- Email: [email protected]
Low power consumption driving for liquid crystal light modulators
Displays 25 (2004) 115–121 www.elsevier.com/locate/displa
Low power consumption driving for liquid crystal light modulators Janez Pirsˇ*, Dusˇan Ponikvar, Bojan Marin, Martin Chambers Jozˇef Stefan Institute, University of Ljubljana, Jamova 39, 1000 Ljubljana, Slovenia Received 28 October 2003; accepted 26 July 2004 Available online 28 August 2004
Abstract A novel liquid crystal display (LCD) driving concept for light shutters is presented, based on the driving signal polarity reversal controlled by the driving voltage integral across the LCD switching element electrodes, as opposed to plain periodic polarity reversal. This driving scheme optimizes the DC driving signal voltage balancing and guarantees compensation of the long-term (DC) component of the driving voltage within one polarity reversal cycle, irrespectively of driving voltage amplitude variations. The possibility of using longer times for the driving signals polarity reversals reduces the LCD switching elements power consumption (w1 mA) compared to standard driving techniques (w100 mA), which is important for portable devices such as welding light shutters. q 2004 Elsevier B.V. All rights reserved. Keywords: Liquid crystals displays; Light valves; Driving schemes; Light shutters; Welding filters
1. Introduction Liquid crystal display (LCD) light shutters have various applications in optics (stereovision, light amplitude/phase modulation, etc.) [1,2,3]. The most widely used application investigated here, is the use of LCD light shutters in human eye protective devices such as automatic welding filters. In this case and actually generally for all portable autonomous devices, improvements in power consumption have large advantages for LCD light shutter based products such as weight reduction, increased product lifetime, comfort, safety, etc. This is especially true in the case of ultra-low power consumption, as light shutters typically face several other extreme performance requirements such as high contrast, high switching speed, low light scattering and high light attenuation uniformity at oblique incident angles [1,2,3]. As a very wide operation temperature range is normally required, twisted nematic as well as various birefringent modes (p-cell, electrically controlled birefringence, etc.) in nematic LCD’s offer the best compromise. The optically
* Corresponding author. E-mail addresses: [email protected] (J. Pirsˇ), [email protected] (M. Chambers). 0141-9382/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.displa.2004.07.006
closed state, characterized by homeotropically oriented nematic liquid crystal molecules between crossed polarizers, provides poor performance at oblique angles of light incidence. However, standard angular compensation methods, based on negative-birefringent c-plates [2,3,4], are used to compensate for this deficiency and provided that high amplitude driving signals are used, an acceptable performance requirements compromise is reached. The use of high amplitude driving signals results in increased power consumption. As the LCD light shutters used in human eye protective devices are typically large, which in turn results in high effective capacitance (w100 nF), the large current consumption due to capacitative elements of the display results in excessive battery size as well as their frequent replacement. This is annoying and can result in device malfunction. Therefore, a new driving signal scheme is required for this and similar applications, which allows for significant reduction of the power consumption.
2. Slow polarity reversal, DC compensated driving scheme The only feasible method to facilitate a significant reduction in power consumption is by using a very slow
116
J. Pirsˇ et al. / Displays 25 (2004) 115–121
polarity reversal driving signal. This allows the average capacitive current through the LCD light shutter to be reduced to a negligible value with respect to the overall device current consumption. The most simple solution for the driving scheme is the present state-of-the-art driving technique introduced by some producers of automatic LCD light filters for welding applications [12]. This driving scheme is a typical ON/OFF technique, using very low frequency (!1 Hz) square wave driving signals with a 50% duty ratio during the optically closed state and no driving signal in the optically open state of the LCD light filter operation. Unfortunately, the synchronization of such a low frequency driving signal with the light stimulus, the time-scale and periodicity of which is often highly irregular and sporadic (for example welding arc) is basically impossible. The effective DC unbalance is inevitable, which in turn results in the increase of the ionic screening of the electric driving signals causing flicker and finally even electrochemical degradation of the device (lifetime). When the driving voltage polarity reversal times become longer than the influence times of the various ionic conductivity effects in the LC and boundary layers (polymer alignment layers, passivation layers,.), ions tend to screen the electric driving voltage resulting in performance degradation (time dependent light attenuation—flickering, memory effects,.). The most significant problems are the low frequency effects generally addressed as the residual DC voltage effects (RDC). The RDC is caused by ionic contamination present in the display via polarization of bound ion pairs [5], drift of ions towards the electrodes (space charge limited regime) [5] leading to a build up of surface charge at the alignment layers interface [8] and subsequent interaction of ions with the alignment layers [7] (ion absorption/desorption in the alignment layer) in addition to generation of new ions through dissociation of neutral molecules [6,9]. All the above-mentioned effects are very sensitive to the DC balance. To avoid RDC problems, it is extremely important that the DC voltage balancing of driving signals is achieved in the shortest time period possible. Standard techniques for DC voltage [10,11] balancing of the driving signals is by connecting one of the electrodes of the LCD light shutter to a potential obtained by averaging the driving signal over a long time period. Such a solution cannot be applied in the case of the LCD light filters, due to the lightswitching request occurring at any time and the time dependence of the driving signal amplitude varying asynchronously with the signal polarity reversals. Therefore, the averaging time constant according to these solutions has to be much longer (several 100 s) than the driving signal time period. However, such a time constant is unacceptably long for fast, intrinsically sporadic switching required for the optical eye protective devices. The proposed new concept assumes the electric driving voltage to be present on the LC light filter all the time in
the optically closed as well as the optically open state. During the optically open state of the LCD light filter, the electric driving voltage is kept just beneath the switching threshold voltage VTH of the LCD, while in the optically closed state its value is higher than the threshold voltage VTH and determined by the required level of light attenuation VSH. Keeping the electric driving voltage just beneath the switching threshold voltage VTH in the optically open state, results in noticeably shorter switching times (of the order of 100 ms) compared to the standard ‘ON/OFF’ switching [12] (see Fig. 6). In order to further increase the speed of the light shutter’s optical response, the latter can be additionally driven with a short (w1 ms) intense (w35 V) electric voltage pulse (see Fig. 3) at the very beginning of the optical switching request to the optically closed state. Such continuous driving of the light shutter opens a possibility for a new approach to driving of the LCD light shutters, where the polarity reversal of the LCD driving signal, necessary for the DC balancing of the electric driving waveform, is controlled by the integral of the LCD driving signal (Fig. 1(b)), rather than by the plain symmetric oscillator (Fig. 1(a)). The driving technique, as shown here, therefore has some major advantages compared to the simple ON/OFF low frequency driving, as used by some producers [12]: † synchronization with sporadic events (like welding flashes) is possible, while preserving the DC balance of the driving electric waveform, † complete DC compensation can be established within one polarity reversal cycle, † driving of the LCD at the threshold voltage VTH before the optical switching occurs, as well as the ease of adding a high voltage switching pulse, results in shorter switching times, while preserving the DC balance of the driving electric waveform. The slow polarity reversal LCD driving scheme that exploits the concept from Fig. 1(b) results in a complete DC compensation of the electric driving signals for the LCD light filters. Therefore, the ionic screening effects, as mentioned before, can be reduced to minimum. This in turn allows the increase in the polarity reversal times to
Fig. 1. (a) Standard LCD driving concept K50/50 square waveform. (b) Proposed ‘integration’ LCD driving concept.
