JP2004341516A - Common anode passive matrix type organic light emitting diode (oled) display, driving circuit therefor, method for precharging same organic light emitting diode, and arrangement - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a precharging circuit which is suitably used for a large-sized screen OLED display arranged with common anode constitution. <P>SOLUTION: Disclosed is a precharging circuit incorporated in a driving circuit of the common anode passive matrix large-sized screen OLED display device to conquer a capacitance characteristic C<SB>OLED</SB>characteristic of an OLED element. A 1st precharging circuit includes a MOSFET element incorporated in an ordinary driving circuit to apply a precharging voltage to the cathode of a given OLED element right before a desired "ON" time, thereby rapidly conquering the C<SB>OLED</SB>. A 2nd precharging circuit includes a method of grounding the cathode of a given OLED element while connecting the anode to a positive voltage right before a desired ON time in an ordinary driving circuit, thereby rapidly conquering C<SB>OLED</SB>. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a drive circuit for a common anode passive matrix large screen organic light emitting diode (OLED) display. In particular, the present invention relates to a precharge circuit for optimizing performance.

BACKGROUND OF THE INVENTION Organic light emitting diode (OLED) technology uses organic light emitting materials that produce intense light of various colors when sandwiched between electrodes and exposed to a DC current. These OLED structures can be combined into pixels or pixels containing a display. OLEDs are also useful in a variety of applications as discrete light emitting elements or active elements in light emitting arrays or displays, such as flat panels in watches, phones, laptop computers, pagers, cell phones, calculators, and the like. Today, the use of light emitting arrays or displays is largely limited to small screen applications as described above.

  The demand for high quality, high resolution large screen display applications has led the industry to look for alternative display technologies that replace older LEDs and liquid crystal displays (LCDs). For example, LCDs cannot provide the bright and high light output, greater vision, high resolution and speed requirements of the large screen display market. In contrast, OLED technology promises bright and vibrant colors with high resolution and wide vision. However, the use of OLED technology in large screen display applications, such as outdoor or indoor stadium displays, large marketing advertising displays and public information displays, is still in development.

  There are several technical challenges associated with using OLED technology in large screen applications. One such challenge is that OLED displays are expected to provide a wide and dynamic range of color, contrast and light intensity in response to various external environmental factors including ambient light, humidity and temperature. It is a point. For example, outdoor displays are required to increase white contrast during the day and black contrast at night. Furthermore, light output must be large in bright sunlight and low in dark and rough weather conditions. The intensity of light emission generated by an OLED device is directly proportional to the amount of current driving the device. Thus, the more light output is required, the more current is supplied to the pixel. Therefore, by limiting the current to the OLED device, light emission can be reduced.

  By definition, a pixel is a single point or unit of a programmable color in a graphic image. However, a pixel may include, for example, an arrangement of red, green, and blue sub-pixels. There are two basic circuit configurations to drive these sub-pixels: a common cathode configuration and a common anode configuration. These configurations differ in that the three sub-pixels are addressed via a common cathode line or via a common anode line. Thus, in a common cathode configuration, the cathodes of the three sub-pixels are electrically connected and commonly addressed. In a common anode configuration, the anodes of the three sub-pixels are electrically connected and commonly addressed.

  Conventional OLED displays typically use a common cathode configuration. In a typical common cathode drive circuit, a current source is located between each individual anode and a positive power supply, and the cathodes are commonly electrically grounded. As a result, current and voltage are not independent of each other, and small voltage fluctuations lead to fairly large current fluctuations, and consequently light output fluctuations. Further, in the common cathode configuration, since the constant current source is based on the positive power supply, small voltage fluctuations also lead to current fluctuations. For these reasons, the common cathode configuration makes it more difficult to accurately control light emission that depends on accurate current control.

  In contrast, in a typical common anode drive circuit, the current source is located between each individual cathode and ground, and the anode is commonly electrically connected to a positive power supply. As a result, the current and voltage are completely independent of each other, and small voltage fluctuations do not lead to current fluctuations, which can eliminate consequent fluctuations in light output. Further, in a common anode configuration, the constant current source is referenced to a ground that does not fluctuate, thereby eliminating any change in current due to the reference. For these reasons, the common anode configuration lends itself to precise control of the light emission required in large screen display applications.

Another consideration in large screen display applications using OLED technology is the physical size of the pixels. The larger emission area is easy to see and lends itself to providing the required wide dynamic range of color, contrast and light intensity. Consequently, an OLED device structure having a larger area than the OLED structure of a conventional small screen display is desirable. In small screen applications, the pixel pitch is typically less than 0.3 mm, and the pixel area is, for example, only 0.1 mm ² . In contrast, in large screen applications, the pixel pitch may be greater than 1.0 mm, which results in a pixel area as large as 0.3 mm ² to 50 mm ² (pitch has a fill factor of 50 mm). % To 10 mm or more). However, as a result of the larger device area, the inherent capacitance (C _OLED ) of larger OLED devices is relatively high compared to smaller OLED structures. Due to this high inherent capacitance, additional charging time is required in operation to reach the working voltage of the OLED device. This charging time limits the on / off ratio of the device and adversely affects the overall display brightness and performance.

