KR20070031924A - Active matrix display devices - Google Patents

Abstract

The active matrix display device comprises an array of display pixels, each pixel comprising a current-driven light emitting display element 2, a drive transistor 22 for driving a current through the display element, a required display pixel output level and An optical feedback element 38 is included to control the drive transistor for driving a substantially constant current through the display element for a period dependent on the optical feedback signal of the optical feedback element. An output configuration including the allowed range of values for the pixel power supply voltage, field periods and pixel drive level is applied to the display. The output configuration changes in response to aging of the display element. In this device, the output configuration changes as the device ages so that the optical feedback system can continue to provide compensation for differential aging of the display element for a longer period of display use.

Description

Active Matrix Display Devices {ACTIVE MATRIX DISPLAY DEVICES}

The present invention relates to an active matrix device, and in particular, but not exclusively, to an active matrix electroluminescent display device having a thin film switching transistor associated with each pixel.

Matrix display devices using electroluminescent display elements are well known. The display device may include, for example, a thin film electroluminescent device using a polymer material or a light emitting diode (LED) using a conventional III-V semiconductor composite. Recent developments in organic electroluminescent materials, especially polymeric materials, have shown their ability to be used in practical use in video display devices. This material generally comprises one or more layers of semiconductor composite polymer sandwiched between a pair of electrodes, one of which is transparent and the other suitable material for injecting holes or electrons into the polymer layer. Is manufactured.

1 shows a known active matrix addressed electroluminescent display device. The display device comprises a panel having a row and column matrix array of regularly spaced pixels represented by block 1, which panel comprises an intersecting set of row (selection) and column (data) address conductors 4 and 6. It comprises an electroluminescent display element 2 with an associated switching means, located at the intersection between. Only a few pixels are shown in this figure for simplicity. In practice, there may be hundreds of pixel rows and columns. The pixel 1 is addressed via a set of row and column address conductors by a peripheral drive circuit comprising a row scanning driver circuit 8 and a column data driver circuit 9 connected to an end of each set of conductors. .

Display devices of this type have display elements that are current-addressed. There are a number of pixel circuits for providing controllable current through the display element, which pixel circuit generally comprises a current source transistor, the gate voltage provided to the current source transistor determining the current passing through the display element. The storage capacitor holds the gate voltage after the addressing step.

For circuits based on polysilicon, there is a variation in the threshold voltage of the transistor because of the statistical distribution of polysilicon grains in the channel of the transistor. However, polysilicon transistors are quite stable under current and voltage stresses so that the threshold voltage remains substantially constant.

Variation in threshold voltage is small in amorphous silicon transistors, at least over a short range on the substrate, but the threshold voltage is very sensitive to voltage stress. Application of a high voltage above the threshold necessary for the drive transistor causes a large variation in the threshold voltage, which varies with the information content of the image being displayed. Therefore, there will be a large difference in the threshold voltage of an amorphous silicon transistor that is always on compared to a silicon transistor that is not an amorphous silicon transistor. This differential aging is a serious problem for LED displays driven with amorphous silicon transistors.

In addition to variations in transistor characteristics, there is also a differential age in the LED itself. This is due to a decrease in the efficiency of the luminescent material after current stressing. In most cases, the more current and charge passes through the LED, the lower the efficiency.

There has been a proposal for voltage-addressed pixel circuits to compensate for aging of LED materials. For example, various pixel circuits have been proposed in which the pixels comprise photosensitive elements. The device responds to the light output of the display device and operates to leak charge stored on the storage capacitor in response to the light output, thereby controlling the integrated light output of the display during the addressing period. 2 shows an example of pixel arrangement for this purpose. Examples of pixel configurations of this type are described in detail in WO 01/20591 and EP 1 096 466.

Drive transistor 22 is controlled by a voltage on the gate, stored on capacitor 24 during the addressing step. During the addressing phase, the required voltage is transferred from the column 6 to the capacitor 24 by the addressing transistor 16 which is turned on only during the addressing phase.

In the pixel circuit of FIG. 2, photodiode 27 discharges the gate voltage stored on capacitor 24. When the gate voltage on the driving transistor 22 reaches the threshold voltage, the EL display element 2 will no longer emit, and the storage capacitor 24 will stop discharging. The rate at which charge leaks from the photodiode 27 is a function of the display element output, so that the photodiode 27 functions as a light-sensitive feedback device. Considering the effect of the photodiode 27, the integrated light output is:

It can be seen that given by.