J. Pirsˇ et al. / Displays 25 (2004) 115–121
the maximum allowed by the LCD technology used (LC materials, polymer orienting layers, boundary glass, etc.) and therefore reducing the power consumption due to the LCD capacitive current to the minimum. The new concept of DC balancing the driving electric signal allows twice the driving signal amplitude, as compared to the techniques presented previously [10,11] using the same supply voltage. This in turn again results in further increase of the switching speed. The basic concept of the proposed driving scheme (as introduced in Fig. 1(b)) is presented by means of the electronic driving circuitry block diagram for the automatic light shutter (Fig. 2(a)) for eye protection applications (e.g. welding light shutter) and corresponding timing diagram of the relevant electric signals (Fig. 2(b)).
Fig. 2. (a) Block diagram for the proposed ‘integration’ LCD driving scheme—the photo detector signal (OP) triggers the voltage selector (VSEL) to switch between different driving voltages corresponding to the optically open (VTH) and optically closed state (VSH), which are led to the LCD driver (VLCD). The voltage difference over the LCD is then integrated by the integrator (INT) to finally provide the signal (T/C) requiring the change of the polarity of the LCD driving signal. (b) Timing diagram of the most relevant electric signals in the LCD driving/control electronics— the photo detector signal (OP) results in switching of the driving voltage amplitude (VLCD1) of the LCD light shutter between the voltages corresponding to the optically open (VTH) and optically closed state (VSH), the time integral (Z) of which in turn controls the change of the polarity of the LCD driving signal.
117
In the case of the automatic light shutters (eye protection applications), the amplitude VLCD of the electric driving signal for the LCD light shutter is controlled by the photodetector circuit with the associated amplifier and comparator, which detects the presence of excessive light and accordingly toggles the voltage selector VSEL to one of the predefined voltages, VTH or VSH by means of the control signal OP. The selected voltage VLCD is present at the input to the LCD driver, which drives the electrodes of the LCD light shutter via its outputs VLCD1 and VLCD2. The LCD driver supplies the selected voltage to one of its outputs, while the other output is connected to the ground. The signal true/complement (T/C) controls, which of the outputs receive the selected voltage and therefore determines the driving signal polarity. The integrator INT calculates the driving voltage time integral applied to the LCD. The integrator output Z is connected to the two comparators COMP0 and COMP1 that toggle the flip-flop FF, when the integrator output voltage reaches either of the comparison levels VR0 or VR1. The output signal T/C from the flip-flop is used to control the LCD driving signal polarity. The timing diagrams presented in Fig. 2(b) illustrate the function of the proposed driving scheme. Initially, the LCD light shutter is in the optically open state, the amplitude of the driving signal VLCD is low VTH and the flip-flop FF is reset, so the line VLCD1 receives the voltage VTH, while the line VLCD2 is at ground. Since the positive driving signal is applied to the LCD and also to the input of the integrator INT, the output signal Z of the integrator increases. Once it reaches the predefined level VR1, the comparator COMP1 provides a set signal S for the flip-flop FF. The output T/C of the flip-flop toggles, and the driving signal polarity reverses. This causes the integrator output signal to decrease, once it drops down to the predefined level VR0, the comparator COMP0 provides a reset signal R for the flip-flop restoring the initial state. The described driving signal time period guarantees the removal of the LCD driving signal DC component. The DC balancing is performed regardless of the driving signal amplitude, as can be seen by analyzing the second period in the timing diagram (Fig. 2 (b)), during which the photodetector triggers the switching of the light shutter into the optically closed state. Here, the driving signal amplitude VLCD increases from the value VTH to VSH due to the activation signal OP from the photo-detector. The amplitude change of the driving signals VLCD1 and VLCD2 results in the increased slope of the integrator output signal Z, and the reference level VR1 is attained quicker. The driving signal polarity is reversed and when the integrator output signal Z again reaches the reference level VR0, the period is concluded. It is a specific advantage of the proposed concept that it can easily allow for the inclusions of a high voltage amplitude switching pulse at any time in order to increase the switching to the optically closed state without losing the DC balance. The ‘integration technique’, as described
118
J. Pirsˇ et al. / Displays 25 (2004) 115–121
Fig. 3. Time dependence of the driving voltage amplitude before (VTH), during (VSW, VSH; during the time intervals TSW and TW, respectively) and after (VTH) the switching. Signal OP, starting at To marks the duration of the signal requesting the optically closed state.
above, therefore allows easy integration of a short (w1 ms) high voltage pulse (w40 V) at the beginning of the optical switching, which further reduces the switching time. The timing diagram presented on Fig. 3 shows a typical time dependence of the driving voltage amplitude over the electrodes of the LCD light shutter (VLCD) before (VTH), during (VSW and VSH) and after (VTH) the intense light flash of the welding arc characterized by the signal OP of the optical detection circuit. As a result of such optimized LCD driving, the switching speed is therefore well below that required by the internationally accepted requirements for the optical personal protecting devices [1], which is 5 ms for switching from a protective shade 3 (‘optically open state’, 12% transmission) to the protective shade 11 (‘optically closed state’, 3.2!10K5 transmission). During the polarity cycling of the LCD, the electronic integration of the signal ensures that no other visible effect, but the increase of the switching speed is noticeable. To briefly summarize, the concept described here is not limited by the time dependence of the driving signal amplitude (Fig. 3) and does not require any synchronization with the optical switching request. This is due to the driving signal integration performing the DC balancing within one driving signal period. Using this driving scheme (high voltage pulse and ‘pre-driving’), the LCD electro-optical response speed can be maximized even at very slow effective driving polarity reversals (see Fig. 4) without losing the DC balance of the driving signal. Contrary to this standard LCD driving technical solutions [11,12] cannot be synchronized with random events like welding arc, if they use low polarity reversal frequency in order to reduce the LCD power consumption. Furthermore, any additional driving voltage amplitude variations necessary to improve the LCD light shutter switching speed (Fig. 3) additionally reduce the DC balance of the LCD driving voltage waveform.