OLED precharge circuits have been developed to help overcome the inherent capacitance of OLEDs in graphic display devices and have been incorporated into existing drive circuits. For example, U.S. Patent No. 6,323,631, entitled "Constant current driver with auto-clamped pre-charge function", discloses a reference bias generator and multiple constant currents. A constant current driver is described that includes a driver cell and has an automatic clamp precharge function, each connected to its reference bias generator to form a respective current mirror. Each constant current driver cell has a switch transistor, a current output transistor, and a precharge transistor. When a constant current is output from the current output transistor to drive the OLED, the precharge transistor is turned on to rapidly precharge the OLED until the gate-source voltage of the precharge transistor becomes smaller than the threshold voltage. It supplies the drain-source current as an additional large current.
US Pat. No. 6,323,631

While the precharge function of U.S. Patent No. 6,323,631 preferably plays a role in quickly precharging an OLED device to optimize performance, the precharge function of U.S. Patent No. 6,323,631 does not. It is designed for use with a common cathode drive circuit and is therefore not suitable for use with a common anode drive circuit in large screen OLED display devices. Another disadvantage of the pre-charge function of US Pat. No. 6,323,631 is that it is designed to handle C _OLED values associated with small pixel areas, such as 0.1 mm ² , and thus has a large pixel area. The associated higher C _OLED value cannot be overcome.

  Accordingly, it is an object of the present invention to provide a precharge circuit suitable for use in large screen OLED displays arranged in a common anode configuration.

It is another object of the present invention to provide a precharge circuit suitable for overcoming the large C _OLED values associated with the large area OLED elements of a large screen OLED display arranged in a common anode configuration and optimizing performance. To provide.

  Another object of the present invention is to provide a precharge circuit that eliminates the influence of fluctuations in characteristics of an OLED element such as capacitance due to fluctuations in a manufacturing process.

SUMMARY OF THE INVENTION The present invention provides a drive circuit for a common anode passive matrix organic light emitting diode (OLED) display that includes at least one OLED having an anode and a cathode, wherein the cathode of the OLED has a first current source and a first current source. Are connected in series. The drive circuit includes means for precharging at least one OLED before closing the switching means.

  The means for precharging the at least one OLED may include a second switching means. The second switching means may include an active switch device, and may include a MOSFET. The MOSFET may be an NMOS transistor device having a voltage and current rating suitable for precharging at least one OLED.

  The second switching means may be branched and coupled in parallel across the first current source. If the second switching means comprises a MOSFET and the MOSFET has a gate, the source may be electrically connected to the precharge voltage. The MOSFET may also have a drain electrically connected to the cathode of the OLED.

  The second switching means comprises a first switch element suitable for coupling the cathode of the OLED to ground and a coupling element suitable for coupling the anode of the OLED to a voltage source substantially corresponding to the normal operating voltage of the OLED. A second switch element. The first switch element and the second switch element may be active switch elements. The active switching device may be a MOSFET transistor having a voltage and current rating suitable for precharging at least one OLED.

  The means for precharging the at least one OLED may further include a second current source coupled in parallel across the first current source. The second current source may be suitable for supplying a current between 50 mA and 800 mA, preferably between 100 mA and 600 mA. The second current source may be substantially the same as or different from the first current source, for example, the second current source may be twice the current supplied by the first current source. It may be suitable to supply between 4 and 4 times the current.

The first current source may be a current source device capable of changing its output current by selecting one of the first and second current references.

  The invention also provides an arrangement comprising an array of OLEDs, each OLED having a common anode, cathode, and drive circuit according to the invention with other OLEDs in the array.

  The present invention further provides a common anode passive matrix organic light emitting diode (OLED) display comprising an array of OLEDs, each OLED having an anode and a cathode, said display including a drive circuit according to the present invention.

  The present invention also provides a method for pre-charging an organic light emitting diode (OLED) of a common anode passive matrix OLED display prior to a desired ON time of the OLED, the method comprising: Charging. The charging step may be performed by applying a precharge voltage to the cathode of the OLED before a desired ON time. Alternatively, this may be accomplished by pulling the cathode of the OLED to a second voltage level while applying the first voltage level to the anode of the OLED, wherein the difference between the first and second voltages is the desired value. Equal to the precharge voltage. In this latter case, the first voltage level may be equal to the desired precharge voltage and the second voltage level may be equal to the ground level. According to yet another alternative embodiment, the charging step may be performed by providing an additional current to the OLED prior to the desired ON time.

  The precharge voltage may be substantially equal to the normal operating voltage of the OLED during the ON time. At low light output, additional gray levels can be realized by selectively switching between the two current sources.

  These and other features, features and advantages of the present invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.

  The same reference numbers in different drawings identify the same or similar elements.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS The present invention will be described with respect to particular embodiments and certain drawings but the invention is not limited thereto but only by the claims. The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

  Furthermore, the terms first, second and third in the description and the claims are used for distinguishing between similar elements, but not for describing a continuous or chronological order. Absent. Where appropriate, the terms are used interchangeably and the embodiments of the invention described herein may operate in a different order than those described or illustrated herein. It is possible.

