Method for maintaining the white colour point in a field-sequential LCD over time

A field sequential liquid crystal display maintains its white colour point through compensation values to at least one colour light emitting diode that illuminates the display. The compensation values may be impedances to control the current or pulsing of the current source according to a pulse width modulation technique. A degradation curve may be used to calculate extrapolate the theoretical forward voltage of the light emitting diode. Additional complexity arises from the need for calculating uptime for multiple light emitting diodes of different colours. Brightness levels may also be factored in.

FIELD OF THE INVENTION

The present invention relates to the field of liquid crystal display and, particularly, to the field of white colour point of a liquid crystal display screen.

BACKGROUND OF THE INVENTION

Field sequential liquid crystal displays (LCD) use three colour light emitting diodes (LED) to provide full colour displays. If the current supplied to the LEDs were finely regulated, the white colour point formed by the three colours would remain the same. Because the LEDs are voltage controlled, over time, the forward voltage (Vf) of each LED varies (increases) so that the calibrated white colour point formed by operation of three colours drifts. Thus, there is a need for a method for maintaining the white colour point for a field sequential LCD.

SUMMARY OF THE INVENTION

In addressing the problem of maintaining the proper white colour point during the life of the LCD, the forward voltages (Vf) of the light emitting diodes for illuminating the LCD are adjusted to calibrate the white colour point established as a combination of the light emitting diode colours. This adjustment may occur through monitoring the ON time and, optionally, brightness of each light emitting diode and comparing a resulting value with thresholds stored in software code, look up tables, arrays, hardwired values, etc.

DETAILED DESCRIPTION

A method and device, especially a mobile station such as a handheld communications device, acts to stabilize a white colour point in a display by compensating for behavioural changes in the light source illuminating the display over time. Preferably, the display is a liquid crystal display and the light source includes light emitting diodes (LEDs) of different colours. The liquid crystal display may be operated at a rate of 30 or more frames per second. The LEDs of the light source preferably will include red, green, and blue colours. Other colour schemes, such as cyan, magenta, and yellow, are contemplated. Although directed to a liquid crystal display per se, the preferred use of the LCD is in a mobile station, such as a wireless portable handheld communications device. Cell phones and pagers are amongst the many handheld devices contemplated.

FIG. 1is a block diagram of a communication system100that includes a mobile station102that communicates through a wireless communication network. Mobile station102preferably includes a visual display112, a keyboard114, and perhaps one or more auxiliary user interfaces (UI)116, each of which is coupled to a controller106. Controller106is also coupled to radio frequency (R.F) transceiver circuitry108and an antenna110.

Typically, controller106is embodied as a central processing unit (CPU) which runs operating system software in a memory component (not shown). Controller106will normally control overall operation of mobile station102, whereas signal processing operations associated with communication functions are typically performed in RF transceiver circuitry108. Controller106interfaces with device display112to display received information, stored information, user inputs, and the like. Keyboard114, which may be a telephone type keypad or full alphanumeric keyboard (e.g., QWERTY or DVORAK), is normally provided for entering data for storage in mobile station102, information for transmission to network, a telephone number to place a telephone call, commands to be executed on mobile station102, and possibly other or different user inputs.

Mobile station102sends communication signals to and receives communication signals from the wireless network over a wireless link via antenna110. RF transceiver circuitry108performs functions similar to those of a base station and a base station controller (BSC) (not shown), including for example modulation/demodulation and possibly encoding/decoding and encryption/decryption. It is also contemplated that RF transceiver circuitry108may perform certain functions in addition to those performed by a BSC. It will be apparent to those skilled in art that RF transceiver circuitry108will be adapted to particular wireless network or networks in which mobile station102is intended to operate.

Mobile station102includes a battery interface (IF)134for receiving one or more rechargeable batteries132. Battery132provides electrical power to electrical circuitry in mobile station102, and battery IF132provides for a mechanical and electrical connection for battery132. Battery IF132is coupled to a regulator.136which regulates power to the device. When mobile station102is fully operational, an RF transmitter of RF transceiver circuitry108is typically keyed or turned on only when it is sending to network, and is otherwise turned off to conserve resources. Similarly, an RF receiver of RF transceiver circuitry108is typically periodically turned off to conserve power until it is needed to receive signals or information (if at all) during designated time periods.