Fig. 4. Simplified block diagram—the photo-detector’s signal (OP) is used to select the LCD driving voltage selector (VSEL) to switch between the voltage below the switching threshold (VTH—optically open state) and to the selected shade level (VSH—optically closed state). The latter is applied to the LCD, and is integrated by the integrator (INT), the output Z of which is compared (COMP1) to the control voltage (VR1). The signal (TG) provided by the comparator (COMP1) to the flip-flop (FF) resets the integrator (INT). The toggle flip-flop (TFF) gives the signal (T/C) controlling the polarity change of the LCD driving signal.
3. Simplified driving scheme The LCD driving concept as presented in Figs. 1 and 2 might appear rather complex to implement. However, assuming the electronic components’ operational performances do not change significantly in time and there is no significant difference between the voltage over the LCD electrodes and the LCD driver input voltage, it is possible to simplify the driving electronics. So the continual comparison loop is reduced to the comparison of the time integrals of two consecutive intervals, during which the LCD driving signal polarity is reversed. Only one comparator COMP1 is needed, with the output TG used to toggle the flip-flop TFF and restart integration (signal RES) from the initial value after every polarity reversal of the driving signal (Fig. 4). In this case, the integration and comparison functions, needed in the proposed LCD driving concept (Figs. 1–4), can be realized with just few electronic components resulting in negligible current consumption. The usual implementation of an integrator requires the use of an operational amplifier, which is not suitable due to its relatively high current consumption. Additionally, the integrator input being permanently connected to the LCD driving signal consumes significant current, since the value of the input resistor is limited to w10 MU for practical reasons. However, the integrator can be implemented by a single integrating capacitor CI, switches SW1 and SW2, a transfer capacitor CT and an amplifier A1 with a gain of C1 and high input resistance, as shown in Fig. 5. The transfer capacitor CT and the switches SW1 and SW2 are used to periodically transfer the charge, which is proportional to the voltage difference VLCD1KVLCD2, into the integrating capacitor CI, and the amplifier (A1) assures the complete transfer of
J. Pirsˇ et al. / Displays 25 (2004) 115–121
119
times much longer than 2 s can be used in the case of very slow LCD driving voltage polarity changes according to the simplified driving concept described above. As the power consumption of the total control and driving electronics including the above described ‘integration polarity control’ can be kept below 10 mA; the capacitive current consumption of the LCD light shutter can be made negligible. As a result of the above-described technical solutions, a standard 3 V Lithium ‘coin’ battery can power an automatic LCD light shutter for human eye protection or for several years. 4. Experiment As described in paragraph 2, the proposed ‘integration, slow polarity reversal’ driving scheme has two important advantages over the standard concept: Fig. 5. Top: schematic diagram of the LCD driving voltage integrator. Bottom: schematic diagram of the LCD driving voltage integrator and comparator circuit.
the charge from the transfer capacitor CT into the integrating capacitor CI. The toggling period of the clock signal CLK for the switches SW1 and SW2 is selected to be two orders of magnitude smaller than the expected period of the LCD polarity reversal in order to avoid integration errors. The amplifier (A1) can be implemented using only two bipolar transistors Q1 and Q2 of opposite polarity. The complete schematic diagram of the integrator and comparator is shown in Fig. 5. The integrator output signal Z is the voltage at the integrating capacitor CI. In this modified LCD driving concept, only one comparator is needed and when the output signal of the integrator reaches the reference level, the flip-flop is toggled and the output signal from the integrator is reset to the initial value, therefore, the integrating capacitor CI is discharged. This is accomplished by a ‘uni-junction’ connection of two transistors Q3 and Q4 of opposite polarity, as shown in Fig. 5. The transistor Q5 is used to form a suitable signal TG for toggling the flip-flop FF. The realization of the electronic driving circuitry results in significant reduction of the current consumption of the LCD light shutters. The current consumption of the additional electronics can be kept very low, typically 1 mA at 3 V battery power supply, and is thus negligible compared to the reduction in the average capacitive current through the LCD light shutter when driven in a conventional manner (w50 Hz). The proposed driving scheme, however, allows for an important increase of the LCD driving voltage polarity reversal times. So the average capacitive current through the typical LCD light shutter, used in the human eye protective devices (equivalent of w100 nF), can be reduced from more than 100 mA in the case of the conventional driving (50 Hz) to about 1 mA, if the driving polarity reversal times can be increased to 2 s. Polarity reversal
† faster switching speeds, † optimized DC balancing and hence maximum possible increase of the LCD driving signal polarity reversal times. The switching response of the LCD was measured for both driving schemes and is shown in Fig. 6(a). The driving
Fig. 6. (a) Optical response (w100 ms) of the LCD light shutter driven according to the proposed driving scheme (black line and bottom left driving voltage timing diagram) compared to the response (w125 ms) to the standard square wave driving signal with 50/50 duty cycle (gray line and top right driving voltage timing diagram). (b) Optical response (12,500 ms) of the LCD light shutter driven by the standard square wave driving signal with 50/50 duty cycle and without the high voltage switching pulse (black line and driving voltage timing diagram).