In this equation, η _PD is the efficiency of the photodiode, which is very uniform across the display, C _s is the storage capacitance, T _F is the frame time, V (0) is the initial gate-source voltage of the drive transistor, and V _T is the threshold voltage of the drive transistor. Therefore, the light output is independent of the EL display element efficiency, thus providing aging compensation. However, V _T will change throughout the display, resulting in non-uniformity.

While there is a refinement of this basic circuit, it remains a problem that the actual voltage-addressed circuitry is still sensitive to threshold voltage variations. Thus, the circuit of FIG. 2 will not compensate for the stress induced threshold voltage variation of the amorphous silicon drive transistor. Furthermore, since the capacitor holding the gate-source voltage is discharged, the driving current for the display element will gradually drop. Thus, the brightness becomes dim. This results in lower average light intensity.

Applicants have proposed alternative optical feedback pixel circuits in which the drive transistors are controlled to provide a constant light output from the display element. For aging compensation, optical feedback is used to change the timing of the operation (especially turn on) of the discharge transistor, which operates to quickly switch off the driving transistor. The timing of the operation of the discharge transistor also depends on the data voltage to be applied to the pixel. In this way, the average light output can be higher than the method of switching off the drive transistor more slowly in response to the light output. Thus, the display element can operate more efficiently. Any drift in the threshold voltage of the drive transistor will appear as a change in the (constant) brightness of the display element. As a result, the modified light output feedback circuit proposed by the applicant compensates for variations in output brightness resulting from both LED aging and drive transistor threshold voltage variations.

Known pixel circuits, in particular the proposed pixel circuits summarized above (and described further below), can provide correction for the differential aging of LED display elements of other pixels, but do not extend the life of the display.

1 shows a known EL display device;

2 illustrates a known pixel design that compensates for differential aging.

3 shows a pixel circuit proposed by the applicant.

4 is a timing diagram for explaining the operation of the circuit of FIG.

5 is a modified view of the circuit of FIG.

6 is a timing diagram for explaining the operation of the circuit of FIG.

7 illustrates device characteristics for the circuit of FIG. 6 for the purpose of describing the operation of the circuit of FIG. 6 in more detail.

8 shows pixel output for one field;

9 illustrates how pixel output cannot be corrected after a more severe aging effect.

10 illustrates how pixel output performance changes over time.

Figure 11 shows a modified pixel circuit of the present invention.

12 shows a first example of a modified thermal circuit for implementing the invention.

13 is a timing diagram for explaining the operation of the circuit of FIG.

14 shows a second example of a modified thermal circuit for implementing the invention.

15 is a timing diagram for explaining the operation of the circuit of FIG.

16 shows a third example of a modified thermal circuit for implementing the invention.

17 is a diagram used to describe an alternative circuit operation of the present invention.

18 illustrates a further example of a pixel circuit that may be modified by the present invention.

19 shows how an amorphous silicon circuit similar to that shown in FIG. 3 can be modified in accordance with the present invention.

According to the invention, there is provided an active matrix display device comprising an array of display pixels, each pixel:

Current-driven light emitting display elements;

A driving transistor for driving a current through the display element;

A pixel circuit comprising an optical feedback element for controlling said drive transistor for driving a substantially constant current through said display element for a period of time dependent upon the required display pixel output level and an optical feedback signal of the optical feedback element; And

Control means for applying an output configuration for the display, the output configuration including at least a permitted range of values, field periods, and pixel drive levels for the pixel power supply voltage, the control means aging the display element; control means adapted to change the output configuration by changing one or more of the values in response to aging.

In this device, the output configuration changes as the device ages, such that the optical feedback system can continue to provide compensation for differential aging of the display element for a longer period of use of the display.

The pixel circuit may include a storage capacitor for storing a voltage to be used for addressing the driving transistor and a discharge transistor for discharging the storage capacitor to switch off the driving transistor. The light-dependent device then controls the timing of the operation of the discharge transistor by changing the gate voltage applied to the discharge transistor in accordance with the light output of the display element. This duty cycle control scheme enables the display element to operate at substantially full brightness, which in turn allows the field period to be reduced to the minimum value desired for larger displays.

A discharge capacitor can be provided between the gate of the discharge transistor and a constant voltage line, and then the light dependent device is for charging or discharging the discharge capacitor.

Each pixel may further include a charging transistor connected between the charging line and the gate of the driving transistor, and each pixel may further include an isolating transistor connected in series with the driving transistor.

