal cell 3 are provided between the data electrode 4 and the counter electrode 7. TFT8
The control terminal (gate) of is connected to the scanning electrode 5,
One of the two controlled terminals (drain) is connected to the data electrode 4 , and the other (source) is connected to one terminal of the liquid crystal cell 3 . Further, the other terminal of the liquid crystal cell 3 is connected to a counter electrode 7, and in this embodiment, the counter electrode 7 is grounded.

The operation of the active matrix liquid crystal display device 100 configured as above will be explained using FIG. 6.

Since the liquid crystal cell 3 deteriorates unless AC voltage is applied,
The counter electrode 7 is held at 0■ as shown by the waveform A, and the voltage of the data electrode 4 is an alternating voltage centered on the potential of the counter electrode 7, as shown by the waveform opening. data electrode 4
The period of the alternating voltage is twice as long as one vertical scanning time (16.7 m5) of a rusk scan in a television image. The scanning electrode 5 is normally maintained at a voltage lower than OV, as shown by waveform C, and becomes high only when the TFT 8 is turned on.

Generally, the time for turning on the TFT 8 is 60 μs.

Then, each time the T F T 8 is turned on, the data on the data electrode 4 at that time is written into the liquid crystal cell 3 as shown by waveform 2.

[Problem to be solved by the invention]

However, in the above-mentioned active matrix liquid crystal display device 100, since the groove of the TFT 8 is longer in the off state than in the on state, each time a negative voltage is applied to the gate of the TFT 8, As shown in FIG. 7, the drain current vs. gate voltage characteristics of the T" After 1000 hours of use, the drain current of TFT8 -
The gate voltage characteristic is shifted to the negative side as shown by the dashed line.

Then, TFT8 is turned off! When a negative potential (V6art) is applied to the scanning electrode 5 to drain the drain, the drain current becomes ■ in the initial state. , 1 hardly flows, but after 1000 hours, a minute train current I LE
AK will flow to liquid crystal cell 3, and liquid crystal cell 3 will flow first.
It becomes impossible to hold the data potential written in. That is,
The conventional active matrix liquid crystal display device 100 has a problem in that the contrast of the liquid crystal cell 3 gradually decreases.

The above-mentioned phenomenon is that when ten voltages are applied to the gate of an a-3i TFT, the TFT turns on and drain current flows, but the accumulated electrons are trapped at the gate insulating film/active layer interface and reach a threshold. The value is shifted to the positive side,
Conversely, when a negative voltage is applied to the gate, the TFT turns off, the drain current drops below 10-'2 A, the trapped electrons are released, and the threshold is shifted to the negative side. This is due to its characteristics.

An object of the present invention is to eliminate shifts in the negative direction of the threshold value due to changes over time in TFTs used in active matrix liquid crystal display devices, and to constantly stabilize the contrast of liquid crystal cells in the TFT liquid crystal display device. An object of the present invention is to provide a driving device for an active matrix liquid crystal display device that can perform the following operations.

[Means to solve the problem]

The structure of an active matrix type liquid crystal display device of the present invention that achieves the above object is shown in FIG.

In the figure, 10 is an active matrix type liquid crystal display device in which a liquid crystal cell is formed by interposing a liquid crystal between a data electrode and its counter electrode, and data given to the data electrode is written into the liquid crystal cell via a TFT. The driving device is shown. An operating time counting means l provided in the driving device 10 counts the operating time of the liquid crystal display device, and a reverse bias circuit 2 calculates a bias voltage opposite to the bias voltage applied to the gate of the TFT when the liquid crystal display device is operating. A bias voltage of a predetermined polarity is applied to the gate of the TFT of the counting means when the liquid crystal display device is not in operation.

At this time, the application time control means 3 provided in the drive device 10 controls the reverse bias circuit 2 according to the counting time 1 and the voltage applied to the reverse bias circuit (2).
control the application time.