120
J. Pirsˇ et al. / Displays 25 (2004) 115–121
schemes used are illustrated in the top right (current, state of the art driving scheme) and bottom left corner (proposed driving schemes) of the figure and also shown schematically in Fig. 1(a) and (b) as well as in the timing diagram in Figs. 2 and 3. The present state-of-the-art scheme runs at 0 V in the optically open state [12] and when the switching occurs, a pulse (35 V for 2 ms) is applied followed by the square wave (4 V, 50 Hz, 50% duty cycle) to keep the required closed state. As the LCD driving cannot be synchronized with the light stimulus, which is completely sporadic, some effective DC unbalance is inevitably generated. Using low driving frequencies also in turn results in ionic screening and related undesirable phenomena. The proposed scheme uses the same voltage amplitudes (VSHw4 V) as the current, state-of-the-art driving scheme, but with polarity reversal times w5 s and in addition uses a ‘predriving’ voltage (w1.5 V amplitude), see Fig. 3. The ‘predriving voltage’ is below the switching threshold VTH of the LCD light shutter, so no noticeable decrease of the light transmission can be seen visually. The use of the ‘predriving’, however, enables the LC molecules to have a higher degree of tilt with respect to the boundary surface so that the switching can be preformed quicker. From Fig. 6(a) it can be seen that the optical response (to 90% of the final light attenuation) using the proposed driving scheme [13] is w100 ms compared to w125 ms, using a standard driving technique (without ‘predriving’), if the high voltage switching pulse (VSWw35 V) is used at the beginning of the optical switching. If the current state-of-the-art scheme [11,12] is used in its original form without the high voltage switching pulse (VSW), the switching time is increased to 12.5 ms (see Fig. 6(b)). The DC compensated LCD light shutter driving scheme allows the use of very slow driving polarity reversals. There is, however, a limit to the ultra-slow voltage polarity reversal times that can be employed. In principle, this concept is only applicable, as long as the RDC voltage build-up effects are linear (see Fig. 7). In this case, the integration can result in perfect DC balancing even, when the LCD driving voltage amplitude varies in time
considerably, which is the case in most LCD light shutters used in human eye protective devices [12] (see Fig. 3). The limitations in very slow polarity reversals of the LCD light shutter driving were evaluated by measuring the time dependence of the RDC voltage build-up. The fact is that, due to the slow nature of the RDC voltage, these effects on the light attenuation of the LCD light shutter are the most noticeable, while the ionic screening effects of the free ions are too fast to be adequately perceived by the human eye. The measurements were performed on sample LCD light shutters using ultra-low RDC polyimide (Nissan SE 4794) alignment layers and commercial low ionic contamination liquid crystals as used in AM LCD’s (Merck MLC 6873). Before every measurement, the LCD light shutters were heated to the isotropic phase (110 8C) for 20 min in order to avoid ‘memory effects’ from previous measurements. After such annealing procedure, the measured LCD light shutters were exposed to a DC voltage UC for the time TC. Immediately after the exposure to a DC voltage, the resulting RDC voltage was evaluated by measuring the light attenuation caused by the LCD light shutter: a square wave signal of 5 Hz without a DC component was used to drive the LCD during measurements with the amplitude selected to induce a relatively light attenuation factor of 65 on the LCD. The first few periods of the driving signal were used to determine the attenuation. The light attenuation variations due to RDC voltage build-up were measured and subsequently converted into an RDC voltage by using the voltage dependent attenuation characteristics of the LCD taken previously. This experiment was repeated for different DC voltages, UC of 1.5, 3, 6 and 9 V, and for different exposure times, TC of 5, 10, 60 and 120 s. Fig. 7 shows the dependence of the absolute value of the RDC voltage on the applied voltage UC for different time periods TC. Here, it can be seen that for up to 5 s and even up to 10 s time period TC, the measured RDC voltage values increase linearly with the applied voltage UC (within error) through the entire driving voltage range, while for the time periods of 60, 120 s and above the linearity can no longer be achieved. This means that: 1. The LCD driving waveforms with relatively high driving voltage amplitudes (up to 9 V) can be used at polarity reversal times as long as 10 s, 2. The time integrals of the DC sections of the LCD driving Ð waveform 0TC UC ðtÞdt can be as high as 9 V!10 s, meaning that substantially higher driving voltage amplitudes (e.g. R40 V), necessary to achieve ultrashort LCD light shutter switching times [12], can be used as long as their time integral is small enough.
Fig. 7. The dependence of the RDC voltage on the exposure of the LCD to the known DC voltage UC for a time TC for UC values (0, 1.5, 3, 6 and 9 V) and TC values (0, 5, 10, 60 and 120 s).
As already discussed above, the capacitive current through the LCD light shutters typically used in eye protection applications can be reduced to 10% of the total device electric power consumption, if the driving voltage polarity changes (cycles) occur slower than within 2 s.
J. Pirsˇ et al. / Displays 25 (2004) 115–121
Fig. 8. Auto-darkening welding filters—the left filter is in the ‘optically closed state’ showing the front side of the display, while the right filter is in the ‘optically open state’ showing the back side of the filter with the control switches for the sensitivity and light transmission number.
This is far below the 10 s confirmed by the experiment, meaning that the power consumption of the LCD light shutter in the personal eye protective devices can be made almost negligible.
121
are kept below 10 s, the basic assumptions of the proposed slow polarity reversal LCD driving scheme are met. As mentioned previously, extending the LCD driving signal polarity reversal times above a few seconds, reduces the capacitive current of the typical LCD light shutter, used in eye protective devices to almost a negligible value (%1 mA) compared to the driving/control electronics power consumption (w5–10 mA). Therefore, using low ionic contamination LCD materials (liquid crystal, polyimide), the light shutters that are driven with the proposed slow polarity reversal driving scheme [13] do not contribute to the overall power consumption of the devices such as in human eye protective light shutters. As a result, a standard 3 V Lithium ‘coin’ battery can power an automatic LCD light filter for human eye protection for several years.
Acknowledgements The authors would like to acknowledge the support of the NATO ‘Science for Peace’ Program through the Research Grant SfP 971874.
5. Conclusion A novel LCD driving concept for LCD light shutters, intensity and phase modulators that can be used in various personal protection devices like welding light filters (Fig. 8) has been described, based on driving signal polarity reversals controlled by the driving voltage integral across the LCD switching element electrodes, as opposed to plain periodic polarity reversals. The driving scheme optimizes the DC voltage balancing of the LCD driving signal, as it guarantees the compensation of the long-term (DC) component of the driving voltage within one polarity reversal cycle, irrespectively of driving voltage amplitude variations. The obtained results clearly show that as long as the average polarity reversals times of the LCD driving voltage
References [1] European standard: EN379, 2002. [2] J. Pirsˇ, S. Pirsˇ, R. Petkovsˇek, M. Bazˇec, P. Bos, SID 02 Digest, p. 53. [3] J. Pirsˇ, M. Bazˇec, A. Vrecko, D. Ponikvar, SID 03 Digest, p. 55.3. [4] T. Uchida et al., SID 89 Digest, pp. 378–381. [5] Y. Nakazono et al., IDW 98, p. 61. [6] T. Chieu, K.H. Yang, Jpn. J. Appl. Phys. 28 (11) (1989) 2240. [7] G. Barbero, et al., J. Appl. Phys. 67 (5) (1990) 2678. [8] R.N. Thurston, et al., J. Appl. Phys. 55 (12) (1984) 4154. [9] H. De Vleeschouwer, et al., Jpn. J. Appl. Phys. 41 (3A) (2000) 1–6. [10] R.A. Erhart, et al., US Patent: US5,852,426, 1998. [11] K. Kutaragi et al., US Patent: US4,205,311, 1980. [12] G. Gunz, et al., European Patent: EP0,550,384, 1994. [13] J. Pirsˇ, B. Marin, D. Ponikvar, S. Pirsˇ, European Patent EP 1,131,669, 2004.
