KR100517664B1 - Active matrix led pixel driving circuit - Google Patents

Abstract

The present invention relates to an active matrix LED pixel drive circuit. This circuit, with a capacitor. A light emitting diode, first and second transistors, and a current switch. Among them, the capacitor is connected between the gate and the source of the first transistor, the second transistor has a source connected to the drain of the first transistor and a gate connected to receive the first voltage, and the first and The second transistor operates in the saturation region, and when the current switch is closed, the first current corresponding to the data signal flows through the first and second transistors to generate a second voltage stored on the capacitor, and the current switch is opened. The second current through the first and second transistors is then generated by the second voltage stored on the capacitor to turn on the light emitting diode.

Description

ACTIVE MATRIX LED PIXEL DRIVING CIRCUIT}

The present invention relates to an active matrix LED pixel driving circuit, and more particularly to an active matrix OLED / PLED pixel driving circuit.

Organic light emitting diodes (OLEDs) or polymer light emitting diodes (PLEDs) are becoming increasingly common in flat panel displays due to their high speed performance, low power consumption and low cost. OLED / PLED displays also have a wider viewing angle than that of conventional liquid crystal displays using backlight systems because the OLED or PLED itself emits light.

There are two classes of LED displays: passive and active matrices. In a passive matrix display, each LED is supplied with a drive current only during one intersyringe of one frame, and then off until the beginning of the intersyringe in the next frame. Each LED emits light between each short syringe that is strong enough to achieve a satisfactory overall illuminance level of each display. Therefore, a large drive current is needed. However, large drive currents not only cause large power consumption, but also shorten the life of the LED.

In contrast, active matrix LED displays do not have the previous disadvantages. The active matrix LED display uses a capacitor that is charged by the drive current during the inter-syringe and maintains the voltage between the syringes of the next frame. These voltages allow the current driven LED to remain on after termination of the syringe. Thus, the LEDs are turned on for a longer time, and the drive current can be further reduced than that of the passive matrix display.

The LEDs in the display can be driven by voltage or current. 1 is a circuit diagram showing one voltage-driven pixel circuit in a conventional active matrix LED display. This circuit comprises transistors 11 and 12, a capacitor 13 and an LED 14. While the gate of the transistor 11 receives the scan signal SS through the scan line, the source of the transistor receives the data signal DS through the data line. This data signal for the pixel is transferred to the gate of the transistor 12 when the transistor 11 is turned on by the scan signal SS. If such a pixel is lit in the current frame, the voltage level of the data signal DS turns on the transistor 12 to produce a drive current that turns on the LED 14 through the transistor 12. . In the meantime, the capacitor 13 is charged to maintain the voltage Vgs. When the scan signal SS turns off the transistor 11 and finishes the transfer of the data signal DS at the end of the syringe period, the voltage Vgs is replaced by the transistor 12 in place of the data signal DS. Maintain the conduction state. However, the pixel circuit in FIG. 1 suffers from drift of the threshold voltage Vt, which can lead to drift of the drive current. The magnitude difference between the drive currents within the pixel results in uneven illuminance on the display panel.

2 is a circuit diagram showing one current-driven pixel circuit in a conventional active matrix LED display. This circuit comprises transistors 21, 22, 23 and 24, a capacitor 25 and an LED 26. While the gate of the transistor 21 receives the scan signal SS through the scan line, the source of the transistor receives the data signal DS through the data line. The gate of the transistor 22 also receives the scan signal SS. When the transistors 21 and 22 are turned on by the scan signal SS, the transistors 23 and 24 operate as a current mirror, whereby the current passing through the transistor 23 is regenerated and the transistor 24 ), To light up the LED (26). In the meantime, the capacitor 25 is charged to maintain the voltage Vgs of the transistor 24. When the scan signal SS turns off the transistors 21 and 22 to terminate the transfer of the data signal DS at the end of the syringe period, the voltage Vgs is applied to the transistor 24 instead of the data signal DS. Maintain conduction.

3A is a circuit diagram illustrating another current-driven pixel circuit in a conventional active matrix LED display. This circuit includes transistors 31, 32, 33, and 34, a capacitor 35, and an LED 36. While the gate of the transistor 31 receives the scan signal SS through the scan line, the source of the transistor receives the data signal DS through the data line. Gates of the transistors 32 and 33 also receive the scan signal SS. When the transistors 31 and 32 are turned on by the scan signal SS and the transistor 33 is turned off, the gate and the drain of the transistor 34 are electrically connected to generate a voltage Vgs and a data line. And a magnitude corresponding to the current passing through the transistor 34. In the meantime, the capacitor 35 is charged to maintain the voltage Vgs. When the scan signal SS turns off the transistors 31 and 32 at the end of the syringe, and the transistor 33 is turned on to terminate the transfer of the data signal DS, the voltage Vgs becomes the data signal DS. Instead, current causes LED 36 to light through transistors 33 and 34. FIG. 3B is a circuit diagram illustrating a modified configuration of the circuit of FIG. 3A. These are similar in circuit operation. PMOS transistor 34 has been replaced with an NMOS transistor, and capacitor 35 has been replaced with transistor 32.

