WO2007034360A2 - Ecrans a matrice active et leurs procedes d'excitation - Google Patents

Ecrans a matrice active et leurs procedes d'excitation Download PDF

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Publication number
WO2007034360A2
WO2007034360A2 PCT/IB2006/053228 IB2006053228W WO2007034360A2 WO 2007034360 A2 WO2007034360 A2 WO 2007034360A2 IB 2006053228 W IB2006053228 W IB 2006053228W WO 2007034360 A2 WO2007034360 A2 WO 2007034360A2
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WIPO (PCT)
Prior art keywords
current source
voltage
output
column
driver
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Application number
PCT/IB2006/053228
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English (en)
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WO2007034360A3 (fr
Inventor
Johannes A. T. M. Van Den Homberg
Hendrik J. Bergveld
Franciscus A. C. M. Schoofs
Eise C. Dijkmans
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Koninklijke Philips Electronics N.V.
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Publication of WO2007034360A2 publication Critical patent/WO2007034360A2/fr
Publication of WO2007034360A3 publication Critical patent/WO2007034360A3/fr

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Classifications

    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G09G3/3611—Control of matrices with row and column drivers
    • G09G3/3685—Details of drivers for data electrodes
    • G09G3/3688—Details of drivers for data electrodes suitable for active matrices only
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2310/00—Command of the display device
    • G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
    • G09G2310/0243—Details of the generation of driving signals
    • G09G2310/0248—Precharge or discharge of column electrodes before or after applying exact column voltages
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2310/00—Command of the display device
    • G09G2310/06—Details of flat display driving waveforms
    • G09G2310/066—Waveforms comprising a gently increasing or decreasing portion, e.g. ramp
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/2007—Display of intermediate tones
    • G09G3/2014—Display of intermediate tones by modulation of the duration of a single pulse during which the logic level remains constant

Definitions

  • This invention relates to active matrix display devices, and particularly to display devices in which an alternative driving scheme is employed, for example Active- Matrix Liquid Crystal Displays (AMLCDs).
  • AMLCDs Active- Matrix Liquid Crystal Displays
  • the invention relates in particular to the integrated driver circuits for such devices.
  • AMLCDs consist of a large number of liquid crystal pixels, the voltage across which determines their transmittance of light.
  • the pixels are arranged in columns and rows.
  • AMLCDs have on-glass Thin-Film Transistors (TFTs) that form a switch between each LCD pixel and its corresponding column line.
  • TFTs Thin-Film Transistors
  • the gates of these TFTs are connected together horizontally, so that a gate driver IC can "enable" the rows in a sequential order.
  • the time during which an individual row is selected is referred to as the line addressing time.
  • a source driver IC applies voltages to the columns that correspond to the desired transmittance of each pixel in the selected row.
  • each output of the source driver IC is a buffered DAC output.
  • FIG. 1 The basic concept of an AMLCD including gate and source driver IC is illustrated in Fig. 1 for a screen resolution of N rows and M columns with an n-bit color depth. This implies that each LC pixel can be driven to one out of 2 n transmittance levels. As an example, the rows are driven sequentially from top to bottom. Alternative orders are possible depending on the applied scanning algorithm. When all rows have been addressed and all pixels have reached the desired transmittance level, a complete frame has been written and the selection of rows repeats for writing the next frame. Depending on the size/resolution of the LCD screen, several gate driver ICs and several source driver ICs are applied in practical realizations.
  • the achieved transmittance of the LCD pixel is determined by the voltage across the capacitance labeled 'LC in Fig. 1.
  • the bottom plates of these LCD pixels are connected to the common electrode with potential V CO m.
  • Non- idealities in the outputs of the source driver ICs such as variations in output level between two adjacent columns with identical digital input, will lead to image artefacts and should be minimized.
  • an additional capacitor is used in parallel to the LC liquid crystal pixel capacitance for stabilization of the pixel voltage.