In one arrangement, power lines are provided for each column of pixels. For example, different power lines can be provided for columns of different color pixels. Such vertical power lines can also be used for monitoring purposes to monitor the aging of display elements. For example, each pixel may further comprise a read transistor for enabling detection of the state of the drive transistor from a column conductor. By detecting the state of the drive transistor at the end of the field period, it may be determined whether the optical feedback system has turned off the drive transistor. If not, this indicates aging of the display element to such an extent that the current operational characteristics of the display do not allow accurate compensation to occur.

In one arrangement, each pixel further includes a read transistor for enabling detection of the state of the drive transistor from the column conductor.

Alternatively, each column of pixels further includes a read transistor for enabling detection of the state of the drive transistor in the column.

The invention also provides a method of driving an active matrix device comprising an array of display pixels, each of the active matrix display devices comprising a pixel circuit comprising a drive transistor, a current-driven light emitting display element, and an optical feedback element. Including,

The method is:

(i) applying an output configuration for the display, the output configuration comprising an allowed range of values, field periods, and pixel drive levels for at least the pixel power supply voltage;

(ii) addressing each pixel by controlling the drive transistor to drive a substantially constant current through the display element for a period dependent on the required display pixel output level and the optical feedback signal of the optical feedback element; And

(iii) monitoring the aging of display elements of the array, changing the output configuration by changing one or more of the values in response to aging of the display elements, and (i) and (ii) for the modified output configuration. Repeating).

The invention will now be described by way of example with reference to the accompanying drawings.

It should be noted that these figures are schematic and not drawn to scale. The relative sizes and ratios of these figures have been exaggerated or reduced in size for clarity and convenience in the figures.

The pixel circuits already proposed by the applicant (but not yet published at the time of filing this invention) will be described first. In this pixel circuit, the driving transistor is driven with a constant gate voltage for a given frame, and the period during which the display element is illuminated takes into account the aging effect of both the LED material and the driving transistor as well as the output of the required brightness.

3 shows an example of the proposed pixel arrangement. The pixel circuit is for use in a display as shown in FIG. The circuit of Figure 3 is suitable for implementation using amorphous silicon n-type transistors.

The gate-source voltage of the drive transistor 22 is again maintained on the storage capacitor 30. However, this capacitor is charged to a fixed voltage from the charging line 32 by the charging transistor 34. Thus, when the display element is to be illuminated, the drive transistor 22 is driven to a constant level independent of data input to the pixel. The brightness is controlled by changing the duty period, in particular by changing the time when the drive transistor is turned off.

The driving transistor 22 is turned off by the discharge transistor 36 which discharges the storage capacitor 30. When the discharge transistor 36 is turned on, the capacitor 30 is quickly discharged and the driving transistor is turned off.

When the gate voltage reaches a sufficient voltage, the discharge transistor is turned on. The photodiode 38 is illuminated by the display element 2 and generates a photocurrent in accordance with the light output of the display element 2. This photocurrent charges the discharge capacitor 40, and at some point in time, the voltage across the capacitor 40 will reach the threshold voltage of the discharge transistor 40, thereby switching on the discharge transistor. This time will depend on the charge originally stored on the capacitor 40 and on the photocurrent, which will therefore depend on the light output of the display element.

Thus, the data signal provided to the pixel on the data line 6 is provided by the address transistor 16 and stored on the discharge capacitor 40. Low brightness is represented by a high data signal (as a result only a small amount of additional charge is needed for the transistor 36 to switch off), and high brightness is represented by a low data signal {as a result a large amount of additional Charge is needed for the transistor 36 to switch off.

Thus, this circuit has optical feedback to compensate for aging of the display element, and also has threshold compensation of the drive transistor 22, because variations in drive transistor characteristics will also result in differences in device element output. This display element output is again compensated by light feedback. For transistor 36, the gate voltage above the threshold is kept very small, so that the threshold voltage variation is much less important.

As shown in FIG. 3, each pixel also has a bypass transistor 42 (T3) connected between the source of the drive transistor 22 and the bypass line 44. This bypass line 44 may be common for all pixels. When the storage capacitor 30 is being charged, it is used to ensure a constant voltage at the source of the drive transistor. This eliminates the dependence of the source voltage on the voltage drop across the display element, which is a function of the flow of current. Thus, the fixed gate-source voltage is stored on the capacitor 30 and when the data voltage is being stored in the pixel, the display element is turned off.

4 shows a timing diagram for the operation of the circuit of FIG. 3 and is used to further explain the circuit operation.