  The word “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. Therefore, the scope of the expression “the device including the means A and B” should not be limited to the device including only the components A and B. That means, with respect to the present invention, that the only relevant components of the device are A and B.

  While the invention will be described primarily with reference to a single display, the invention is not so limited. For example, the display may be expandable to form a larger array, for example, via tiling or the like. Thus, the invention may include an assembly of an array of pixels, e.g., they may be tiled displays, modules made of tiled arrays that are tiled into supermodules. May be included. Thus, the term display relates to a set of addressable pixels within an array or group of arrays. Some display units or "tiles" can be positioned adjacent to one another to form a larger display. That is, when a plurality of display element arrays are arranged in physical proximity, they appear as a single image.

In one aspect of the invention, a precharge circuit is provided that can be incorporated into a drive circuit of a common anode passive matrix OLED display device to overcome the inherent capacitance characteristic _COLED inherent in OLED devices. The display may be a large screen display. Specifically, in one aspect of the invention, the first precharge circuit of the invention applies a precharge voltage to the cathode of a given OLED element just prior to a desired "on" time, thereby causing the OLED Charges the device quickly. The second precharge circuit of the present invention applies a precharge voltage to the anode of a given OLED element immediately prior to the desired on-time while simultaneously pulling the cathode to ground, thereby rapidly charging the OLED element. I do. The third charging circuit of the present invention simply supplies additional current to the OLED device just prior to the desired on-time, thereby rapidly charging the OLED device. Finally, the fourth precharge circuit of the present invention includes a single current source device whose output current can be rapidly changed by selecting a low or high current reference to rapidly charge the OLED device. can do.

  FIG. 1 is a schematic diagram of an OLED array circuit 100, showing a portion of a typical common anode passive matrix large screen OLED array and associated drive circuits. OLED array circuit 100 includes an OLED array 110 formed from a plurality of OLEDs 112 (as is well known, each having an anode and a cathode) arranged in a row and column matrix. For example, OLED array 110 is formed from OLEDs 112a, 112b, 112c, 112d, 112e, 112f, 112g, 112h, and 112j arranged in a 3x3 array, and the anodes of OLEDs 112a, 112b, and 112c are electrically connected to ROW LINE1. The anodes of OLEDs 112d, 112e and 112f are electrically connected to ROW LINE2, and the anodes of OLEDs 112g, 112h and 112j are electrically connected to ROW LINE3. Further, the cathodes of OLEDs 112a, 112d and 112g are electrically connected to COLUMN LINE A, the cathodes of OLEDs 112b, 112e and 112h are electrically connected to COLUMN LINE B, and the cathodes of OLEDs 112c, 112f and 112j are connected to COLUMN LINE C. It is electrically connected. Each OLED 112 represents a pixel of a black and white display or a sub-pixel of a color display (usually red, green or blue, but any color variation is acceptable. The sub-pixels are grouped together geometrically. To form a single addressable full-color pixel, for example, 112a-112c may be red, green and blue, respectively). As is well known, OLEDs emit light when forward biased with an appropriate current supply.

A positive voltage (+ V _LED ), which is typically between 5 volts (i.e. the normal working voltage across the OLED + the voltage on the current source which is typically 0.7 V) and 15-20 volts, can be used for multiple switches. It is electrically connected to the lines ROW LINE1, ROW LINE2, and ROW LINE3 of each row via 114a, 114b, 114c. Switches 114a, 114b, 114c are conventional active switching devices such as MOSFET switches or transistors having suitable voltage and current ratings. Specifically, the positive voltage + V _LED is electrically connected to ROW LINE1 via switch 114a, to ROW LINE2 via switch 114b, and to ROW LINE3 via switch 114c. Column lines COLUMN LINE A, COLUMN LINE B, and COLUMN LINE C are driven by another constant current source, i.e., a plurality of current sources (I _SOURCE ) 116a, 116b, 116c. Specifically, current source I _SOURCE 116a drives COLUMN LINE A, current source I _SOURCE 116b drives COLUMN LINE B, and current source I _SOURCE 116c drives COLUMN LINE C. Switch 118a is connected in series between current source I _SOURCE 116a and ground. Switch 118b is connected in series between current source I _SOURCE 116b and ground. Switch 118c is connected in series between current source I _SOURCE 116c and ground. The current sources _ISOURCE 116a, 116b, 116c are conventional current sources that can supply a constant current, typically in the range of 5 mA to 50 mA. Examples of constant current devices include Toshiba TB62705 (8-bit constant current LED driver with shift register and latch function) and Silicon Touch ST2226A (PWM controlled constant current driver for LED devices). Switches 118a, 118b, 118c are typically included in current source integrated circuits and consist of conventional active switching elements such as MOSFET switches or transistors having suitable voltage and power supply ratings.

  The matrix of OLEDs 112a-112j in OLED array circuit 100 is arranged in a common anode configuration. For example, for each color pixel on ROW LINE2, the anodes of each sub-pixel 112d-112f are all connected to the same row of lines. In this way, the current source is referenced to ground, the current and voltage are independent of each other, and the emission of light is better controlled.

