Driving circuit and method for pixel unit, pixel unit and display apparatus

This disclosure relates to the technical field of display driving, and in particular, to a driving circuit and method for a pixel unit, the pixel unit and a display apparatus.

An AMOLED (Active Matrix Organic Light-Emitting Diode) is able to emit light as it is driven by a current generated by a driving TFT in saturation; that is to say, the AMOLED is driven by the current to emit light. FIG. 1A is a schematic diagram showing a pixel structure of an AMOLED of an existing elementary current type, and FIG. 1B is a timing sequence chart corresponding to the same. As shown in FIG. 1A , the pixel structure of the AMOLED of the existing elementary current type includes an OLED (Organic Light-Emitting Diode), transistors T 1 , T 2 , T 3 , and T 4 , and a storage capacitor Cst, wherein T 1 is a driving thin film transistor, T 2 , T 3 and T 4 are controlling thin film transistors; a gate of T 2 and a gate of T 3 are connected to a control line for outputting a control signal CN 1 , and a gate of T 4 is connected to a control line for outputting a control signal CN 2 . A driving current Idata is directly applied to the pixel structure of the AMOLED of the existing elementary current type externally to determine a voltage across the storage capacitor Cst, thus generating a driving current Ioled for driving the OLED to emit light. In the pixel structure of the AMOLED of the existing elementary current type, Ioled is equal to Idata, and since bled must be in the range of the operating current of the OLED, i.e., Ioled must be a relatively small current, Idata should be relatively small. As the storage capacitor Cst is a large capacitor, the speed for charging the storage capacitor Cst is relatively slow; especially at a low grey level, it takes a long time to charge the storage capacitor Cst. Therefore, the pixel structure of the AMOLED of the existing elementary current type is not suitable for an AMOLED display of high definition and high refresh frequency.

The embodiments of the present disclosure provide a driving circuit and method for a pixel unit, the pixel unit and a display apparatus, for solving the problem that the current driving technique for the pixel unit is not suitable for an AMOLED display of high definition and high refresh frequency since such a driving technique causes a relatively slow speed for charging the storage capacitor Cst; especially at a low grey level, it takes a long time to charge the storage capacitor Cst.

An embodiment of the present disclosure provides a driving circuit for a pixel unit including:

a light-emitting device with a first terminal connected to a first level;

a sixth transistor with a drain connected to a second terminal of the light-emitting device;

a first transistor with a drain connected to a source of the sixth transistor and a source connected to a second level;

a storage capacitor with a first terminal connected to a gate of the first transistor, a first control node and a third control node respectively and a second terminal connected to the second level;

a second transistor with a source connected to the second level and a gate connected to the third control node; and

a switching unit, being connected to a data signal terminal for supplying a data signal current, a scan signal terminal for supplying a scan signal, the first control node, the third control node and a drain of the second transistor respectively, and further being connected to the source of the sixth transistor via a second control node, for controlling the data signal current to charge the storage capacitor.

In an example, the switching unit comprises a third transistor, a fourth transistor and a fifth transistor, wherein:

the third transistor has a source connected to the third control node, a drain connected to the second control node, and a gate connected to the scan signal terminal;

the fourth transistor has a source connected to the second control node, a drain connected to the data signal terminal, and a gate connected to the scan signal terminal; and

the fifth transistor has a source connected to the first control node, a drain connected to the data signal terminal, and a gate connected to the scan signal terminal.

In an example, the first, second, and sixth transistors have equal threshold voltages.

In an example, the first, second, third, fourth, fifth and sixth transistors are of N type thin film transistors; the first level is a high level, and the second level is a low level; the first terminal of the light-emitting device is an anode of the light-emitting device, and the second terminal of the light-emitting device is a cathode of the light-emitting device.

In another example, the first, second, third, fourth, fifth and sixth transistors are of P type thin film transistors; the first level is a low level, and the second level is a high level; the first terminal of the light-emitting device is a cathode of the light-emitting device, and the second terminal of the light-emitting device is an anode of the light-emitting device.

In a further example, the first, second, and sixth transistors are of N type thin film transistors, and the third, fourth and fifth transistors are of P type thin film transistors; the first level is a high level, and the second level is a low level; the first terminal of the light-emitting device is an anode of the light-emitting device, and the second terminal of the light-emitting device is a cathode of the light-emitting device.

