KR100655778B1 - Active matrix oled driving circuit with current feedback - Google Patents

Abstract

An AMOLED(Active Matrix Organic Light Emitting Diode) driving circuit using current feedback is provided to overcome brightness nonuniformity in a voltage driving method caused due to nonuniformity of a driving TFT(Thin Film Transistor) and to exactly drive the low current by current comparison using a precise current mirror. An AMOLED driving circuit using current feedback includes a pixel circuit(100) having an OLED and a driving transistor(T11) driving the OLED; a current mirror(110) comparing current flowing in the OLED of the pixel circuit with input current; constant current sources(130a,130b) and an input current source(140) connected to each other at one side of the current mirror in parallel; a driving current stabilizing unit(120) connected to the other side of the current mirror to stabilize pixel current by stabilizing a first parasitic capacitor(CD) and a first parasitic resistance(RD) formed on a first connection line connecting the pixel circuit and a driving circuit of the pixel circuit; and a loop characteristic compensating unit(150) of which an output stage is connected to a second parasitic capacitor(CG) and a second parasitic resistance(RG), which are formed on a second connection line for connecting the pixel circuit and the driving circuit, and an input stage is connected between the constant current source and the input current source so that a feedback loop is formed, and compensating the gain of the feedback loop and the characteristic of bandwidth according to the characteristic change of the driving transistor.

Description

AMOLD LED driving circuit using current feedback {ACTIVE MATRIX OLED DRIVING CIRCUIT WITH CURRENT FEEDBACK}

1 illustrates an OLED driving circuit using a conventional current feedback.

2 illustrates an AMOLED driving circuit using current feedback in accordance with the present invention.

3 is an operation timing diagram of FIG. 2.

4A to 4H are pixel structure diagrams that can be driven through an AMOLED driving circuit using current feedback according to the present invention.

5 is another configuration example of the current mirror of FIG. 2;

6 is a complementary circuit block diagram of the present invention.

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

100: pixel circuit 110: current mirror

120: drive current stabilizer 130a, 130b: constant current source

140: input current source 150: loop characteristic compensation unit

160: zero insertion unit 170: gain control unit

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flat panel display driving circuit, and more particularly, to a current program type AMOLED driving circuit using current feedback for solving brightness irregularities due to nonuniformity of elements constituting a display system.

OLED (Organic Light Emitting Diode) is considered as a very promising display device along with LCD and PDP. In particular, it has the advantages of being the thinnest, lightest in weight, and very good in color reproduction.

OLEDs have a characteristic of controlling their brightness as a current, unlike LCDs that use brightness to control voltage.

Recently, as a flat panel display having a larger, more color reproduction, and a faster response speed is required, a flat panel display in the form of an active matrix is replaced with a flat panel display in the form of an active matrix. The trend is to shift.

An active matrix OLED display distributes a large number of OLED elements in two dimensions and controls current by sequentially approaching a thin-film transistor (TFT) that can integrate each OLED onto a glass substrate.

In the conventional active matrix OLED display, the digital graphic data input for each pixel is converted into an analog voltage through digital-to-analog conversion, and then applied as a voltage between the gate and the source of the driving TFT connected in series with the OLED, The method of maintaining this voltage through a storage capacitor has been adopted.

In this way, uniformity control of the current and even brightness of the OLED is due to the non-uniformity of the TFT, which makes it a process based on amorphous silicon (a-Si) or poly-silicone (p-Si). There is a problem that is very difficult.

In order to solve the problem of the voltage driving method, a pixel structure using a current source or a current mirror has been proposed.

In the current-driven pixel, it not only meets the basic characteristics of the OLED showing brightness proportional to the current, but also has the advantage of uniform brightness control despite the nonuniformity of the driving TFT because the current is input.

However, this driving method has a problem that it takes a very long time until the current of the OLED is stabilized when driving a small current. This problem is serious as the parasitic resistance and capacitance of the line constituting the pixel becomes larger due to the enlargement of the panel, and it is known that such a current driving method is very difficult to use in a large panel.

The current feedback OLED drive circuit which combines the above-mentioned voltage drive and current drive is described with reference to US Pat. No. 6,433,488.

1 shows an OLED driving circuit of US 6,433,488. In the pixel circuit, the driving TFT T1 is connected in series with the OLED, and a plurality of switching TFTs T2 to T4 are provided. When high and a specific pixel is selected, the TFTs T2 and T3 are turned on, and current is programmed in the OLED.

The time a pixel is selected depends on the size of the panel and the number of frames per second. At the end of the scan time of a specific pixel, the scan signal selects the next pixel, the TFT (T2), (T3) is turned off, and the TFT (T4) is turned on so that the pre-programmed OLED current is from the power supply Vs until the next program time. Flow.

