JP2009139552A - Driving circuit for light emitting element, display provided therewith, and driving method for light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving circuit for a light emitting element, which minimizes a change of an output current with respect to a difference in characteristics between two transistors constituting a current mirror, while maintaining an advantage of a current mirror type current programming pixel circuit common in a source. <P>SOLUTION: This driving circuit is provided with a current mirror circuit including the two transistors connected respectively to a gate and the source in common, and a holding capacitor with one terminal fixed in a potential, and with the other terminal connected to the common gate of the current mirror circuit, holds an input current of the current mirror circuit as a voltage of the holding capacitor, and supplies a current according to the voltage of the holding capacitor to the light emitting element connected to the common source. The driving circuit is provided with a switch for cutting a drain current of the transistor in an output side of the current mirror circuit, during the supply of the input current to the current mirror circuit. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a display such as an organic EL (Electro Luminescence) active matrix display, a driving circuit for a light emitting element used in the display, and a driving method for the light emitting element.

  In recent years, electronic devices using organic semiconductor materials have been widely developed, and the development of organic EL, organic TFT (Thin Film Transistor), organic solar cells, etc., which are light emitting elements, has been reported. Among them, the organic EL display is regarded as promising as the technology that is closest to practical use.

  The structure of the organic EL display panel is classified into a passive matrix and an active matrix. Since the passive matrix is premised on blinking driving, it is difficult to obtain a high-luminance display panel in an organic EL element in which there is a serious tradeoff between luminance and lifetime. On the other hand, in the active matrix, it is not always necessary to drive to blink, and an operation close to lighting is possible. Therefore, it is considered effective for extending the life of the organic EL. How to overcome drift is a major issue.

  For this reason, a voltage programming method, a current programming method, and the like have been proposed, and attempts have been made mainly to correct variations in TFT and characteristic drift (mainly threshold drift) (see, for example, Patent Document 1 and Patent Document 2). .

  In addition, attempts have been made to perform more precise correction (correction of TFT mobility change, etc.) by a current programming method (see, for example, Patent Document 3 and Patent Document 4).

  Furthermore, by using a current mirror circuit to pass current through the organic EL element, an attempt to correct the TFT and organic EL element drift even if the TFT has insufficient saturation characteristics (it cannot function as a constant current source). (See, for example, Patent Document 5).

As shown in FIG. 4, a current programming pixel circuit including a current mirror circuit disclosed in Patent Document 5 has a program current supplied to an OLED via one of the two transistors T3 and T4 constituting the current mirror. And a predetermined voltage is programmed to the holding capacitor Cs. Thereby, the characteristic drift of T3 and OLED can be corrected, and the drive voltage can be suppressed low (power consumption can be suppressed low).
US Pat. No. 6,229,506 Japanese Patent No. 2,784,615 US Pat. No. 6,373,454 US Pat. No. 6,501,466 International Publication No. 2005 / 029,455 Pamphlet

  However, when the characteristics of T3 and T4 shift in the circuit of FIG. 4, there is a problem that not only the correction does not work but rather the correction is applied in the opposite direction.

  When the threshold voltage of T4 becomes higher than T3, the drain current of T4 during the period in which the program current flows through T3 decreases, and the voltage between the terminals of the OLED decreases, so that the voltage of the storage capacitor Cs decreases. As a result, during EL driving, in addition to a decrease in T4 drain current due to an increase in the threshold voltage of T4, a decrease in drain current due to a decrease in the gate potential of T4 is added, thereby further reducing the EL driving current.

  This is a problem peculiar to the current mirror circuit 13 of FIG. 4 in which the outputs of T3 and T4 are both connected to the OLED. As described above, if the threshold of T4 is shifted and the driving force is reduced, the drain potential of T3 is lowered through the common output (source terminal) of T3 and T4, and the program current is supplied to T3 with a gate potential smaller than normal. It will flow. To correct the threshold voltage increase at T4, a larger gate potential must be programmed, but the opposite smaller gate potential is programmed.

  As described above, the current mirror type current programming pixel circuit having a common source causes a change in current when the characteristics of the two transistors constituting the current mirror are shifted, and displays a luminance greatly deviating from the programmed luminance. Will be done.

  An object of the present invention is to provide a light emitting device that keeps the advantages of a current mirror type current programming pixel circuit having a common source and suppresses a change in output current due to a difference in characteristics of two transistors constituting the current mirror. Drive circuit and method thereof, and a display using the same.

