Driving passive-matrix LCDs with low hardware complexity and reduced supply voltage

Transcription

1 Driving passive-matrix LCDs with low harware complexity an reuce supply voltage K. G. Panikumar T. N. Ruckmongathan Abstract In passive-matrix liqui-crystal isplays (LCDs), multiplexing is achieve by using the intrinsic non-linear characteristics of the liqui-crystal material. If the electro-optic characteristic is steeper than necessary for the matrix isplay, the selection ratio nee not be maximize. Instea, the selection ratio can be reuce to match the electro-optic characteristics of the isplay. This leas to a reuction in the supply voltage of the rive electronics. We have consiere the possibility of using aressing techniques with low harware complexity along with isplays having steep electro-optic characteristics. Supply voltages for these techniques are compare with that of multi-line aressing (MLA). The supply voltages of the Hybri Aressing Technique (HAT), Improve Hybri Aressing Technique-S3 (IHAT-S3), an Improve Hybri Aressing Technique-S4 (IHAT-S4) are lower than that of MLA for the lower range of N. These hybri aressing techniques with lower harware complexity are a better choice for riving passive-matrix LCDs, especially in portable equipment. Keywors Passive-matrix LCDs, aressing technique, multi line aressing, hybri aressing supply voltage. 1 Backgroun TABLE 1 Column voltages for the various hybri aressing techniques. FIGURE 1 Electro-optic characteristics curve (V on > V u ). The intrinsic non-linearity of the electro-optic effect is exploite to rive passive-matrix liqui-crystal isplays (LCDs). During the last few ecaes, the aressing techniques for passive-matrix LCDs were optimize to achieve the maximum selection ratio. Selection ratio (SR) is efine as the ratio of the RMS voltage across an ON pixel (V on ) to that of an OFF pixel (V off ). Passive-matrix LCDs have been replace by active-matrix LCDs in high-informationcontent applications requiring a large matrix size. Passivematrix supertwiste nematic (STN) isplays are now being use in mobile telephones, personal igital assistants (PDAs), an other meium- an low-information isplays. In these applications, the matrix size is moerate an the electro-optic characteristics can be sharper than necessary. Supply voltage is another important parameter to be consiere while using LCDs in portable evices. Several aressing techniques with low supply voltage have been propose in the past (HAT, 1 IHAT, IHAT-S3, an IHAT-S4 3 ). The supply voltage of an aressing technique is etermine by the maximum swing in the aressing waveforms. Supply voltage is a minimum when the maximum swings in the row an column waveforms are equal. Supply voltage is a minimum for the case of the Improve Hybri Aressing Technique (IHAT) as well as Multi-Line Aressing (MLA) 4 when s = N 1/. Here, N is the total number of lines being scanne in a matrix LCD an s is the number of rows in the subgroups. The supply-voltage requirement can be further The authors are with the Raman Research Institute, Liqui Crystal Laboratory, Sir C. V. Raman Ave., Saashivanagar, Bangalore Inia; telephone , fax , ruck@rri.res.in. Copyright 00 Society for Information Display /0/ $1.00 Journal of the SID 10/4,

2 TABLE The coefficents α, β, γ, an δ for the various hybri aressing techniques. where an q is the total number of time intervals in the orthogonal matrix use in the case of MLA. reuce by following the scheme propose by Kuijk (1999). 5 Here, the electro-optic characteristic is steeper than necessary for the matrix size. The ifference between the RMS voltage across the ON an OFF pixels is more than necessary for multiplexing N rows of the matrix LCD as shown in Fig. 1. The voltage across the ON pixel can be lowere to V u without affecting the contrast of the isplay. Hence, the selection ratio is reuce to V u /V t an is referre to as the reuce selection ratio. This will help to lower the supply voltage of the rive electronics without losing contrast. We have consiere HAT, IHAT-S3, an IHAT-S4 with a low harware complexity of rivers. The selection ratios of these techniques are lower than the maximum value ue to the restrictions impose on the number of voltage levels in the column waveforms. A lower selection ratio is not a problem since the electro-optic characteristic of the liqui crystal is steep enough for the small or moerate number of rows being multiplexe. The number of voltage levels in the column waveforms is restricte to two, three, an four in the cases of HAT, IHAT-S3, an IHAT-S4, respectively, as compare to (s + 1) in the case of IHAT or MLA 4. The supply-voltage requirements of these techniques are compare with that of the multi-line aressing technique 5 when the selection ratio is reuce to match the electro-optic characteristic. Hybri aressing techniques In the hybri aressing techniques, the N rows in the matrix are ivie into (N/s) non-intersecting subgroups with each subgroups consisting of s aress lines. At a given instant of time, one subgroup is selecte with voltages corresponing to an s bit row select pattern. Here, a row select pattern correspons to one of the s binary patterns. The amplitues of these voltages are either +V r for logic 0 an V r for logic 1. The remaining (N s) unselecte rows are groune. The ata to be isplaye in the selecte subgroup in any column is also an s bit wor with logic 0 representing an OFF pixel an logic 1 for an ON pixel. The column voltage is ecie by the number of mismatches 1,3 364 Panikumar et al. / Driving passive-matrix LCDs