  • the bottom plate of this capacitor can be either connected to V CO m, a separate electrode or an adjacent row-line. This capacitor has been omitted in Fig. 1 for simplicity.
  • a prerequisite for any LCD driving scheme is that each pixel is driven with an
  • the source driver IC should address the pixel with a voltage of +V g reyievei during one frame and with a voltage of-V grey ievei during the next frame. This is commonly referred to as frame inversion.
  • the transmittance of the LCD pixel is not sensitive to the sign of the applied voltage.
  • a polarity signal is used in addition to the digital transmittance level signals to determine the sign of the analogue voltage at the outputs of the source driver IC. This signal will toggle between positive and negative from frame to frame for each pixel.
  • the translation of the desired transmittance of the LCD pixel into an output voltage of the source driver IC occurs through a so-called gamma curve.
  • This gamma curve is highly non- linear. Since AC drive is required and the complete effective gamma curve is usually asymmetrical (for example caused by the asymmetric signal injection via the gate electrodes of the TFTs), separate gamma curves are used for positive and negative driver output voltages, respectively.
  • the gamma curves should be programmable in a practical device.
  • a common way of implementing the DAC function in the source driver IC is by using resistor ladders and a selection matrix.
  • a tap is selected from the ladder implementing the positive gamma curve or the negative gamma curve. This is shown in Fig. 2 for one row and one column. As can be seen in Fig. 2, both the positive (20) and negative ladder (22) have
  • each selection matrix (24) selects one of these levels from either the positive or negative ladder, so 2 n+1 lines are fed from the ladders to the selection matrix.
  • the ladders are placed centrally on the IC, whereas the 2 n+1 lines are fed over the entire IC, with one selection matrix being used for pairs of columns, because hardware can be shared between adjacent columns when they have opposite polarities (26).
  • the color depth for the display is implemented in the voltage domain. This means that when the color depth is increased, the number of voltage levels is increased with a factor of 2 for each additional bit. As a consequence, the size of the selection matrices doubles for each extra bit. This is a disadvantage of this set-up.
  • a corresponding "grey-scale voltage selecting circuit" 32 provides a ramp voltage on the drains of all TFTs in that line. This means that the TFT is now used to sample the correct voltage level on the liquid crystal cell by means of using the corresponding pulse-width for driving the gate of the TFT. Therefore, the voltage on the pixel tracks the ramp voltage until the TFT switch is opened by the pulse-width signal, after which the voltage remains stable until the new voltage is written in the next frame. Since the gamma curve is non- linear, the ramp voltage does not need to be a linear ramp, but can be any sort of curve.
  • Fig. 4 shows a pixel circuit for using a ramp voltage, which is tracked by the pixel and then sampled. In series with the regular TFT (40), which conducts during the complete line addressing time, an additional TFT (42) is used to sample the wanted value of a ramp voltage on the pixel.
  • the sampling switch has been moved into the source driver IC 50 in the form of a transmission gate 52 (NMOS and PMOS switches in parallel), because the additional sampling TFT in Fig. 4 leads to a decrease in light throughput when placed in each pixel and the TFT has poor performance compared to transistors realized in IC-technology silicon.
  • the pixel configuration is then the same as in Fig. 1.
  • the silicon area of these source driver ICs tends to become unacceptably high using existing topologies/architectures and driving schemes, like the driving method using resistor ladders shown in Fig. 2.
  • the silicon area needed to implement a color depth of the LCD screen of n bits scales with 2 n . This means that for each additional bit the number of resistor taps doubles, as do the number of switches in the selection matrices, each having a track connected to it that is routed over the entire IC. Since this is inherent to any driving architecture using resistor ladders, this architecture is fundamentally unsuitable for realization of low silicon-area driver ICs.
  • a voltage signal with a certain waveform is offered to each column that includes all values to which the column should be charged to cover all possible transmittance values of the addressed pixel.
  • An example of such a waveform is a ramp voltage.