The driving circuit portion of FIG. 1 includes a current mirror composed of PMOS transistors P1 and P2 for copying a reference current Iref determined through a circuit (not shown) for converting digital graphic data into a current, a pixel, That is, a current mirror composed of PMOS transistors P3 and P4 radiating current flowing through the OLED, and a current comparator composed of NMOS transistors N1 and N2 comparing the two currents, and amplify the result of the current comparison. Is composed of an amplifying NMOS transistor N3.

When the scan signal Scan is high in the driving circuit, i.e., at the program time, if I _OLED < The drain voltage of the transistor N3 increases so that the gate voltage of the pixel driving TFT T1 increases so that the OLED current increases.

On the other hand, if I _OLED > Iref, the gate current of the driving TFT T1 is operated to be low, thereby reducing the OLED current. The circuit enables the operation such that I _OLED = Iref through such a negative feedback operation.

In order for the circuit to operate normally, the loop gain must be large enough to compensate for errors, the negative feedback loop must be stable, and the bandwidth of the loop should be sufficient to stabilize at the desired current within a sufficiently fast time.

However, in an actual OLED display panel, parasitic resistance and capacitance exist in the connection line between the driving circuit and the pixel circuit. As the size of the panel increases, the parasitic resistance and capacitance increase.

In FIG. 1, this corresponds to a portion connecting the drain of the PMOS transistor P3 and the TFT T3 and a portion connecting the drain of the NMOS transistor N3 and the TFT T2.

Considering the parasitic resistance and capacitance of this part, the circuit of Fig. 1 is difficult to obtain sufficient bandwidth and stability, and sufficient loop gain cannot be obtained with an amplifier composed only of the NMOS transistor N3.

In addition, since the simple curret mirror of FIG. 1 has a large error, it is difficult to drive a very small current.

  In addition, in the pixel circuit of FIG. 1, the area occupied by the TFT is increased by using one driving TFT and three switching TFTs, that is, four TFTs, resulting in a problem of low aperture ratio of the pixel.

SUMMARY OF THE INVENTION The present invention has been made in view of this point, and an object of the present invention is to provide an AMOLED driving circuit using current feedback, which enables accurate current driving using current feedback to overcome brightness unevenness of the driving TFT.

Another object of the present invention is to provide an AMOLED driving circuit using current feedback to ensure loop stability and to realize sufficient loop gain and fast settling speed.

It is still another object of the present invention to provide an AMOLED driving circuit using current feedback having a pixel structure in which the number of TFTs constituting pixels can be reduced to increase the aperture ratio.

AMOLED driving circuit using a current feedback according to the present invention for achieving the above object, the pixel circuit including an OLED and a driving transistor for driving the same; A current mirror for comparing an input current with a current flowing through the OLED of the pixel circuit; A constant current source and an input current source connected in parallel to one side of the current mirror; A driving current stabilizer connected to the other side of the current mirror and stabilizing pixel current by stabilizing a first parasitic resistor and a first parasitic capacitor present in a first connection line connecting the pixel circuit and the driving circuit of the pixel circuit; And an output terminal of the second parasitic resistor and a second parasitic capacitor in the second connection line connecting the pixel circuit and the driving circuit, the input terminal of which is connected between the constant current source and the input current source so that the entire loop And a loop characteristic compensator configured to compensate for gain and bandwidth characteristics of the entire negative feedback loop according to the characteristic change of the driving transistor for driving the OLED of the pixel circuit.

The pixel circuit includes an OLED having an anode terminal connected to a supply voltage; A driving transistor having a drain terminal connected to the cathode of the OLED and a source terminal connected to ground; A storage capacitor connected between the gate terminal and the ground of the driving transistor; And first and second switching transistors each having a source terminal connected to the gate terminal and the drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal.

According to another embodiment of the pixel circuit, an OLED; A driving transistor having a drain terminal connected to a cathode terminal of the OLED and a source terminal connected to ground; A storage capacitor connected between the gate terminal and the ground of the driving transistor; A first switching transistor having a source terminal connected to a gate terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A second switching transistor having a source terminal connected to an anode terminal of the OLED and controlled on / off by a pixel selection signal input to a gate terminal; And a source terminal connected to an anode terminal of the OLED and a source terminal of the second switching transistor, and a drain terminal connected to a supply voltage so as to operate opposite to the pixel selection signal input to the gate terminal. And a third switching transistor controlled on / off by a bar signal.

According to another embodiment of the pixel circuit, an OLED having an anode terminal connected to a supply voltage; Drive transistors; First and second switching transistors each having a source terminal connected to a gate terminal and a drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A storage capacitor connected between the gate terminal and the ground of the driving transistor; A source terminal is connected to a drain terminal of the driving transistor and a source terminal of a second switching transistor, and a drain terminal is connected to a source terminal of the OLED, and the pixel selection signal input to a gate terminal is connected to the source terminal of the driving transistor. And a third switching transistor controlled on / off by a pixel selection bar signal operating in reverse.