Low power consumption driving for liquid crystal light modulators Janez Pirsˇ*, Dusˇan Ponikvar, Bojan Marin, Martin Chambers Jozˇef Stefan Institute, University of Ljubljana, Jamova 39, 1000 Ljubljana, Slovenia Received 28 October 2003; accepted 26 July 2004 Available online 28 August 2004
Abstract A novel liquid crystal display (LCD) driving concept for light shutters is presented, based on the driving signal polarity reversal controlled by the driving voltage integral across the LCD switching element electrodes, as opposed to plain periodic polarity reversal. This driving scheme optimizes the DC driving signal voltage balancing and guarantees compensation of the long-term (DC) component of the driving voltage within one polarity reversal cycle, irrespectively of driving voltage amplitude variations. The possibility of using longer times for the driving signals polarity reversals reduces the LCD switching elements power consumption (w1 mA) compared to standard driving techniques (w100 mA), which is important for portable devices such as welding light shutters. q 2004 Elsevier B.V. All rights reserved. Keywords: Liquid crystals displays; Light valves; Driving schemes; Light shutters; Welding filters
1. Introduction Liquid crystal display (LCD) light shutters have various applications in optics (stereovision, light amplitude/phase modulation, etc.) [1,2,3]. The most widely used application investigated here, is the use of LCD light shutters in human eye protective devices such as automatic welding filters. In this case and actually generally for all portable autonomous devices, improvements in power consumption have large advantages for LCD light shutter based products such as weight reduction, increased product lifetime, comfort, safety, etc. This is especially true in the case of ultra-low power consumption, as light shutters typically face several other extreme performance requirements such as high contrast, high switching speed, low light scattering and high light attenuation uniformity at oblique incident angles [1,2,3]. As a very wide operation temperature range is normally required, twisted nematic as well as various birefringent modes (p-cell, electrically controlled birefringence, etc.) in nematic LCD’s offer the best compromise. The optically
* Corresponding author. E-mail addresses: [email protected] (J. Pirsˇ), [email protected] (M. Chambers). 0141-9382/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.displa.2004.07.006
closed state, characterized by homeotropically oriented nematic liquid crystal molecules between crossed polarizers, provides poor performance at oblique angles of light incidence. However, standard angular compensation methods, based on negative-birefringent c-plates [2,3,4], are used to compensate for this deficiency and provided that high amplitude driving signals are used, an acceptable performance requirements compromise is reached. The use of high amplitude driving signals results in increased power consumption. As the LCD light shutters used in human eye protective devices are typically large, which in turn results in high effective capacitance (w100 nF), the large current consumption due to capacitative elements of the display results in excessive battery size as well as their frequent replacement. This is annoying and can result in device malfunction. Therefore, a new driving signal scheme is required for this and similar applications, which allows for significant reduction of the power consumption.
2. Slow polarity reversal, DC compensated driving scheme The only feasible method to facilitate a significant reduction in power consumption is by using a very slow
116
J. Pirsˇ et al. / Displays 25 (2004) 115–121
polarity reversal driving signal. This allows the average capacitive current through the LCD light shutter to be reduced to a negligible value with respect to the overall device current consumption. The most simple solution for the driving scheme is the present state-of-the-art driving technique introduced by some producers of automatic LCD light filters for welding applications [12]. This driving scheme is a typical ON/OFF technique, using very low frequency (!1 Hz) square wave driving signals with a 50% duty ratio during the optically closed state and no driving signal in the optically open state of the LCD light filter operation. Unfortunately, the synchronization of such a low frequency driving signal with the light stimulus, the time-scale and periodicity of which is often highly irregular and sporadic (for example welding arc) is basically impossible. The effective DC unbalance is inevitable, which in turn results in the increase of the ionic screening of the electric driving signals causing flicker and finally even electrochemical degradation of the device (lifetime). When the driving voltage polarity reversal times become longer than the influence times of the various ionic conductivity effects in the LC and boundary layers (polymer alignment layers, passivation layers,.), ions tend to screen the electric driving voltage resulting in performance degradation (time dependent light attenuation—flickering, memory effects,.). The most significant problems are the low frequency effects generally addressed as the residual DC voltage effects (RDC). The RDC is caused by ionic contamination present in the display via polarization of bound ion pairs [5], drift of ions towards the electrodes (space charge limited regime) [5] leading to a build up of surface charge at the alignment layers interface [8] and subsequent interaction of ions with the alignment layers [7] (ion absorption/desorption in the alignment layer) in addition to generation of new ions through dissociation of neutral molecules [6,9]. All the above-mentioned effects are very sensitive to the DC balance. To avoid RDC problems, it is extremely important that the DC voltage balancing of driving signals is achieved in the shortest time period possible. Standard techniques for DC voltage [10,11] balancing of the driving signals is by connecting one of the electrodes of the LCD light shutter to a potential obtained by averaging the driving signal over a long time period. Such a solution cannot be applied in the case of the LCD light filters, due to the lightswitching request occurring at any time and the time dependence of the driving signal amplitude varying asynchronously with the signal polarity reversals. Therefore, the averaging time constant according to these solutions has to be much longer (several 100 s) than the driving signal time period. However, such a time constant is unacceptably long for fast, intrinsically sporadic switching required for the optical eye protective devices. The proposed new concept assumes the electric driving voltage to be present on the LC light filter all the time in
the optically closed as well as the optically open state. During the optically open state of the LCD light filter, the electric driving voltage is kept just beneath the switching threshold voltage VTH of the LCD, while in the optically closed state its value is higher than the threshold voltage VTH and determined by the required level of light attenuation VSH. Keeping the electric driving voltage just beneath the switching threshold voltage VTH in the optically open state, results in noticeably shorter switching times (of the order of 100 ms) compared to the standard ‘ON/OFF’ switching [12] (see Fig. 6). In order to further increase the speed of the light shutter’s optical response, the latter can be additionally driven with a short (w1 ms) intense (w35 V) electric voltage pulse (see Fig. 3) at the very beginning of the optical switching request to the optically closed state. Such continuous driving of the light shutter opens a possibility for a new approach to driving of the LCD light shutters, where the polarity reversal of the LCD driving signal, necessary for the DC balancing of the electric driving waveform, is controlled by the integral of the LCD driving signal (Fig. 1(b)), rather than by the plain symmetric oscillator (Fig. 1(a)). The driving technique, as shown here, therefore has some major advantages compared to the simple ON/OFF low frequency driving, as used by some producers [12]: † synchronization with sporadic events (like welding flashes) is possible, while preserving the DC balance of the driving electric waveform, † complete DC compensation can be established within one polarity reversal cycle, † driving of the LCD at the threshold voltage VTH before the optical switching occurs, as well as the ease of adding a high voltage switching pulse, results in shorter switching times, while preserving the DC balance of the driving electric waveform. The slow polarity reversal LCD driving scheme that exploits the concept from Fig. 1(b) results in a complete DC compensation of the electric driving signals for the LCD light filters. Therefore, the ionic screening effects, as mentioned before, can be reduced to minimum. This in turn allows the increase in the polarity reversal times to
Fig. 1. (a) Standard LCD driving concept K50/50 square waveform. (b) Proposed ‘integration’ LCD driving concept.