[For production]

According to the driving device for an active matrix type liquid crystal display device of the present invention, a reverse bias is applied to the gate of the TFT when the active matrix type liquid crystal display device is not in operation, depending on the operating time of the active matrix type liquid crystal display device.
Since the drain current-gate voltage characteristic of the TFT is returned to the initial position by the reverse bias during non-operation by the amount shifted in the negative direction, the drain current-gate voltage characteristic of the TFT is always maintained at the initial position. Therefore, the TFT does not deteriorate and the drain current when the TFT is off does not increase, so the contrast of the liquid crystal cell does not deteriorate.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 2 shows the structure of an embodiment of a driving device for an active matrix type liquid crystal display device according to the present invention, and the same members as in the prior art are given the same reference numerals.

In FIG. 2, 100 is an active matrix type liquid crystal display device, and the display panel 60 includes liquid crystal cells 3 that are turned on and off by TFTs (thin film transistors).
A large number of electrodes are provided between data electrodes 4 and scanning electrodes 5 which are arranged in a matrix. Each line of the data electrode 4 is supplied with a voltage by a data electrode driver 40, and each line of the scan electrode 5 is supplied with a voltage by a scan electrode driver 5.
The voltage is given by 0.

Reference numeral 10 denotes a drive circuit for the active matrix type liquid crystal display device 100, and this drive circuit 10 includes an operation clock circuit 20 of the liquid crystal display device 100, and a drive circuit 10 for the liquid crystal display device 100.
00 non-operating control circuit 30, and a changeover switch S.
They are connected to the active matrix liquid crystal display device 100 via WI and SW2, respectively. These changeover switches SWI and SW2 are each equipped with contacts a, b, and c, and contact a is the operating tense mode. The contact C is connected to the I11 circuit 20, the contact C is connected to the non-operating control circuit 30, and the contact C is connected to the active matrix liquid crystal display device 100.

The selector switches SWI and SW2 are switched in conjunction with the power switch PSW, so that when the power switch PS- is on, contacts a and contact C are connected, and when the power switch pst11 is off, the contacts are connected. The contact point C is configured to connect with the contact point C. Therefore, the operating control circuit 20 controls the data electrode driver 40 and the scanning electrode driver 50 when the power switch PSW is on, and the non-operating control circuit 30 controls the data electrode driver 40 and the scanning electrode driver 50 when the power switch PSW is off.
The data electrode driver 40 and the scan electrode driver 50 are controlled.

This embodiment is provided with another changeover switch SW3 that is linked to the power switch PSW, and when the power switch PS is turned on, contacts a and C are connected.
The structure is such that when the power switch PSW is turned off, the contact C is connected.

Contact a is connected to a power supply circuit 11 connected to a high level "■" power supply, for example, a power switch/7-inch PSW,
Contact point C is grounded, and contact point C is connected to a timer counter 31 of the control circuit 30 during non-operation.

The timer counter 31 counts the operating time of the active matrix liquid crystal display device 100, and when the power switch PSW is turned on, the contact C becomes a high level "1 (").
When this happens, time counting starts, and when power switch PSK is turned off and contact C is grounded, time counting is stopped. The time counted by the timer counter 31 is input to a time control circuit 32, and this time control circuit 32 is activated by a reverse bias application circuit 34 in response to the time count value input from the timer counter 31. According to the predetermined reverse bias voltage applied to the matrix type liquid crystal display device W100, the driving time of the evening/timer 33 is calculated from a built-in conversion table (described later), and the calculated driving time is output to the timer 33. do.

The timer 33 has a function of controlling on/off of the reverse bias application circuit 34, and turns on the reverse bias application circuit 34 for the drive time instructed by the time control circuit 32.
The reverse bias voltage from the reverse bias application circuit 34 is applied to the data electrode driver 4 via the changeover switches SWI and SW2.
0 and is applied to the scan electrode driver 50. When the time instructed by the time control circuit 32 has elapsed, the timer 33 turns off the reverse bias application circuit 34 and resets the count value of the timer counter 31. When the power switch PS- is turned on, the timer counter 31 From O to active mud IJ type liquid crystal display R1
00 operation time can be counted.