According to another embodiment of the pixel circuit, an OLED; A drain terminal is connected to the cathode terminal of the OLED, and a source transistor is connected to ground; First and second switching transistors each having a source terminal connected to a gate terminal and a drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A storage capacitor connected between the gate terminal and the ground of the driving transistor; And a fourth switching on / off controlled by a pixel selection bar signal which is connected to a source terminal of the OLED terminal, a drain terminal of the drain terminal of the OLED, and an opposite operation of the pixel selection signal input to the gate terminal of the OLED. It is preferable that the transistor is configured.

According to another embodiment of the pixel circuit, an OLED; A driving transistor having a drain terminal connected to a cathode terminal of the OLED and a source terminal connected to ground; A storage capacitor connected between the gate terminal and the ground of the driving transistor; A first switching transistor having a source terminal connected to a gate terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A second switching transistor having a source connected to an anode of the OLED and controlled on / off by a pixel selection signal input to a gate; And a drain terminal is connected to an anode terminal of the OLED and a source terminal of a second switching transistor, and a source terminal is connected to a supply voltage, and is controlled on / off by the pixel selection signal input to a gate terminal. A third switching transistor; It is preferable that it consists of.

According to another embodiment of the pixel circuit, an OLED having an anode terminal connected to a supply voltage; Drive transistors; First and second switching transistors each having a source terminal connected to a gate terminal and a drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A storage capacitor connected between the gate terminal and the ground of the driving transistor; And a drain terminal is connected to a drain terminal of the driving transistor and a source terminal of the second switching transistor, and a source terminal is connected to a source terminal of the OLED, and controlled on / off by the pixel selection signal. And a third switching transistor.

According to another embodiment of the pixel circuit, an OLED; A drain terminal is connected to the cathode terminal of the OLED, and a source transistor is connected to ground; The drive transistor is direct. First and second switching transistors each having a source terminal connected to the gate terminal and the drain terminal and controlled on / off by a pixel selection signal input to the gate terminal; A storage capacitor connected between the gate terminal and the ground of the driving transistor; And a fourth switching transistor connected to the anode terminal of the OLED, the source terminal connected to a supply voltage, and controlled on / off by the pixel selection signal input to the gate terminal. .

According to another embodiment of the pixel circuit, an OLED having a cathode end connected to ground; A driving transistor having an anode end of the OLED connected to a source end; A storage capacitor connected between the gate terminal and the ground of the driving transistor; First and second switching transistors each of which has a source terminal connected to the gate terminal and the drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A source terminal is connected to a drain terminal of the driving transistor and a source terminal of the second switching transistor, and a drain terminal is connected to a supply voltage so that pixel selection operates in a manner opposite to the pixel selection signal input to the gate terminal. And a third switching transistor controlled on / off by a bar signal.

According to another embodiment of the pixel circuit, an OLED having a cathode end connected to ground; A driving transistor having an anode end of the OLED connected to a source end; A storage capacitor connected between the gate terminal and the ground of the driving transistor; First and second switching transistors each of which has a source terminal connected to the gate terminal and the drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A drain terminal is connected to a drain terminal of the driving transistor and a source terminal of the second switching transistor, and a source terminal is connected to a supply voltage to control on / off by the pixel selection signal input to a gate terminal. And a third switching transistor.

The current mirror may include one of the first and second PMOS transistors having gates connected to each other, and the third and fourth PMOS transistors having the same shape as the first current mirror. It is preferable that it is the caswood current mirror which laminated | stacked the 2nd current mirror which consists of.

In addition, the current mirror has first and second PMOS transistors having a source terminal connected to a supply voltage and a gate terminal connected to each other, and a first current mirror formed at a drain terminal of each of the first and second PMOS transistors. A second current mirror is formed of third and fourth PMOS transistors having a source terminal connected thereto and a predetermined bias voltage connected thereto, and a second current mirror is formed at the gate terminal of the first and second PMOS transistors. It is preferable that the drain end is connected.

The driving current stabilizer may include an NMOS transistor having a drain terminal connected to the current mirror; A differential amplifier connected to a gate terminal of the NMOS transistor, a predetermined voltage is input to a non-inverting input terminal, and an inverting input terminal connected to a first connection line of the pixel circuit and a driving circuit; And a constant current source connected in series with a source terminal of the NMOS transistor and having the same value as the constant current source.

Preferably, the loop characteristic compensator has an output terminal connected to the second connection line, a predetermined voltage is applied to a non-inverting input terminal, and an inverting input terminal is configured as a variable gain amplifier connected between the constant current source and an input current source.