J. Pirsˇ et al. / Displays 25 (2004) 115–121
the maximum allowed by the LCD technology used (LC materials, polymer orienting layers, boundary glass, etc.) and therefore reducing the power consumption due to the LCD capacitive current to the minimum. The new concept of DC balancing the driving electric signal allows twice the driving signal amplitude, as compared to the techniques presented previously [10,11] using the same supply voltage. This in turn again results in further increase of the switching speed. The basic concept of the proposed driving scheme (as introduced in Fig. 1(b)) is presented by means of the electronic driving circuitry block diagram for the automatic light shutter (Fig. 2(a)) for eye protection applications (e.g. welding light shutter) and corresponding timing diagram of the relevant electric signals (Fig. 2(b)).
Fig. 2. (a) Block diagram for the proposed ‘integration’ LCD driving scheme—the photo detector signal (OP) triggers the voltage selector (VSEL) to switch between different driving voltages corresponding to the optically open (VTH) and optically closed state (VSH), which are led to the LCD driver (VLCD). The voltage difference over the LCD is then integrated by the integrator (INT) to finally provide the signal (T/C) requiring the change of the polarity of the LCD driving signal. (b) Timing diagram of the most relevant electric signals in the LCD driving/control electronics— the photo detector signal (OP) results in switching of the driving voltage amplitude (VLCD1) of the LCD light shutter between the voltages corresponding to the optically open (VTH) and optically closed state (VSH), the time integral (Z) of which in turn controls the change of the polarity of the LCD driving signal.
117
In the case of the automatic light shutters (eye protection applications), the amplitude VLCD of the electric driving signal for the LCD light shutter is controlled by the photodetector circuit with the associated amplifier and comparator, which detects the presence of excessive light and accordingly toggles the voltage selector VSEL to one of the predefined voltages, VTH or VSH by means of the control signal OP. The selected voltage VLCD is present at the input to the LCD driver, which drives the electrodes of the LCD light shutter via its outputs VLCD1 and VLCD2. The LCD driver supplies the selected voltage to one of its outputs, while the other output is connected to the ground. The signal true/complement (T/C) controls, which of the outputs receive the selected voltage and therefore determines the driving signal polarity. The integrator INT calculates the driving voltage time integral applied to the LCD. The integrator output Z is connected to the two comparators COMP0 and COMP1 that toggle the flip-flop FF, when the integrator output voltage reaches either of the comparison levels VR0 or VR1. The output signal T/C from the flip-flop is used to control the LCD driving signal polarity. The timing diagrams presented in Fig. 2(b) illustrate the function of the proposed driving scheme. Initially, the LCD light shutter is in the optically open state, the amplitude of the driving signal VLCD is low VTH and the flip-flop FF is reset, so the line VLCD1 receives the voltage VTH, while the line VLCD2 is at ground. Since the positive driving signal is applied to the LCD and also to the input of the integrator INT, the output signal Z of the integrator increases. Once it reaches the predefined level VR1, the comparator COMP1 provides a set signal S for the flip-flop FF. The output T/C of the flip-flop toggles, and the driving signal polarity reverses. This causes the integrator output signal to decrease, once it drops down to the predefined level VR0, the comparator COMP0 provides a reset signal R for the flip-flop restoring the initial state. The described driving signal time period guarantees the removal of the LCD driving signal DC component. The DC balancing is performed regardless of the driving signal amplitude, as can be seen by analyzing the second period in the timing diagram (Fig. 2 (b)), during which the photodetector triggers the switching of the light shutter into the optically closed state. Here, the driving signal amplitude VLCD increases from the value VTH to VSH due to the activation signal OP from the photo-detector. The amplitude change of the driving signals VLCD1 and VLCD2 results in the increased slope of the integrator output signal Z, and the reference level VR1 is attained quicker. The driving signal polarity is reversed and when the integrator output signal Z again reaches the reference level VR0, the period is concluded. It is a specific advantage of the proposed concept that it can easily allow for the inclusions of a high voltage amplitude switching pulse at any time in order to increase the switching to the optically closed state without losing the DC balance. The ‘integration technique’, as described
118
J. Pirsˇ et al. / Displays 25 (2004) 115–121
Fig. 3. Time dependence of the driving voltage amplitude before (VTH), during (VSW, VSH; during the time intervals TSW and TW, respectively) and after (VTH) the switching. Signal OP, starting at To marks the duration of the signal requesting the optically closed state.
above, therefore allows easy integration of a short (w1 ms) high voltage pulse (w40 V) at the beginning of the optical switching, which further reduces the switching time. The timing diagram presented on Fig. 3 shows a typical time dependence of the driving voltage amplitude over the electrodes of the LCD light shutter (VLCD) before (VTH), during (VSW and VSH) and after (VTH) the intense light flash of the welding arc characterized by the signal OP of the optical detection circuit. As a result of such optimized LCD driving, the switching speed is therefore well below that required by the internationally accepted requirements for the optical personal protecting devices [1], which is 5 ms for switching from a protective shade 3 (‘optically open state’, 12% transmission) to the protective shade 11 (‘optically closed state’, 3.2!10K5 transmission). During the polarity cycling of the LCD, the electronic integration of the signal ensures that no other visible effect, but the increase of the switching speed is noticeable. To briefly summarize, the concept described here is not limited by the time dependence of the driving signal amplitude (Fig. 3) and does not require any synchronization with the optical switching request. This is due to the driving signal integration performing the DC balancing within one driving signal period. Using this driving scheme (high voltage pulse and ‘pre-driving’), the LCD electro-optical response speed can be maximized even at very slow effective driving polarity reversals (see Fig. 4) without losing the DC balance of the driving signal. Contrary to this standard LCD driving technical solutions [11,12] cannot be synchronized with random events like welding arc, if they use low polarity reversal frequency in order to reduce the LCD power consumption. Furthermore, any additional driving voltage amplitude variations necessary to improve the LCD light shutter switching speed (Fig. 3) additionally reduce the DC balance of the LCD driving voltage waveform.