  • the density in the used time grid increases with a factor of two, but this can be realized in silicon without scaling the area with a factor of two.
  • an active matrix display device comprising: an array of pixels arranged in rows and columns; a column driver circuit for providing pixel drive signals to the columns of pixels, wherein the column driver circuit comprises an array of current source circuits, a respective current source circuit being provided for each column of pixels, wherein each current source circuit comprises:
  • - a storage capacitor connected to the output of the output driver; and - means for controlling the time during which the current source supplies current to or drains current from the storage capacitor (72), thereby determining the output voltage of the output driver,
  • the device further comprises a mapping means for deriving from a pixel drive level a digital value which represents a time period for the control of the supply switch of each current source circuit.
  • This arrangement provides a current source circuit for each column, and this facilitates the application of inversion patterns, as each column is controlled independently.
  • the control signals that are needed by the current source circuits are timing control signals, rather than signals to be sampled. These control signals can easily be provided across a large area array without loss of information.
  • the means for controlling the time is preferably a supply switch.
  • the use of an output driver enables the resulting drive voltage to be independent of the characteristics of the column of pixels, as the pixel column is not used to derive the drive voltage.
  • the mapping means preferably implements a single mapping function for use in providing the digital values for all current source circuits.
  • the control circuit preferably comprises a look-up table (LUT).
  • LUT look-up table
  • Each current source circuit may comprise a precharge switch for connecting the output driver output to a predetermined voltage. This defines the starting point from which each column voltage is charged (or discharged) by the current source circuit.
  • the current source of each current source circuit may comprise a unipolar current source, and in this case, the reference voltage is below the lowest pixel drive voltage or above the highest pixel drive voltage.
  • the current source of each current source circuit can comprise a bidirectional current source, and in this case the reference voltage is between the lowest pixel drive voltage and the highest pixel drive voltage.
  • the current source of each current source circuit can supply or drain a constant current over time.
  • the invention also provides a method of driving the pixels of an active matrix display device comprising an array of pixels arranged in rows and columns, the method comprising: deriving a digital value representing a time period for each column from a pixel drive level for each column using a common mapping of pixel drive levels to digital values; driving an array of current source circuits, with a respective current source circuit for each column of pixels, each current source circuit being driven for the time value corresponding to the respective digital value; varying the output voltage of an output driver associated with the respective current source circuit using the current source circuit charge flow; and driving the pixels with the resulting output driver output voltage.
  • Fig. 1 shows a known AMLCD screen with a resolution of N rows and M columns;
  • Fig. 2 shows the known use of resistor ladders to implement the DAC function in a source driver IC
  • Fig. 3 shows a pixel configuration and driving circuit for a driving scheme of US 6,567,062;
  • Fig. 4 shows a block diagram of another known pixel arrangement
  • Fig. 5 shows a block diagram of another known pixel arrangement
  • Fig. 6 is used to explain the principles underlying a proposed drive scheme of the applicant
  • Fig. 7 is used to explain the principles underlying the drive scheme of the invention
  • Fig. 8 shows a first detailed embodiment of the invention using a unipolar current source for each column
  • Fig. 9 shows waveforms for explaining the operation of the circuit of Fig. 8;
  • Fig. 10 shows a second detailed embodiment of the invention using a bipolar current source for each column;
  • Fig. 11 shows waveforms for explaining the operation of the circuit of Fig. 10.
  • Figs. 12A and 12B show alternative ways to provide precharging.
  • the applicant has proposed (but not yet published) a column driver circuit in which a current source circuit is used to provide charge to each column for a selected time period. This time period gives rise to an amount of charge which in turn leads to a desired end voltage on the column.
  • a look-up table (LUT) is shared by all columns, and individual counters are present at each column for the conversion of a digital value from the LUT to time.
  • the current source circuits can, however, be individually calibrated. This provides efficient use of silicon area for the column driver circuit whilst enabling accurate control of the pixel brightness output.