It is preferable to further include a switch connecting the inverting input terminal and the output terminal of the variable gain amplifier so that the variable gain amplifier can be used as an amplifier having a gain of 1 when no pixel is selected.

In addition, the loop characteristic compensator includes a zero insertion unit for mitigating deterioration of frequency characteristics of the entire driving circuit due to parasitic resistance and parasitic capacitance present in the first and second connection lines between the driving circuit and the pixel circuit. It is preferable to.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the following examples are merely to illustrate the present invention is not limited to the contents of the present invention.

2 illustrates an AMOLED driving circuit using current feedback according to the present invention.

As shown, a current mirror 110 for comparing the input current and the current flowing through the pixel circuit 100, the OLED of the pixel circuit 100, a constant current source connected in parallel to each other on one side of the current mirror 110 An input current source 140 corresponding to the digital graphic data 130a and the other side of the current mirror 110, and the driving circuits of the pixel circuit 100 and the pixel circuit 100 (a and b in FIG. 2). A driving current stabilizer 120 for quickly stabilizing the pixel current by quickly stabilizing the parasitic resistance RD and the parasitic capacitor CD existing between the left side and the driving circuit of the pixel circuit 100 and the pixel circuit 100. The output terminal is connected to the parasitic resistance (RG) and the parasitic capacitor (CG) that is present between the parasitic resistance (RG), and the input terminal is connected between the constant current source (130a) and the input current source (140) to form a whole negative loop. And the driving transistor T11 of the pixel circuit 100. It consists of a loop characteristic compensation section 150 compensates the gain and bandwidth characteristics of the overall negative-feedback loop in accordance with the property change.

In addition, a zero point insertion unit 160 for inserting a zero point into the loop characteristic compensator 150 to improve loop stability is connected between the input current source 140 and the upper loop characteristic beam 150. The gain control unit 170 is configured to adjust the gain of the loop characteristic compensation unit 150 by using the input current source 140 as an input.

The pixel circuit 100 has an N-type TFT driving transistor T11 connected in series with the OLED, and an anode terminal of the OLED is connected with a supply voltage VDD. In addition, a source terminal or a drain terminal of the switching transistor T12 and the switching transistor T13, which are N-type TFTs, which are controlled on and off by the pixel selection signal SEL, are provided at the gate terminal and the drain terminal of the driving transistor T11. One end of each is connected, and the other end of the transistor T12 and the transistor T13 is connected to the driving circuit through connection lines line1 and line2.

Parasitic resistance and parasitic capacitance exist in the connection lines line1 and line2, and the parasitic resistance RD and the parasitic capacitor CD are connected to the drain terminal of the driving transistor T11 through the transistor T13. ) And a parasitic resistor RG and a parasitic capacitor CG, respectively, at a portion connected to the gate terminal of the driving transistor T1 through the transistor T12.

In addition, a storage capacitor CS is connected between the gate terminal of the driving transistor T11 and the ground to maintain the gate voltage of the driving transistor T11 after the pixel selection signal SEL is finished.

The current mirror 110 includes a current mirror having a source terminal connected to a supply voltage VDDA and a gate terminal connected to each other, and a transistor M12 connected to a gate terminal and a drain terminal of the PMOS transistors M11 and M12. And PMOS transistors M13 and M14, which form a current mirror of the same type, are stacked to form a precise cascode current mirror.

The driving current stabilizer 120 includes an NMOS transistor M15 and a differential amplifier AMP2. The drain terminal of the NMOS transistor M15 is connected to the drain terminal of the PMOS transistor M14 of the current mirror 110. The constant current source 130b having the same value as the constant current source 130a connected to the drain terminal of the PMOS transistor M13 is connected in series to the source terminal of the NMOS transistor M15, and the NMOS transistor M15. The connection line line1 is connected between the source terminal of the ()) and the constant current source 130b.

In addition, the differential amplifier AMP2 has an output terminal thereof connected to a gate terminal of the NMOS transistor M15, a non-inverting input terminal (+) receives a reference voltage VB2, and an inverting input terminal (−) connected to the connection line. is connected to (line1).

The loop characteristic compensator 150 is composed of a variable gain amplifier AMP1, the output of which is connected to the line line2, a reference voltage VB1 is input to a non-inverting input terminal (+), and an inverting input terminal. (−) Is connected between the drain terminal of the PMOS transistor M13 of the current mirror 110 and the input current source 140.

In addition, the zero insertion unit 160 is connected between the output terminal of the variable gain amplifier AMP1 and the inverting input terminal (-), and between the output terminal of the variable gain amplifier AMP1 and the inverting input terminal (-). A switch SW1 that is controlled to be switched by the pixel selection bar signal SEL_b that is operated opposite to the selection signal SEL is connected.