The power line has a switched voltage applied to it. Plot 50 shows the voltage. While writing data to the pixel, power supply line 26 is switched low, resulting in drive transistor 22 being turned off. This enables the bypass transistor 42 to provide a good ground reference.

The control lines for the three transistors 16, 34, 42 are connected together, and when the power supply line is low, all three transistors are turned on. This shared control line signal is shown as plot 52.

Turning on the transistor 16 has the effect of charging the discharge capacitor 40 to the data voltage. Turning on transistor 34 has the effect of charging storage capacitor 30 from charge line 32 to a constant charging voltage, turning on transistor 42 bypasses display element 2 and drives it. It has the effect of fixing the source voltage of the transistor 22. As shown in plot 54, data (hatched areas) is applied to the pixels during this time.

The circuit is an n-type only arrangement and is therefore suitable for amorphous silicon implementation.

FIG. 5 shows n-type and p-type circuits suitable for implementation using low temperature polysilicon processes and using n-type and p-type devices.

The drive transistor 22 is implemented as a p-type device. Since the source is now connected to the power supply line, the storage capacitor 30 is connected between the power supply line 26 and the gate of the driving transistor 22. Similarly, the discharge transistor 36 is a p-type device, so that the discharge capacitor 40 is connected between the power supply line 26 and the gate of the transistor 36. In this circuit, charge is removed from the capacitor 40 by the photodiode 38, causing a drop in the gate voltage of the discharge transistor 36 until the discharge transistor 36 is turned on.

The charging transistor 34 is also a p-type device and is connected between the gate and the ground of the driving transistor 22. Thus, the charging operation performed by the transistor 34 is to charge the capacitor until the maximum power supply voltage is across it. This keeps the gate of the drive transistor 22 at ground and turns the drive transistor to the maximum (since it is a p-type device).

Thus, basically, this circuit operates in the same way as the circuit with an adaptation to allow the use of p-type transistors.

The blocking transistor 62 allows the display element 2 to be turned off during the addressing step, so that black performance is preserved. In FIG. 5 this may of course be an n-type device, but this is a p-type device.

As shown in FIG. 6, when the p-type transistor 62 is low, the gate control signal 56 turns on the p-type transistor 62, and when the transistor passes an addressing period, the transistor 62 ) Is turned off, while transistors 16 and 34 are turned on (by a reverse signal of 56).

The overall lifetime of an OLED display remains the most important factor for this type of display, especially for blue LED pixels. Therefore, any treatment that enables extended life is important.

The present invention relates to the control of pixel circuits of the type described above during their lifetime in order to obtain an extended lifetime while maintaining the benefits of compensated differential aging. The main factors affecting display life are supply voltage, frame time and data voltage range. The present invention relates to the control of these parameters to obtain the best possible display life with minimal differential aging.

The present invention extends the life of a display using an optical feedback compensation system, but determines when the optical feedback system has reached its limit of its correction capability, and then changes the output configuration for the display. This output configuration includes values for the pixel power supply voltage, the field duration of the pixel drive level, and the allowed range. By changing one or more of these parameters, the correction capability is extended.

To illustrate the method and circuit modification of the present invention, it is useful to analyze the operation of the circuit described above in more detail. For this purpose, FIG. 7 shows the circuit of FIG. 6 with device values indicated for analysis purposes. Subscript 1 is associated with drive transistor 22 (and denoted as T _D ) and subscript 2 is associated with discharge transistor 36 (and denoted as T _S ).

The current provided to the OLED by T _D can be written as I ₁ = f (V ₁ , V _DS ), the luminance of the OLED is L = η _LED I ₁ / A _LED , and η _LED is the Efficiency, A _LED is a region of pixel aperture. Since T _S is a perfect switch, I ₁ = H (V ₂ -V _T2 ), where H can be assumed to be a step function that is zero until V ₂ is equal to V _T2 . The differential equation describing the circuit operation is given by equation (2).

The first part of the equation pair comes from the discharge of capacitance C ₁ , the second part is an η _LED whose efficiency is in units of A / Cd, and by a photodiode having an area APD From the charging of C ₂ . Since H is a step function, this connected equation can be easily solved. The solution for V ₁ is simply:

And t _ON is the time for the circuit to emit light as shown in FIG. 8.

Since V ₁ (t) is fixed until t _ON is reached, V _DS (t) can also be found.

V _P is the supply voltage, V _LED is the OLED anode voltage, and V _TLED is the threshold voltage of the OLED. This can be easily solved for V ₂ .

