Patent Application
20110080441
Not Applicable
1. Field of the Invention
The present invention relates to organic light emitting diodes (OLEDs). In particular, the present invention relates to a pulse mode OLED pixel or sub-pixel driver.
2. Description of Prior Art
An OLED device typically includes a stack of thin layers formed on a substrate. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between a cathode and an anode. The light-emitting layer may be selected from any of a multitude of fluorescent and phosphorescent organic solids. Any of the layers, and particularly the light-emitting layer, also referred to herein as the emissive layer or the organic emissive layer, may consist of multiple sublayers.
In a typical OLED display, either the cathode or the anode is transparent or semitransparent. The films may be formed by evaporation, spin casting, chemical self-assembly or any other appropriate polymer film-forming techniques. Thicknesses typically range from a few monolayers (i.e., a single, closely packed layer of atoms or molecules, perhaps as thin as one molecule), up to about 1000 to 2,000 angstroms.
Protection of an OLED display against oxygen and moisture can be achieved by encapsulation of the device. The encapsulation can be obtained by means of a single thin-film layer surrounding the OLED situated on the substrate.
High resolution active matrix displays may include millions of pixels and sub-pixels that are individually addressed by the drive electronics. The drive electronics for each sub-pixel can have several semiconductor transistors and other integrated circuit (IC) components. Each OLED may correspond to a pixel or a sub-pixel, and therefore these terms are used interchangeably hereinafter.
In an OLED device, one or more layers of semiconducting organic material are sandwiched between two electrodes. An electric current is applied across the device, causing negatively charged electrons to move into the organic material(s) from the cathode. Positive charges, typically referred to as holes, move in from the anode. The positive and negative charges meet in the center layers (i.e., the semiconducting organic material), combine, and produce photons. The wave-length—and consequently the color—of the photons depends on the electronic properties of the organic material in which the photons are generated.
The color of light emitted from the organic light emitting device can be controlled by the selection of the material used to form the emissive layer. White light may be produced by generating blue, red and green lights simultaneously. Other individual colors, different than red, green and blue, can be also used to produce in combination a white spectrum. The precise color of light emitted by a particular structure can be controlled both by selection of the organic material, as well as by selection of dopants in the organic emissive layers. Alternatively, filters of red, green or blue (or other colors), may be added on top of a white light emitting pixel. In further alternatives, white light emitting OLED pixels may be used in monochromatic displays.
Pixel drivers can be configured as either current sources or voltage sources to control the amount of light generated by the OLEDs in an active matrix display.
AMOLED displays are normally driven with constant luminance over a full frame cycle. A pixel is typically programmed once each frame period and the data is held constant by a storage capacitor (for analog pixels) or register (for digital pixels) until the next frame cycle when the pixel data is refreshed. This is known as a hold-type display in contrast to an impulse display like a cathode ray tube (CRT).
A new drive scheme is provided for OLED displays that uses a pulsed drive mode. The pulsed drive mode results in a reduced duty cycle for pixel operation. The peak OLED current is increased correspondingly to maintain a constant average luminance over the frame period so that there is no brightness loss. The method, system and computer-readable medium according to the present innovation uses a blanking signal to set the OLED pixel to black by discharging a capacitive element prior to re-programming the OLED pixel during a next synchronization cycle.
A method is provided for driving an organic light emitting diode (OLED) pixel. The method includes receiving a first control signal corresponding to a first time duration and energizing the OLED pixel for a second time duration shorter than the first time duration. The method also includes setting the OLED pixel to black after the second time duration until an end of the first time duration.
In the method, the first control signal may define a first intensity level for the OLED pixel for the first time duration. The method may further includes transforming the first control signal into a drive signal having a second intensity level, the second intensity level being approximately equal to the first intensity level multiplied by the first time duration and divided by the second time duration.
The step of transforming may include providing a synchronization signal at a beginning of a cycle having the first time duration, and providing a ramp signal beginning at substantially a same time as the synchronization signal. The step of transforming may also include providing a second control signal for modulating the ramp signal and having a third time duration. The third time duration is based on the first intensity level of the first control signal. The second control signal may begin at substantially the same time as the synchronization signal. The drive signal may include the ramp signal when the second control signal is high, and, when the second control signal is low, the drive signal may be substantially constant at a value of the ramp signal at a transition of the second control signal from high to low.