Mobile station102operates using a Subscriber Identity Module (SIM)140which is connected to or inserted in mobile station102at a SIM interface (IF)142. SIM140is one type of a conventional “smart card” used to identify an end user (or subscriber) of mobile station102and to personalize the device, among other things. Without SIM140, the mobile station terminal is not fully operational for communication through the wireless network. By inserting SIM140into mobile station102, an end user can have access to any and all of his/her subscribed services. SIM140generally includes a processor and memory for storing information. Since SIM140is coupled to SIM IF142, it is coupled to controller106through communication lines144. In order to identify the subscriber, SIM140contains some user parameters such as an International Mobile Subscriber Identity (IMSI). An advantage of using SIM140is that end users are not necessarily bound by any single physical mobile station. SIM140may store additional user information for the mobile station as well, including datebook (or calendar) information and recent call information.

Mobile station102may consist of a single unit, such as a data communication device, a multiple-function communication device with data and voice communication capabilities, a personal digital assistant (PDA) enabled for wireless communication, or a computer incorporating an internal modem. Alternatively, mobile station102may be a multiple-module unit comprising a plurality of separate components, including but in no way limited to a computer or other device connected to a wireless modem. In particular, for example, in the mobile station block diagram ofFIG. 1, RF transceiver circuitry108and antenna110may be implemented as a radio modem unit that may be inserted into a port on a laptop computer. In this case, the laptop computer would include display112, keyboard114, one or more auxiliary UIs116, and controller106embodied as the computer's CPU. It is also contemplated that a computer or other equipment not normally capable of wireless communication may be adapted to connect to and effectively assume control of RF transceiver circuitry108and antenna110of a single-unit device such as one of those described above. Such a mobile station102may have a more particular implementation as described later in relation to mobile station202ofFIG. 2.

FIG. 2is a detailed block diagram of a preferred mobile station202. Mobile station202is preferably a two-way communication device having at least voice and advanced data communication capabilities, including the capability to communicate with other computer systems. Depending on the functionality provided by mobile station202, it may be referred to as a data messaging device, a two-way pager, a cellular telephone with data messaging capabilities, a wireless Internet appliance, or a data communication device (with or without telephony capabilities). Mobile station202may communicate with any one of a plurality of fixed transceiver stations200within its geographic coverage area.100141Mobile station202will normally incorporate a communication subsystem211, which includes a receiver, a transmitter, and associated components, such as one or more (preferably embedded or internal) antenna elements and, local oscillators (LOs), and a processing module such as a digital signal processor (DSP) (all not shown). Communication subsystem211is analogous to RF transceiver circuitry108and antenna110shown inFIG. 1. As will be apparent to those skilled in field of communications, particular design of communication subsystem211depends on the communication network in which mobile station202is intended to operate.

Network access is associated with a subscriber or user of mobile station202and therefore mobile station202requires a Subscriber Identity Module or “SIM” card262to be inserted in a SIM IF264in order to operate in the network. SIM262includes those features described in relation toFIG. 1. Mobile station202is a battery-powered device so it also includes a battery IF254for receiving one or more rechargeable batteries256. Such a battery256provides electrical power to most if not all electrical circuitry in mobile station202, and battery IF254provides for a mechanical and electrical connection for it. The battery IF254is coupled to a regulator (not shown) which provides power V+to all of the circuitry.

Mobile station202includes a processor238(which is one implementation of controller106ofFIG. 1) which controls overall operation of mobile station202. Communication functions, including at least data and voice communications, are performed through communication subsystem211. Processor238(e.g., a microprocessor or processing circuit or core) also interacts with additional device subsystems such as a display222, a flash memory224, a random access memory (RAM)226, auxiliary input/output (I/O) subsystems228, a serial port230, a keyboard232, a speaker234, a microphone236, a short-range communications subsystem240, and any other device subsystems generally designated at242. Some of the subsystems shown inFIG. 2perform communication-related functions, whereas other subsystems may provide “resident” or on-device functions. Notably, some subsystems, such as keyboard232and display222, for example, may be used for both communication-related functions, such as entering a text message for transmission over a communication network, and device-resident functions such as a calculator or task list. Operating system software used by processor238is preferably stored in a persistent store such as flash memory224, which may alternatively be a read-only memory (ROM) or similar storage element (not shown). Those skilled in the art will appreciate that the operating system, specific device applications, or parts thereof, may be temporarily loaded into a volatile store such as RAM226.