3 between the row select pattern an the ata in the selecte subgroup. The number of mismatches i is given by s i = Â a j ks+ j, j= 1 where (a 1,a,a 3,...a s ) is an s bit row select pattern an ks+j is the ata in the k th subgroup [k = 0,1,,... (N/S 1)]. Table 1 gives the column voltages for the various hybri aressing techniques. The mismatches for all the columns in the matrix are compute an transferre to the column river. Then both the row an column voltages are applie simultaneously to the matrix isplay for a time uration τ. The process is repeate with another row select pattern by selecting the same subgroup or a ifferent subgroup. A cycle is complete when all the subgroups (N/s) are selecte with all the s row-select patterns once. The isplay is refreshe continuously by repeating this cycle. Expressions for the RMS voltage across the ON an OFF pixels are of the form shown in Eqs. () an (3): Von = Voff = avr + bvr Vc + gvc, avr - bvr Vc + gvc. (1) () (3) The coefficients α, β, γ, a n δ for the various hybri aressing techniques are given in Table. With a steep electro-optic characteristic, the selection ratio can be reuce to V u /V t. Hence, the reuce selection ratio is Vu SRreuce = = k. (4) Vt The conition for the reuce selection ratio is etermine as follows: SR reuce Von = = V off This expression can be simplifie to r r c c r r c c av + bv V + gv av - bv V + gv x x SRreuce = a + b + g = ax - bx + g wherein x = V r /V c. Solving for x, we get b x = a F I HG K J ± The RMS voltage across the OFF pixels is controlle to be near V th in orer to get a goo contrast ratio in the isplay. Hence, L NM k + 1 b k -1 a F HG k + 1 k -1 k, I K JO QP - = g. a k. (5) TABLE 3 The coefficients α, β, γ, an δ of the hybri aressing techniques for two iffernet values of s. Journal of the SID 10/4,

4 TABLE 4 The minimum supply voltage (normalize to V th ) with reuce selection ratios for various values of s. ax - bx + g Voff = Vc = Vth, V c = V ax - bx + g (6) The supply voltage is etermine by the maximum swing in the aressing waveforms. The expressions for the supply voltage are Vsup = Vc for Vr Vc, Vsup = The coefficients α, β, γ, an δ of the hybri aressing techniques for two ifferent values of s are shown in Table 3. The column voltages of IHAT-S3 an IHAT-S4 have several possible values epening on the grouping of mismatches. 3 We have consiere the grouping leaing to the highest selection ratio. Both the grouping of mismatches an the corresponing voltage levels 3 are also shown in Table 3. Supply voltage is a minimum when the maximum swings in the aressing waveforms (row an column) are equal (when x = V r /V c is 1); that is, when Table 4 shows the minimum supply voltage (normalize to V th ) with reuce selection ratios for various values of s. th Vth for Vr Vc, ax - bx + g Vsup = Vr = xvc for Vr Vc, Vsup = x Vth for Vr Vc. ax - bx + g F I HG K J - k + 1 g = b a. k -1. (7) (8) (9) 3 Results an iscussions Supply voltages of HAT, IHAT, IHAT-S3, an IHAT-S4 with a reuce selection ratio are compare with the results of Kuijk 5 for MLA. The IHAT has the same reuction in supply voltage as that of the MLA technique. 5 This is expecte since the row-select patterns of MLA are a subset of the row select patterns of IHAT. 6 The number of time intervals to complete a cycle is the only parameter that iffers between IHAT an MLA while all other parameters like supply voltage, selection ratio, etc., are the same for IHAT an MLA. Two ifferent liqui-crystal mixtures were consiere to compute the supply voltage for the hybri aressing techniques. Liqui-crystal mixture 1 (LC1), suitable for multiplexing 100 lines, i.e. (V u /V t ) = , an liqui-crystal mixture (LC), capable of multiplexing 40 lines, i.e., (V u /V t ) = , were consiere for the analysis. Supply voltages for HAT, IHAT-S3, an IHAT-S4 were calculate for ifferent matrix sizes (N). The minimum supply voltage TABLE 5 The number of aress lines for which supply voltages of HAT, IHAT-S3, an IHAT-S4 are almost equal to that of MLA (N eqs ). 366 Panikumar et al. / Driving passive-matrix LCDs