  • Fig. 6, shows in schematic form the manner in which columns of pixels are driven.
  • Fig. 6 shows a single current source circuit 60 (which functions as a column driver) having a current source 62 and a supply switch 64 for controlling the time during which the current source supplies current to or drains current from the column.
  • the bidirectional current source 62 and switch 64 are of course merely a schematic representation of the function, and they may also be implemented by two unipolar current sources each having a switch.
  • the switch function is not necessarily implemented as a series switch with the current source but can be implemented as part of the output interface of the current source.
  • a digital value representing time is derived from a pixel drive level, and a common mapping is used for all columns in obtaining the digital values from the pixel drive levels. This digital data is converted locally into a time period using a local counter (not shown in Fig. 6).
  • Each column-driver 60 has to drive a capacitive load of column and pixel and the voltage value to which this load capacitance C load must be driven corresponds to a certain amount of charge stored in C load -
  • the desired voltage end value across the capacitor can be reached.
  • Time t ⁇ y depends on the desired transmittance level.
  • the capacitor starts with a known charge due to a pre-charge (Pc) to a precharge voltage level Vp re - C h a rg e that is applied at the beginning of the line addressing time.
  • the current source lin t can either sink or source current, as shown schematically in Fig. 6.
  • a constant value current source is shown in Fig. 6, leading to a ramp voltage on the capacitor, as shown in the lower part of Fig. 6, both for charging and discharging the capacitance.
  • the scheme is not, however, limited to a constant current.
  • the main advantage compared to the resistor-ladder architecture of Fig. 2 is that the color depth is not implemented in the voltage domain, which means that the silicon area does not scale with 2 N . Both I ⁇ t and t ⁇ y determine the charge on C load - This means that color depth can be implemented in the current and/or time domain. For varying values of lin t the voltage on C load will have a different shape than shown in Fig. 6.
  • An additional advantage is that multiple ladder-tap voltages no longer need to be transported from IC to IC in case of cascaded ICs. Instead, a simple digital LUT can be used in each IC, translating the desired transmittance level of a pixel into a combination of lin t and tgr ey . This enhances the programmability of the gamma curves.
  • the digital LUT may instead be provided off-chip as a central resource which provides functionality to all column driver ICs.
  • One current source is also used for each column, so that there is no common ramp signal.
  • Any inversion scheme can be implemented simply by defining the current direction per column. The problem of a common ramp voltage being fed over the complete large-width IC, which is susceptible to noise pick-up, also is avoided.
  • Local calibration loops can be used to make sure that the voltage waveform spanning all gamma voltages, generated by integrating Ijn t in the column and pixel capacitance C load , reaches a single (or multiple) defined intermediate value(s) during the line addressing time. Local calibration loops can also be used for cases wherein the load capacitance seen at the individual driver outputs differs from the anticipated value. This calibration essentially involves adjusting the charge delivered at the output to ensure the correct column end voltage.
  • This drive scheme can be applied to a conventional LCD panel as shown in Fig. 1, with one common electrode.
  • the approach can of course be applied to other active- matrix LCD panel configurations as well.
  • the current source 62 is used during a fixed amount of time tgr ey . This means that even when the switch and column have series resistance (which is always the case), the correct amount of charge is fed to the column and pixel.
  • This drive scheme requires only the value of the capacitance of column and pixel (C load ) to be known to achieve the correct transmittance of the pixel.
  • the value of t ⁇ y can be determined from the values of I mt , C loa d and the desired end voltage on the pixel V en d, which depends on the desired transmittance level via the non-linear gamma curve, according to: f _ C ⁇ load V* end ( ⁇ ⁇ l grey ⁇ j ' ⁇ l )
  • C load - Q oad represents the capacitive load of the column and addressed pixel as seen from the driver output, and this makes the method dependent on the characteristics of the connected LCD screen. This works well for LCD screens where this load capacitance can be described accurately enough in the system.