Processor238, in addition to its operating system functions, preferably enables execution of software applications on mobile station202. A predetermined set of applications which control basic device operations, including at least data and voice communication applications, will normally be installed on mobile station202during its manufacture. A preferred application that may be loaded onto mobile station202may be a personal information manager (PIM) application having the ability to organize and manage data items relating to the user such as, but not limited to, instant messaging (IM), e-mail, calendar events, voice mails, appointments, and task items. Naturally, one or more memory stores are available on mobile station202and SIM262to facilitate storage of PIM data items and other information.

The PIM application preferably has the ability to send and receive data items via the wireless network. In a preferred embodiment, PIM data items are seamlessly integrated, synchronized, and updated via the wireless network, with the mobile station user's corresponding data items stored and/or associated with a host computer system thereby creating a mirrored host computer on mobile station202with respect to such items. This is especially advantageous where the host computer system is the mobile station user's office computer system. Additional applications may also be loaded onto mobile station202through network200, an auxiliary I/O subsystem228, serial port230, short-range communications subsystem240, or any other suitable subsystem242, and installed by a user in RAM226or preferably a non-volatile store (not shown) for execution by processor238. Such flexibility in application installation increases the functionality of mobile station202and may provide enhanced on-device functions, communication-related functions, or both. For example, secure communication applications may enable electronic commerce functions and other such financial transactions to be performed using mobile station202.

In a data communication mode, a received signal such as a text message, an e-mail message, or web page download will be processed by communication subsystem211and input to processor238. Processor238will preferably further process the signal for output to display222, to auxiliary I/O device228or both as described further herein below with reference toFIGS. 3 and 4. A user of mobile station202may also compose data items, such as e-mail messages, for example, using keyboard232in conjunction with display222and possibly auxiliary I/O device228. Keyboard232is preferably a complete alphanumeric keyboard and/or telephone-type keypad. These composed items may be transmitted over a communication network through communication subsystem211.

For voice communications, the overall operation of mobile station202is substantially similar, except that the received signals would be output to speaker234and signals for transmission would be generated by microphone236. Alternative voice or audio I/O subsystems, such as a voice message recording subsystem, may also be implemented on mobile station202. Although voice or audio signal output is preferably accomplished primarily through speaker234, display222may also be used to provide an indication of the identity of a calling party, duration of a voice call, or other voice call related information, as some examples.

Serial port230inFIG. 2is normally implemented in a personal digital assistant (PDA)-type communication device for which synchronization with a user's desktop computer is a desirable, albeit optional, component. Serial port230enables a user to set preferences through an external device or software application and extends the capabilities of mobile station202by providing for information or software downloads to mobile station202other than through a wireless communication network. The alternate download path may, for example, be used to load an encryption key onto mobile station202through a direct and thus reliable and trusted connection to thereby provide secure device communication.

Short-range communications subsystem240ofFIG. 2is an additional optional component which provides for communication between mobile station202and different systems or devices, which need not necessarily be similar devices. For example, subsystem240may include an infrared device and associated circuits and components, or a Bluetooth™ communication module to provide for communication with similarly-enabled systems and devices. Bluetooth™ is a registered trademark of Bluetooth SIG, Inc.

In accordance with an embodiment, mobile station202is a multi-tasking handheld wireless communications device configured for sending and receiving data items and for making and receiving voice calls. To provide a user-friendly environment to control the operation of mobile station202, an operating system resident on station202(not shown) provides a GUI having a main screen and a plurality of sub-screens navigable from the main screen.