Then, since this will be the time T _S is switched on, that is, V ₂ (t) = V _T2 , t _ON can be found. The average luminance of the circuit is:

T _F is given by frame time. Therefore, when t _ON <T _F ,

When it is assumed that T _S is a perfect switch, this shows that the circuit has nothing to do with the efficiency of the OLED and the parameters of the driving TFT (T _D ). Parameters that can be used to control the brightness are voltage {V ₂ (0)} and frame time (T _F ).

If t _ON > T _F , however, an error occurs in the circuit's differential aging correction capability. In this case the luminance error is:

This value is positive, ie, because the end of the frame time has been reached, as shown in Figure 9, this circuit provided too much illumination.

This error needs to be less than or equal to zero, that is, ΔL ≤ 0,

Assumptions can be made for the driving TFT T _D. When T _D is driven in its linear region, the lowest power consumption of the circuit can be achieved:

Can be assumed, β is the trans-conductance parameter of T _D. Simple model for OLED, i.e.

Assuming

Replace with in Equation 4:

or

τ is

Is the time constant given by.

As the OLED ages, η _LEDs and α will decrease, which reduces the τ and thus the initial OLED voltage, which is necessary to provide sufficient brightness within the frame time and to ensure that the circuit turns off within the frame. Will increase. Since T _D is in its linear region, the power supply will be slightly above the OLED voltage. Therefore, the power supply or frame time needs to be increased, or the data voltage range needs to be reduced as the OLED decays.

Pixel usage for the AMPLED display is shown in FIG. 10. This shows the probability of using pixels on the lifetime P (T _P ) = T _P / T _max over time T. T _P is the overall pixel on-time and T _max is the maximum possible time for the pixel to be on. Three plots show the probability of any pixel having a given on-time, and each plot represents a pixel for a display of a different age.

The expansion in pixel usage (i.e. pixel on-time) at the beginning of the display life T1 is very small and therefore the visible effects of burn-in will be negligible. On the lifetime of the display (T2 then T3), the distribution will be wider and the burn-in effect will be more severe.

This shows that the effects of burn-in (ie, differential aging of LED display elements) will not be significant at the beginning of the display life, so that the optical feedback compensation scheme will not require the maximum frame period to perform differential aging compensation. will be.

As a result, at the beginning of the display life, the display can be operated at low power supply V _P , and burn-in will not occur. This will reduce heating and therefore reduce OLED decay. As the display ages, the expansion in pixel usage will become more serious and a correction process of the light feedback will be needed. this is:

(A) increasing the power source (as a result sufficient light output can be provided within the field period); And / or

(B) increasing the frame time (as a result more time is available to provide a compensated integrated light output for all pixels), and / or

(C) would require reducing the data voltage range (as a result no pixels are driven to full brightness output).

The treatment A will enable constant luminance during that lifetime, resulting in greater heat generation and thus shorter lifetime. Processing (B) and (C) will reduce the brightness over the lifetime without burn-in. For example, by increasing the frame time, the frame rate will be reduced and, of course, the average light output will be reduced, which can also result in flicker.

The present invention involves adjusting the supply voltage, and / or frame time and / or data voltage range during the lifetime of the display so that differential aging compensation can be achieved for the extended lifetime of the display.

In a preferred embodiment, the power lines are arranged to run vertically with separate power supplies for the red, green and blue display elements. Each power source can be adjusted for the voltage operation of each color, thus reducing overall power consumption and improving lifetime.

In order to implement control of display operating characteristics, the distribution of pixels in the display needs to be determined. For a display with a vertical power supply line, this can be accomplished by sensing the state of the voltage on the storage capacitor with respect to the drive transistor gate voltage C ₁ in FIG. 7.

If C ₁ is fully charged at the end of the field period, there is not enough light to turn off the pixel. In this case, the present invention recognizes that it is necessary to increase the power supply voltage, increase the frame time, or reduce the data voltage range for this pixel.

The present invention involves sensing the state of every pixel and then determining whether any one of the three processes needs to be implemented.

FIG. 11 illustrates a modification to the pixel circuit of FIG. 7 that allows the conduction state of the drive transistor to be sensed and then provides an indication of the voltage on C1. The pixel circuit includes an additional transistor 70, which is gated by the same control line as the blocking transistor 62, but operates in a complementary manner. This circuit allows the state of the voltage on C1 to be sensed from the column and requires one additional TFT, but no additional column or address line.