  • the capacitance of a liquid crystal is voltage-dependent, making it a non- linear device.
  • the capacitance C load which is the sum of the column and pixel capacitance, may also become voltage-dependent, i.e. a non- linear capacitance.
  • the numerator in equation (1) changes to the voltage-dependent charge on the load capacitance to obtain end voltage V en d, i-e. Q(Vend)- This possible non-linearity has to be taken into account in the lookup table determining the value for tgr ey for each transmission level.
  • the look-up table translates a desired transmittance level into a desired end voltage V end via the non- linear gamma curve.
  • Voltage V end is in turn translated into a tgr ey value, assuming a fixed and known value for I mt , or into an integrated charge value I mt tgr ey
  • tgr ey assuming a fixed and known value for I mt
  • I mt tgr ey an integrated charge value
  • C load may become increasingly non- linear, e.g. due to cross-talk problems, making it more elaborate to describe the relation Q(V end ) accurately in a look-up table.
  • Variations in temperature will increasingly induce variations in C load across the increasingly large LCD screen. This may imply that C load , as seen at a single driver output connected to a single column, may also become different for different addressed rows.
  • the use of calibration schemes to address all of these issues becomes increasingly complex.
  • Fig. 7 shows the basic concept underlying the drive scheme of the invention.
  • the driving method is made independent of the LCD screen characteristics by integrating the output current of a driver output in an on-chip capacitor Csi, instead of integrating charge on the column and pixel capacitance C load - Since the capacitance Csi is realized on the driver IC, with close control over its value, the integration time tgr ey can now be determined according to equation (1) without the need to include any information on the LCD screen characteristics, i.e. Csi replaces C load - In fact, the output of the driver becomes a voltage with an end value V en d, which is the result of storing a charge Iinttgrey on the on-chip capacitor Csi. The actual column and pixel capacitance C load , as seen at the driver output, now becomes irrelevant in the definition of the transmission level of the liquid crystal.
  • the circuit comprises a current source 70 and supply switch 78 for controlling the coupling of the current source 70 to an output driver 71.
  • the output driver has a feedback path including a storage capacitor 72 (CsO as we U as a precharge switch 74 in parallel with the storage capacitor 74.
  • the current source 70 is selectively coupled by the switch 78 to the inverting input of the output driver 71, and the non- inverting input is connected to a reference voltage Vr e f.
  • the output of the output driver is connected to the column, represented as the load capacitance C loa d to the common electrode voltage V CO m. As shown in the timing diagram of Fig.
  • the pre-charge of the capacitor 72 is needed to start off from a known charge at the start of the integration time.
  • the output driver circuit 71 ensures equal voltages at its non inverting '+' and inverting '-' inputs.
  • the capacitor can be precharged using a short-circuiting method, and the resulting output voltage Vc then starts from V re f at the beginning of each line addressing time tun e -
  • the value of the charge to which the capacitor is pre-charged is irrelevant, as long as its value is defined.
  • the output voltage of the output driver is kept constant at value V end -
  • the polarity of the end voltage is determined by the direction of the current source, which can either sink or source current out of or into the capacitor 72. Which current direction is used depends on the polarity signal that is used to determine the voltage polarity of the addressed pixel.
  • a constant current source lin t has been assumed for simplicity, leading to a ramp voltage at the output driver output.
  • current lin t may be made variable during the line addressing time, for instance to implement part of the depth-depth/ transmission- level resolution.
  • the charge on Csi (Iin t tgr ey ) is the accuracy-determining variable, and the level-depth/transmission- level resolution can therefore be implemented both in the amplitude domain (Ijn t ) and in the time domain (tgr ey ).
  • the approach of the invention is to apply a voltage to the columns using output driver circuitry (for example in the form of a buffer circuit), rather than applying a current to the columns of the LCD screen.
  • This use of an output driver circuit to provide a voltage to the columns of the display is essentially the same as existing voltage-addressed architectures.
