CN1241163C - How to drive the display - Google Patents

Abstract

A method for driving a display. The method is a voltage driving circuit for an organic light emitting diode display. The present invention sets a data voltage to a negative data voltage that is maintained for a predetermined time during a frame. When the scan voltage is set at a high voltage value, the negative data voltage is supplied to the gate of the driving thin film transistor, and the gate maintains a negative gate voltage for a holding time. This will keep the threshold voltage of the driving thin film transistor unchanged. Therefore, the present invention can maintain the original brightness of the organic light emitting diode after long-term use, so the service life of the display can be effectively increased.

Description

How to drive the display

technical field

The invention relates to a driving method of a display.

Background technique

The earliest moving images that humans can see are movies in the form of documentaries. Afterwards, the invention of the cathode ray tube (Cathode Ray Tube, referred to as CRT) successfully derived commercial TV sets and became a must-have household appliance for every family. With the development of science and technology, the application of CRT has been extended to desktop monitors in the computer industry, making the CRT popular for nearly decades. However, all types of displays made of CRTs face the problem of radiation, and because of the structure of the internal electron gun, the display is bulky and takes up space, which is not conducive to thinness and light weight.

Due to the above problems, researchers have started to develop so-called flat panel displays (Flat Panel Display). This field includes liquid crystal display (Liquid Crystal Display, referred to as LCD), field emission display (Field Emission Display, referred to as FED), organic light emitting diode (Organic Light Emitting Diode, referred to as OLED), and plasma display (Plasma Display Panel, referred to as PDP).

Wherein, the organic light-emitting diode is also called an organic electroluminescence display (OELD for short), which is a self-luminous element. Because the characteristics of OLED are DC low voltage drive, high brightness, high efficiency, high contrast value, and light and thin, and its luminous color is from red (Red, referred to as R), green (Green, referred to as G), and blue (Blue, referred to as G). B) The degree of freedom from the three primary colors to white is high, so OLED is regarded as the focus of the development of new flat panels in the next era. OLED technology not only has the advantages of thinness and high resolution of LCD, but also the advantages of active light emission, fast response speed and power-saving cold light source of LED, and also has many advantages such as wide viewing angle, good color contrast effect and low cost. Therefore, OLEDs can be widely used in backlights of LCDs or signage boards, mobile phones, digital cameras, and personal digital assistants (PDAs).

From the perspective of driving methods, OLED can be divided into two types: passive matrix driving method and active matrix driving method. The advantage of passive matrix OLED is that the structure is very simple and does not need to be driven by thin film transistors (Thin Film Transistor, referred to as TFT), so the cost is low, but its disadvantage is that it is not suitable for high-resolution image quality applications, and it is in the direction of large-scale When the panel is developed, there will be problems such as increased power consumption, reduced component life, and poor display performance. The advantages of active-matrix OLEDs are not only applicable to large-scale active-matrix drive methods, but also its wide viewing angle, high brightness, and fast response characteristics cannot be ignored, but its cost will be slightly lower than that of passive-matrix OLEDs. high.

According to different driving methods, flat panel displays can be further divided into voltage-driven and current-driven types. The voltage-driven type is usually used in TFT-LCD, that is, different voltages are input to the data lines to achieve different gray scales, so as to achieve the purpose of full color. The voltage-driven TFT-LCD has the advantages of mature technology, stability, and low cost. The current-driven type is usually used in OLED displays, that is, different currents are input to the data lines to achieve different gray scales, so as to achieve the purpose of full color. However, this way of driving pixels with current requires the development of new circuits and ICs, and thus requires huge costs. Therefore, if the OLED is driven by the voltage driving circuit of the TFT-LCD, the cost will be greatly reduced. However, when the OLED is driven by the voltage driving circuit of the TFT-LCD, the threshold voltage (Threshold Voltage) of the driving TFT will drift in the long-term operation, and the threshold voltage will increase. The formula of the drain current of TFT in the saturation region is: I _d = (1/2) × μ _n × C _ox × (W/L) × (V _gs -V _th ) ² , where the electron mobility μ _n and The gate capacitance C _ox per unit area is a constant value, V _th is the threshold voltage of the TFT, W is the channel width of the TFT, and L is the channel length of the TFT. It can be seen from the formula that when the threshold voltage increases, the driving current flowing between the drain and the source of the driving TFT will decrease. Since the driving current is used to drive the OLED to make the OLED emit light, when the driving current decreases, the brightness of the OLED will decrease accordingly.