The present invention configured as described above will be described with the timing chart of FIG.

First, we look at the pixel before it is selected to be programmed with a new current, that is, during the illumination period.

Since the pixel selection signal SEL is low, the transistors T12 and T13, which are switching transistors, are turned off, and the driving transistors T11 and OLED are programmed to flow during the previous program period.

In addition, the same current I1 flows through both kinds of the current mirror 110.

The switch SW1 between the inverting input terminal (-) and the output terminal of the variable gain amplifier Amp1 is turned on by the pixel selection bar signal SEL_b. Accordingly, the variable gain amplifier Amp1 operates as an amplifier having a gain of 1 and is variable. The output line of the gain amplifier AMP1, that is, the voltage of the connection line line2, becomes the same voltage VB1 as the non-inverting input terminal (+) of the variable gain amplifier AMP1. At this time, the same voltage VB1 is also charged in the parasitic capacitor CG connected to the connection line line2.

The voltage VB1 may be selected at design time, but the voltage VB1 may be set to a voltage close to the threshold voltage of the driving transistor T11 to help stabilize the small driving current.

Similarly, the voltage of the connection line line1 becomes the voltage VB2 by the differential amplifier AMP2 and the transistor M15, and the parasitic capacitor CD connected to the connection line line1 is also charged with the same voltage VB2. .

Since the voltage VB2 becomes the cathode voltage of the OLED in the program period, the following relation must be satisfied.

VB2> VDD-Von, OLE expression (1)

Here, Von and OLED are voltage drops of the OLED when the OLED starts to turn on.

When this relation is satisfied, the current can be prevented from flowing through the OLED during the program period, and all the pixel currents flow through the driving transistor T11 to enable accurate current programming.

When the program period starts and the pixel selection signal SEL becomes high, the transistors T12 and T13 are turned on so that the connection line line2-the driving transistor T11-the connection line line1-the current mirror 110 are turned on. Variable gain amplifier AMP1-An entire loop consisting of a connection line line2 is formed.

Then, when the desired pixel driving current IDATA is input by the input current source 140, the drain current of the PMOS transistor M13 of the current mirror 110 becomes I1 + IDATA, and the drain current of the PMOS transistor M14 is still present. Since I1, the voltage at the drain terminal side node C of the PMOS transistor M13 is lowered.

Accordingly, the voltage of the connection line line2, that is, the gate voltage of the driving transistor T11 is increased by the variable gain amplifier Amp1, thereby increasing the Ipixel which is a current flowing through the connection line line1.

When the loop operates stably, the current mirror 110 is controlled to have the same current, i.e., I1 + IDATA = I1 + Ipixel, so that Ipixel = IDATA.

The differential amplifier Amp2 and the transistor M15 are stabilized so that the voltage of the connection line line1 is almost unchanged at the beginning of the program period, thereby reducing the time required for the drive current to stabilize.

In the absence of the differential amplifier Amp2 and the transistor M15, since the current charging and discharging the parasitic capacitor CD is a small pixel current, it takes a long time to stabilize the voltage of the parasitic capacitor CD.

Therefore, the poor loop composed of the differential amplifier Amp2 and the transistor M15 should be designed so that the gain and bandwidth are sufficiently large.

On the other hand, representative of the entire loop formed of the connection line (line2)-driving transistor (T11)-connection line (line1)-current mirror 110-node (C)-variable gain amplifier (AMP1)-connection line (line2) In view of the position of the pole, the pole fA made up of the equivalent resistance of the parasitic capacitor CG and the connection line line2, the pole fB made up of the equivalent resistance of the parasitic capacitor CD and the connection line line1, and And the pole fC composed of the equivalent resistance of the node C and the parasitic capacitor.

The equivalent resistance of the connection line line2 can be viewed as the sum of the parasitic resistance RG and the output resistance of the variable gain amplifier Amp1.

In addition, the equivalent resistance of the connection line line1 may be regarded as the sum of the resistance of the parasitic resistance RD and the source of the transistor M15. The resistance of the source of the transistor M15 is a differential amplifier ( Since the negative feedback circuit using Amp2 has a small value, the equivalent resistance of the connection line line1 can be considered to be almost equal to the parasitic resistance RD.

The equivalent resistance of the node C may be regarded as a parallel connection of the constant current source 130a, the output resistance of the input current source 140, and the resistor viewed through the drain of the PMOS transistor M13 of the current mirror 110.

The relative positions of the three poles depend on the design of the variable gain amplifier Amp1, the differential amplifier Amp2 and the constant current source 130a, the input current source 140, and the current mirror 110.

To ensure the stability of the entire loop, all the poles except the dominant pole must be located above the frequency where the loop gain is less than zero.