In another example, the first, second, and sixth transistors are of P type thin film transistors, and the third, fourth and fifth transistors are of N type thin film transistors; the first level is a high level, and the second level is a low level; the first terminal of the light-emitting device is a cathode of the light-emitting device, and the second terminal of the light-emitting device is an anode of the light-emitting device.

In an example, the light-emitting device is an EL or an OLED.

An embodiment of the present disclosure provides a driving method for a pixel unit, which is applied to the driving circuit for the pixel unit, and the driving method comprising:

step for charging the pixel unit: turning on a switch for controlling the data signal current to charge the storage capacitor until the voltage across the storage capacitor does not rise any more;

step for the light-emitting device emitting light: turning off the switch for controlling the data signal current, and turning on a switch for controlling the light-emitting device to make the light-emitting device to emit light, wherein the current flowing through the light-emitting device is in direct proportion to the data signal current.

In an example, the current flowing through the light-emitting device is equal to the sum of the current flowing through the first transistor and that flowing through the second transistor in the driving circuit for the pixel unit.

An embodiment of the present disclosure provides a pixel unit including the driving circuit for pixel unit as mentioned above.

An embodiment of the present disclosure provides a display apparatus including a plurality of the pixel units as mentioned above.

Compared to the prior art, the driving circuit and method for the pixel unit, the pixel unit and the display apparatus provided by the embodiments of the present disclosure can enable a relatively large scale ratio of the data signal current Idata to the current Ioled flowing through the light-emitting device, which ensures that Ioled is in the range of the operating current of the light-emitting device and expedites the speed for charging the storage capacitor since Idata can be a relatively large current. In addition, for the leak current of the storage capacitor Cst, the driving circuit and method for the pixel unit, the pixel unit and the display apparatus provided by the embodiments of the present disclosure have an excellent negative feedback function and thus ensure the stable operation of the circuit.

FIG. 1A is a schematic diagram of the pixel circuit structure of an AMOLED of the existing elementary current type;

FIG. 1B is a timing sequence chart of the pixel circuit structure shown in FIG. 1A ;

FIG. 2 is a schematic block diagram of a driving circuit for a pixel unit according to an embodiment of the present disclosure;

FIG. 3A is a circuit diagram of a first example of the driving circuit for a pixel unit according to an embodiment of the present disclosure;

FIG. 3B is a timing sequence chart of the circuit shown in FIG. 3A ;

FIG. 4A is a circuit diagram of a second example of the driving circuit for a pixel unit according to another embodiment of the present disclosure;

FIG. 4B is a timing sequence chart of the circuit shown in FIG. 4A ;

FIG. 5A is a circuit diagram of a third example of the driving circuit for a pixel unit according to an embodiment of the present disclosure;

FIG. 5B is a timing sequence chart of the circuit shown in FIG. 5A ;

FIG. 6 is an equivalent circuit diagram of the circuit shown in FIG. 3A in a first stage;

FIG. 7 is an equivalent circuit diagram of the circuit shown in FIG. 3A in a second stage; and

FIG. 8 shows a circuit simulation result of the circuit shown in FIG. 3A in the second stage.

Hereinafter, in order to make the technical problem to be solved, the technical solutions and the advantages of the embodiments of the present disclosure clearer, detailed descriptions will be given by combining the accompanying drawings with the detailed embodiments of the present disclosure.

As shown in FIG. 2 , an embodiment of the present disclosure provides a driving circuit for a pixel unit comprising:

a light-emitting device (in FIG. 2 , an OLED is taken as an example of the light-emitting device) with a first terminal connected to a first level;

a sixth transistor T 6 with a drain connected to a second terminal of the light-emitting device;

a first transistor T 1 with a drain connected to a source of the sixth transistor T 6 and a source connected to a second level;

a storage capacitor Cst with a first terminal connected to a gate of the first transistor T 1 , a first control node G and a third control node Q respectively and a second terminal connected to the second level;

a second transistor T 2 with a source connected to the second level and a gate connected to the third control node G; and

a switching unit, being connected to a data signal terminal Idata for supplying a data signal current, a scan signal terminal Scan for supplying a scan signal, the first control node G, the third control node Q and a drain of the second transistor T 2 respectively, and further being connected to the source of the sixth transistor T 6 via a second control node P, for controlling the data signal current to charge the storage capacitor Cst.