In operation, to activate (turn on) any given OLED 112a-112j, its associated row line ROW1, ROW LINE2, ROW LINE3 and column line COLUMN LINE A, COLUMN LINE B, COLUMN LINE. C are activated by simultaneously closing their associated switches 114a, 114b, 114c and 118a, 118b, 118c. In the first example, to turn on the OLED 112b, a positive voltage + V _LED is applied to the ROW LINE1 by closing the switch 114a, and at the same time, a constant current is passed through the current source I _SOURCE 116b by closing the switch 118b. COLUMN LINE B. In this manner, OLED 112b is forward biased and current flows through OLED 112b. When the device threshold voltage (typically 1.5-2 volts) is reached across OLED 112b, OLED 112b begins to emit light. OLED 112b is lit as long as switch 114a and switch 118b are closed. To deactivate OLED 112b, switch 118b is opened. In the second example, to turn on the OLED 112g, by closing the switch 114c, a positive voltage + V _LED is applied to the ROW LINE3, and at the same time, by closing the switch 118a, the constant current becomes COLUMN via the current source I _SOURCE 116a.
LINE A. In this way, OLED 112g is forward biased and current flows through OLED 112g. When the device threshold voltage (typically 1.5-2 volts) is reached across OLED 112g, OLED 112g begins to emit light. OLED 112 is on as long as switch 114c and switch 118a are closed. To deactivate OLED 112g, switch 118a is opened.

Along a line ROW LINE1, ROW LINE2, ROW LINE3 of a given row, any one or more OLEDs 112a-112j can be activated at a given time. In contrast, only one OLED 112 can be activated at any given time along a given column of lines COLUMN LINE A, COLUMN LINE B, COLUMN LINE C. Thus, an entire image is created from successively or randomly selecting each row of the OLED array 110 by closing its corresponding switch 114a-114c. In each row, a current with a certain strength and a certain duration causes diodes 112a-112c, 112d-112f, 112g- on the row to be opened and closed by switches 118a, 118b, 118c by current sources 116a, 116b, 116c. Sent through 112j, the correct intensity is displayed at each pixel or subpixel. Switches 114a, 114b, 114c are closed as long as that row is selected and open when the next row is selected. All switches 118a, 118b, 118c open before the next row is selected. Details of the operation of a given OLED 112a-112j are described below with reference to FIGS. 2A and 2B.

FIG. 2A is a schematic diagram of the OLED drive circuit 200, showing a typical drive circuit of a single OLED 112 in the OLED array circuit 100 of FIG. The OLED drive circuit 200 includes a switch 114, an OLED 112, a current source I _SOURCE 116 and a switch 118 all arranged in series between the positive voltage + V _LED and ground, as shown in FIG. 2A. OLED drive circuit 200 further includes a capacitor 210 arranged in parallel with OLED 112. Capacitor 210 indicates the element capacitance (C _OLED ) of _OLED 112. In the area of the structure of _OLED 112, a typical value of C _OLED may be greater than 500 pF, relative to a typical _OLED value of 5 pF for a small OLED structure used in small screen OLED display applications. High. Physical values of C _OLED with the capacitance of any additional lines of the package is considered to be negligible for the purposes of this description, to achieve a satisfactory display performance must overcome. Voltage V _OLED indicates the voltage potential across _OLED 112, and voltage V _ISOURCE indicates the voltage potential across current source I _SOURCE 116 and switch 118 connected in series.

FIG. 2B is a graph 250 of the voltage potential V _ISOURCE across the current source 116 and the switch 118 connected in series from the time t 0 when the switches 114 and 118 are closed to the time t 2 when the switch 118 is opened, and the OLED drive circuit 200. The operation of FIG. At time t0, the value of V _ISOURCE is equal to the positive voltage + V _LED and begins to slowly drop toward the working voltage (V _WORKING ) of _OLED 112 due to the relatively high capacitance value C _OLED of _OLED 112. The OLED begins to light slightly when the threshold level or threshold voltage is reached (the OLED's threshold voltage is the voltage across the OLED that is sufficient to light it, and the normal operating voltage across the OLED Or the working voltage is higher than this threshold voltage). V _ISOURCE reaches the working voltage of OLED 112 at time t1. The period between t0 and t1 indicates the charging time T _CHARGE of the capacitor 210 of the OLED 112. The transition of the voltage from t0 to t1 is linear. This is because the current output of the current source I _SOURCE 116 is constant. At time t1, OLED 112 begins to emit its full light and continues to emit light for a predetermined time, OLED emission time T _ON , as long as switches 114 and 118 are closed. OLED112 is inactivated by opening the switch 118, followed by V _ISOURCE returns to rapidly + V _LED value. The OLED 112 is off during the period from t2 to the next t0, that is, the OLED off time or the period _TOFF . Therefore, the cycle time T _CYCLE is represented by T _CHARGE + T _ON + T _OFF . As shown in graph 250, T _CHARGE indicates the time spent when switches 114 and 118 are closed and capacitor 210 is charging, but OLED 112 is not yet emitting light at the desired emission level. . This leads to an increase in T _CYCLE , which reduces the achievable T _ON / T _FF ratio and limits the achievable performance of the OLED drive circuit 200. FIGS. 3A, 3B, 4, 5 and 6 below illustrate a method for minimizing or eliminating the T _CHARGE time and minimizing the T _CYCLE by performing a precharge operation on the capacitor 210.