However, since the parasitic capacitors CG and CD that determine the poles have very large values and are determined by the panel size and the pixel structure, it may be difficult to secure stable loop characteristics. At this time, it is possible to improve the performance by adding a zero insertion unit 160 for adding a zero point to the variable gain amplifier (Amp1).

On the other hand, as the driving current changes, the driving transistor T11 changes its operating point, that is, its transconductance and output resistance, so that the gain and bandwidth of the entire loop change to affect stability and operating speed. .

To compensate for this, the gain of the variable gain amplifier Amp1 is adjusted through the gain controller 170. That is, the gain of the variable gain amplifier Amp1 is increased when the driving current of the OLED is small, and the gain of the variable gain amplifier Amp1 is decreased when the driving current of the OLED is large.

4 illustrates a pixel circuit of various structures driven by an AMOLED driving circuit using current feedback according to the present invention.

This is because, in FIG. 2, when the voltage VB2 cannot be determined as shown in Equation (1) or when the Von and OLED of Equation (1) are not known correctly, an additional switch is added to prevent current flowing to the OLED during the program period. to be.

4A shows a switching transistor T4 which connects a cathode of the OLED to the drain of the driving transistor T11 and a transistor T13 which connects a pixel current to the driving circuit to the anode of the OLED, which is another N-type TFT. Is connected between the connection point of the transistor T13 and the OLED and the supply voltage VDD. The gate signal of the transistor T14 is controlled on / off by the pixel selection bar signal SEL_b.

FIG. 4B is for switching, which is composed of another N-type TFT between the anode of OLED and the connection point of the drain of transistor T13 and transistor T11, connecting the anode of OLED to the supply voltage VDD as in the case of FIG. The transistor T14 is connected.

The gate signal of the transistor T14 is controlled on / off by the pixel select bar signal SEL_b which operates in opposition to the pixel select signal SEL.

4C is a switching transistor connected to the cathode of the OLED at the junction of the transistor T13 and the drain of the driving transistor T11, as in the case of FIG. 2, and another N-type TFT between the anode of the OLED and the supply voltage VDD. It is the structure which connected (T14). The gate signal of the transistor T14 is controlled on / off by the pixel select bar signal SEL_b which operates in opposition to the pixel select signal SEL.

FIG. 4D replaces the transistor T14 with a P-type TFT in the structure of FIG. 4A, and the gate signal of the transistor T14 is connected to the pixel selection signal SEL which is a control signal of the transistors T12 and T13. It is a structure that is controlled by on / off.

4E replaces the transistor T14 with a P-type TFT in the structure of FIG. 4B, and the gate signal of the transistor T14 is applied to the pixel select signal SEL which is a control signal of the transistors T12 and T13. By the on / off control.

4F replaces the transistor T14 with a P-type TFT in the structure of FIG. 4C, and the gate signal of the transistor T14 is connected to the pixel selection signal SEL which is a control signal of the transistors T12 and T13. By the on / off control.

4G is a structure in which the OLED is connected between the source terminal of the driving transistor T11 and the ground in the structure of FIGS. 4A to 4C. Operation of the driving transistors T11 and the transistors T12-T14 is described with reference to FIGS. 4A to 4C. Same as FIG. 4C.

4H shows that the transistor T14 is replaced with a P-type TFT in the structure of FIG. 4G, and the gate signal of the transistor T14 is controlled by the pixel selection signal SEL which is a control signal of the transistors T12 and T13. On / off controlled structure.

5 illustrates another embodiment of the current mirror 110 of FIG. 2.

Instead of directly connecting the drain and gate of the PMOS transistors M12 and M14, the gate of the PMOS transistor M12 and the drain of the PMOS transistor M14 are connected, and the gates of the PMOS transistors M13 and M14 are connected. In the structure connected to the same bias voltage (Vbias), the operation in the present invention is the same as FIG.

  FIG. 6 illustrates a transistor having the complementary relationship with FIG. 2 by changing the transistor of the driving circuit and the pixel circuit 100 of FIG. 2 to the N type as the P type and the P type as the N type. The operation of the 100 may be understood by those skilled in the art from FIG. 2, and thus the detailed description thereof is weak.

As described above, although described with reference to a preferred embodiment of the present invention, those skilled in the art various modifications of the present invention without departing from the spirit and scope of the invention described in the claims below Or it may be modified.

As described above, the present invention has the following effects.

First, by adopting the current feedback method, it overcomes the brightness unevenness in the voltage driving method due to the nonuniformity of the driving TFT, and in implementing the current feedback method, it is possible to accurately drive very small current through current comparison using a precise current mirror. do.