The first of the present invention is a first transistor to which an input current is supplied from a drain, a second transistor having a drain connected to a power source and a source connected to a source of the first transistor, and one terminal Is connected to the gate of the first transistor and the potential of the other terminal is fixed, holds the input current as a voltage of the storage capacitor, and the second transistor A driving circuit for a light emitting element that supplies a current corresponding to a voltage from the power source to a light emitting element connected to the common source,
The current of the second transistor is cut off while the input current is supplied to the first transistor.

The second of the present invention is a current mirror circuit including two transistors whose gate and source are connected in common, and the potential of one terminal is fixed, and the other terminal is connected to the common of the current mirror circuit. A holding capacitor connected to the gate, holding an input current of the current mirror circuit as a voltage of the holding capacitor, and connecting a current corresponding to the voltage of the holding capacitor to the commonly connected sources. A method of driving a light emitting element to be supplied to the light emitting element,
While the input current is supplied to the current mirror circuit, the drain current of the transistor on the output side of the current mirror circuit is cut off by a switch.

  According to the present invention, while maintaining the advantages of a current mirror type current programming pixel circuit having a common source, it is possible to suppress a change in output current due to a deviation in characteristics of two transistors constituting the current mirror. .

  An example of the organic semiconductor element of the present invention will be described below as an embodiment.

  FIG. 1 schematically shows an embodiment of the present invention, and is a block diagram showing a pixel circuit for one pixel of a display device. A driving circuit having a plurality of intersecting data lines 11 and a plurality of selection lines 12 and including a current mirror circuit 13, a switching circuit 14, and a storage capacitor 15 is provided at the intersection, and two currents flowing through the current mirror circuit are provided. Are joined to flow to the organic EL element (an example of a light emitting element) 16. One terminal of the storage capacitor 15 is fixed to the ground, and the other terminal is connected to the current mirror circuit 13. A power line switch (on / off means 17) for turning on / off the current from the power source 18 is provided between the power source 18 and the current mirror circuit 13. The on / off means 17 may be included in the pixel circuit or outside the pixel circuit.

  The current mirror circuit 13 is a circuit that has two current paths inside and is set so that the currents flowing through them are always the same or the ratio is constant. The current in one current path is input from the external signal source 10 and the current in the other current path is output to the outside, thereby being used as a current amplifier. At this time, the input from the external signal source 10 is referred to as an input current, and the output from the outside is referred to as an output current.

  FIG. 2 shows a display device in which the pixel circuits are arranged in a matrix, and shows four pixels. In this example, the data line 11 has a variable current source 19-1 as means for giving luminance information, and has a logic circuit 19-2 and a control circuit 21 together. A case where a variable voltage source and a variable current source are provided as means for providing luminance information can be considered.

  FIG. 3 shows the pixel circuit of FIG. 2 in more detail. As the switching circuit 14, two switches of a switch 14-1 for voltage programming and a switch 14-2 for flowing a program current to the organic EL element 16 are provided. The current mirror circuit 13 of the present embodiment includes a first transistor 31 (input side transistor) constituting an input current path and a second transistor 32 (output side transistor) constituting an output current path, and these two transistors The gate and the source are connected in common. Further, a common source of the first transistor 31 and the second transistor 32 is connected to the organic EL element 16. A power line switch 17-1 is provided as the power current on / off means 17.

  The current signal generated by the signal source 10 flows through the data line 11 and the switch 14-1 to the drain of the first transistor 31 that is an input side transistor, and is supplied from the source to the organic EL element 16. Since the gates and sources of the two transistors 31 and 32 are common, the same current flows through the second transistor as long as the characteristics of the threshold voltage and conductance of the two transistors 31 and 32 are the same.

  In the program step of programming a predetermined voltage value as luminance information into the holding capacitor 15, as shown in (1) in FIG. 3, when the switching circuit 14 is turned on and an input current flows into the holding capacitor 15, the power line switch ( The power supply current on / off means 17) is turned off to cut off the current to the second transistor 32 in the current mirror circuit 13.

  After the programming step, the process proceeds to a light emitting step in which a current from the power source 18 is supplied to the second transistor 32 connected to the power source 18. In the light emitting step, as shown in (2) in FIG. 3, both the switches 14-1 and 14-2 of the switching circuit 14 are turned off and the power line switch (power supply current on / off means 17) is turned on. . As a result, an output current is supplied to the second transistor 32 in the current mirror circuit 13 and supplied from the source to the organic EL element 162.

  Since the power line switch is turned off (1) in the programming step, the potential of the common source terminal (point A in FIG. 3) is determined only by the input current supplied by the first transistor 31.