4A is a circuit diagram showing another current-driven pixel circuit in a conventional active matrix LED display. This circuit includes transistors 41, 42, 43, and 44, a capacitor 45, and an LED 46. While the gate of the transistor 41 receives the scan signal SS through the scan line, the source of the transistor receives the data signal DS through the data line. Gates of the transistors 42 and 43 also receive the scan signal SS. By the scan signal SS, when the transistors 41 and 42 are turned on and the transistor 43 is turned off, the gate and the drain of the transistor 44 are electrically connected, and the voltage Vgs of the transistor 44 ) Is generated and has a magnitude corresponding to the current passing through the data lines, transistors 41 and 44 and OLED 46. The capacitor 45 is charged to maintain the voltage Vgs. When the scan signal SS turns off the transistors 41 and 42 at the end of the syringe, and the transistor 43 is turned on to terminate the transfer of the data signal DS, the voltage Vgs becomes the data signal DS. In place of), a current passing through the transistor 44 is generated to light up the OLED 46. 4B is a circuit diagram illustrating a modified configuration of the circuit of FIG. 4A. These are similar in circuit operation. PMOS transistor 44 has been replaced with an NMOS transistor, and capacitor 45 has been replaced with transistor 42.

Fig. 5 is a circuit diagram showing an equivalent circuit of all the current-driven pixel circuits described above. The equivalent circuit includes a transistor 51, a capacitor 52, a current switch 53, and an LED 54. Current switch 53 includes three switches 531-533 and a data line connected to a current source (not shown). The switches 531-533 are controlled by the scan signal SS. A current source coupled with the data line provides the current that drives the LED 54.

At the start of the scan cycle, switches 531-532 are closed and switches 533 are open. When this pixel is lit in the current frame, the current I of the data signal DS flows through the transistor 51 and charges the capacitor 52 to maintain the voltage Vgs. When the scan signal opens the switches 531-532 and closes the switch 533, the voltage Vgs generates a current I 'through the transistor 51 to replace the data signal DS (LED). 54).

The current-driven pixel circuit described above still suffers from the disadvantages resulting from channel length modulation, even though the drift of the threshold voltage does not significantly affect the uniformity of illumination. As shown in FIG. 6, the curves L2 ₁ , L2 ₂ and L2 ₃ show the IV characteristics of the three transistors (with the gate and drain connected) for different threshold voltages during the syringe. Line L1 represents the IV characteristic of the current source connected to the data line for the transmission of the data signal DS. The magnitude of the drain-source current and gate-source voltage of the steady state transistor during the syringe can be derived from the intersections a1, b1 and c1 of the curves L2 ₁ , L2 ₂ and L2 ₃ and the line L1. . As shown in FIG. 6, curves L3 ₁ , L3 ₂ and L3 ₃ represent IV characteristics of three transistors (with gate and drain separated from each other) outside the syringe. Curve L4 represents the IV characteristic of the LED. The magnitude of the drain-source current and the gate-source voltage of the steady state transistor outside the syringe can be derived from the intersection points a2, b2 and c2 of the curves L3 ₁ , L3 ₂ and L3 ₃ and the line L4. . The change in channel length results in non-overlapping of curves L3 ₁ , L3 ₂ and L3 ₃ so that the magnitude of the drain-source current of the three transistors is the same during the syringe, but different outside the syringe. Go to the value. Thus, the illuminance on the display panel is uneven due to the change in channel length, even if the threshold voltages of the transistors are the same.

It is an object of the present invention to provide an active matrix OLED / PLED pixel drive circuit that eliminates the undesirable effects resulting from channel length variations.

The present invention provides an active matrix LED pixel drive circuit. This circuit includes a capacitor; A light emitting diode; As a first and a second transistor, a capacitor is connected between the gate and the source of the first transistor, the second transistor has a source connected to the drain of the first transistor and a gate connected to receive the first voltage, and the first The first and second transistors, by voltage, operate in the saturation region; A current switch controlled by a scanning signal, wherein when the current switch is closed, a first current corresponding to the data signal flows through the first and second transistors to generate a second voltage stored on the capacitor, and the current switch is opened. And a second current passing through the first and second transistors is generated by a second voltage stored on the capacitor to turn on the light emitting diode.