Second, the large parasitic resistance and capacitance in the connection line that delivers the current of the pixel to the driving circuit for current comparison can be quickly stabilized by the negative feedback circuit using a large gain and wide bandwidth amplifier to stabilize the pixel current. By shortening and varying the gain of the amplifier in the entire negative feedback loop, it is possible to prevent deterioration of the gain and bandwidth characteristics of the entire negative feedback loop due to the characteristic change of the driving transistor according to the magnitude of the driving current.

Third, by allowing zero insertion in the entire negative feedback loop, the parasitic capacitance and resistance of the line connecting the driving circuit and the pixel are alleviated to limit the performance of the stability and bandwidth of the entire negative feedback loop. By providing the freedom of selecting the reference voltage properly, the pixel structure can be made up of only three TFTs, thereby increasing the aperture ratio of the display.

Claims (18)

A pixel circuit including an OLED and a driving transistor for driving the OLED; A current mirror for comparing an input current with a current flowing through the OLED of the pixel circuit; A constant current source and an input current source connected in parallel to one side of the current mirror; A driving current stabilizer connected to the other side of the current mirror and stabilizing pixel current by stabilizing a first parasitic resistor and a first parasitic capacitor present in a first connection line connecting the pixel circuit and the driving circuit of the pixel circuit; And An output terminal is connected to a second parasitic resistor and a second parasitic capacitor present in a second connection line connecting the pixel circuit and the driving circuit, and an input terminal is connected between the constant current source and the input current source to form a whole negative loop. A loop characteristic compensator configured to compensate for gain and bandwidth characteristics of the entire negative feedback loop according to a characteristic change of a driving transistor for driving the OLED of the pixel circuit; AMOLED driving circuit using a current feedback comprising a. The method of claim 1, wherein the pixel circuit An OLED whose anode end is connected to a supply voltage; A driving transistor having a drain terminal connected to the cathode of the OLED and a source terminal connected to ground; A storage capacitor connected between the gate terminal and the ground of the driving transistor; And First and second switching transistors each of which has a source terminal connected to the gate terminal and the drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; AMOLED driving circuit using a current feedback, characterized in that consisting of. The method of claim 1, wherein the pixel circuit OLED; A driving transistor having a drain terminal connected to a cathode terminal of the OLED and a source terminal connected to ground; A storage capacitor connected between the gate terminal and the ground of the driving transistor; A first switching transistor having a source terminal connected to a gate terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A second switching transistor having a source terminal connected to an anode terminal of the OLED and controlled on / off by a pixel selection signal input to a gate terminal; And A source terminal is connected to a connection terminal between an anode terminal of the OLED and a source terminal of a second switching transistor, and a drain terminal is connected to a supply voltage so as to operate opposite to the pixel selection signal input to a gate terminal. A third switching transistor controlled on / off by a signal; AMOLED driving circuit using a current feedback, characterized in that consisting of. The method of claim 1, wherein the pixel circuit An OLED having an anode end connected to a supply voltage; Drive transistors; First and second switching transistors each having a source terminal connected to a gate terminal and a drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A storage capacitor connected between the gate terminal and the ground of the driving transistor; And A source terminal is connected to the drain terminal of the driving transistor and a source terminal of the second switching transistor, and a drain terminal is connected to the casing terminal of the OLED, and is opposite to the pixel selection signal input to the gate terminal. A third switching transistor controlled on / off by an operating pixel selection bar signal; AMOLED driving circuit using a current feedback, characterized in that consisting of. The method of claim 1, wherein the pixel circuit OLED; A drain terminal is connected to the cathode terminal of the OLED, and a source transistor is connected to ground; First and second switching transistors each having a source terminal connected to a gate terminal and a drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A storage capacitor connected between the gate terminal and the ground of the driving transistor; And A fourth switching for on / off controlled by a pixel selection bar signal which is connected to a source terminal of the OLED, a drain terminal of the drain terminal of the OLED, and an opposite operation of the pixel selection signal input to the gate terminal of the OLED; transistor; AMOLED driving circuit using a current feedback, characterized in that consisting of. The method of claim 1, wherein the pixel circuit OLED; A driving transistor having a drain terminal connected to a cathode terminal of the OLED and a source terminal connected to ground; A storage capacitor connected between the gate terminal and the ground of the driving transistor; A first switching transistor having a source terminal connected to a gate terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A second switching transistor having a source connected to an anode of the OLED and controlled on / off by a pixel selection signal input to a gate; And A drain terminal is connected to an anode terminal of the OLED and a source terminal of a second switching transistor, and a source terminal is connected to a supply voltage and controlled on / off by the pixel selection signal input to a gate terminal. A third switching transistor; AMOLED driving circuit using a current feedback, characterized in that consisting of. The method of claim 1, wherein the pixel circuit An OLED having an anode end connected to a supply voltage; Drive transistors; First and second switching transistors each having a source terminal connected to a gate terminal and a drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A storage capacitor connected between the gate terminal and the ground of the driving transistor; And A drain terminal is connected to a drain terminal of the driving transistor and a source terminal of the second switching transistor, and a source terminal is connected to a source terminal of the OLED and controlled on / off by the pixel selection signal. 