In an embodiment of the present disclosure, examples are shown in FIG. 3A , FIG. 4A or FIG. 5A , wherein

the switching unit includes a third transistor T 3 , a fourth transistor T 4 and a fifth transistor T 5 , wherein:

the third transistor T 3 has a source connected to the third control node Q, a drain connected to the second control node P, and a gate connected to the scan signal terminal Scan;

the fourth transistor T 4 has a source connected to the second control node P, a drain connected to the data signal terminal Idata, and a gate connected to the scan signal terminal Scan; and

the fifth transistor T 5 has a source connected to the first control node G, a drain connected to the data signal terminal Idata, and a gate connected to the scan signal terminal Scan.

In a first example, as shown in FIG. 3A , the first, second, third, fourth, fifth and sixth transistors T 1 , T 2 , T 3 , T 4 , T 5 and T 6 are of N type thin film transistors; meanwhile, the first level is a high level VDD, and the second level is a low level VSS; the first terminal of the light-emitting device is an anode of the light-emitting device, and the second terminal of the light-emitting device is a cathode of the light-emitting device. In such a case, the light-emitting device is a top emitting device. FIG. 3B is a timing sequence chart of the driving circuit for a pixel unit shown in FIG. 3A when driving the pixel unit, wherein the signal {circle around ( 1 )} in FIG. 3B corresponds to the charging stage of the storage capacitor Cst, and the signal {circle around ( 2 )} corresponds to the light-emitting stage of the light-emitting device.

In a second example, as shown in FIG. 4A , the first, second, third, fourth, fifth and sixth transistors T 1 , T 2 , T 3 , T 4 , T 5 and T 6 can also be of P type thin film transistors; meanwhile, the first level is a low level VSS, and the second level is a high level VDD; the first terminal of the light-emitting device is a cathode of the light-emitting device, and the second terminal of the light-emitting device is an anode of the light-emitting device. In such a case, the light-emitting device is a bottom emitting device. FIG. 4B is a timing sequence chart of the driving circuit for the pixel unit shown in FIG. 4A when driving the pixel unit, wherein the signal {circle around ( 1 )} in FIG. 4B corresponds to the charging stage of the storage capacitor Cst, and the signal {circle around ( 2 )} corresponds to the light-emitting stage of the light-emitting device.

In a third example, as shown in FIG. 5A , the first, second, and sixth transistors T 1 , T 2 , and T 6 are of N type thin film transistors, and the third, fourth and fifth transistors T 3 , T 4 and T 5 are of P type thin film transistors; meanwhile, the first level is a high level VDD, and the second level is a low level VSS; the first terminal of the light-emitting device is an anode of the light-emitting device, and the second terminal of the light-emitting device is a cathode of the light-emitting device. FIG. 5B is a timing sequence chart of the driving circuit for the pixel unit shown in FIG. 5A when driving the pixel unit, wherein the signal {circle around ( 1 )} in FIG. 5B corresponds to the charging stage of the storage capacitor Cst, and the signal {circle around ( 2 )} corresponds to the light-emitting stage of the light-emitting device.

In a fourth example, the first, second, and sixth transistors T 1 , T 2 , and T 6 are of P type thin film transistors, and the third, fourth and fifth transistors T 3 , T 4 and T 5 are of N type thin film transistors; meanwhile, the first level is a high level VDD, and the second level is a low level VSS; the first terminal of the light-emitting device is a cathode of the light-emitting device, and the second terminal of the light-emitting device is an anode of the light-emitting device (not shown in the drawings).

Besides the above forms of the low temperature polysilicon (LTPS) transistors, the first, second, third, fourth, fifth and sixth transistors T 1 , T 2 , T 3 , T 4 , T 5 and T 6 can also be of oxide transistors, oxide TFTs, organic transistors or organic TFTs.

The light-emitting device can be of EL, OLED and the like.