FIG. 3A is a schematic diagram of an OLED precharge circuit 300 according to a first and preferred embodiment of the present invention. The OLED precharge circuit 300 is provided by the OLED drive circuit 20 shown in FIG. 2A.
0, except that a MOSFET 310 arranged in parallel with the current source I _SOURCE 116 is added. Specifically, the drain of MOSFET 310 is electrically connected directly to the cathode of OLED 112, the source of MOSFET 310 is connected to precharge voltage + V _PRE-CHARGE, and the gate of MOSFET ₃₁₀ is electrically connected to precharge control voltage V _{PRECHARGE-CONTROL} . Connected. MOSFET 310 may be any conventional MOS transistor device having suitable voltage and current ratings for this application. However, MOSFET 310 represents any suitable active switching device.

FIG. 3B is a graph 350 of V _ISOURCE and + V _PRE-CHARGE from the time t0 when the precharge operation starts to the time t2 when the switch 118 is opened, and shows the operation of the OLED precharge circuit 300. (Note that the graphs of V _ISOURCE and + V _PRE-CHARGE are not drawn to scale with one another along the voltage axis. Graph 350 is for general voltage transition and timing only.) in t0, MOSFET 310 is turned on by applying a voltage V _{PRECHARGE-CONTROL} to the gate, the voltage V _{PRECHARGE-CONTROL,} sufficiently positive to saturate with respect to the source of the MOSFET 310 (voltage = V _PRECHARGE) MOSFET 310 It is. The MOSFET 310 is connected to a source capable of sinking a current of typically 100 to 600 ma. Further, at time t0, switch 114 is closed and switch 118 is open. As a result, current flows through OLED 112 for a short time via the electrical path created by MOSFET 310. This time must be long enough to build up a voltage across the capacitor 210 of the OLED 112 that approaches the working voltage of the OLED 112. Once this voltage builds up across the capacitor 210 of the OLED 112, the MOSFET 310 is turned off. That is, the + V _PRE-CHARGE voltage at the cathode of the OLED is removed, while the switch 118 is closed, whereby normal operating current from the current source I _SOURCE 116 flows through the OLED 112 and light is emitted.

Graph 350 of FIG. 3B shows that when V _{PRECHARGE-CONTROL} is applied (MOSFET ₃₁₀ is on), switch 114 is closed and switch 118 is open, the value of V _ISOURCE is equal to + V _LED at t0. . Subsequently, V _ISOURCE drops sharply toward the working voltage of OLED 112 due to + V _PRE-CHARGE charging capacitor 210 quickly. At t1, V _{PRECHARGE-CONTROL} is removed and switch 118 is closed. The period between t0 and t1 indicates the charging time T _CHARGE of the capacitor 210 of the OLED 112. At time t1, OLED 112 begins to emit normal light and continues to emit light for a predetermined time T _ON as long as switches 114 and 118 are closed. Thus, switch 114 is closed for at least a time equal to the charging time T _CHARGE + OLED emission time T _ON , and switch 118 is closed for a time equal to the OLED emission time T _ON . OLED112 is deactivated by opening the switch 118, followed by V _ISOURCE returns rapidly to a value of + V _LED. The OLED 112 remains off during the period from t2 to the next t0, that is, during the OLED off time T _OFF off. Therefore, the cycle time T _CYCLE is represented by T _CHARGE + T _ON + T _OFF .

Referring to the graph 250 and the graph 350 of FIG. 3A in FIG. 2A, the normal charging time T _CHARGE of OLED pre-charge circuit 300 in the range from 50 ns 12 ns typically charge time T _CHARGE of OLED drive circuit 200 in the range of 65s of 25ns It can be seen that it is greatly reduced as compared with. As a result, the T _CYCLE of the OLED precharge circuit 300 is much shorter than the T _{CYCLE of the} OLED drive circuit 200, while achieving the same T _ON time. As a result, the achievable T _ON / T _OFF ratio of the OLED 112 in the OLED precharge circuit 300 is increased compared to the on / off ratio of the OLED 112 in the OLED drive circuit 200, and the overall performance is improved. .

It is important to balance the operation of OLED precharge circuit 300 so that the charging time T _CHARGE is minimized and the step of precharging is stopped when the working voltage of OLED 112 is reached. As a result, if the timing of the precharge is too long, excessive current will flow through the OLED 112, leading to excessive light emission. As a precautionary measure, it is desirable to terminate the precharge operation just before V _ISOURCE reaches the normal operating voltage of 0.7 volts. For example, V _{PRECHARGE CONTROL} may be removed when V _ISOURCE reaches 1.5 volts. This leads to a slightly longer charging time T _CHARGE , which means that the voltage transition from 1.5 volts to 0.7 volts will cause the current source I _CHARGE without the help of + V _{PRE-CHARGE. This} is due to the current supplied by _SOURCE 116.