3 switching transistors; AMOLED driving circuit using a current feedback, characterized in that consisting of. The method of claim 1, wherein the pixel circuit OLED; A drain terminal is connected to the cathode terminal of the OLED, and a source transistor is connected to ground; First and second switching transistors each having a source terminal connected to a gate terminal and a drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A storage capacitor connected between the gate terminal and the ground of the driving transistor; And A fourth switching transistor having a drain terminal connected to an anode terminal of the OLED, a source terminal connected to a supply voltage, and controlled on / off by the pixel selection signal input to a gate terminal; AMOLED driving circuit using a current feedback, characterized in that consisting of. The method of claim 1, wherein the pixel circuit An OLED whose cathode end is connected to ground; A driving transistor having an anode end of the OLED connected to a source end; A storage capacitor connected between the gate terminal and the ground of the driving transistor; First and second switching transistors each having a source terminal connected to a gate terminal and a drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A source terminal is connected to a drain terminal of the driving transistor and a source terminal of the second switching transistor, and a drain terminal is connected to a supply voltage so that pixel selection operates in a manner opposite to the pixel selection signal input to the gate terminal. A third switching transistor controlled on / off by a bar signal; AMOLED driving circuit using a current feedback, characterized in that consisting of. The method of claim 1, wherein the pixel circuit An OLED whose cathode end is connected to ground; A driving transistor having an anode end of the OLED connected to a source end; A storage capacitor connected between the gate terminal and the ground of the driving transistor; First and second switching transistors each of which has a source terminal connected to the gate terminal and the drain terminal of the driving transistor and controlled on / off by a pixel selection signal input to the gate terminal; A drain terminal is connected to a drain terminal of the driving transistor and a source terminal of the second switching transistor, and a source terminal is connected to a supply voltage to control on / off by the pixel selection signal input to a gate terminal. A third switching transistor; AMOLED driving circuit using a current feedback, characterized in that consisting of. The method of claim 1, wherein the current mirror is Any one of the first and second PMOS transistors having gates connected to each other includes a second current including a third and fourth PMOS transistors having the same shape as the first current mirror in a first current mirror having a gate and a drain terminal connected thereto. AMOLED driving circuit using current feedback, characterized in that the casing current mirror is a laminated mirror. The method of claim 1 wherein the current mirror is A first current mirror is formed of first and second PMOS transistors having a source terminal connected to a supply voltage and a gate terminal connected to each other, and a source terminal connected to drain terminals of the first and second PMOS transistors, respectively. And a second current mirror formed of the third and fourth PMOS transistors having a predetermined bias voltage connected to the gate terminal, and the drain terminal of the fourth PMOS transistor connected to the gate terminal of the first and second PMOS transistors. AMOLED driving circuit using current feedback. The method of claim 1, wherein the driving current stabilizer An NMOS transistor having a drain terminal connected to the current mirror; A differential amplifier connected to a gate terminal of the NMOS transistor, a predetermined voltage is input to a non-inverting input terminal, and an inverting input terminal connected to a first connection line of the pixel circuit and a driving circuit; And  A constant current source connected in series with a source terminal of the NMOS transistor and having the same value as the constant current source; AMOLED driving circuit using a current feedback, characterized in that consisting of. 15. The AMOLED driving circuit according to claim 13, wherein the predetermined voltage applied to the non-inverting input terminal of the differential amplifier is a voltage such that the OLED is not turned on during the programming of the current to the pixel. The method of claim 1, wherein the loop characteristic compensation unit The output terminal is connected to the second connection line, a predetermined voltage is applied to the non-inverting input terminal, and the inverting input terminal is composed of a variable gain amplifier connected between the constant current source and the input current source. in. 16. The method of claim 15, further comprising a switch connecting the inverting input terminal and the output terminal of the variable gain amplifier so that the variable gain amplifier can be used as an amplifier having a gain of 1 when no pixel is selected. AMOLED driving circuit using feedback. 16. The AMOLED driving circuit according to claim 15, further comprising a gain controller for adjusting the gain of the variable gain amplifier. The method of claim 1, wherein the loop characteristic compensator is configured to mitigate deterioration of frequency characteristics of the entire driving circuit by parasitic resistance and parasitic capacitance present in the first and second connection lines between the driving circuit and the pixel circuit. AMOLED driving circuit using a current feedback, characterized in that the zero insertion portion.

Images