The step of transforming may further include setting the drive signal to substantially zero with another synchronization signal after the second time duration.
In the method, the first control signal may be associated with a first frame period. The first frame period may be one of immediately consecutive frame periods, and each frame period may have a frame duration substantially identical to the first time duration. The synchronization signal may be periodic and a first number of the synchronization signals may occur during any frame period. The first number may be three or more. The OLED pixel may be set to black in response to a second one of the synchronization signals. The second one of the synchronization signals may follow the first one of the synchronization signals by a second number of synchronization signals. The second number may be less than the first number.
The setting of the drive signal to substantially zero may include discharging a capacitive element in a drive circuit that holds the drive signal.
The synchronization signal may be periodic and correspond to a horizontal synchronization signal, and the first time duration may include a frame in which the horizontal synchronization signal pulses once for each row in a display. The method may be performed for other OLED pixels in a same row as the OLED pixel using the synchronization signal.
The synchronization signal may be periodic and correspond to a vertical synchronization signal, and the first time duration may include a frame in which the vertical synchronization signal pulses once for each column in a display. The method may be performed for other OLED pixels in a same column as the OLED pixel using the synchronization signal.
The second intensity level may be approximately equal to between 5 and 10 times the first intensity, and the second time duration may be approximately equal to between 10 and 20 percent of the first time duration.
The first intensity level and the second intensity level may be measured in candela per square meter, and the first time duration and the second time duration may be measured in milliseconds.
An organic light emitting diode (OLED) pixel system is provided that is driven based on a first control signal defining a first intensity level for a first time duration. The system includes an arrangement for receiving the first control signal and an arrangement for transforming the first control signal into a drive signal having a second intensity level. The second intensity level is approximately equal to the first intensity level multiplied by the first time duration and divided by a second time duration. The second time duration is shorter than the first time duration. The system also includes an arrangement for energizing the OLED pixel for the second time duration based on the drive signal.
The OLED pixel system may include an arrangement for setting the OLED pixel to black during that portion of the first time duration that does not correspond to the second time duration.
In the OLED pixel system, the transforming arrangement may include an arrangement for providing a synchronization signal at a beginning of a cycle having the first time duration, and an arrangement for providing a ramp signal beginning at substantially the same time as the synchronization signal. The transforming arrangement may also include an arrangement for providing a second control signal for modulating the ramp signal and having a third time duration, the third time duration being based on the first intensity level of the first control signal. The second control signal may begin at substantially the same time as the synchronization signal. The drive signal may include the ramp signal when the second control signal is high, and, when the second control signal is low, the drive signal is substantially constant at a value of the ramp signal at a transition of the second control signal from high to low.
In the OLED pixel system, the transforming arrangement may further include an arrangement for setting the drive signal to substantially zero with another synchronization signal after the second time duration.
In the OLED pixel system, the setting of the drive signal to substantially zero may include discharging a capacitive element in a drive circuit that holds the drive signal.
A computer-readable medium having stored thereon computer-executable instructions is provided. The computer-executable instructions cause a processor to perform a method when executed. The method is for driving an organic light emitting diode (OLED) pixel based on a first control signal defining a first intensity level for a first time duration. The method includes receiving the first control signal and transforming the first control signal into a drive signal having a second intensity level. The second intensity level is approximately equal to the first intensity level multiplied by the first time duration and divided by a second time duration. The second time duration is shorter than the first time duration. The method also includes energizing the OLED pixel for the second time duration based on the drive signal.
a is a timing diagram illustrating a standard OLED pixel drive signal compared to a pulse drive signal in accordance with an exemplary embodiment;
b is a timing diagram showing signals for different rows of an OLED array using a pulse drive in accordance with an exemplary embodiment;
The pulse mode drive scheme provides a shortened active or “on” duration for the OLED pixel or sub-pixel, which may be controllable to be anything from 1 to 99% of the frame time. The minimum pulse duty may be limited by the peak current capability of the pixel drive circuit. In a Super Extended Graphics Array (SXGA) for example, the peak current may be limited by the voltage range of the CMOS drive circuit to about 5-10 times a nominal value. As a result, the pulse duty should not be less than 10-20% to keep the same average luminance.
