CA2773699A1 - External calibration system for amoled displays - Google Patents

Abstract

Disclosed is a system consist of measurement algorithm, measurement circuits, and data processing for improving the display uniformity.

Description

FIELD OF THE INVENTION
The present invention generally relates to improving the spatial and/or temporal non-uniformity of a display.
SUMMARY OF INVENTION
The disclosed techniques provide fast and accurate measurement of the panel non-uniformity and correct for it.
ADVANTAGES
It can help improve the display uniformity and lifetime despite instability and non-uniformity of individual devices and pixels.

The current-comparator used to compare the pixel current against a reference current is shown in Figure 1. The functionality of the different blocks are listed as following 1. The front-end stage which provides the following features:
a. Fully differential implementation for low-noise performance.
b. Low input impedance.
c. Leakage and noise cancellation.
d. Insensitive to clock timing jitter for higher accuracy.
e. No need for separate timing to set the panel-line voltage.

2. The Pre-Amp stage:
a. Offset and flicker noise cancellation.

3. Quantizer a. Single bit implementation for inherent linearity.

4. Slew Enhancement Circuit a. Class-B operation for extremely low-power consumption.
Figure 2 illustrates the timing of current comparator (CCMP) driving scheme with respect to pixel programming sequence in a frame time. Prior to the measurement of a pixel, the reference current sources integrated in the EIC are programmed. The device under test (OUT) which is a pixel of interest in this case (either drive TFT or OLED of the sub-pixel) is programmed and, subsequently, in-pixel compensation is carried out in advance of a measurement. In-pixel compensation and programming of the reference current sources can take place in parallel.
Pixel measurement starts by activating CLK1 and CLK2jcAL, 1= While both clock phase are high, current comparator prepares itself for a measurement by removing offset and noise of its internal circuitry.
CCMP is ready to accept the pixel current when Phi3 goes down (tpix). The pixel current from the panel that flows in through the EIC switch matrix is sampled at the falling edge of Phil. While CLK1 and CLK2 are both low, the reference current is applied to the current comparator (tu(G). At the rising edge of CLK3, a comparison is performed between the pixel current and the reference current. Momentarily, a digitized output corresponding to the comparison result is ready to be sampled during t .LCH= The digital outputs from 24 readout channels are then loaded into a parallel-to-serial converter which will be shifted out to the FPGA in serial format.

Figure 3 shows a state machine for extracting the pixel values based on the output of a current comparator.
Here the output of the current comparator (CMP) goes to 0 if the pixel current is larger than a reference current and the output goes to '1' otherwise.
The state machine can be a normal state machine in which increases the pixel values by a step if the CMP is '1' and reduce the pixel value by a step if CMP is '0'. Here another state is added to the normal state machine for searching larger deviation in pixel values using search algorithm.
In two cases, it is faster to use the search state: for initial calibration of the panel and when the change in the pixel value is too large.
Here, when the step size for calibration passes a certain threshold while the system is in state S. the system goes to the search state. One way of implementing search is to divide the step value after each measurement to get closer the real value.
For example, if the real value is 35 and the initial pixel value is 128 and the step value is 64: the following steps will occurs 128 > 35 4 pixel value = 128-64= 64 and step = 32 64>35 4 pixel value = 64-32 = 32 step = 16 32<35 4 Pixel value = 32+16 = 48 step = 8 48>35 4 Pixel value = 48-8 = 40 step = 4 40>35 4 Pixel value = 40-4 = 36 step = 2 36>35 4 Pixel value = 36-2 = 34 step = 1 34<35 4 Pixel value = 34+1= 35 The video data coming into the MaxlifeTM system is modified to compensate for non-uniformities due to aging of TFT and OLED materials in each sub-pixel. A block diagram of the data path is shown in Figure 4. Incoming video data, i.e. GreyscaleIN, is converted to an equivalent current, i0LED, using the "Gamma Correction" module. This current is adjusted by multiplying by a scaling factor provided by the "OLED
Compensation" module, which accounts for any aging the OLED has experienced.
This adjusted current, i'OLED, is used by the "Hysteresis Compensation" module, which corrects for the effects of changes in greyscales of a sub-pixel from one frame to the next. The resulting current, icomp, is input to the "TFT
Compensation" module which performs a look-up to determine the greyscale to get the required current by taking in account the mobility and TFT aging of each sub-pixel being programmed. In addition to the TFT aging compensation, the "Dynamic Effect" of VOLED on the programming of the sub-pixel's drive transistor is added at the end.