Fig. 4. Simplified block diagram—the photo-detector’s signal (OP) is used to select the LCD driving voltage selector (VSEL) to switch between the voltage below the switching threshold (VTH—optically open state) and to the selected shade level (VSH—optically closed state). The latter is applied to the LCD, and is integrated by the integrator (INT), the output Z of which is compared (COMP1) to the control voltage (VR1). The signal (TG) provided by the comparator (COMP1) to the flip-flop (FF) resets the integrator (INT). The toggle flip-flop (TFF) gives the signal (T/C) controlling the polarity change of the LCD driving signal.
3. Simplified driving scheme The LCD driving concept as presented in Figs. 1 and 2 might appear rather complex to implement. However, assuming the electronic components’ operational performances do not change significantly in time and there is no significant difference between the voltage over the LCD electrodes and the LCD driver input voltage, it is possible to simplify the driving electronics. So the continual comparison loop is reduced to the comparison of the time integrals of two consecutive intervals, during which the LCD driving signal polarity is reversed. Only one comparator COMP1 is needed, with the output TG used to toggle the flip-flop TFF and restart integration (signal RES) from the initial value after every polarity reversal of the driving signal (Fig. 4). In this case, the integration and comparison functions, needed in the proposed LCD driving concept (Figs. 1–4), can be realized with just few electronic components resulting in negligible current consumption. The usual implementation of an integrator requires the use of an operational amplifier, which is not suitable due to its relatively high current consumption. Additionally, the integrator input being permanently connected to the LCD driving signal consumes significant current, since the value of the input resistor is limited to w10 MU for practical reasons. However, the integrator can be implemented by a single integrating capacitor CI, switches SW1 and SW2, a transfer capacitor CT and an amplifier A1 with a gain of C1 and high input resistance, as shown in Fig. 5. The transfer capacitor CT and the switches SW1 and SW2 are used to periodically transfer the charge, which is proportional to the voltage difference VLCD1KVLCD2, into the integrating capacitor CI, and the amplifier (A1) assures the complete transfer of
J. Pirsˇ et al. / Displays 25 (2004) 115–121
119
times much longer than 2 s can be used in the case of very slow LCD driving voltage polarity changes according to the simplified driving concept described above. As the power consumption of the total control and driving electronics including the above described ‘integration polarity control’ can be kept below 10 mA; the capacitive current consumption of the LCD light shutter can be made negligible. As a result of the above-described technical solutions, a standard 3 V Lithium ‘coin’ battery can power an automatic LCD light shutter for human eye protection or for several years. 4. Experiment As described in paragraph 2, the proposed ‘integration, slow polarity reversal’ driving scheme has two important advantages over the standard concept: Fig. 5. Top: schematic diagram of the LCD driving voltage integrator. Bottom: schematic diagram of the LCD driving voltage integrator and comparator circuit.
the charge from the transfer capacitor CT into the integrating capacitor CI. The toggling period of the clock signal CLK for the switches SW1 and SW2 is selected to be two orders of magnitude smaller than the expected period of the LCD polarity reversal in order to avoid integration errors. The amplifier (A1) can be implemented using only two bipolar transistors Q1 and Q2 of opposite polarity. The complete schematic diagram of the integrator and comparator is shown in Fig. 5. The integrator output signal Z is the voltage at the integrating capacitor CI. In this modified LCD driving concept, only one comparator is needed and when the output signal of the integrator reaches the reference level, the flip-flop is toggled and the output signal from the integrator is reset to the initial value, therefore, the integrating capacitor CI is discharged. This is accomplished by a ‘uni-junction’ connection of two transistors Q3 and Q4 of opposite polarity, as shown in Fig. 5. The transistor Q5 is used to form a suitable signal TG for toggling the flip-flop FF. The realization of the electronic driving circuitry results in significant reduction of the current consumption of the LCD light shutters. The current consumption of the additional electronics can be kept very low, typically 1 mA at 3 V battery power supply, and is thus negligible compared to the reduction in the average capacitive current through the LCD light shutter when driven in a conventional manner (w50 Hz). The proposed driving scheme, however, allows for an important increase of the LCD driving voltage polarity reversal times. So the average capacitive current through the typical LCD light shutter, used in the human eye protective devices (equivalent of w100 nF), can be reduced from more than 100 mA in the case of the conventional driving (50 Hz) to about 1 mA, if the driving polarity reversal times can be increased to 2 s. Polarity reversal
† faster switching speeds, † optimized DC balancing and hence maximum possible increase of the LCD driving signal polarity reversal times. The switching response of the LCD was measured for both driving schemes and is shown in Fig. 6(a). The driving
Fig. 6. (a) Optical response (w100 ms) of the LCD light shutter driven according to the proposed driving scheme (black line and bottom left driving voltage timing diagram) compared to the response (w125 ms) to the standard square wave driving signal with 50/50 duty cycle (gray line and top right driving voltage timing diagram). (b) Optical response (12,500 ms) of the LCD light shutter driven by the standard square wave driving signal with 50/50 duty cycle and without the high voltage switching pulse (black line and driving voltage timing diagram).