  • the approach of the invention provides all of the advantages outlined above in connection with Fig. 6.
  • the circuitry of Fig. 7 does not scale with a factor 2 in silicon area for each additional bit in color depth.
  • the output driver 71 maintains its inputs at virtual ground. This means that rail-to-rail inputs are not required, which makes the output driver circuitry simpler than buffer circuitry used in voltage-addressing schemes.
  • the output driver 71 can thus be realized in a smaller silicon area.
  • the reference voltage V re f may be chosen at a convenient level for optimum DC settings. Since the DC voltage at the '-' input of the output driver is at V re f, it can be chosen such that the realization of the current source becomes as simple as possible, allowing enough voltage headroom.
  • the buffers need to be able to handle all gamma voltages, both of the positive and negative gamma curve, and this requires the buffer input circuitry to be rail-to-rail.
  • the drive scheme of the invention again avoids the use of a common ramp signal, thereby enabling the implementation of any inversion scheme and local calibration loops.
  • the drive scheme of this invention is screen- independent and does not need any information on the column capacitance C load as input for the system. This becomes beneficial when the screen performance and area/size increases, making it increasingly difficult to include accurate information on C load in the system.
  • Local calibration can be implemented, for example, by calibrating the charge delivered to the storage capacitor Csi to compensate for the actual values of lin t and Csi being different from the anticipated values.
  • the value of C load is irrelevant in this calibration process.
  • the calibration is not aimed at adapting the driver-IC characteristics to the screen characteristics, but is instead aimed at overcoming matching problems on the integrated circuit.
  • Fig. 8 shows first possible embodiment of the invention using a unipolar current source 80 for each column.
  • Fig. 8 shows a single column 82 and a single schematic pixel circuit 84 within the column.
  • the switch 78 is controlled by a look-up table 86, and this look-up table may be shared between all column driver circuits.
  • the range of voltages to which the column must be driven is V N , O to Vp )0 , which are the required voltages for a black state for the two polarity schemes (for a 'normally- white' LCD screen).
  • V N , O to Vp )0 the required voltages for a black state for the two polarity schemes (for a 'normally- white' LCD screen).
  • the reference voltage the starting voltage
  • the reference voltage the starting voltage
  • the current source lin t 80 is shown to flow in the upward direction as an illustration, leading to a rising voltage waveform at the output of the output driver 71 from Vr e f to Vp )0 . Assuming a constant value for 1 ⁇ , a voltage ramp will occur at the output of the output driver circuit. Other embodiments where the current is variable, i.e. color depth resolution is placed both in the amplitude (lin t ) and time (tgr ey ) domain, can also be envisioned, as mentioned earlier. Of course, the current may also flow in the downward direction.
  • the reference voltage should then be chosen greater than or equal to Vp )0 as indicated in the Figure.
  • the integration time period tgr ey is obtained from the look-up table 86 based on the desired transmittance information.
  • the look-up table 86 includes the entire gamma curve.
  • the translation can be done based on a constant-cycle-time clock, or based on a variable-cycle-time clock.
  • the reference voltage V re f may be chosen for optimum DC settings of the circuits.
  • V re f With current lin t flowing in the indicated direction, and a rising voltage waveform is obtained at the output driver output, the value of V re f should be less than V N , O - In that case, the voltage waveform after pre-charge will include all gamma voltages ranging from V N , O to Vp )0 .
  • Other pre-charge schemes may also be used, as long as the capacitor starts with a defined charge and as long as the resulting voltage waveform at the output driver output includes all gamma voltages in the range. This allows more freedom in choosing V re f. In the embodiment depicted in Fig. 8, the column and pixel are also pre- charged to Vr e f, since the output driver output always remains connected to the column.
  • the initial charge on C load can be chosen to be able to reach the desired end voltage in time. Whether this is needed depends on the available line addressing time and the value of the load capacitance C loa d-
  • Fig. 9 shows possible waveforms to clarify the idea.