For more clarity, please refer to FIG. 1 , which shows a circuit diagram of a pixel in a display that drives an OLED in a voltage-driven manner. The pixel includes voltage driving circuit 102 and OLED (104). The aforementioned voltage driving circuit 102 includes a TFT1 (106), a capacitor C (108), and a TFT2 (110). Wherein, the TFT2 (110) is called a driving thin film transistor, and is used to generate a driving current for driving the OLED (104), so as to make the OLED (104) emit light. The drain of TFT1 (106) is coupled to the data voltage (V _data ); the gate of TFT1 (106) is coupled to the scanning voltage (V _scan ); the source of TFT1 (106) is coupled to the capacitor C (108 ). The first end and the gate of TFT2 (110). The drain of TFT2 (110) is coupled to the supply voltage (V _DD ); the source of TFT2 (110) is coupled to the anode of OLED (104), wherein the supply voltage (V _DD ) is usually a positive voltage, and the voltage source Provided. The second end of the capacitor C ( 108 ) is coupled to a power supply with potential V _ref . And the cathode of OLED (104) is coupled to ground.

For the timing diagram between V _DD , V _scan , V _data , and the gate voltage (V _2g ) of TFT2 (110) in an existing driving method of voltage driving circuit 102, please refer to FIG. 2 . Firstly, it should be noted that when V _scan is set at a high voltage value, TFT1 ( 104 ) will be turned on. When V _scan is set at a low voltage value, TFT1 ( 104 ) will be turned off. In addition, it should be noted that the time when V _scan appears a high voltage value and a low voltage value is called a frame (Frame) time (that is, T shown in Figure 2), and the time of a frame is usually 1 /60 seconds, that is, the frequency is 60Hz, and one picture will form a picture of one pixel. It can be seen from FIG. 2 that when V _scan is at a high voltage value, V _data is at a high voltage value, so V _2g is always maintained at a positive voltage, and V _2g is gradually increased. As a result of the gradual increase of V _2g , more trap charges will be accumulated in the oxide layer of the gate of the TFT2 ( 110 ), so that the threshold voltage of the TFT2 ( 110 ) will drift and the threshold voltage will rise. As a result, the driving current flowing between the drain and the source of the TFT2 (110) will be reduced, thus reducing the brightness of the OLED (104).

Contents of the invention

In view of this, the present invention proposes a driving method for improving the threshold voltage drift of the driving thin film transistor. In the present invention, the data voltage is set as a negative data voltage maintained for a predetermined time during one frame period. And when the scan voltage is set at a high voltage value, the negative data voltage will be supplied to the gate of the driving thin film transistor, so that the gate maintains a negative gate voltage for a holding time. This will cause the oxide layer of the gate electrode of the driving thin film transistor to release trap charges, so that the threshold voltage of the driving thin film transistor will not rise.

To achieve the above and other objectives, the present invention provides a method for driving a display. This driving method is used for voltage driving circuits in displays of organic light emitting diodes. The display includes a plurality of pixels, and the image of each pixel is composed of a frame, and the frame has an original frequency. The driving method is characterized in that: during this frame period, the data voltage is set as a negative data voltage maintained for a predetermined time; and when the scanning voltage is set at a high voltage value, the negative data voltage is supplied to the driving thin film transistor The grid, and the gate maintains a negative gate voltage for a holding time.

In an embodiment of the present invention, the predetermined time can be adjusted.

In an embodiment of the invention, the holding time is different from the predetermined time. And the frequency of the picture will be greater than the original frequency.

In another embodiment of the invention, the holding time is the same as the predetermined time. And the frequency of the picture is the same as the original frequency.

In the embodiment of the present invention, the driving method of the present invention will not attenuate the driving current generated by driving the thin film transistor. And the driving current is used to drive the organic light-emitting diode, so that the organic light-emitting diode emits light.

In an embodiment of the invention, the drain of the driving thin film transistor is coupled to a supply voltage. The supply voltage is provided by a voltage source.

In an embodiment of the present invention, the source of the driving TFT is coupled to the anode of the OLED.

In an embodiment of the present invention, the cathode of the OLED is coupled to the ground.

To sum up, the present invention sets the data voltage as a negative data voltage for a predetermined time during a frame period. And when the scan voltage is set at a high voltage value, the negative data voltage will be supplied to the gate of the driving thin film transistor, so that the gate maintains a negative gate voltage for a holding time. This will cause the oxide layer of the gate electrode of the driving thin film transistor to release trap charges, so that the threshold voltage of the driving thin film transistor will not rise. Therefore, the present invention can keep the brightness of the organic light emitting diode at the original brightness after a long time of use, thus effectively increasing the service life of the display.