Thus, in the present invention, one transistor is connected in series with the transistor through which the LED drive current flows. The gate bias voltage is applied to two transistors, which operate in the saturation region. The I-V characteristic curves of these saturated transistors are more similar to each other than transistors operating in the linear region, which reduces current transfer and increases the uniformity of illumination.

The invention will be more fully understood from the detailed description given below and the accompanying drawings which are given by way of illustration only and thus are not intended to limit the invention.

Fig. 7 is a circuit diagram showing a current-driven pixel circuit of an active matrix LED display according to the first embodiment of the present invention. The circuit includes transistors 71-74, 77, and 78, capacitor 75, and LED 76. While the gate of the transistor 71 receives the scan signal SS through the scan line, the source of the transistor receives the data signal DS through the data line. The gate of the transistor 72 also receives the scan signal SS. When the transistors 71 and 72 are turned on by the scan signal SS, the transistors 73 and 74 operate as current mirrors so that the current passing through the transistor 73 is regenerated and flows through the transistor 74. The LED 76 is turned on. In the meantime, the capacitor 75 is charged and maintains the voltage Vgs of the transistor 74. When the scan signal SS turns off the transistors 71 and 72 to end the transfer of the data signal DS at the end of the syringe, the voltage Vgs is replaced by the transistor 74 in place of the data signal DS. Keep continuity. This circuit differs from the conventional circuit in that transistors 77 and 78 are connected in series with transistors 74 and 73, respectively. Gates of transistors 77 and 78 receive a bias voltage V _bias , causing transistors 73 and 74 to operate in the saturation region.

Optionally, as shown in FIG. 7A, transistor 78 can be removed, which removal does not result in changes in circuit performance and operation.

8 is a circuit diagram showing a current-driven pixel circuit of an active matrix LED display according to a second embodiment of the present invention. The circuit includes transistors 81-84, and 87, a capacitor 85, and an LED 86. While the gate of the transistor 81 receives the scan signal SS through the scan line, the source of the transistor receives the data signal DS through the data line. Gates of the transistors 82 and 83 also receive the scan signal SS. When the transistors 81 and 82 are turned on by the scan signal SS, and the transistor 83 is turned off, a voltage Vgs is generated to correspond to a current corresponding to the current through the data line and the transistor 84. Have In the meantime, the capacitor 85 is charged to maintain the voltage Vgs. When the scan signal SS turns off the transistors 81 and 82 at the end of the syringe, and the transistor 83 is turned on to terminate the transfer of the data signal DS, the voltage Vgs becomes the data signal DS. Current causes LED 86 to light through transistors 83, 84, and 87 instead. This circuit differs from the conventional circuit in that the transistor 87 is connected to the drain of the transistor 84. The gate of transistor 87 receives a bias voltage (V _bias ), causing transistors 84 and 87 to operate in the saturation region.

9 is a circuit diagram showing a current-driven pixel circuit of an active matrix LED display according to a third embodiment of the present invention. The circuit includes transistors 91-94, and 97, capacitor 95, and LED 96. While the gate of the transistor 91 receives the scan signal SS through the scan line, the source of the transistor receives the data signal DS through the data line. Gates of the transistors 92 and 93 also receive the scan signal SS. When the transistors 91 and 92 are turned on by the scan signal SS, and the transistor 93 is turned off, the voltage Vgs of the transistor 94 is generated to generate a data line, transistors 91, 97 and 94. ) And a size corresponding to the current passing through the OLED 96. The capacitor 95 is charged to maintain the voltage Vgs. When the scan signal SS turns off the transistors 91 and 92 and ends the transfer of the data signal DS by turning on the transistor 93 at the end between the syringes, the voltage Vgs becomes the transistor 94. By generating a current passing through it, the OLED 96 is turned on in place of the data signal DS. This circuit differs from the conventional circuit in that transistor 97 is connected to the drain of transistor 94. The gate of transistor 97 receives a bias voltage (V _bias ), causing transistors 94 and 97 to operate in the saturation region.

10 is a circuit diagram showing an equivalent circuit of a current-driven pixel circuit according to previous embodiments of the present invention. This circuit includes transistors 101 and 105, a capacitor 102, a current switch 103 and an LED 104. The current switch 103 includes three switches 1031-1033 and data lines connected to a current source (not shown). The switches 1031-1033 are controlled by the scan signal SS. A current source coupled with the data line supplies the current that drives the LED 104. The gate of transistor 105 receives a bias voltage (V _bias ), causing transistors 101 and 105 to operate in the saturation region.