In an example, the first, second, and sixth transistors T 1 , T 2 , and T 6 have equal threshold voltages. For instance, if an ELA (Excimer Laser Annealing) process is employed, the first, second and sixth transistors T 1 , T 2 and T 6 can be placed on a same horizontal position within one pixel when the layout of the pixel circuit is designed, so that these three transistors can be located within the same horizontal laser beam in manufacturing process to ensure the threshold voltage Vth 1 of the first transistor T 1 , the threshold voltage Vth 2 of the second transistor T 2 , and the threshold voltage Vth 6 of the sixth transistor T 6 to be equal and thus to be considered as Vth.

In the above driving circuits for the pixel unit, the definitions of the source and the drain of the transistor are not fixed, and they vary with the variation of the voltages. For an N type thin film transistor, a drain is arranged at a location of high voltage, and a source is arranged at a location of low voltage; the case is contrary for a P type thin film transistor.

In addition, an embodiment of the present disclosure further provides a driving method for a pixel unit, which is applied to the above driving circuit for the pixel unit provided by the embodiment of the present disclosure, and the driving method comprises two steps of A 1 -A 2 as follows:

The step A 1 is for charging the pixel unit: controlling the data signal current Idata to charge the storage capacitor Cst until the voltage across the storage capacitor Cst does not rise any more; at this time, no data current flows into the storage capacitor Cst, and the voltage across the storage capacitor corresponds to the data current flowing through the transistor under the control.

Wherein, the first transistor T 1 and the second transistor T 2 are in saturation, and the sum of the saturation current of the first transistor T 1 and that of the second transistor T 2 is equal to the data signal current Idata.

That is, the step A 1 can be implemented as follows: turning on a switch for controlling the data signal current, and using the data signal current Idata to charge the storage capacitor Cst until the sum of the saturation current of the first transistor T 1 and that of the second transistor T 2 is equal to the data signal current Idata.

The step A 2 is for the light-emitting device emitting light: turning off the switch for controlling the data signal current, and meantime turning on a switch for controlling the light-emitting device so that the current Ioled flowing through the light-emitting device is in direct proportion to the data signal current Idata.

Wherein, the step A 2 can be implemented as follows:

turning off the switch for controlling the data signal current, and meantime turning on the switch for controlling the light-emitting device, so that the first transistor T 1 and the second transistor T 2 operate in a linear operating area and the current flowing through the light-emitting device is equal to the sum of the current flowing through the first transistor T 1 and that flowing through the second transistor T 2 .

The above steps of A 1 -A 2 can be carried out by the switching unit and the corresponding transistors in the above driving circuit for the pixel unit provided by the embodiments of the present disclosure. Hereinafter, detailed descriptions will be given to the operating principle of the driving circuit for the pixel unit provided by the embodiment of the present disclosure by taking FIG. 3A as an example.

A first stage is the stage for charging the storage capacitor Cst.

FIG. 3B is a timing sequence chart of the driving circuit for the pixel unit as shown in FIG. 3A . FIG. 6 is an equivalent circuit diagram of the driving circuit for the pixel unit shown in FIG. 3B when the storage capacitor Cst is being charged. FIG. 6 corresponds to the signal {circle around ( 1 )} shown in FIG. 3B .

With reference to FIG. 3A (only the circuit structure is shown in FIG. 3A , but the states of the individual transistors when being turned on are not shown), the third, fourth, and fifth thin film transistors T 3 , T 4 , and T 5 are turned on, and the data signal current Idata charges the storage capacitor Cst via the node G.

At this time, the node G and node P are connected together, and the gate-source voltage of the sixth thin film transistor T 6 is equal to 0, and thus the sixth thin film transistor T 6 is turned off. Meanwhile, the node P and node Q are connected together, and the first thin film transistor T 1 and the second thin film transistor T 2 serve as diodes, and the individual voltage values of the first thin film transistor T 1 and the second thin film transistor T 2 have the following relationship:

Vgs=Vds=Vst=Vg−VSS

wherein, Vgs represents the gate-source voltage of a thin film transistor, Vds represents the source-drain voltage of the thin film transistor, Vst represents the voltage across the storage capacitor, and Vg represents the voltage at node G.

At this time, both the first thin film transistor T 1 and the second thin film transistor T 2 are in saturation.