Description of drawings

In order to make the above-mentioned and other objects, features and advantages of the present invention more obvious and understandable, the preferred embodiments are specifically cited below, together with the accompanying drawings, and are described in detail as follows:

FIG. 1 shows a circuit diagram for driving a pixel in an OLED display in a voltage-driven manner;

FIG. 2 is a timing diagram among V _DD , V _scan , V _data , and V _2g of a current driving method of a voltage driving circuit;

FIG. 3 shows a timing diagram among V _DD , V _scan , V _data , and V _2g of a display driving method according to a preferred embodiment of the present invention; and

FIG. 4 is a timing diagram among V _DD , V _scan , V _data , and V _2g of a display driving method according to another preferred embodiment of the present invention.

Explanation of symbols in the figure:

10: Pixel

102: Voltage drive circuit

104: OLED

106: TFT1

108: capacitance

110: TFT2

Detailed ways

Please refer to FIG. 1 again, the display driving method of the present invention is used in the voltage driving circuit 102 in FIG. 1 , and the driving method of the present invention will be described below based on FIG. 1 .

The display driving method of the present invention is used for OLED displays. In the display driving method according to a preferred embodiment of the present invention, the difference between the supply voltage (V _DD ), the scan voltage (V _scan ), the data voltage (V _data ), and the gate voltage (V _2g ) of the TFT2 (110) Please refer to Figure 3 for the timing diagram. It can be seen from FIG. 3 that in this embodiment, the frequency of one picture is increased from the original 60 Hz to 120 Hz. That is to say, the time of one picture is shortened from 1/60 second to 1/120 second. So if during a frame, when V _scan is set to a high voltage value and V _data is a positive voltage, then during the next frame, when V _scan is set to a high voltage value, V _data will maintain a constant value. Negative voltage for a predetermined time, where the predetermined time is the same as the time that V _scan remains at a high voltage value. That is, every other screen time (1/120 second), V _data will be reversed interactively.

In this embodiment, it is assumed that during the first frame period, when V _scan is set to a high voltage value, V _data is set at a positive voltage (for example, 5V). The operating principle of this embodiment is that during the first frame, when V _scan is set to a high voltage value, TFT1 (106) will be turned on, and positive V _data will be supplied to the gate of TFT2 (110). And make V _2g maintain a positive value voltage (for example, 5V) for one frame time (1/120 second). At this time, the oxide layer of the gate of the TFT2 (110) accumulates trap charges. During the second frame, when V _scan is set to a high voltage value, TFT1 (106) will be turned on, and negative value V _data (for example -5V) will be supplied to the gate of TFT2 (110), so that V _2g maintains a negative voltage (for example -5V) for one screen time (1/120 second). At this time, the oxide layer of the gate of the TFT2 (110) will release trapped charges. During subsequent screens, the above-mentioned method will be repeated.

In this embodiment, since V _2g will maintain a negative voltage for one picture time (1/120 second) during two picture times (1/60 second), so the gate of TFT2 (110) can be made The oxide layer releases trapped charges, so that the threshold voltage of the TFT2 (110) does not rise, but remains near the original threshold voltage value. The formula of the drain current of TFT in the saturation region is: I _d = (1/2) × μ _n × C _ox × (W/L) × (V _gs -V _th ) ² , where the electron mobility μ _n and The gate capacitance C _ox per unit area is a constant value, V _th is the threshold voltage of the TFT, W is the channel width of the TFT, and L is the channel length of the TFT. It can be seen from the formula that since the threshold voltage of TFT2 (110) will not increase, the driving current of TFT2 (110) will not decrease. Therefore, the OLED (104) can still maintain the original brightness under long-term operation, so that the life of the display can be effectively extended.

In the display driving method according to another preferred embodiment of the present invention, among the supply voltage (V _DD ), the scan voltage (V _scan ), the data voltage (V _data ), and the gate voltage (V _2g ) of the TFT2 (110) Please refer to Figure 4 for the timing diagram. It can be seen from FIG. 4 that the frequency of a picture in this embodiment is the same as the original 60 Hz. That is to say, the time of one frame is still maintained at 1/60 second. In this embodiment, during each frame, when V _scan is set to maintain a high voltage value for T (as shown in FIG. 4 ), V _data will be maintained for a period of time T ₁ (as shown in FIG. 4 . ) negative voltage, and maintain a positive voltage for a period of time T ₂ (as shown in FIG. 4 ).