This module maps incoming greyscale to the required pixel luminance. The OLED
converts current to light; therefore, the luminance output of the OLED can be defined by a current. Most monitors, printers and the internet use the sRGB standard colour space, which has a gamma of 2.2.
Using a luminance meter, the luminance for every input greyscale from 0 to 255 (for 8-bit input) is measured to produce a panel luminance response curve. The video input into the system has a Gamma of 2.2, complying to the sRGB standard. Each OLED colour may result in a different luminance response curve.
The gamma correction is performed digitally by adjusting the input greyscale using a look-up table. The incoming greyscale is modified to correct the luminance values such that the panel displays a gamma of approximately 2.2. Effectively, the gamma correction table pre-distorts the input greyscales such that the greyscale to luminance gamma curve is approximately 2.2. Since each colour has a different luminance response there are 3 look-up tables in total, one for each colour.
For example, it could translates 8-bit input values to 10-bits output values representing a current.
OLED devices age when they are conducting current. As a result of this aging, the required voltage applied to the anode of the OLED for a given current increases, i.e. VOLED
increases, and the amount of current required to emit a given luminance also increases, i.e. OLED
efficiency decreases. There is a relationship between changes in VOLED and efficiency. The relationship between VOLED and the OLED
efficiency is dependent on the level of stress used to age the OLED;
therefore, the OLED stress history is needed to accurately correlate the change in VOLED to the change in OLED
efficiency. The "OLED
Compensation" module takes the AVOLED and stress history of the sub-pixel and calculates the new current required for the desired luminance. The method of using this information is called Adaptive Compensation Technology (ACT). More information on ACT can be found at www.google.com/patents/20110191042.pdf.
The stress history for each sub-pixel is determined by looking at the video content being displayed and maintaining some kind of averaging of the data. The averaging can be calculated with the formula such as:
kavg x S(ti ¨ 1) + I(ti) St(ti) =
kavg(VOLED) + 1 Where St is the average stress value, I is the present stress value, and kavg is a function of VOLED.
TFT hysteresis is the dependence the current of the TFT has on the previous voltage biasing conditions.
For example, if the transistor gate voltage is high and then is dropped to a mid voltage, the current is greater than if the gate voltage was low and raised to the same mid voltage value. Since OLED's are current driven devices, this difference in current will be visible on the panel. A typical test case is to have a checkerboard pattern of black and white squares, and then put a mid-greyscale flat-field. The squares which were white will be brighter than the squares which were black. To compensate for hysteresis, an adjustment to the pixel current is made based on the bias history compared to the present stress value.
Since pixel stress history is being calculated for OLED compensation, the results of this stress history module can also be used in hysteresis calculations.
!comp = H x i'OLED
H = f(St ¨ i'OLED) The value of H is a function of the difference between stress history, St, and the compensated OLED
current, i'OLED. If the stress history and the present stress value are the same, then H = 1, which leaves the value the same because no hysteresis will occur. When the difference is negative, then H> 1, i.e.
present stress value is greater than the historic value because it went from a low greyscale to a high greyscale and needs to be increased a little. Similarly when positive, H < 1.
The input to this module is the required OLED current. This current is mapped to a greyscale, but this mapping must take in account the aging and mobility of the TFT. TFT aging occurs when the TFT
undergoes voltage biasing stress and results in a voltage threshold shift, AVt. This voltage threshold shift and mobility are stored for each sub-pixel. As shown in Figure 5, a static table is used to select the current-to-greyscale (or voltage) for a given mobility and voltage threshold for each sub-pixel. For getting exact curve for a given mobility and VT, one can use either nearest-neighbor, bilinear, or bicubic techniques. The greyscale required for programming is then determined from this relationship.
The VG programmed on the TFT is affected by VOLED. Its relationship is dependent on the ratio between the gate-to-drain capacitance (Cl) and the gate-to-source capacitance (CGA).
As AVOLED increases, the amount of extra voltage programmed needs to be increased.