After the storage capacitor Cst is charged, the following relationship holds true:

I data =I ds1 +I ds2

wherein I ds1 denotes the saturation current of the first thin film transistor T 1 , and I ds2 denotes the saturation current of the second thin film transistor T 2 .

For the first thin film transistor T 1 , the following relationship holds true:

wherein k 1 denotes a constant relating to the design size and the process of the first thin film transistor T 1 .

For the second thin film transistor T 2 , the following relationship holds true:

wherein k 2 denotes a constant relating to the design size and the process of the second thin film transistor T 2 .

It can be obtained from the above equations (1) and (2):

It can be obtained by combining the above equations:

A second stage is the stage for the light-emitting device emitting light.

FIG. 7 is an equivalent circuit diagram of the driving circuit for the pixel unit when the OLED enters the stage for emitting light. FIG. 7 corresponds to the signal {circle around ( 2 )} shown in FIG. 3B .

With reference to FIG. 3A , the third, fourth and fifth thin film transistors T 3 , T 4 , and T 5 are turned off, and the OLED enters the stage for emitting light.

Let Vp denote the voltage at node P before the third, fourth, and fifth thin film transistors T 3 , T 4 and T 5 are turned off, and Vp′ denote the voltage at node P after the third, fourth, and fifth thin film transistors T 3 , T 4 and T 5 are turned off.

Before the third, fourth, and fifth thin film transistors T 3 , T 4 and T 5 are turned off, Vp=Vq=Vg, wherein Vq denotes the voltage at node Q, and Vg denotes the voltage at node G.

After the third, fourth, and fifth thin film transistors T 3 , T 4 and T 5 are turned off, for all the gate voltages of the first, second, and sixth thin film transistors T 1 , T 2 and T 6 Vg=Vst, and for both the source voltage of the second thin film transistor T 2 and that of the first thin film transistor T 1 Vs=VSS, and the drains of the first and second thin film transistors T 1 and T 2 are connected to the source of the sixth thin film transistor T 6 at a connection point, the voltage at which being equal to Vp′. From the above connection relationship, it can be known that, after the third, fourth and fifth transistors T 3 , T 4 , and T 5 are turned off, the voltage Vp′ is pulled down rapidly (refer to the point A shown in FIG. 8 ), so that the circuit enters another steady operating state.

In the operating state, the voltage at node P decreases to Vg−Vp′≧Vth. For the sixth thin film transistor T 6 , the following relationship holds true:

Vgs=Vg−Vp′≧Vth   (4)

Thus, the sixth thin film transistor T 6 will be turned on.

From the above equation (4), it can be obtained

Vg≧Vp′+Vth   (5)

Further, it can be obtained that

Vg−VSS≧Vp′−VSS+Vth   (6)

At this time, for the sixth thin film transistor T 6 , Vgs=Vg−Vp′, Vds=VDD−Vp′, since Vg<VDD, and thus for the sixth thin film transistor T 6 , Vds>Vgs, in turn Vds>Vgs−Vth. It can be known from the above that the sixth thin film transistor T 6 operates in a saturated operating area.

For the first and second thin film transistors T 1 and T 2 , Vgs=Vg−VSS, Vds=Vp′−VSS, thus the following equation holds true for the first thin film transistor T 1 :

Vgs≧Vds+Vth   (7)

Therefore, the first thin film transistor T 1 operates in a linear operating area, and for the same reason, the second thin film transistor T 2 also operates in a linear operating area. It can be known from the above that the first and second thin film transistors T 1 and T 2 operates in a linear operating area during the stage for the OLED emitting light.

Additionally, since VDD>Vg, VDD−Vp′>Vg−Vp′,

wherein VDD represents the positive voltage of the voltage source which drives the OLED to emit light.

The following equation can be obtained from the operating state of the first thin film transistor T 1 :

The following equation can be obtained from the operating state of the second thin film transistor T 2 :

Where I d1 , I d2 represent the drain-source current of T 1 and T 2 when operating in a linear area respectively.