Transistor 70 is connected in series with the drive transistor, and there is a connection to the power supply line through the drive transistor, which can be detected if the drive transistor is turned on.

Transistor 70 is turned on only when a particular row of pixels is being addressed. Thus, within any column, only one pixel has a transistor 70 that is always turned on, and the state of C ₁ can be determined for individual pixels.

FIG. 12 shows the sensing circuit in the column driver, and FIG. 13 shows the timing of the pixel driver line and column driver switches M1, M2 and M3 of FIG. 12 with the high state closed.

Just before the pixel is addressed (ie at the end of the previous field period), the column is pre-charged to a low voltage by closing the switch M3.

M3 is then opened and M2 is closed to measure the state of the thermal voltage. If C ₁ is not discharged, the heat will be charged to high voltage because the driving TFT is on, whereas C ₁ will be discharged, and then heat will be kept at low voltage since the driving TFT is OFF. Thus, charging the thermal voltage represents an on drive transistor, which indicates that the optical feedback system could not provide full correction.

The state of the column is then stored in memory. M2 is then opened and M1 is closed so that the column is charged to the next data voltage. The normal addressing step follows, and the present invention is implemented as an additional step in an addressing period having a period corresponding to the period of the control pulse for M2. This period should simply be sufficient for charging the thermal capacitance by the power line passing through the on drive transistor and may be on the order of several microseconds.

At the end of the field time all pixels will be sensed and a number of schemes can be used to control the display parameters in response to the date collected.

In one way, if any pixel has a storage capacitor C ₁ , which is not discharged at the end of the field time, a correction process is performed. As summarized above, this correction process is:

(i) increasing the power line voltage by ΔV after each field until no heat is detected high, and / or

(ii) increasing the frame time by ΔV after each field until no heat is detected high, and / or

(iii) reducing the data voltage range by ΔV after each field until no heat is detected high.

In an alternative control scheme, correction processing can be used only when the pixels exceeding the predetermined number N have an undischarged capacitor C ₁ at the end of the field time.

A correction scheme based on individual pixels does not allow any burn-in, but this may be undesirable as there may be pixel errors. Thus, a correction scheme that allows for the level of burn-in specified by the predetermined number N is desirable.

14 illustrates another method for achieving pixel state sensing and requires an additional transistor 80 per column. The low potential line for the pixel is arranged to run parallel to the column, and an additional transistor 80 selectively connects the low potential line to the low potential voltage source (ground).

In this arrangement, the low potential line can be low pre-charged. During the sensing operation, this line is disconnected from the low voltage source by the transistors so that the voltage on the line is monitored. In this arrangement, if the storage capacitor C ₁ is discharged, the discharge transistor T _S is used to charge the low potential column line high. If capacitor C ₁ is discharged, this is because the optical feedback system has turned on the discharge transistor. As a result, there is a conductive path from the power supply line through the discharge transistor and the charge transistor 34 (which is on for the field period).

In this case, the discharge transistor T _S will be the threshold voltage, so that the charging time will be very long. Therefore, this method is best implemented when sufficient time is available, for example whenever the display is turned off.

A timing diagram for sensing is shown in FIG. 15 for the case where heat is charged with the high voltage generated when C ₁ is discharged. 15 shows the case where a pixel is immediately addressed after sensing. This arrangement again allows the state of each pixel discharge transistor to be determined for the maximum addressing period of the display.

The storage of the thermal state in the circuit can be performed in analog or digital mode.

16 illustrates an analog implementation. If M2 is closed (see FIG. 12), if the heat is charged high, current will flow through transistor T _M. Any other row that passes high will also cause a current to flow through T _M for that column, so that the current on the measurement line (if shared between all columns) will be the sum of all the rows that go high. This represents the analysis for the combination of pixels within a row. A value corresponding to this current can be stored and accumulated with the current generated for every other row in the display. The resulting value can then be used to adjust power and frame time and the like.

The digital method can use a latch at the output of the column driver shift register to store the sensed value on the column and generate a clock. This value is then accumulated and input into the decision logic to adjust the appropriate parameter.

In the above example, the sensing function is described as occurring just before the line is re-addressed. This can be extended to any time in the frame period. For example, it may be desirable to limit the duty cycle of the LED display device so that it does not exceed 50%. By emitting the display elements with higher brightness and with shorter duty cycles, the lifetime of the display can also be extended. In this case, the sensing function may occur in the middle of the field period, during part of the field period when there is no light output.

If each addressing step includes a sensing period, any line (eg, row conductor) can be used for sensing, while other lines are used for addressing. The addressed and sensed lines can be controlled by a row driver with two outputs as shown in FIG.

