EP0666555B1 - Driving method for DHF-LCD - Google Patents

Description

The invention relates to a control method for the pixels one DHF type liquid crystal cell (DHF-LCD). DHF LCDs are in the European patent EP 0 309 774 B1.

Liquid crystal cells of the DHF type can be operated in two different modes: In the asymmetrical mode, the cell is arranged between crossed polarizers in such a way that the transmission is minimal for a certain, eg negative voltage-U ₀ and maximum for + U ₀ . In symmetrical mode, the cell is arranged so that the transmission is minimal for 0 V applied voltage and increases for positive and negative voltages.

In asymmetric mode, the cell is more sensitive, i.e. of the electro-optical effect is around twice as large as in the symmetrical Mode. There is a risk that in the event of not being free of DC voltage Triggering electrochemical processes or Polarization charges in the orientation layers of the Liquid crystal cell are generated. Both effects can be phantom images to lead. In symmetrical mode this can be avoided by alternating a picture with positive tension and with drives negative voltage.

In both modes, it is important to get from a grayscale as quickly as possible to be able to switch to another. In symmetrical mode this is more difficult because with the same change in gray value, a larger one Liquid crystal movement is necessary.

Important applications of DHF-LCDs require an active matrix control, i.e. each pixel are semiconductor elements (transistors or Diodes) assigned, which enable the display to be multiplexed.

The invention has for its object a control to indicate with the lowest possible voltage in combination with an active matrix achieves the shortest possible switching time of DHF-LCDs becomes.

This is achieved according to the invention in that each pixel before the Supply of a data pulse brought to a predetermined voltage becomes.

Fig. 1 ein Ersatzschaltbild eines DHF-Pixels

Fig. 2 ein Pulsdiagramm für eine Form des Ansteuerpulses

Fig. 3 ein Pulsdiagramm einer alternativen Form des Ansteuerpulses

Fig. 4 ein Pulsdiagramm einer weiteren alternativen Form des Ansteuerpulses.

Exemplary embodiments of the invention are described below with reference to the accompanying drawings. Show it:

Fig. 1 is an equivalent circuit diagram of a DHF pixel

Fig. 2 is a pulse diagram for a form of the drive pulse

Fig. 3 is a pulse diagram of an alternative form of the drive pulse

4 shows a pulse diagram of a further alternative form of the control pulse.

In the equivalent circuit diagram of a DHF pixel shown in FIG. 1, the static capacitance C _{s is} the capacitance at which the director does not move. C _hx describes the fact that the deformation of the ferroelectric helix is associated with a charge (polarization charge). R _hx describes the associated friction losses. For liquid crystal _mixtures with high spontaneous polarization, C _hx is several _times larger than C _s . The charging of C _s is fast and only limited by the output impedance of the voltage source used. The charging time of C _hx, however, is determined by τ = R _hx C _hx .

If a DHF cell is activated with an active matrix, a low-impedance signal is present at the pixel during the row addressing time t _z (typically 64 µsec). Then the pixel is isolated until the next image (typically 40 ms). During this time, the charge that has flowed to the pixel in the row addressing time is distributed among the two capacitors so that they are charged to the same voltage. If the resulting charge on C _{hx is} large enough to cause the desired deformation, there are no problems. This is especially the case if the characteristic time τ is several times shorter than t _z (then C _hx is charged directly and C _s has no meaning) and / or if the voltage used is so high that sufficient charge after the charge equalization is stored on C _hx .

Since τ becomes longer than permissible, especially at low temperatures, relatively high voltages must be used. This is mainly because C _{s is} significantly smaller than C _hx . In order to bring enough charge to C _s (which then flows to C _hx when the charge is _equalized ), a correspondingly higher charge voltage is required.

However, high charge voltages are difficult to reconcile with Active matrix technology. Control methods that the necessary Being able to reduce tension is therefore preferable.

If a voltage source with the maximum voltage U ₀ is available, the charge Q ₀ = C _s U _{0 is} stored on the pixel with a very short activation time τ ₀ (C _hx is not significantly charged). If you wait τ a few times, Q _{0 has been distributed} over the two capacities. This cycle can be repeated several times (n times), with C _hx being charged further and further. The total time during which a pixel is addressed is nτ ₀ . Since τ ₀ can be chosen to be very short, ie nτ ₀ <t _z , the total time t _z permissible for multiplexing is not exceeded, it is only distributed over a plurality of shorter times that are separate from one another.

To ensure that a DHF cell operates without DC voltage ensure the polarity of the control from image to image switch. The pixel must therefore be discharged before the new one Information can be registered. This happens with pulses that Before writing the data (gray values) on an entire line will. Essentially three variants of such pulses are suitable which are shown in Figures 2 to 4 are outlined. In these figures is the one created Voltage U and the charge Q on the pixel and four periods 1-4 shown. During the times before (time period 1) and after (time period 4) the control of the pixel is isolated, i.e. the applied voltage is not Are defined.

The pre-pulses shown in FIG. 2 below during the time segment 2 unload the pixel so that the data pulses during period 3 only still need to charge to the new grayscale value.

The pre-pulses shown in FIG. 3 below during the time segment 2 preload the pixel to a suitable value. The data pulses then less charge must be added or during the time period 3 dissipate. The data pulses have the same polarity as the precharge pulses.

The pre-pulses shown in the middle and below in FIG. 4 during the time segment 2 charge the pixel to the maximum voltage. The data pulses during the time period 3 then discharge the pixel to the desired gray value. This can be done in two ways: either (1) by amplitude modulation as in FIGS. 2 and 3, ie by pulses of different amplitudes (center of FIG. 4), the full voltage swing of -U ₀ ... U ₀ can be used, or (2) by pulse width modulation, ie by pulses of maximum voltage U ₀ but different pulse lengths (Fig. 4 below). With this type of control, the polarity does not have to change depending on the gray level as with amplitude modulation.

Prepulses with maximum voltage saturate the pixel, which is a danger the crosstalk of data information of other lines, which during the Pre-charging pulse are addressed, reduced.

Claims (6)

1. A triggering method for a liquid crystal cell of DHF type, characterized in that each pixel is bringing to a predetermined voltage before a data pulse is supplied.
2. A triggering method according to claim 1, characterized in that the pixels are brought to the voltage 0 V line-wise.
3. A triggering method according to claim 1, characterized in that the pixels are brought line-wise to a voltage of the same polarity as the data pulse.
4. A triggering method according to claim 1, characterized in that the pixels are charged line-wise to the maximum voltage and the data pulses in the form of amplitude-modulated signals utilise the full voltage swing between a maximum positive and maximum negative voltage.
5. A triggering method according to claim 1, characterized in that the pixels are charged line-wise to the maximum voltage and the data pulses consist of pulse-width-modulated pulses of maximum amplitude but opposite polarity.
6. A triggering method according to claim 1, characterized in that the data pulse consists of a plurality of consecutive pulses at intervals, wherein the interval being equal to or greater than the characteristic charging time of the DHF-helix.

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