The equation can be obtained from the above (8) and (9):

wherein, Ioled denotes the current flowing through the OLED when the circuit operates,

it can be obtained from the above equations (10) and (11):

It can be obtained from the operating state of T 6 :

The following set of equations can be obtained when the above equations (1), (8) and (13) are combined together:

wherein, Vgs denotes the gate-source voltage of the first thin film transistor T 1 . Since Vgp′=Vgs−Vp′s, in other words, in equations (1) and (8), Vp′s=Vgs−Vgp′, the solution can be obtained by solving the equations (1) and (8):

It can be obtained by substituting (14) into (13):

It can be obtained by substituting

I ds ⁢ ⁢ 1 = k 1 ⁢ Idata k 1 + k 2 and I d ⁢ ⁢ 1 = k 1 ⁢ I oled k 1 + k 2

into (15):

Wherein, k 1 , k 2 and k 6 represent the constants relating to the design size and the process of the first, second and sixth thin film transistors T 1 , T 2 and T 6 respectively.

since

wherein μ,Cox represent the constants relating to the process, W represents the channel width of the TFT, and L represents the channel length of the TFT, wherein W and L can be constants to be optionally designed.

It can be known from the equation (16) that Idata is in direct proportion to Ioled and has a function for amplifying Ioled, and there is a large scale ratio of Idata to Ioled,

Hence, there is a large value of Idata within the operating range of Ioled, and thus it is possible to charge the storage capacitor Cst rapidly.

On the other hand, since a thin film transistor is not an ideal switch, a certain leak current still exists after the third thin film transistor T 3 and the fifth thin film transistor T 5 are turned off, and a variation is thus generated in the grey level voltage signal stored in the storage capacitor Cst during the time of one frame due to the existence of the leak current, thus resulting in a variation of the driving current. The driving circuit for the pixel unit provided by the embodiments of the present disclosure has a negative feedback function for the leak current of the storage capacitor Cst to suppress the distortion of the driving current, and the detailed explanation is given as below:

when there is a leak current in the storage capacitor Cst, if the voltage Vg at the node G decreases due to the leak current, while Vp′ remains unchanged, Vgp′ will decrease. As Vgp′ represents the voltage Vgs of the sixth thin film transistor T 6 , it can be known from the equation (13) that Ioled will decrease, and thus Id 1 will decrease naturally as Ioled decreases.

It can be derived from the equation (14)

that Vgp′ would increase as Id 1 decreases, thus suppressing the decrease of Ioled. Actually, the decrease of the voltage Vg at the node G results in the decrease of Id 1 , and the decrease of Id 1 in turn causes the voltage Vp′ at the node P to decrease, thus suppressing the decrease of Vgp′.

It can be seen that, compared to the prior art, the driving circuit for the pixel unit provided by the embodiments of the present disclosure can enable a relatively large scale ratio of the data signal current Idata to the current Ioled flowing through the light-emitting device, which ensures that Ioled is within the range of the operating current of the light-emitting device and Idata can be a relatively large current, thus expediting the speed for charging the storage capacitor. In addition, the driving circuit for the pixel unit provided by the embodiments of the present disclosure further has an excellent negative feedback function for the leak current of the storage capacitor Cst and thus ensures the stable operation of the circuit.

Additionally, an embodiment of the present disclosure further provides a pixel unit including any one of the driving circuits for the pixel unit provided by the embodiments of the present disclosure as above.

Additionally, an embodiment of the present disclosure further provides a display apparatus including the above pixel unit provided by the embodiments of the present disclosure as above.

With reference to the above analysis, it can be seen that, compared to the prior art, the pixel unit and the display apparatus provided by the embodiments of the present disclosure can enable a relatively large scale ratio of the data signal current Idata to the current Ioled flowing through the light-emitting device, which ensures that bled is within the range of the operating current of the light-emitting device, and Idata can be a relatively large current, thus expediting the speed for charging the storage capacitor. In addition, the driving circuit for the pixel unit provided by the embodiments of the present disclosure further has an excellent negative feedback function for the leak current of the storage capacitor Cst and thus ensures the stable operation of the circuit.

The above descriptions have illustrated the preferable embodiments of the present disclosure, and it should be pointed out that those skilled in the art can make several improvements and modifications on the embodiments without departing from the principle of the present disclosure as described, and such improvements and modifications should be considered within the protection scope of the present disclosure.