Furthermore, when the OLED emission time T _ON = 0, no precharge operation must be performed, and the OLED 112 is not turned on when it is not desired. In this case, both switch 118 and MOSFET 310 remain open. If the precharge operation is possible when the OLED emission time T _ON = 0, the OLED 112 may start to light. This is because + V _PRE-CHARGE reaches a level high enough to light the OLED 112 slightly. Therefore, in order to avoid undesired lighting of the OLED 112, the precharge operation may be eliminated when the OLED emission time T _ON = 0.

That is, referring to FIGS. 3A and 3B, just prior to the desired T _ON time, a precharge voltage (+ V _PRE-CHARGE ) from a source capable of sinking a suitable amount of current is applied to the OLED via MOSFET 310. Applied to the cathode. Thus, the capacitor 210 is charged rapidly through a high current through the MOSFET 310, rather than through the normal current source (I _SOURCE 116).

FIG. 4 is a schematic diagram of an OLED precharge circuit 400 according to a second embodiment of the present invention. The OLED precharge circuit 400 is the same as the OLED drive circuit 200 of FIG. 2 except that the voltage + V _OLED is electrically connected to the anode of the _OLED 112 via the switch 410, and the cathode of the _OLED 112 is electrically connected via the switch 412. In that it can be grounded. Switches 410 and 412 are conventional active switching elements such as MOSFET switches or transistors having suitable voltage and current ratings.

In operation, just before the desired OLED emission time T _ON (see FIG. 3B), the anode of the OLED 112 is brought to the normal operating voltage (+ V _OLED ) across the _OLED 112 for a short time by closing the switch 410, while at the same time the cathode of the _OLED 112 Closes the switch 412 to short-circuit the ground. In this way, charge builds up quickly across the capacitor 210. After a predetermined time (ie, charging time T _CHARGE in FIG. 3B), switches 114 and 118 are closed and switch 412 is opened, which causes the + V _LED to be applied to the anode of OLED 112 and current source I Normal operating current is supplied via _SOURCE 116. As a result, OLED 112 begins its normal operation (ie, T _ON in FIG. 3B).

Similar to the OLED precharge circuit 300 of FIG. 3A, the T _CHARGE of the OLED precharge circuit 400 is typically in the range of 12 ns to 50 ns, compared to the charging time T _CHARGE of the OLED drive circuit 200 which is typically in the range of 25 ns to 65 s. Thus, it is greatly reduced. As a result, the T _CYCLE of the OLED precharge circuit 400 is significantly shorter than the T _{CYCLE of the} OLED drive circuit 200, while achieving an equivalent OLED emission time T _ON . As a result, the achievable T _ON / T _OFF ratio of the OLED 112 in the OLED precharge circuit 400 is increased compared to the achievable on / off ratio of the OLED 112 in the OLED drive circuit 200, and the overall performance is increased. improves.

That is, referring to FIG. 4, just prior to the desired OLED emission time T _ON, a precharge voltage (+ V _OLED ) is applied to the anode of _OLED 112, while the cathode of _OLED 112 is pulled to ground. Thus, capacitor 210 is charged quickly, not through the normal current source (I _SOURCE 116), but through the straight connection of + _VOLED and cathode to ground.

FIG. 5 is a schematic diagram of an OLED precharge circuit 500 according to a third embodiment of the present invention. The OLED precharge circuit 500 is identical to the OLED drive circuit 200 of FIG. 2, but as shown in FIG. 5, an additional current source (ie, a current source I with an associated series-connected switch 512). _SOURCE 510) is connected in parallel with the current source I _SOURCE 116. Current source I _SOURCE 510 is a conventional current source that can supply a constant current typically in the range of 100 mA to 600 mA. Switch 512 is a conventional active switch element such as a MOSFET switch or transistor having suitable voltage and current ratings.

In operation, just prior to the desired T _ON time (see FIG. 3B), switches 114, 118 and 512 are all closed, thus providing a current source along with the normal current supplied via current source I _SOURCE 116. Additional current is available via I _SOURCE 510. As a result of this additional current being available, the charging time of capacitor 210 (ie, T _CHARGE in FIG. 3B) is reduced. In this way, charge builds up quickly across the capacitor 210. After a predetermined time (ie, T _CHARGE in FIG. 3B), switch 512 is opened, thereby allowing only normal operating current via current source I _SOURCE 116. As a result, OLED 112 begins its normal operation (ie, T _ON in FIG. 3B).

Similar to the OLED precharge circuit 300 in FIG. 3A and the OLED precharge circuit 400 in FIG. 4, the OLED drive circuit in which the charging time T _CHARGE of the OLED precharge circuit 500 is usually 12 ns to 50 ns, and usually 25 ns to 65 s. Compared to the charging time T _{CHARGE of} 200, it is greatly reduced. As a result, the T _CYCLE of the OLED precharge circuit 400 is significantly shorter than the T _{CYCLE of the} OLED drive circuit 200, while achieving an equivalent OLED emission time T _ON . As a result, the achievable T _ON / T _OFF ratio of the OLED 112 in the OLED precharge circuit 500 is increased compared to the achievable on / off ratio of the OLED 112 in the OLED drive circuit 200, and the overall performance is increased. Is improved.