The liquid crystal display cell222is shown in greater detail inFIG. 3in which a light source formed from multiple LEDs322,324,326is used as a backlight. Preferably, the LCD is a field sequential liquid crystal display (FS LCD). LCD controller316provides a voltage to the common electrode(s)308and the active elements310of the active matrix. The active elements are preferably thin film transistors. The common electrode(s)308and active elements are supported on substrates306and312, respectively. Alternatively, the LCD may be a passive matrix. The LCD preferably contains a brightness enhancing film or layer304to optimize the distribution of light for a viewer and a diffusing layer. As the preferred liquid crystal material is super twisted nematic, polarizers302and314are used. The LCD controller316sets the pixel grey scale of the LCD. An optional processor318may coordinate synchronization of the LCD controller316with the light source controller320. Preferably, the LCD controller316and the processor318are integrated into a single device317, which may simply be referred to as an LCD controller having the capability of controlling a light source controller320. The light source may be implemented by using red, green, and blue LEDs322,324,326. In a specific embodiment, four green, four red, and two blue LEDs are used to provide full colour and/or black and white display. The LED controller320may sequence the three colours or may simultaneously energize LEDs of all of the colours and terminate power to the LEDs simultaneously. Other combinations of LEDs are contemplated. The light guide328may have a tapered block construction and may have approximately a trapezoidal, cross sectional form to more evenly distribute the light into the LCD. The light guide may also have uneven areas330,332that scatter the light so as to avoid shadowing effects in the LCD image. Although uneven area330is shown to project out from the surface of the light guide328and uneven area332is shown to project inward to the surface of the light guide328, the uneven areas may be arranged differently so long as the arrangement effectively scatters the light from the LEDs322,324,326. The uneven areas may be abraded, molded, corrugated, chemically etched, or the like. Preferably, to maximize the utilization of light, the LEDs322,324,326and the light guide328are partially enclosed by a reflector such that the only opening is fully bounded by the light transmissive area of the LCD.

FIG. 4illustrates an embodiment of the LCD controller402and LCD430for the method. The LED controller may be internally adapted to provide a sequence of lights each centered on a specific wavelength according to the LEDs energized, followed by light generated simultaneously from all LEDs or at least two LEDs generating light centered on two different wavelengths. InFIG. 4, in synchronization with the LED controller, the LCD controller402creates a grey scale pattern for each light centred on a specific wavelength according to column driver440(source driver) according to data and control signals410and row selectors450(gate driver) from a data bit line and a LOAD LINE clock in a X-Y matrix arrangement. For a red light pattern, only pixels selectable by the column driver440may be set to a variable transmissive state to provide a desired grey scale pattern. Pixels that do not have a red component of light are turned off. For green and blue light patterns, similar procedures are followed. When all red, green, and blue colours are transmitted through a given pixel, that pixel may have a white or whitish appearance because of the blending of the three primary colours perceived by a viewer. Advantages in using the light source to determine colours include elimination of a colour filter layer, thus enhancing brightness of the display by reducing a light absorbing layer, and increasing the resolution as only one pixel is needed to provide full colour instead of separate red, green, and blue pixels. The size of a pixel is allowed to increase while resolution is improved; in other words, using the light source and not the LCD to determine colour optimizes substrate real estate usage.

FIG. 5illustrates a colour only mode in which either the entire display screen is in colour or the non-colour portion of the display screen is in the off state. In operation, pixel grey scale is achieved through pulses written to a pixel during scanning. Each colour frame502is divided into three parts (or fields)504,506,508for the three colours in full colour mode. Each pixel to be illuminated by a specific colour of light achieves a grey scale value from a pulse pattern into the source of the thin film transistor providing charge to the pixel. The pulse pattern (i.e., colour scans) includes multiple high and/or low pulses for each pixel. One pulse is applied to each colour pixel during a scan of the colour region that includes the colour pixel. During the colour region scan (or sweep)532, the actual scanning occupies most of the time allotted530for a given colour. It is the successive scans of the colour pixels during a frame that establishes a grey scale value. A smaller portion of the time allotted in a scan period is idle time534. During most of the scan period, the light source is turned off514. In alternative embodiments, the light source may remain on for most or all of the scan period and/or the actual scanning may occupy a different portion of the time allotted for a given colour. Once the final grey scale value for a row or line of pixels is fairly well established, the light source (e.g., light emitting diode) is turned on512. In some embodiments, during the light source turn on time, the common electrode of the display is inverted from a first voltage bias level522to a second voltage bias level524to prevent charge buildup in the liquid crystal that would degrade performance and damage the display. The inversion of the common electrode voltage occurs for each colour for each frame. Thus, for a red, green, and blue pixel LCD, the common electrode voltage is inverted three times. Other inversion modes are contemplated such as line inversion and pixel inversion. In line inversion, a given line may be alternately supplied through the source driver with voltages from a first set of a polarity and then supplied with voltages from a second set of a polarity opposite to that of the first set; that is, a non-inverting pair of voltages may be applied and an inverting pair of voltages may later be applied. In pixel inversion, alternate columns may be supplied for each row with voltage sets of opposing polarities.