  • the top plot in Fig. 9 shows the column voltage returning to the pre-charge voltage at the end of each line addressing time, and shows the column being charged to a voltage lying alternately in the two polarity ranges.
  • the second plot in Fig. 9 shows the control of the precharge switch 74 (see Fig. 8), the third shows the control of the current source switch 78 (see Fig. 8), and the bottom plot shows a polarity control signal.
  • FIG. 10 A possible embodiment with a bipolar current source is shown in Fig. 10. Similar considerations, concerning the choice of V re f, implementation of pre-charge for Csi, and pre-charge of the columns apply for this embodiment.
  • the precharge has again been implemented by short-circuiting Csi, as before, which means that V re f will appear at the output driver output during pre-charging.
  • V re f between the voltage values of the negative and positive gamma curves, e.g. equal to V CO m as indicated in the right hand part of Fig.
  • the voltage waveform at the output driver output will include all voltages between V CO m and V N , O for negative polarities and all voltages between V CO m and Vp )0 for positive polarities.
  • Other embodiments achieving this with a bipolar current source can also be envisioned.
  • Fig. 11 The main voltage waveforms for this embodiment are illustrated in Fig. 11, showing the same plots and desired transmittance levels as in Fig. 9.
  • the pre- charge level V co m can be an intermediate level, in the middle between the negative and positive gamma curves. This is more efficient, especially for positive gamma voltages, since the capacitor no longer needs to be charged starting from voltage Vpre-charge lower than V N , O - Fig. 11 shows that the pre-charge is more efficient, since the variations in the column voltage are reduced.
  • a time grid that is a factor of two coarser compared to the time grid required for a unipolar source can now be used. Since the slopes of the ramps are less, the signals tgr ey and/or the value of lint have also changed. The influence of variations of parameters on the performance of the driver circuit is also made equal for negative and positive gamma curves. This leads to less image flicker. Any error accumulates during time integration, so that the error is largest at the end of the line addressing time tun e - In the case of a unipolar current source, the ramp has to run all the way from V N , O to Vp )0 , and therefore the error is larger for the positive gamma curve than for the negative gamma curve.
  • the LUT in the embodiments above can actually include two sub-LUTs, one implementing the negative gamma curve and one implementing the positive gamma curve. Which sub-LUT is used for a certain frame depends on the desired polarity, hence on the value of V po i.
  • the value of the current source lin t can also be made variable. In this way, any voltage waveform can be generated.
  • the LUT is then used to translate the desired transmittance level into a combination of lin t and tgr ey .
  • a single mapping operation can still be provided for all column driver current source circuits.
  • the charging capacitor does not have to be charged by a pure current source, and the current source can be implemented as a voltage source with a series impedance, providing the series resistance does not become dominant or significant compared to the load capacitance.
  • the output driver can be implemented with either low output impedance (for example an operational amplifier (opamp)) or with high output impedance (for example and operational transconductance amplifier (OTA)).
  • the gain should be high enough to guarantee a virtual ground at the inverting input and a correct output voltage irrespective of the load impedance, when the load is within the expected range of possible output loads to be experienced in operation of the circuit.
  • the output of the driver is a voltage, and the voltage level is determined by the current integration step.
  • the output driver provides isolation between the pixel columns and the circuit providing the output drive level.
  • each current source circuit comprises a precharge switch for connecting the output driver output to the reference voltage, to define the starting point from which each column voltage is charged (or discharged) by the current source circuit.
  • the output driver output could be connected to the common voltage V CO m (in case the current source is bi-directional), with the non- inverting input of the output driver still at V re f.
  • This approach can be implemented with a feedback loop including a voltage comparator.
  • the any pre-charge method can be used which sets the output voltage of the output driver to a pre-defined voltage, such that the capacitor is pre-charged.
  • the use of a shorting switch across the capacitor, as shown in the detailed examples above, is merely one example, and which sets the starting point at V re f.