That is, referring to FIG. 5, just prior to the desired T _ON time, the capacitor 210 is available to the OLED 112 via the current source I _SOURCE 510, rather than only via the normal current source (I _SOURCE 116). Charges quickly with additional current.

The charging time T _CHARGE used for pre-charging greatly affects the performance of the display. If the precharge time T _CHARGE is long, the maximum light output is limited, and if the compensation is made by increasing the current level, the lowest light output increases, and the gradation is eliminated. High quality displays require a large number of gray levels, and therefore require a high digital resolution or a current source operating at a certain number of possible output values or high clock speeds. A single current pulse (one clock cycle) produces light only if the threshold is reached within that pulse, for example, half the time of the clock cycle. If f _C is the clock frequency, the shortest t2 to t0 is 1 / f _C. For example, with a 40 MHz clock, the precharge time T _CHARGE must be as short as 12 ns. Usually the OLED diode and large C _OLED 500 pF to operate in the range of 9 to 15 volts, it is necessary at least 375mA of precharge current (C _OLED ^* dV / dt), but this is quite high. However, the requirement to reach the precharge state within the clock pulse period is that the two current sources 116, 5
Using 10 can be overcome.

FIG. 5A shows a possible result of using two current sources 116 and 510 as in FIG. Current source 510 can transmit, for example, twice the current of current source 116. This means that V _{ISOURCE of} current source 510 reaches the threshold voltage in half the time of V _ISOURCE of current source 116. As a result, when both voltage sources 510, 116 operate simultaneously, V _ISOURCE reaches the threshold in one third of the time. In the lower part of FIG. 5A, the corresponding current I _OLED through _OLED 112 is for t ₂ -t ₀ equal to the time required for both current sources 510, 116 to reach the threshold together. Shown. The area under the current curve is a measure for the emitted light. As shown, V _ISOURCE for each current source 510,116 does not reach the threshold as per separately necessarily time, three possible values of the light output, the V _OLED (Figure 5A is reached shown ) Is generated as long as the diode 112 is high enough to start emitting light. Extending this principle, at low light output values, very accurate gray scale can be achieved by varying the on-time of one or two current sources 510,116. Further, by turning on both current sources 510 and 116, a high current can be realized with a high light output.

FIG. 6 is a schematic diagram of an OLED precharge circuit 600 according to a fourth embodiment of the present invention. OLED precharge circuit 600 is identical to OLED drive circuit 200 of FIG. 2 except that current source I _SOURCE 116 selects its lower or higher current reference via switches 612 and 614, respectively, to reduce its output current. The difference is that it is replaced by a current source I _SOURCE 610, which is a single current source device that can be changed quickly. Switches 612 and 614 are conventional active switching elements such as MOSFET switches or transistors having suitable voltage and current ratings.

In operation, referring to FIGS. 3B and 6, during the charging time T _CHARGE , switches 114, 118 and 612 are closed and switch 614 is open. This provides a high current reference to current source I _SOURCE 610 and charges capacitor 210 quickly. When the precharge operation is completed, switch 612 is opened and switch 614 is closed. This provides a low current reference to current source I _SOURCE 610. As a result, current source I _SOURCE 610 drops rapidly to its normal constant operating current. Switches 114, 118 and 614 are closed for the duration of the OLED emission time T _ON and normal operation occurs. At time t2, switch 118 is opened and OLED emission time T _ON ends.

Finally, the precharge circuit of the present invention to overcome the adverse effect on performance due to C _OLED, variation of any process affecting the C _OLED also not affect the overall display performance of the OLED. Therefore, the precharge circuit of the present invention eliminates the influence of fluctuations in the characteristics of the OLED element such as capacitance due to fluctuations in the manufacturing process.

FIG. 2 is a schematic diagram of an OLED array circuit showing a portion of a common anode passive matrix large screen OLED array and associated drive circuits. FIG. 2A is a schematic diagram of an OLED drive circuit showing a drive circuit of a single OLED in the OLED array circuit of FIG. 1, and FIG. 2B is a graph of V _ISOURCE , _showing the operation of the OLED drive circuit of FIG. 2A. Show. FIG. 3A is a schematic diagram of an OLED precharge circuit according to a first and preferred embodiment of the present invention, and FIG. 3B shows the operation of the OLED precharge circuit of FIGS. 3A, 4, 5 and 6. It is a graph of _VISOURCE and + V _PRE-CHARGE . FIG. 4 is a schematic diagram of an OLED precharge circuit according to a second embodiment of the present invention. FIG. 9 is a schematic diagram of an OLED precharge circuit according to a third embodiment of the present invention. FIG. 6 shows the function and time of voltage and current in the use of two current sources as in FIG. 5. FIG. 9 is a schematic diagram of an OLED precharge circuit according to a fourth embodiment of the present invention.