  The voltage itself programmed in the storage capacitor 15 varies depending on the current-voltage characteristic of the organic EL element 16 and the characteristic change of the first transistor 31. However, this change is independent of the characteristic change of the second transistor 32. In addition, as long as the characteristics of the first transistor 31 and the characteristics of the second transistor 32 are the same, the output current flowing through the second transistor when the power switch 17 is turned on (2) does not change. The current flowing through the EL element 16 does not change.

  When a change occurs in which the characteristics of the first transistor 31 and the characteristics of the second transistor 32 are different, the output current is changed. However, the amount of change is the amount of change according to the difference in characteristics of the two transistors with respect to the first transistor 31, from which the amount of change common to the first transistor 31 is subtracted. Since the characteristic change of the second transistor 32 is not affected as it is, the amount of change in the output current can be suppressed to that extent.

  FIG. 4 is a current programming pixel circuit without the power line switch 17 of FIG. 3 and is shown as a comparative example. As described above, in this case, even when the characteristics of T3 corresponding to the first transistor and T4 corresponding to the second transistor are changed without deviation, the input current and the output current are not the same. . Assuming that the threshold voltages of T3 and T4 increase by the same amount, the programmed gate potential increases, so that the output current increases during light emission after programming is completed. When the characteristics of T3 and T4 shift, there is a problem that not only the correction is not effective, but rather the correction is applied in the opposite direction. This occurs because current is always supplied from T4, and the potential at point A in FIG. 4 is not determined only by the program current passing through T3, but also depends on the current flowing through T4.

  Further, the transistors 31 and 32 constituting the current mirror circuit 13 and the switching circuit 14 may be thin film transistors.

  Further, the current mirror circuit 13 may be composed of two thin film transistors.

  Further, a switch may be provided on the power supply line as means for turning on and off the current from the power supply 18 to transistors other than the first transistor 31 constituting the current mirror circuit 13.

  In addition, as a means for turning on and off the current from the power supply 18 to transistors other than the first transistor 31 constituting the current mirror circuit 13, a switch may be provided between the power supply line and the other transistors than the first transistor. .

  Further, as a means for turning on and off the current from the power supply to transistors other than the first transistor 31 constituting the current mirror circuit 13, a switch may be provided in the gate electrode portion of the transistors other than the first transistor 31.

  FIG. 5 is a schematic diagram illustrating a pixel circuit according to the first embodiment. 5 is obtained by replacing the switches 14-1 and 14-2 and the power switch 17-1 of the switching circuit 14 of FIG. 3 with TFTs.

  FIG. 6 is a voltage application timing diagram for representing a driving state based on FIG.

A SPICE simulation was performed based on FIG. The characteristics of TFT were fitted using SPICE model Level 3, and the characteristics of OLED were fitted using a combination of a diode model and a capacitor. The gate width was set to 40 μm for T1, T3, and T4 and 200 μm for T2. In any of T1 to T4, an n-channel FET model was used, and carrier mobility was set to 0.7 cm ² / Vs, a gate insulating film thickness was 0.3 μm, and a gate length was 5 μm. The power supply voltage was set to 15V.

Based on these parameters, the value of the current flowing through the organic EL when only the threshold voltage of T2 was shifted from 0V by 1V in the positive direction was calculated. In addition, a driving mode (A mode: the present invention) in which no current flows through T2 during voltage programming by the current source and a driving mode (B mode: conventional) in which current flows through T2 during voltage programming by the current source were compared. The results are shown in Table 1.
In Table 1, ΔVth is a threshold voltage shift of the TFT T2, Ioled is a current flowing through the organic EL element 16, and ΔIoled is a relative variation with respect to Ioled when there is no threshold voltage shift.

  As is clear from this result, the driving method (A mode) according to the present invention has a smaller current decrease rate (Δ Ioled) flowing in the organic EL with respect to the threshold voltage shift (ΔVth) of T2, and is stronger against the threshold voltage shift of T2. I understand that.

  Furthermore, the same effect could be confirmed by test pattern prototyping using a-Si TFTs.

FIG. 7 shows an example in which a power switch T5 for turning on / off the power supply current is arranged in each pixel.
A timing chart for driving this pixel circuit is shown in FIG. The power supply line switch T5 is controlled by the signal of the selection line 2, is cut off during programming, and is closed during the light emission period.

  When current is supplied to one or more pixel circuits from one power line, the power switch T5 is arranged in each pixel as in this embodiment, so that data of other pixel circuits can be transmitted through the power line during programming. Current can be prevented from flowing.

  FIG. 9 shows a configuration in which a power switch T6 is arranged between the gates of the current mirror TFTs (T1, T2) in each pixel instead of the switch T5 for turning on / off the power supply current in the second embodiment. FIG. 10 shows a timing chart for driving this pixel circuit.