17 shows a row driver 8 with two outputs A and B. At any time, one output A is used to address the pixel row and the other output B is used for the sensing function. The two outputs are staggered from each other by a fraction of the field period so that the sensing operation occurs after the illumination of the pixels in the row is complete.

As shown in the timing diagram, the addressing period 82 for each row includes two parts. One portion 84 is used for the sensing function and the other portion 86 is used for the addressing function.

During the sensing operation, the thermal conductor is initially high impedance (“high Z”), but is driven low to ensure that the pixel is off. During the pixel addressing operation, the row pulse 86 corresponds to the usual timing of the data signal on the column conductor. During each field period, each column is therefore used twice, once for detection and once for addressing.

The preferred implementation described above uses vertical power lines. However, horizontal power lines can also be used. In this case, the current flowing on the horizontal power line can be sensed at an appropriate time, and the adjustment is made in the same manner as described above.

The above description relates to the implementation of the present invention in one particular optical feedback pixel design. There are various possible alternative implementations of the optical feedback system to which the invention can be applied.

FIG. 18 shows a modification to the pixel circuit of FIG. 17, in which an additional transistor 90 is provided between the gate and the ground line of the discharge transistor 36, and an optical feedback system is used to switch off the display element. Increases discharge when in operation.

If the circuit was switched off, the TFT 90 makes it possible to drive the heat low, so the circuit shown in Fig. 18 can also be used for sensing.

Although an example of the amorphous silicon optical feedback circuit is shown in Fig. 3, the example of the present invention uses a polysilicon driving TFT. One variant of FIG. 3 connects the photodiode to the charging line 32 such that the power supply line 26 is only connected to the drive transistor 22. This allows the power supply line 26 to be switched so that the display element can be turned off during the addressing step. This blackens the contrast of the pixel drive. This also allows the bypass transistor to be omitted. An implementation of the present invention for this type of circuit is shown in FIG. 19.

FIG. 19, with the modification outlined above, corresponds essentially to FIG. 3, wherein an additional transistor switch 100 is connected between the anode and column lines of the display element to enable the sensing operation to be performed.

In the above example, the control parameter includes a power supply voltage. This may be the voltage provided to the power supply line 26, but control of the display may also be achieved by modifying the voltage on the charging line 32. This charging line voltage is one of the pixel power supply voltages. Thus, the pixel power supply voltage includes the charge line 32 voltage (where it is separate from the main power supply line) and the power supply line 26 voltage.

The above example is a common-cathode implementation, where the anode side of the LED display element is patterned, and the cathode side of all LED elements share an electrode that is not common patterned. This is a presently preferred implementation as a result of the materials and processes used in the manufacture of LED device arrays. However, patterned cathode designs are being implemented, which can simplify pixel circuitry.

In the above example, optical feedback is used to compensate for the aging of the LED material and the drive transistor. In the case of amorphous silicon driving transistors where the variation in threshold voltage is very large, some electrical threshold voltage compensation may be required. This can be accomplished by maintaining the gate-source voltage for the drive transistors on the two capacitors in series, the storage capacitor and the threshold capacitor. A discharge capacitor for turning off the discharge transistor is arranged to short the storage capacitor. This circuit can then provide the drive voltage level on the storage capacitor 30 and store the drive transistor threshold voltage on the threshold capacitor.

There are many variations and refinements to the optical feedback system described above.

In this example, the light dependent element is a photodiode, but the pixel circuit can be designed using phototransistors or photoresistors. Circuits using various transistor semiconductor technologies are shown. For example, many variations are possible, such as crystalline silicon, hydrogenated amorphous silicon, polysilicon and even semiconductor polymers. This is intended to be all within the scope of the invention as claimed. The display device can be an LED device, an organic LED device, phosphors including materials, and other light emitting structures.

Adjustment to the display configuration may be to change the configuration for every pixel. For example, when the frame time changes, this will be appropriate. However, adjustments to the display configuration may be for individual groups of pixels, especially for columns of pixels. Thus, different power supply voltages can be applied to different columns. This variation in voltage may require that the image data be processed. In particular, the aging of LED display elements may not have a linear effect over all output levels, and one function may need to be applied to the pixel data for the adjusted columns. Instead, the voltage change can be performed for complete display, in which case pixel data processing may not be required.

One or more of the processes described above can be applied in any combination to change the output configuration.