Explanation of reference numerals

  100 OLED array circuit, 112 OLED, 116 first current source, 118 first switching means.

Claims (25)

  A drive circuit for a common anode passive matrix organic light emitting diode (OLED) display including at least one OLED (112) having an anode and a cathode, wherein the cathode of the OLED (112) is connected to a first current source (116). ) And a first switching means (118), the driving circuit including means for precharging the at least one OLED (112) before closing the switching means (118). Drive circuit for passive matrix organic light emitting diode (OLED) display.   The drive circuit for a common anode passive matrix organic light emitting diode (OLED) display according to claim 1, wherein said means for precharging said at least one OLED comprises a second switching means.   3. The driving circuit for a common anode passive matrix organic light emitting diode (OLED) display according to claim 2, wherein said second switching means comprises an active switching element.   4. The drive circuit for a common anode passive matrix organic light emitting diode (OLED) display according to claim 3, wherein the active switching element comprises a MOSFET.   5. The common anode passive matrix organic light emitting diode (OLED) display of claim 4, wherein said MOSFET is an NMOS transistor device having a voltage and current rating suitable for precharging said at least one OLED (112). Drive circuit for   6. A common anode passive matrix organic light emitting diode (OLED) display according to any of claims 2 to 5, wherein the second switching means is shunted and coupled in parallel across the first current source (116). Drive circuit for   7. A common anode passive matrix organic light emitting diode (OLED) according to claim 6, wherein said MOSFET (310) has a gate and a source is electrically connected to a precharge voltage. ) Drive circuit for the display.   The common anode passive matrix organic light emitting diode of claim 7, wherein said MOSFET (310) has a drain, said drain of said MOSFET (310) being electrically connected to said cathode of said OLED (112). (OLED) A drive circuit for a display.   The second switching means substantially corresponds to a first switch element (412) suitable for coupling the cathode of the OLED (112) to ground, and a normal operating voltage of the OLED (112). A common anode passive matrix organic light emitting diode (OLED) according to any of claims 2 to 5, comprising a second switch element (410) suitable for coupling the anode of the OLED (112) to a voltage supply. Drive circuit for display.   The drive circuit for a common anode passive matrix organic light emitting diode (OLED) display according to claim 9, wherein the first switch element (412) and the second switch element (410) are active switch elements. The active switching device is a MOSFET transistor having a voltage and current rating suitable for precharging the at least one OLED (112).
A drive circuit for a common anode passive matrix organic light emitting diode (OLED) display according to claim 1.   6. The method of any of claims 2 to 5, wherein the means for precharging the at least one OLED (112) further comprises a second current source (510) coupled in parallel across the first current source. A drive circuit for a common anode passive matrix organic light emitting diode (OLED) display according to claim 1.   13. A common anode passive matrix organic light emitting diode according to claim 12, wherein the second current source (510) is suitable for supplying a current between 50 mA and 800 mA, preferably between 100 mA and 600 mA. (OLED) A drive circuit for a display.   14. A common anode passive matrix organic light emitting diode (OLED) display according to any of claims 12 or 13, wherein the second current source (510) is substantially identical to the first current source (116). Drive circuit for   14. The method according to claim 12, wherein the second current source (510) is suitable for supplying between two and four times the current supplied by the first current source. A drive circuit for the described common anode passive matrix organic light emitting diode (OLED) display.   6. The current source device according to claim 2, wherein the first current source (610) is a current source device whose output current can be changed by selecting one of the first and second current references. A drive circuit for the described common anode passive matrix organic light emitting diode (OLED) display. Arrangement
An array of OLEDs (112), each OLED (112) having a common anode and cathode with other OLEDs in the array, the arrangement further comprising:
An arrangement comprising the drive circuit according to claim 1.   17. A common anode passive matrix organic light emitting diode (OLED) display comprising an array of OLEDs (112), wherein each OLED (112) has an anode and a cathode, the display according to any of the preceding claims. A common anode passive matrix organic light emitting diode (OLED) display including a drive circuit. A method for precharging an organic light emitting diode (OLED) of a common anode passive matrix OLED display prior to a desired ON time of the OLED,
A method for precharging an organic light emitting diode (OLED), comprising charging the OLED before the desired ON time.   20. The method for precharging an organic light emitting diode (OLED) according to claim 19, wherein said charging is performed by applying a precharge voltage to said cathode of said OLED prior to said desired ON time. .   The charging is performed by pulling the cathode of the OLED to a second voltage level while applying a first voltage level to the anode of the OLED, wherein a difference between the first and second voltages is provided. 20. The method for precharging an organic light emitting diode (OLED) according to claim 19, wherein is equal to a desired precharge voltage.   22. The method for precharging an organic light emitting diode (OLED) according to claim 21, wherein the first voltage level is equal to the desired precharge voltage and the second voltage level is a ground level.   20. The method for precharging an organic light emitting diode (OLED) according to claim 19, wherein said charging is performed by providing an additional current to said OLED prior to said desired ON time.   23. A method for precharging an organic light emitting diode (OLED) according to any of claims 20 to 22, wherein the precharge voltage is substantially equal to a normal operating voltage of the OLED (112) during an ON time. .   25. The method for pre-charging an organic light emitting diode (OLED) according to claim 24, wherein additional gray levels are achieved at low light output by selectively switching between two current sources.

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