120
J. Pirsˇ et al. / Displays 25 (2004) 115–121
schemes used are illustrated in the top right (current, state of the art driving scheme) and bottom left corner (proposed driving schemes) of the figure and also shown schematically in Fig. 1(a) and (b) as well as in the timing diagram in Figs. 2 and 3. The present state-of-the-art scheme runs at 0 V in the optically open state [12] and when the switching occurs, a pulse (35 V for 2 ms) is applied followed by the square wave (4 V, 50 Hz, 50% duty cycle) to keep the required closed state. As the LCD driving cannot be synchronized with the light stimulus, which is completely sporadic, some effective DC unbalance is inevitably generated. Using low driving frequencies also in turn results in ionic screening and related undesirable phenomena. The proposed scheme uses the same voltage amplitudes (VSHw4 V) as the current, state-of-the-art driving scheme, but with polarity reversal times w5 s and in addition uses a ‘predriving’ voltage (w1.5 V amplitude), see Fig. 3. The ‘predriving voltage’ is below the switching threshold VTH of the LCD light shutter, so no noticeable decrease of the light transmission can be seen visually. The use of the ‘predriving’, however, enables the LC molecules to have a higher degree of tilt with respect to the boundary surface so that the switching can be preformed quicker. From Fig. 6(a) it can be seen that the optical response (to 90% of the final light attenuation) using the proposed driving scheme [13] is w100 ms compared to w125 ms, using a standard driving technique (without ‘predriving’), if the high voltage switching pulse (VSWw35 V) is used at the beginning of the optical switching. If the current state-of-the-art scheme [11,12] is used in its original form without the high voltage switching pulse (VSW), the switching time is increased to 12.5 ms (see Fig. 6(b)). The DC compensated LCD light shutter driving scheme allows the use of very slow driving polarity reversals. There is, however, a limit to the ultra-slow voltage polarity reversal times that can be employed. In principle, this concept is only applicable, as long as the RDC voltage build-up effects are linear (see Fig. 7). In this case, the integration can result in perfect DC balancing even, when the LCD driving voltage amplitude varies in time
considerably, which is the case in most LCD light shutters used in human eye protective devices [12] (see Fig. 3). The limitations in very slow polarity reversals of the LCD light shutter driving were evaluated by measuring the time dependence of the RDC voltage build-up. The fact is that, due to the slow nature of the RDC voltage, these effects on the light attenuation of the LCD light shutter are the most noticeable, while the ionic screening effects of the free ions are too fast to be adequately perceived by the human eye. The measurements were performed on sample LCD light shutters using ultra-low RDC polyimide (Nissan SE 4794) alignment layers and commercial low ionic contamination liquid crystals as used in AM LCD’s (Merck MLC 6873). Before every measurement, the LCD light shutters were heated to the isotropic phase (110 8C) for 20 min in order to avoid ‘memory effects’ from previous measurements. After such annealing procedure, the measured LCD light shutters were exposed to a DC voltage UC for the time TC. Immediately after the exposure to a DC voltage, the resulting RDC voltage was evaluated by measuring the light attenuation caused by the LCD light shutter: a square wave signal of 5 Hz without a DC component was used to drive the LCD during measurements with the amplitude selected to induce a relatively light attenuation factor of 65 on the LCD. The first few periods of the driving signal were used to determine the attenuation. The light attenuation variations due to RDC voltage build-up were measured and subsequently converted into an RDC voltage by using the voltage dependent attenuation characteristics of the LCD taken previously. This experiment was repeated for different DC voltages, UC of 1.5, 3, 6 and 9 V, and for different exposure times, TC of 5, 10, 60 and 120 s. Fig. 7 shows the dependence of the absolute value of the RDC voltage on the applied voltage UC for different time periods TC. Here, it can be seen that for up to 5 s and even up to 10 s time period TC, the measured RDC voltage values increase linearly with the applied voltage UC (within error) through the entire driving voltage range, while for the time periods of 60, 120 s and above the linearity can no longer be achieved. This means that: 1. The LCD driving waveforms with relatively high driving voltage amplitudes (up to 9 V) can be used at polarity reversal times as long as 10 s, 2. The time integrals of the DC sections of the LCD driving Ð waveform 0TC UC ðtÞdt can be as high as 9 V!10 s, meaning that substantially higher driving voltage amplitudes (e.g. R40 V), necessary to achieve ultrashort LCD light shutter switching times [12], can be used as long as their time integral is small enough.
Fig. 7. The dependence of the RDC voltage on the exposure of the LCD to the known DC voltage UC for a time TC for UC values (0, 1.5, 3, 6 and 9 V) and TC values (0, 5, 10, 60 and 120 s).
As already discussed above, the capacitive current through the LCD light shutters typically used in eye protection applications can be reduced to 10% of the total device electric power consumption, if the driving voltage polarity changes (cycles) occur slower than within 2 s.
J. Pirsˇ et al. / Displays 25 (2004) 115–121
Fig. 8. Auto-darkening welding filters—the left filter is in the ‘optically closed state’ showing the front side of the display, while the right filter is in the ‘optically open state’ showing the back side of the filter with the control switches for the sensitivity and light transmission number.
This is far below the 10 s confirmed by the experiment, meaning that the power consumption of the LCD light shutter in the personal eye protective devices can be made almost negligible.
121
are kept below 10 s, the basic assumptions of the proposed slow polarity reversal LCD driving scheme are met. As mentioned previously, extending the LCD driving signal polarity reversal times above a few seconds, reduces the capacitive current of the typical LCD light shutter, used in eye protective devices to almost a negligible value (%1 mA) compared to the driving/control electronics power consumption (w5–10 mA). Therefore, using low ionic contamination LCD materials (liquid crystal, polyimide), the light shutters that are driven with the proposed slow polarity reversal driving scheme [13] do not contribute to the overall power consumption of the devices such as in human eye protective light shutters. As a result, a standard 3 V Lithium ‘coin’ battery can power an automatic LCD light filter for human eye protection for several years.
Acknowledgements The authors would like to acknowledge the support of the NATO ‘Science for Peace’ Program through the Research Grant SfP 971874.
5. Conclusion A novel LCD driving concept for LCD light shutters, intensity and phase modulators that can be used in various personal protection devices like welding light filters (Fig. 8) has been described, based on driving signal polarity reversals controlled by the driving voltage integral across the LCD switching element electrodes, as opposed to plain periodic polarity reversals. The driving scheme optimizes the DC voltage balancing of the LCD driving signal, as it guarantees the compensation of the long-term (DC) component of the driving voltage within one polarity reversal cycle, irrespectively of driving voltage amplitude variations. The obtained results clearly show that as long as the average polarity reversals times of the LCD driving voltage
References [1] European standard: EN379, 2002. [2] J. Pirsˇ, S. Pirsˇ, R. Petkovsˇek, M. Bazˇec, P. Bos, SID 02 Digest, p. 53. [3] J. Pirsˇ, M. Bazˇec, A. Vrecko, D. Ponikvar, SID 03 Digest, p. 55.3. [4] T. Uchida et al., SID 89 Digest, pp. 378–381. [5] Y. Nakazono et al., IDW 98, p. 61. [6] T. Chieu, K.H. Yang, Jpn. J. Appl. Phys. 28 (11) (1989) 2240. [7] G. Barbero, et al., J. Appl. Phys. 67 (5) (1990) 2678. [8] R.N. Thurston, et al., J. Appl. Phys. 55 (12) (1984) 4154. [9] H. De Vleeschouwer, et al., Jpn. J. Appl. Phys. 41 (3A) (2000) 1–6. [10] R.A. Erhart, et al., US Patent: US5,852,426, 1998. [11] K. Kutaragi et al., US Patent: US4,205,311, 1980. [12] G. Gunz, et al., European Patent: EP0,550,384, 1994. [13] J. Pirsˇ, B. Marin, D. Ponikvar, S. Pirsˇ, European Patent EP 1,131,669, 2004.