When the syringe stem starts, the switches 1031 and 1032 are closed and the switch 1033 is open. When this pixel is lit in the current frame, the current I of the data signal DS flows through the transistors 101 and 105 and charges the capacitor 102 to maintain the voltage Vgs. When the scan signal opens the switches 1031 and 1032 and causes the switch 1033 to close, the voltage Vgs generates a current I 'through the transistor 101 to replace the data signal DS. (104) is turned on.

Through comparison of the equivalent circuit diagram of Figs. 5 and 10, in the present invention, the transistor 105 is additionally connected in series with the transistor 101, a bias voltage is applied to the gate of the transistor 105, so that two transistors It can be seen that is operating in the saturation region. Transistor 105 suppresses fluctuations in drain-source voltage of transistor 101, which reduces current movement due to channel length variations. The I-V characteristic curves of transistors 101 and 105 are more similar to each other. Therefore, even with transistors having different threshold voltages, it is possible to obtain display panels with uniform illuminance.

The foregoing description of the preferred embodiment of the present invention has been presented for purposes of illustration and description. Obvious modifications and variations are possible from the disclosure. The embodiments have been selected and described in order to provide the best explanation of the principles of the invention, and therefore their practical application will enable those skilled in the art to use the invention in various embodiments and in various modifications suitable for the particular use contemplated. do. All such modifications and variations are intended to fall within the scope of the present invention as determined by the appended claims, interpreted in accordance with the scope of their fair, legal and just rights.

1 is a circuit diagram illustrating one voltage-driven pixel circuit of an active matrix LED display.

2, 3A, 3B, 4A, and 4B are circuit diagrams showing another current-driven pixel circuit of an active matrix LED display.

5 is a circuit diagram showing an equivalent circuit of the current-drive circuit.

6 is a graph showing I-V characteristic curves of transistors, data signal current sources, and LEDs.

7, 7A, 8 and 9 are circuit diagrams showing current-driven pixel circuits of an active matrix LED display according to an embodiment of the invention.

10 is a circuit diagram showing an equivalent circuit of the current-driven pixel circuit according to the embodiment of the present invention.

<Explanation of symbols for the main parts of the drawings>

71-74,77,78,81-84,87,91-94,97,101,105: Transistor

75,85,95,102: Capacitor 76,86,96,104: LED

103: current switch SS: scan signal

DS: Data Signal V _bias : Bias Voltage

Vgs: Gate-source voltage of the transistor

Claims (4)

Active matrix LED pixel driving circuit, Capacitors, With light emitting diode, As a first and a second transistor, a capacitor is connected between the gate and the source of the first transistor, the second transistor has a source connected to the drain of the first transistor and a gate connected to receive the first voltage, and the first The first and second transistors are driven by a voltage to operate in the saturation region; A current switch controlled by a scanning signal, wherein when the current switch is closed, a first current corresponding to the data signal flows through the first and second transistors to generate a second voltage stored on the capacitor, and the current switch is opened. And a current switch when the second current through the first and second transistors is generated by a second voltage stored on the capacitor to turn on the light emitting diode. The method of claim 1, wherein the current switch, A first switch controlled by the scan signal and having a first stage coupled to receive a data signal; A second switch controlled by the scan signal and connected between the second end of the first switch and the gate of the first transistor; A third transistor having a gate connected to the gate of the first transistor, a source connected to the source of the first transistor, and a drain connected to the second end of the first switch, And the first and third transistors act as current mirrors to generate a first current through the first and second transistors when the first and second switches are closed. The method of claim 1, wherein the current switch, A first switch controlled by the scan signal and having a first stage coupled to receive a data signal, and a second stage coupled to the drain of a second transistor; A second switch controlled by the scan signal and connected between the second end of the first switch and the gate of the first transistor; A third switch controlled by the scan signal and connected between the light emitting diode and the drain of the second transistor, And said first current is generated when the first and second switches are closed and the third switch is open. The method of claim 1, wherein the current switch, A first switch controlled by the scan signal and having a first stage coupled to receive a data signal, and a second stage coupled to the drain of a second transistor; A second switch controlled by the scan signal and connected between the drain of the second transistor and the gate of the first transistor; A third switch controlled by the scan signal, the third switch having a first end coupled to receive a third voltage and a second end coupled to the drain of a second transistor, And said first current is generated when the first and second switches are closed and the third switch is open.