The pulse mode drive scheme may offer several benefits over a standard continuous drive scheme. First, the pulse mode drive scheme may provide a fast motion response time. Motion blur artifacts on liquid crystal display (LCD) and OLED displays may primarily be a result of the hold-type displaying method used rather than response time (See T. Kurita, “Moving Picture Quality Improvement for Hold-Type AM-LCDs”, 2001 SID Digest, pp. 986-989 (2001)). A conventional matrix display holds the image data for the entire duration of a frame until re-programmed at the start of the next frame. In contrast, a CRT display may have an impulse response in which the luminance decays very quickly within a small fraction of the frame period, and therefore the duty cycle over one frame period may be low. This may result in a smoother perceived image because the human eye may track the expected image motion better. By simulating a CRT response using a reduced pulse duration, the OLED motion response may provide a considerable improvement.
Second, the pulse mode drive scheme may provide a reduced storage capacitor requirement. A limitation to miniaturization of the pixel size may be the need for a large data storage capacitor within the pixel area. The storage capacitor may occupy more than 30% of the pixel area because it needs to hold the data over the long frame time without any substantial loss of signal. If the hold time is reduced by 80% for example, the storage capacitor can also be reduced, enabling a significant miniaturization in the pixel pitch without a loss of performance. This may provide a path to higher density arrays and/or smaller display size using the same silicon technology.
Third, the pulse mode drive scheme may provide an extended temperature operation. At higher temperatures, the parasitic leakage currents in a pixel driver may tend to discharge the storage capacitor faster than at room temperature or lower temperatures. This may result in a deterioration of image brightness and quality at high temperatures. By using the pulse mode drive scheme, the signal loss in the storage capacitor may be reduced within a frame time, and therefore the display may be able to perform to a higher temperature specification.
a illustrates timing diagram 200 including vertical sync (VS) pulse signal 210, standard OLED pixel drive signal 220 and pulse OLED pixel drive signal 230 in accordance with an exemplary embodiment. VS pulse signal 210 provides timing pulses 212, 213 and 214 indicating beginnings of frames. For instance, timing pulse 212 begins frame duration 218, which is illustrated in the graph of pulse OLED pixel drive signal 230. In a conventional pixel drive system, all of the rows are rewritten during a frame and are held at a constant voltage or current until rewritten in the next frame. For instance, row 1 of a pixel array may be written immediately after pulse 212 in frame duration 218 and may be rewritten immediately after pulse 213, and subsequently rewritten again after pulse 214. Standard OLED pixel drive signal 220 illustrates the conventional pixel drive scheme for conventional signal 225. In this case, the OLED pixel or sub-pixel driven according to this conventional scheme is always being energized. Alternatively, a conventional OLED drive signal may vary in intensity when rewritten in each frame, i.e., at regular intervals after each pulse 212, 213 and 214. In this case, the conventional OLED would still be constantly energized (except when the pixel is dark or black due to the signal being dark for that pixel or sub-pixel), but at different levels. The reprogramming of an individual OLED pixel or sub-pixel would occur at regular intervals after a VS pulse according to the row number of the OLED pixel or sub-pixel.
Pulse OLED pixel drive signal 230 illustrates a pixel drive signal according to an exemplary embodiment that is synchronized with VS pulse signal 210, and is therefore energized immediately after, or at the same time as, VS pulse signal 210. Alternatively, as discussed above, a pulse OLED pixel drive signal may be energized at regular intervals after VS pulse signal 210. Pixel drive signal 232 represents the signal for an OLED pixel or sub-pixel for frame duration 218 following VS pulse signal 210. Pixel drive signal 232 has a pulse duration 216, which is less than frame duration 218. Consequently, the pixel drive signal 232 has a greater intensity (i.e., an increased luminance) relative to conventional signal 225 so that an average luminance of a pixel driven by pixel drive signal 232 over frame duration 218 is equal to an average luminance of a pixel driven by conventional signal 225 over frame duration 218. After pulse duration 216, the OLED pixel or sub-pixel is reset to black for black period 236.