  • Fig. 12 shows in simplified form two examples of alternative ways to provide precharging.
  • an operational amplifier is used in the feedback path as a voltage comparator.
  • the amplifier is turned on (for example using the bias currents)
  • the reference voltage V R results at the inverting input of the output driver 71, and a corresponding charge is held on the capacitor.
  • an operational transconductance amplifier (OTA) is used in the feedback path. This delivers a current 120 to the capacitor until the output of the driver is at the common voltage Vcom supplied to the OTA.
  • a look up table is used for deriving time values from the pixel values.
  • Mathematical expressions may instead be used for this purpose, for describing the non- linear relation between transmission level and voltage of a liquid crystal.
  • a supply switch is used to control the supply of current. This may be a part of the current source circuit itself, and other controlling means may be used.
  • the invention is of particular advantage for source driver ICs for AMLCD panels, and enables production of simple, small- silicon-area source drivers for displays with moderate color depths.
  • the invention can also be used to realize higher color depths, without the dramatic increase in silicon area.

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • General Physics & Mathematics (AREA)
  • Theoretical Computer Science (AREA)
  • Control Of Indicators Other Than Cathode Ray Tubes (AREA)
  • Control Of El Displays (AREA)
  • Liquid Crystal Display Device Control (AREA)

Abstract

L'invention porte sur un écrans à matrice active comportant un réseau de pixels disposés en rangées et colonnes et un circuit de pilotage de colonnes fournissant les signaux d'excitation des pixels des colonnes. Ledit circuit comprend: un réseau de circuits de sources de courant; à raison d'un par colonne de pixel, et à raison d'une source de courant (70; 80; 90) par circuit; un pilote de sortie (71), une capacité (72) reliée à la sortie du pilote de sortie; et un commutateur d'alimentation (78) réglant le temps pendant lequel la source de courant alimente la capacité (72) ou en tire du courant, et déterminant par-là la tension de sortie du pilote de sortie. L'écran comporte en outre un système de mappage (86) permettant de tirer du niveau d'excitation d'un pixel une valeur numérique représentant le temps de commutation du commutateur d'alimentation (78) pour chaque circuit de source de courant.
PCT/IB2006/053228 2005-09-19 2006-09-12 Ecrans a matrice active et leurs procedes d'excitation WO2007034360A2 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
EP05108610 2005-09-19
EP05108610.6 2005-09-19

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WO2007034360A2 true WO2007034360A2 (fr) 2007-03-29
WO2007034360A3 WO2007034360A3 (fr) 2007-07-05

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6331844B1 (en) * 1996-06-11 2001-12-18 Kabushiki Kaisha Toshiba Liquid crystal display apparatus
US20020145584A1 (en) * 2001-04-06 2002-10-10 Waterman John Karl Liquid crystal display column capacitance charging with a current source
WO2005024773A1 (fr) * 2003-09-10 2005-03-17 Koninklijke Philips Electronics N. V. Circuit de pilotage d'un affichage a cristaux liquides
WO2006092757A2 (fr) * 2005-03-02 2006-09-08 Koninklijke Philips Electronics N.V. Dispositifs d'affichage a matrice active et procedes d'entrainement de ceux-ci

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6331844B1 (en) * 1996-06-11 2001-12-18 Kabushiki Kaisha Toshiba Liquid crystal display apparatus
US20020145584A1 (en) * 2001-04-06 2002-10-10 Waterman John Karl Liquid crystal display column capacitance charging with a current source
WO2005024773A1 (fr) * 2003-09-10 2005-03-17 Koninklijke Philips Electronics N. V. Circuit de pilotage d'un affichage a cristaux liquides
WO2006092757A2 (fr) * 2005-03-02 2006-09-08 Koninklijke Philips Electronics N.V. Dispositifs d'affichage a matrice active et procedes d'entrainement de ceux-ci

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