The operating principle of this embodiment is that during a frame time, when V _scan is set to a high voltage value and during time _T1 , TFT1 (106) will be turned on, and negative V _data will be supplied to TFT2 (110) gate, and make V _2g maintain the negative value voltage of time T ₁ . At this time, the oxide layer of the gate of the TFT2 (110) will release trapped charges. When V _scan is set to a high voltage value and during time _T2 , TFT1 (106) will be turned on, and positive V _data will be supplied to the gate of TFT2 (110), so that V _2g maintains a positive voltage , and maintain until the V _scan of the next screen is set to a high voltage value. At this time, the oxide layer of the gate of the TFT2 (110) will accumulate trap charges. During subsequent screens, the above-mentioned method will be repeated.

In this embodiment, because during each frame, when V _scan is set to a high voltage value, V _data has a negative voltage for a period of T ₁ , so V _2g can be maintained for a time T ₁ . value voltage. Therefore, the oxide layer of the gate of the TFT2 (110) can release trapped charges, so that the threshold voltage of the TFT2 (110) will not rise, thus keeping the threshold voltage of the TFT2 (110) near the original threshold voltage value. The formula of the drain current of TFT in the saturation region is: I _d = (1/2) × μ _n × C _ox × (W/L) × (V _gs -V _th ) ² , where the electron mobility μ _n and The gate capacitance C _ox per unit area is a constant value, V _th is the threshold voltage of the TFT, W is the channel width of the TFT, and L is the channel length of the TFT. It can be seen from the formula that since the threshold voltage of TFT2 (110) will not increase, the driving current of TFT2 (110) will not decrease. Therefore, the OLED (104) can still maintain the original brightness under long-term operation, so that the life of the display can be effectively extended.

In addition, comparing the embodiments of FIG. 3 and FIG. 4, it can be known that in the embodiment of FIG. 3, V _2g maintains a negative voltage for a longer time, so the oxide layer of the gate of TFT2 (110) can release more Therefore, the effect of improving the threshold voltage drift of the TFT2 (110) is better, but the frame frequency needs to be increased to twice the original frame frequency.

To sum up, the present invention sets the data voltage as a negative data voltage for a predetermined time during a frame period. And when the scanning voltage is set at a high voltage value, the negative data voltage will be supplied to the gate of the driving thin film transistor, so that the gate maintains a negative gate voltage for a holding time. This will cause the oxide layer of the gate electrode of the driving thin film transistor to release trap charges, so that the threshold voltage of the driving thin film transistor will not rise. Therefore, the present invention can keep the brightness of the organic light emitting diode at the original brightness after a long time of use, thus effectively increasing the service life of the display.

Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art may make various modifications and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be defined by the claims in combination with the specification and drawings.

Claims (11)

1. the driving method of display, this driving method is a Voltag driving circuit that is used for a display of an organic LED, and this display comprises a plurality of picture elements, and the portrait of each these picture element is made up of a picture, this picture has an original frequency, it is characterized in that:

During this picture, be to keep the negative value data voltage of the schedule time with data voltage sets; And

When scanning voltage is set in high-voltage value, this negative value data voltage will be supplied to the grid of a drive thin film transistors, and make this grid keep the negative value grid voltage of retention time.

2. the driving method of display as claimed in claim 1, it is characterized in that: this schedule time can be adjusted.

3. the driving method of display as claimed in claim 1, it is characterized in that: this retention time and this schedule time are inequality.

4. the driving method of display as claimed in claim 3, it is characterized in that: the frequency of this picture is greater than this original frequency.

5. the driving method of display as claimed in claim 1, it is characterized in that: this retention time is identical with this schedule time.

6. the driving method of display as claimed in claim 5, it is characterized in that: the frequency of this picture is identical with this original frequency.

7. the driving method of display as claimed in claim 1, it is characterized in that: the drive current that this driving method will make this drive thin film transistors produce can not decayed.

8. the driving method of display as claimed in claim 1 is characterized in that: the drain electrode of this drive thin film transistors is coupled to a supply voltage.

9. the driving method of display as claimed in claim 8, it is characterized in that: this supply voltage is to be provided by a voltage source.

10. the driving method of display as claimed in claim 1, it is characterized in that: the source electrode of this drive thin film transistors is coupled to the positive pole of this organic LED.

11. the driving method of display as claimed in claim 1 is characterized in that: the negative pole of this organic LED is coupled to ground.

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