The control means for changing the display operating characteristics can be of conventional design and will control the voltage and / or timing operation of the row and column addressing circuitry, which control means are schematically shown in FIG. 1 with reference numeral 10. . For implementations where the voltage level is changed, conventional circuitry can be used to adjust the power supply level, eg, column driver power supply, display power supply, or pixel charging line power supply level.

The sensing operation of the display configuration and the implementation of the control will be apparent to those skilled in the art.

Various other variations will be apparent to those skilled in the art.

The present invention is applicable to active matrix devices, and in particular, but not exclusively, to active matrix electroluminescent display devices having thin film switching transistors associated with each pixel.

Claims (18)

An active matrix display device comprising an array of display pixels, wherein each pixel is A current-driven light emitting display element 2; A drive transistor 22 for driving a current through the display element; An optical feedback element 38 for controlling the driving transistor for driving a substantially constant current through the display element for a period dependent on the required display pixel output level and the optical feedback signal of the optical feedback element. Pixel circuits; And Control means for applying an output configuration for the display, the output configuration comprising at least a value for the allowed range of the pixel power supply voltage, the field period and the pixel drive level, wherein the control means Control means (10) adapted to change the output configuration by changing one or more of the values in response to aging of the device. 2. The active matrix display device of claim 1, wherein the pixel circuit comprises a storage capacitor (30; C ₁ ) for storing a voltage to be used for addressing for the drive transistor (22). 3. The pixel circuit according to claim 2, wherein the pixel circuit includes a discharge transistor 36 (T2) for discharging the storage capacitor to switch off the driving transistor 22, and the optical feedback element 38 comprises the display element. An active matrix display device for controlling the timing of the operation of the discharge transistor (36; T2) by changing the gate voltage applied to the discharge transistor in accordance with the light output of (2). 4. An active matrix display device according to claim 3, wherein the optical feedback element (38) controls the timing of switching of the discharge transistor (36; T2) from an off state to an on state. 5. Active matrix display device according to claim 3 or 4, wherein the optical feedback element (38) comprises a discharge photodiode. A discharge capacitor (40; C ₂ ) is provided between the gate of said discharge transistor (36; T2) and a constant voltage line, and said optical feedback element is provided with said discharge capacitor. To charge or discharge the active matrix display device. 7. Active matrix display device according to any of the preceding claims, wherein the drive transistor (22) is connected between a power supply line (26) and the display element (2). 8. Active matrix display device according to claim 7, wherein the storage capacitor (30; C ₁ ) is connected between the gate and the source of the drive transistor (22). 9. An active matrix display device according to any of the preceding claims, wherein each pixel further comprises a charging transistor (34) connected between the gate and the charging line of the driving transistor. 10. The active matrix display device according to any one of the preceding claims, wherein each pixel further comprises an isolating transistor (62) connected in series with the drive transistor (22). The active matrix display device of claim 1, wherein a power line is provided for each column of pixels. The active matrix display device of claim 11, wherein different power lines are provided for columns of different color pixels. 13. Active according to any of the preceding claims, wherein each pixel further comprises a readout transistor (70; 100) for enabling detection of the state of the drive transistor (22) from a thermal conductor. Matrix display device. 13. An active matrix display device according to any of the preceding claims, wherein each column of pixels further comprises a read transistor (80) for enabling detection of the state of the drive transistor in the column. 15. Active matrix display device according to any of the preceding claims, wherein the current-driven light emitting display element (2) comprises an electroluminescent display element. A method of driving an active matrix device comprising an array of display pixels, the method comprising: In the method of driving an active matrix display, each display pixel comprises a pixel circuit comprising a driving transistor 22, a current-driven light emitting display element 2, and an optical feedback element 38. (i) applying an output configuration for the display, the output configuration comprising at least a permitted range of values, field periods, and pixel drive levels for the pixel power supply voltage; (ii) controlling the drive transistor 22 to drive a substantially constant current through the display element 2 for a period dependent on the required display pixel output level and the light feedback signal of the optical feedback element 38. Addressing each pixel; And (iii) monitoring the aging of the display of the array, changing the output configuration by changing one or more of the values in response to aging of the display element, and (i) and (ii) for the modified output configuration. Repeating the method of driving an active matrix device. 17. The method of claim 16, wherein monitoring the aging of display elements of the array comprises monitoring an on or off state of the drive transistor 22 at the beginning or end of a field period. Way. 18. The method of claim 17, wherein if more than a predetermined number of drive transistors (22) are turned on at the end of the field period, the output configuration is changed.