FIG. 3 shows the waveform of the data electrode 4 (drain voltage V of the TFT 8) and the scanning electrode 5 when the active matrix liquid crystal display device 100 is operated by the non-operating control circuit 20 in the embodiment shown in FIG. (gate voltage of TFT 8 G), drive waveform of liquid crystal cell 3 (source voltage VS of TFT 8), and waveform of counter electrode 7 are shown. Since the liquid crystal cell 3 deteriorates unless an alternating current voltage is applied, the counter electrode 7 is maintained at OV in this embodiment as well, and the voltage of the data electrode 4 has a waveform of ■. As shown in this counter electrode 7
It is an alternating voltage centered on the potential of . The period of the alternating voltage of the data electrode 4 is one vertical scanning time (16.7 m5) of a rusk scan in a television image, as in the conventional device.
It is twice as much.

For example, the drain voltage of TFT8. Assuming that is an alternating voltage of ±5V and gate voltage 6 is normally -10V, which is the voltage that generates a +IOV pulse when writing data, then T
The source voltage ■ and the drain voltage ■ of FT8 are. Same as ±I
This becomes an alternating voltage of OV. In this state, the gate stress voltage of TFT8 is ■. is the gate-source voltage VCS and the gate-drain voltage V (, p average value Vco= (V
6s+VcD)/2, and in this embodiment, the average value of the gate stress voltage is -10V.

By the way, the shift of the drain current-gate voltage characteristic of the TFT from the initial state to the negative side is caused by the gate stress voltage V
It is known that it is proportional to G to the a power and is activated by temperature. Therefore, in this embodiment, the reverse bias voltage applied from the reverse bias applying circuit 34 of the non-operating control circuit 30 to the active matrix liquid crystal display device 100 is set to +30V, which is approximately three times the gate stress voltage VG.

As a result, when the liquid crystal display device is not in operation, the v during operation is reduced.
, = 3 times the reverse bias voltage vG = -10V = +3
0V is applied. Then, the speed at which the drain current-gate voltage characteristic of TFT 8 shifts to the negative side from the initial state when the liquid crystal display device is not operating is approximately 9 times (30"/10
2), and if the gate bias time during non-operation is set to 1/9 of that during operation, the shift in the characteristics of the TFT 8 can be completely canceled.

Next, the operation of the driving device for the active matrix liquid crystal display device configured as shown in FIG. 2 will be explained using FIG. 4.

When the power switch PS- is turned on at time t0,
Changeover switch SWI, SW2. SW3 contacts a-c
is connected to the active matrix liquid crystal display device 10.
0 starts the display operation, and the timer counter 31 counts the operation time. At time t, power switch P is turned on.
When SW is turned off, the changeover switches SWI SW2.
Contacts b-c of SW3 are connected, and the active matrix liquid crystal display device 100 becomes inactive, and
The timer counter 31 also stops counting. Assuming that the count value of the timer counter 31 at this time is 90, this value is input to the time control circuit 32, and the time control circuit 32 uses this count value and the value of the reverse bias voltage of the non-operating control circuit 30. Timer 33 using a conversion table (not shown)
calculates the time for operating the reverse bias application circuit 34,
This time is output to the timer 33. The timer 33 counts only this instructed time, during which time the bias application circuit 34 is operated to apply a reverse bias voltage to the TFT 8.

The conversion table is a combination of the gate stress voltage applied to the TFT 8 when the active matrix liquid crystal display device 100 is in operation, and the reverse bias voltage applied to the TFT from the non-operating control circuit 30 when the liquid crystal display device 100 is not operating. This is to determine how much time to operate the timer 33 with respect to the time counted by the timer counter 31, and the time control circuit 32
calculates the timer drive time based on this conversion table.