Pixel drive signal 233 represents the signal for an OLED pixel or sub-pixel for a frame duration following VS pulse signal 213. Pixel drive signal 233 has a pulse duration equal to pulse duration 216, which is less than frame duration 218, and consequently has a greater intensity relative to conventional signal 225. An average luminance of a pixel driven by pixel drive signal 233 over the frame duration is equal to an average luminance of a pixel driven by conventional signal 225 over the frame duration. After the pulse duration, the OLED pixel or sub-pixel is reset to black for black period 237. Similarly, pixel drive signal 234 represents the signal for an OLED pixel or sub-pixel for a frame duration following VS pulse signal 214. Pixel drive signal 234 has a pulse duration less than frame duration 218, and consequently has a greater intensity relative to conventional signal 225. An average luminance of a pixel driven by pixel drive signal 234 over the frame duration is equal to an average luminance of a pixel driven by conventional signal 225 over the frame duration. After the pulse duration, the OLED pixel or sub-pixel is reset to black for black period 238.
A timing diagram for implementing a pulse mode drive, for example in an SXGA, is shown in
b is timing diagram 240 for different rows of an OLED array using a pulse drive in accordance with an exemplary embodiment.
In a conventional system, active pixel signal 275 would remain at this substantially constant value until rewritten, namely after the writing of the frame is finished and n−1 rows of the next frame are written, namely frame duration 218 as shown in
In this manner, the pixel associated with row_n signal 270 is black for a period during each frame duration 218, and therefore may have an opportunity to cool, which may have a beneficial impact on the life cycle and characteristics of the pixel. Consequently, the brightness of the pixel may have to be increased, which may be achieved by a standard adjustment of the brightness, which may be accomplished by increasing the time at which square wave 272 is high or by changing the rate of increase of the ramp pulses of ramp signal 260. The number w, which represents an integer value less than the number of rows in the array, may be determined by experimentation, and may be any of 10%, 20%, 40%, 50% or 80% of a number of rows of an array, and in particular, may be any integer number from one to the number of rows minus one.
Row_n+1 signal 280 may provide a window to a next ramp pulse of ramp signal 260, thereby providing a ramping portion to active pixel signal 285, which is shown superimposed on row_n+1 signal 280. Active pixel signal 285 ramps up according to the ramp pulse while square wave 282 is high and then holds the final, highest value of the ramp pulse upon the end of square wave 282 (i.e., when row_n+1 signal 280 goes low). Active pixel signal 285 then maintains a substantially constant value, as supported by capacitive elements in the drive circuit, until reset pulse 286 causes active pixel signal 285 to reset to black.
Row_n+w signal 290 may provide a window to a later ramp pulse of ramp signal 260, thereby providing a ramping portion to active pixel signal 294, which is shown superimposed on row_n+w signal 290. Active pixel signal 294 ramps up according to the ramp pulse while square wave 292 is high and then holds the final, highest value of the ramp pulse upon the end of square wave 292 (i.e., when row_n+w signal 290 goes low). Active pixel signal 294 then maintains a substantially constant value until a reset pulse causes active pixel signal 294 to reset to black.
Row_n+w+1 signal 295 may provide a window to a later ramp pulse of ramp signal 260, thereby providing a ramping portion to active pixel signal 298, which is shown superimposed on row_n+w+1 signal 295. Active pixel signal 298 ramps up according to the ramp pulse while square wave 296 is high and then holds the final, highest value of the ramp pulse upon the end of square wave 296 (i.e., when row_n+w+1 signal 296 goes low). Active pixel signal 296 then maintains a substantially constant value until a reset pulse causes active pixel signal 296 to reset to black.
While only a limited number of preferred embodiments of the present invention have been disclosed for purposes of illustration, it is obvious that many modifications and variations could be made thereto. It is intended to cover all of those modifications and variations which fall within the scope of the present invention, as defined by the following claims.
This application claims the benefit of U.S. Provisional Application No. 61/278,302 filed Oct. 5, 2009, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61278302 | Oct 2009 | US |