FIG. 6represents a more specific embodiment. An output shift register (e.g., serial in/parallel out shift register) may be used for scanning the display screen. The shift register contains initialization values for the gate shift register. It preferably contains a one-hot encoding of the starting line number of display screen. (As used in an embodiment, one-hot encoding refers to a single active bit that is shifted through the shift register such that only one line at a time of pixels is written to from the source driver.) The shift register is loaded and then used to sweep the display. A LINE CLOCK rate is relatively high; for example, a 10 MHz clock rate may be used. The storage elements may be latches618,626that latch data on the rising or falling edges of a clock, D type flip flops, or the like. A counter602may be used to hold the number of lines in the display screen.

FIG. 7illustrates an overview of the embodiment of a method corresponding to the display scanning system. In the general method, initialization occurs704(e.g., registers are initialized) and the three colour fields are cycled through706-710through successive scans during a frame.

FIG. 8illustrates a more detailed embodiment of a scan for a field. The gate line driver is shifted once804. The load pattern is deasserted806. A new source pattern is loaded807. The source lines on the display matrix are driven808. The line count is reduced by one810. As long as the counter does not expire (e.g., the line count remains greater than zero in a count down mode)812, scanning resumes at step804.

A field sequential liquid crystal display maintains its white colour point through compensation values to at least one colour light emitting diode that illuminates the display. A degradation curve may be used to calculate extrapolate the theoretical forward voltage of the light emitting diode. Additional complexity arises from the need for calculating uptime for multiple light emitting diodes of different colours. Brightness levels may also be factored in.

FIG. 9illustrates an embodiment of a general method for determining the application of compensation to a light emitting diode of a single colour A according to the time of use or a more complicated function of time of use and brightness per use. It is to be understood that in a colour display, there will be two or more light emitting diodes of different colours—for example, red, green, and blue—or one or more light emitting diode that produces two or more colours. Colour A, as used here, may be any colour—including red, green, or blue. LED compensation is preferably performed through pulse width modulation (PWM) techniques or through current control. A determination is periodically made as to whether a light emitting diode is turned on904. If so, then the time of use value is adjusted to correspond to the time the light emitting diode has been turned on906. For example, the time of use value may be expressed as Σi=1k{unit time Δt} where the unit time At may be uniform or non-uniform in duration. A degradation curve may be used to calculate or extrapolate the theoretical forward voltage of an LED based on usage time. An algorithm may be used to keep track of display “uptime” and to insert Vf compensation values as required to pull a white point back to a specified value. In another embodiment, a more complicated function value is adjusted and stored in which the function correlates time of use and intensity of the light emitting diode being monitored to determine a cumulative intensity-time value. In this embodiment, the display brightness level must be tracked. For example, the cumulative intensity-time value may be expressed as Σi=1k{intensity during unit time I * unit time Δt} where the unit time Δt may be uniform or non-uniform in duration. Because LEDs of different colours (e.g., red, green, blue) are likely to be used, there is additional complexity for calculating uptime in a field sequential LCD since the amount of ON versus OFF time for red, green, and blue is different. Through multiple LEDs having two or more different colours, a synergy may arise that further complicates the adjustment values to maintain the white colour point. Thresholds are stored for determining the amount of compensation to be applied to the LED. The thresholds may be stored in a data structure, an array, a look up table (e.g., an aging table), or the like. If the time of use value or the cumulative intensity-time value for the light emitting diode exceeds a first threshold908and is less than or equal to a second threshold level910, then a first compensation element or arrangement is turned on912. A compensation element/ arrangement may be resistive or capacitive in effect and includes one or more passive and/or active components, such as a resistor, a capacitor, or a transistor. In the case of PWM techniques, the compensation arrangement may entail the processor altering a set of pulses applied to the LED being controlled. For example, the number of pulses may be varied in a unit interval of time. If the time of use value or the cumulative intensity-time value for the light emitting diode exceeds a second threshold level910, but not a third threshold level914, a second compensation element or arrangement is switched on916. In this case, the first compensation element or arrangement may be switched off or may remain switched on. If the time of use value or the cumulative intensity-time value for the light emitting diode exceeds a third threshold, then the third compensation element or arrangement is switched on918. Either or both of the first and second compensation elements or arrangements may be switched off in this case.