5 FIGURE Supply voltage (normalize to V th ) vs. N for HAT an MLA when s = 3 an SR = FIGURE 5 Supply voltage (normalize to V th ) vs. N for HAT an MLA when s = 5 an SR = FIGURE 3 Supply voltage (normalize to V th ) vs. N for HAT an MLA when s = 3 an SR = is achieve when the maximum swings in the row an column waveforms are equal. Table 4 gives the number of lines being multiplexe when the supply voltage is a minimum (N min ) for hybri aressing N min (HA) an multi-line aressing N min (MLA). Supply voltages of HAT, IHAT-S3, an IHAT-S4 are less than that for MLA over certain ranges of N. Figures 13 show the supply voltage (normalize to V th ) vs. the number of aress lines (N) for two s values in the case of HAT, IHAT-S3, an IHAT-S4 as compare to MLA. 5 The supply voltage of MLA when the selection ratio is a maximum has also been plotte for comparison. The number of aress lines for which supply voltages of HAT, IHAT-S3, an IHAT-S4 are almost equal to that of MLA (N eqs ) are shown in Table 5. The supply voltage of hybri aressing is lower than that of MLA when N (the number of lines being multiplexe) is less than N eqs. A goo reuction in supply voltage is achieve when N is less than N min (HA). The percentage reuction in supply voltage com- FIGURE 4 Supply voltage (normalize to V th ) vs. N for HAT an MLA when s = 5 an SR = FIGURE 6 Supply voltage (normalize to V th ) vs. N for IHAT-S3 an MLA when s = 4 an SR = Journal of the SID 10/4,

6 FIGURE 7 Supply voltage (normalize to V th ) vs. N for IHAT-S3 an MLA when s = 4 an SR = FIGURE 9 Supply voltage (normalize to V th ) vs. N for IHAT-S3 an MLA when s = 6 an SR = pare with MLA 5 is almost constant when N is less than or equal to N min (HA): Vsup( MLA) - Vsup( HA) percentage reuction = 100. Vsup( MLA) This reuction in supply voltage is plotte in Fig. 14 for the various hybri aressing techniques. Table 6 gives the maximum reuction (percentage) in supply voltage compare to the MLA 5 technique when N is equal to N min (HA). The supply voltage for line-by-line aressing may also be reuce by lowering the selection ratio to match the electro-optic characteristics. The supply voltages of the Alt an Pleshko Technique 7 (APT) an the Improve Alt an Pleshko Technique 8 (IAPT) are the same for lower values of N (in the region where the reuce row select pulse V r(reuce) V c(reuce) ). The supply voltages for APT an IAPT are almost equal to that of HAT as shown in Figs. 15 an 16 in the region V r(reuce) V c(reuce). The supply voltage is FIGURE 10 Supply voltage (normalize to V th ) vs. N for IHAT-S4 an MLA when s = 5 an SR = FIGURE 8 Supply voltage (normalize to V th ) vs. N for IHAT-S3 an MLA when s = 6 an SR = FIGURE 11 Supply voltage (normalize to V th ) vs. N for IHAT-S4 an MLA when s = 5 an SR = Panikumar et al. / Driving passive-matrix LCDs

7 TABLE 6 The maximum reuction (percentage) in supply voltage compare to the MLA techniqe when N is equal to N min (HA). FIGURE 1 Supply voltage (normalize to V th ) vs. N for IHAT-S4 an MLA when s = 7 an SR = a minimum when the amplitues of row an column waveforms are equal (when N = 19 an N = 30 for the liqui-crystal mixtures LC1 an LC, respectively). A comparison of the lowere supply voltages of hybri aressing techniques with that of APT, IAPT, an MLA with reuce selection ratios is shown in Figs. 15 an 16. The hybri aressing techniques show a goo reuction in supply voltage. The aressing techniques which have the lowest supply voltage an the range of N over which the supply voltage is low are given in Tables 7 an 8 for LC1 an LC, respectively. The hybri aressing technique (HAT) has low harware complexity (two voltage levels in the column waveforms an three voltage levels in the row waveforms) an has the lowest supply voltage for the lower values of N. IHAT-S3 an S4 have the lowest supply voltage for the mi-range of N, while IHAT as well as MLA have the lowest supply voltage for the higher values of N. Active aressing 9 wherein all the rows are selecte simultaneously requires the same supply voltage as APT. The harware complexity of column rivers an the controller is high for active aressing, so this technique is not attractive for practical implementation even though the reuce supply voltage is also same as that of APT. The row an column waveforms of HAT, IHAT-S3, an IHAT-S4 were generate using the waveform-generator WFG 500 for various values of s an N in orer to verify the results experimentally. The RMS voltage across the ON an OFF pixels were measure using the HP 3467A, a logging multimeter capable of measuring true RMS voltage. Tables 9 11 show the measure RMS voltages an the percentage of error as compare to the theoretical value for ifferent values of N an s for the cases of HAT, IHAT-S3, an IHAT-S4. The RMS voltages an the selection ratios obtaine from these measurements agree within ±0.8% of the theoretical values. The hybri aressing techniques have a lower supply voltage. The harware complexity of the column rivers of these techniques is lower than that for IHAT an MLA. It is important to note that the higher number of time intervals to complete a cycle for the hybri aressing techniques compare to MLA is not a isavantage. In fact, the Haamar as well as the Walsh matrices are subsets of the FIGURE 13 Supply voltage (normalize to V th ) vs. N for IHAT-S4 an MLA when s = 7 an SR = FIGURE 14 Percentage reuction in supply voltage vs. number of aress lines (N ) in comparison with MLA (Ref. 5). Journal of the SID 10/4,