For example, if the gate stress voltage when the liquid crystal display device 100 is in operation is equal to the reverse bias voltage when it is not in operation,
The time control circuit 32 outputs a value exactly the same as the count value of the timer counter 31 to the timer 33. In this embodiment, as described above, since the reverse bias voltage during non-operation is three times the gate stress voltage during operation, the time control circuit 32 outputs the count value 90 of the timer counter 31.
10, which is 1/9 of the value, is output to the timer 33.

In this way, 1/9 of the time from time t0 to Ll
At time t2, when the time has elapsed, the timer 33 stops the operation of the reverse bias applying circuit 34, and sends a reset signal to the timer counter 31 to clear the previously counted value. By applying the reverse bias voltage to TFT 8 from time t1 to time t2, the characteristics of TFT 8 are shifted in the positive direction, and the shift in the characteristics of TFT 8 in the negative direction while active matrix liquid crystal display Wff100 is operating is completely canceled. will be done.

In the above-mentioned embodiment, a reverse bias voltage is applied to the TFT from the non-operating control circuit 30 immediately after the power switch is turned off every time the active matrix liquid crystal display device 100 is operated. It is not necessarily the active matrix liquid crystal display device 100 that applies the bias.
The operation time of the active matrix liquid crystal display device 100 is not limited to immediately after the end of the operation, but a limit is set in advance on the operation time of the active matrix liquid crystal display device 100, and after that time limit has elapsed, the operation time is not limited to immediately after the end of the operation. It is also possible to cancel the shift ffi by applying a reverse bias voltage to the TFT.

〔Effect of the invention〕

As explained above, according to the driving device for an active matrix type liquid crystal display device of the present invention, the characteristics of the TFTs shifted during the operation of the active matrix type liquid crystal display device can be
Since it can be completely canceled while the device is not operating, it is possible to completely eliminate the display deterioration of the active matrix type liquid crystal display device, thereby realizing a highly reliable active matrix type liquid crystal display device with no display deterioration over a long period of time. It has the effect of being able to

[Brief explanation of the drawing]

FIG. 1 is a principle block diagram of an active matrix type liquid crystal display device of the present invention, FIG. 2 is a circuit diagram showing the configuration of an embodiment of a driving device for an active matrix type liquid crystal display device of the present invention, and FIG. FIG. 2 is a waveform diagram showing an example of driving the display device; FIG. 4 is a waveform diagram showing an example of the operation of the control circuit in FIG. 2;
Figure 5 is a circuit diagram showing the configuration of a conventional active matrix liquid crystal display device, Figure 6 is a driving waveform diagram of the circuit in Figure 5, and Figure 7 is a diagram showing the drain current-gate voltage characteristics of a thin film transistor from its initial state. It is a line diagram explaining a shift. DESCRIPTION OF SYMBOLS 1... Operating time counting means, 2... Application time control means, 3... Reverse bias means, 4... Data electrode, 5... Scanning electrode, 7...
Counter electrode, 8...TPT, 10... Drive circuit, 20... Control circuit during operation, 30... Control circuit during non-operation, 31... Timer counter, 32... Time control Circuit, 33...Timer, 34...
...Reverse bias application circuit, 100...Active matrix liquid crystal display device, S
WI~SW3...Choice switch, PSen...Power switch.

Claims (1)

[Claims] An active matrix liquid crystal display in which a liquid crystal is interposed between a data electrode and its counter electrode to form a liquid crystal cell, and data given to the data electrode is written into the liquid crystal cell via a TFT. A drive device (10) for the device, comprising: an operation time counting means (1) for counting the operation time of the liquid crystal display device; a reverse bias circuit (2) that applies a predetermined bias voltage of opposite polarity to the applied bias voltage to the gate of the TFT; a counting time of the counting time (1); A driving device for an active matrix liquid crystal display device, comprising an application time control means (3) for controlling the application time of the reverse bias circuit (2) according to an applied voltage.