  Prior to programming, an initialization period is set for determining an initial voltage of the gate of the transistor T2. During the initialization period, the data current is set to 0, both the selection lines 1 and 2 are set to the H level, and the switching transistors T3 and T4 and the power switch T6 of the switching circuit 14 are turned on. As a result, the gate potentials of the transistors T1 and T2 become the GND level.

  Thereafter, when programming is performed with only the selection line 1 set to H, since T6 is off, the gate of T2 is maintained at GND and T2 is kept off. Therefore, as in the second embodiment, current from other pixel circuits on the same power line is cut off. During the light emission period, T6 is turned on to make the gate voltage of T2 equal to the voltage of the storage capacitor 15, and a current flows from the power supply line to the organic EL element 16.

  FIG. 11 shows an example in which two power supply lines are prepared, a switch for turning on / off the power supply current is separately provided, and two current-side drive transistors T2 and T2 'are arranged. In this case, the current value to be passed through the organic EL can be selected by making the gate widths of T2 and T2 'different. A timing chart for driving this pixel circuit is shown in FIG. In this example, a driving method in which the power source current is supplied only to T2 is shown, but it is also possible to supply only T2 'or both T2 and T2'.

It is a block diagram which shows the concept of the pixel circuit which concerns on embodiment of this invention. It is a figure which shows the concept of the circuit arrangement | positioning which concerns on embodiment of this invention. It is a figure which shows the concept of the pixel circuit which concerns on embodiment of this invention. It is an example of the conventional pixel circuit. 1 is a circuit diagram illustrating a pixel circuit according to Example 1 of the invention. FIG. It is a figure which shows the voltage application timing of the drive circuit which concerns on Example 1 of this invention. It is a circuit diagram which shows the pixel circuit which concerns on Example 2 of this invention. It is a figure which shows the voltage application timing of the drive circuit which concerns on Example 2 of this invention. It is a circuit diagram which shows the pixel circuit which concerns on Example 3 of this invention. It is a figure which shows the voltage application timing of the drive circuit which concerns on Example 3 of this invention. It is a circuit diagram which shows the pixel circuit which concerns on Example 4 of this invention. It is a figure which shows the voltage application timing of the drive circuit which concerns on Example 4 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Data line 12 Selection line 13 Current mirror circuit 14 Switching circuit 15 Holding capacity 16 Organic EL element 17 On / off means 18 Power supply 21 Control circuit 31 1st transistor 32 2nd transistor 33 Power supply line switch 51 Power supply line

Claims (6)

A first transistor to which an input current is supplied from a drain; a second transistor in which a drain is connected to a power source and a source is connected to a source of the first transistor; and one terminal of the first transistor A holding capacitor connected to the gate and having a fixed potential at the other terminal, holding the input current as a voltage of the holding capacitor, and the second transistor supplying a current according to the voltage of the holding capacitor. A driving circuit for a light emitting element that supplies power to a light emitting element connected to the common source,
A drive circuit for a light emitting element, wherein the current of the second transistor is cut off while the input current is supplied to the first transistor. It is a drive circuit of the light emitting element of Claim 1, Comprising:
The gate of the second transistor is connected to the gate of the first transistor, the drain of the second transistor is connected to the power supply through a switch, and the input current is supplied to the first transistor. The drive circuit of the light emitting element is characterized in that the switch is cut off during the operation. It is a drive circuit of the light emitting element of Claim 1, Comprising:
The gate of the second transistor is connected to the gate of the first transistor via a switch, and the switch is cut off while the input current is supplied to the first transistor. A driving circuit of a light emitting element characterized by the above. The drive circuit of the light emitting element according to claim 1 is a display in which a plurality of the power supplies are arranged in common,
The gate of the second transistor is connected to the gate of the first transistor, and the drain of the second transistor is connected to the power supply via a switch provided in each driving circuit, and the first transistor The display is characterized in that the switch is cut off while the input current is supplied to the display. The drive circuit of the light emitting element according to claim 1 is a display in which a plurality of the power supplies are arranged in common,
The gate of the second transistor is connected to the gate of the first transistor via a switch, and the switch is cut off while the input current is supplied to the first transistor. Display. A current mirror circuit including two transistors whose sources are connected in common, and a holding capacitor in which the potential of one terminal is fixed and the other terminal is connected to one gate of the two transistors of the current mirror circuit A driving method of a light emitting element that holds an input current of the current mirror circuit as a voltage of the holding capacitor and supplies a current corresponding to the voltage of the holding capacitor from the commonly connected source to the light emitting element. There,
A driving method of a light emitting element, wherein a drain current of a transistor on an output side of the current mirror circuit is cut off while an input current is supplied to the current mirror circuit.