FIG. 11illustrates an embodiment of a general method for a process for determining the white point compensation of a field sequential liquid crystal display. In step1102, a white point is calibrated at the factory. For a red, green, blue colour scheme in which red, green, blue light emitting diodes are used, the calibrated may be set by the following equations:
RT=XCseconds
GT=YCseconds
BT=ZCseconds
At some point, later or earlier than step1102, an ageing table is created, step1104, for the particular model, sampled batches, or individual field sequential liquid crystal displays. An exemplary ageing table is presented below:

RTGTBT1hourΔ1Ω1φ110hoursΔ2Ω2φ21000hoursΔ3Ω3φ310,000hoursΔ4Ω4φ4
After steps1102and1104, through actual usage of the FS LCD, the white colour point is compensated automatically. For example, when usage time is greater than or equal to one hour but less than 10 hours, the R, G, B values may be set as RT=XC+Δ2; GT=YC+Ω2; and BT=ZC+Φ2.

FIG. 10illustrates a block diagram of an arrangement of a current compensation scheme for a light emitting diode of one of the three colours. It is to be understood that light emitting diodes of one or both of the other colours will similarly be compensated for behavioural changes over the lifespan of the LED. InFIG. 10, light emitting diode LED1may have series compensation A or parallel compensation B or both. The switches SWA and SWB may be implemented as complementary metal oxide semiconductor field effect transistors (CMOS FET) or as another active circuit element. A processor1002controls a switch internally or externally, such as one of switches SWA1, SWA2, and SWN. In an embodiment, only one switch of the A switches may be activated (i.e., turned) or two or more switches may be activated through processor1002or other control circuitry. Because it is not desirable to keep an LED on continuously, it is necessary that the current path from power +V through a current limiting resistor RES be interruptible, so a switch is always required at the power receiving end of the LED. The activated switch permits compensation element(s) A to modify the current and voltage applied to LED1. In an embodiment, it may be desirable to have one of the compensation elements A to have negligible resistance and capacitance such as through the absence of any impedance element CEA. Additionally or largely alternatively to series compensation elements A, compensation elements B may be placed in parallel with LED1. Processor1002or other control circuitry may also be used to control switching of switches SWB1, SWB2, through SWBN to activate compensation elements CEB1, CEB2, and CEBN. It is to be understood thatFIG. 10may be varied so as there may be a single switch A or multiple switches A in conjunction with zero or more switches B. Other compensation arrangements are contemplated. Preferably, processor1002and the compensation circuitry for the light emitting diode or diodes are incorporated within the same integrated circuit. Alternatively, processor1002and the compensation circuitry may be formed separately in which case the processor may control the switches through various interface circuitry through addressing information or may directly control the switches. In the case of pulse width modulation (PWM), the processor may directly control an LED without an impedance element by controlling the number of uniform pulses per unit time or by altering the pulse width of one or more pulses in a pulse train.

The above-described embodiments of the present application are intended to be examples only. Those of skill in the art may effect alterations, modifications and variations to the particular embodiments without departing from the scope of the application. The invention described herein in the recited claims intends to cover and embrace all suitable changes in technology.