8 TABLE 7 Aressing techniques which have the lowest supply voltage an the range of N over which the supply voltage is low for LC1. TABLE 8 Aressing techniques which have the lowest supply voltage an the range of N over which the supply voltage is low for LC. 370 Panikumar et al. / Driving passive-matrix LCDs

9 FIGURE 15 Supply voltage vs. N for various aressing techniques inicating the range wherein the supply voltage is the lowest for LC1. FIGURE 16 Supply voltage vs. N for various aressing techniques inicating the range wherein the supply voltage is the lowest for LC. matrix corresponing to the Raemacher functions. For example, the Raemacher functions for selecting four rows at a given instant of time have 16 row-select patterns (4 16 matrix). This matrix can be interprete as four orthogonal matrices of 4 4. Hence, the large number of row-select patterns just correspons to using more than one orthogonal matrix or set of functions. This helps in increasing the brightness uniformity of the pixels an ecreases the harware complexity of the column rivers. In summary, the hybri aressing techniques with lower harware com- TABLE 9 The measure RMS voltages an the percentage of error compare to the theoretical value for ifferent values of N an s for HAT. Journal of the SID 10/4,

10 TABLE 10 The measure RMS voltages an the percentage of error compare to the theoretical value for ifferent values of N an s for IHAT-S3. TABLE 11 The measure RMS voltages an the percentage of error compare to the theoretical value for ifferent values of N an s for IHAT-S4. 37 Panikumar et al. / Driving passive-matrix LCDs

11 plexity an reuce supply voltage are a better choice for riving passive-matrix LCDs, especially in portable equipment such as mobile phones an PDAs. References 1 T N Ruckmongathan an N V Mahusuana, New aressing techniques for multiplexe liqui crystal isplays, Proc SID 4, No. 3, 59-6 (1983). T N Ruckmongathan, A generalize aressing technique for RMS responing matrix LCDs, Proc 18th IDRC, (1988). 3 T N Ruckmongathan, An aressing technique with reuce harware complexity, SID Intl Symp Digest Tech Papers, (1994). 4 S Ihara et al, A color STN-LCD with improve contrast, uniformity, an response times, SID Intl Symp Digest Tech Papers, 3-35 (199). 5 K E Kuijk, Minimum-voltage riving of STN LCDs by optimize multiple row aressing, Proc EuroDisplay 99, (1999). 6 T N Ruckmongathan, Novel aressing methos for fast responing LCDs, Reports Res Lab Asahi Glass Co Lt 43 (1), (1993). 7 P M Alt an P Pleshko, Scanning limitations of liqui crystal isplays, IEEE Trans Electron Dev ED-1, (1974). 8 H Kawakami et al, Matrix aressing technology of twiste nematic liqui crystal isplay, Conf Rec Biennial Display Research Conf, (1976). 9 T J Scheffer an B Clifton, Active aressing metho for high-contrast vieo-rate STN isplays, SID Intl Symp Digest Tech Papers, 8-31 (199). K. G. Panikumar receive his M.Sc egree in electronics from the University of Mysore, Mysore, Inia, in He is currently working towar his Ph.D. egree at the Raman Research Institute, Bangalore, Inia. His research interests are aressing techniques an controllers for passive-matrix LCDs. He is a stuent member of SID. T. N. Ruckmongathan s area of research is the aressing of passive-matrix LCDs. He has invente several aressing techniques for riving rms-responing LCDs an ferroelectric LCDs. The Improve Hybri Aressing Technique he propose at IDRC 88 was the first of several multi-line aressing techniques that were use in proucts like PDAs an mobile telephones. He is an associate professor at the Raman Research Institute. In the past, he was a visiting professor at Chalmers University of Technology, Sween (1998), Guest Researcher at Asahi Glass Co., Japan ( ), an an LCD specialist at Philips, The Netherlans ( ). Journal of the SID 10/4,