JP4118971B2 - Multi-tone LCD driver - Google Patents

Description

（２）式で表される電流ΔＩ_i は中間タップに接続されるγ補正電圧発生回路４_i （ｉ＝１〜ｎ−１）での電力消費Ｐ_i をもたらす。
【０００９】
Ｐ_i ＝Ｖ_i ×ΔＩ_i ＝Ｖ_i （ΔＶ_i+1 −ΔＶ_i ）／Ｒｔ
（ｉ＝１…，ｎ−１） ………… （３）
また、回路４_n 及び４₀ 内でそれぞれ消費される電力は、電源電圧をＶ_ccとすれば
Ｐ_n ＝（Ｖ_cc−Ｖ_n ）Ｉ_n
Ｐ₀ ＝Ｖ₀ Ｉ₁ ………… （４）
ドライバ１内部でのＲ−ＳＴＲＩＮＧ２そのものが消費する電力もある。このＲ−ＳＴＲＩＮＧ２そのものの電力消費はＲ−ＳＴＲＩＮＧ２の値が大きい程少なくなるのは明らかであるが、ドライド１の出力インピーダンスとＬＣＤパネルの容量との合成時定数がＬＣＤパネルを充放電する時間に対して十分小さい値でないと、ＬＣＤに適正な電圧を与えることができなくなる。これはコントラストの低下やクロストークの増大となって表示品位を低下させる。そのため、端子、タップ５_i （ｉ＝０〜ｎ）に電圧源を接続することは単にγ補正のための入力としてだけでなく、Ｒ−ＳＴＲＩＮＧの抵抗値を大きくし、かつドライバの出力インピーダンスを下げるという側面がある。
【００１０】
ドライバ１の出力インピーダンスは選択されるスイッチＳＷの位置によって変化する。スイッチＳＷにより分割されるＲｔの分割比をｋとすると、出力インピーダンスＲout は抵抗（１−ｋ）ＲｔとｋＲｔの並列抵抗値と、オン抵抗ｒの和となり、以下の（５）式で表される。

と置くと、ｋ＝１／２が得られる。このときＲout が最大になり、その値は（７）式となる。
【００１１】
〔Ｒout 〕max ＝（Ｒｔ／４）＋ｒ ………… （７）
一般に、Ｒ−ＳＴＲＩＮＧ方式のドライバでは出力インピーダンスの最大値とＬＣＤの容量との時定数が１水平時間Ｈに対して十分小さくなるように設定される。
【００１２】
【発明が解決しようとする課題】
従来のＲ−ＳＴＲＩＮＧ方式のドライバはＮ＝ｍ×ｎ＝６４個の抵抗が全て等しい値（Ｒ）になっているため、外部からγ補正用の端子、タップ５_i に電圧を印加して、γ特性を補正していた。このγ補正のために各中間のタップ５_i には（２）式で表される電流ΔＩ_i がγ補正電圧発生回路４_i （ｉ＝１〜ｎ−１）に流れるため、その電流ΔＩ_i ×出力電圧Ｖ_i ＝Ｐ_i は回路４_i での電力消費となる。また、この不平衡電流ΔＩ_i はストリング抵抗器を流れるので、それだけストリング抵抗器の消費電力が増加する。
【００１３】
この発明は、γ補正電圧発生回路４_i （ｉ＝１〜ｎ−１）に吸込まれる電流ΔＩ_i をほぼゼロにして、ドライバ１のストリング抵抗器及び電源回路４の消費電力を低減することを目的としている。
【００１４】
【課題を解決するための手段】
（１）請求項１の発明は、直流電圧を直列接続されたＮ個（Ｎは３以上の整数）のストリング抵抗器により分割して、Ｎ個の階調電圧を得、入力ディジタルデータに基づいて階調電圧の１つを選択して液晶素子に出力すると共に、ストリング抵抗器のＮ−１個の接続点よりｎ−１個（１≦ｎ−１＜Ｎ−１）の接続点を抽出して、それらの抽出された接続点よりタップを導出し、それらのタップ（５₁ 〜５_n-1 ）及び一連のストリング抵抗器の両端の端子（５₀ ，５_n ）にγ補正電圧（Ｖ₀ ，Ｖ₁ …，Ｖ_n ）を印加するようにした多階調液晶ドライバに関する。
【００１５】
請求項１では特に、γ補正電圧を、前記隣接の端子、タップ間の電位差（ΔＶ_j ）が全て相等しくなるように印加すると共に、それら隣接の端子、タップ間のストリング抵抗器の抵抗値（Ｒｔ_j ）を全てほぼ等しく設定する。
（２）請求項２の発明では、前記（１）において、Ｎ個の各ストリング抵抗器の抵抗値Ｒ（ｐ）が、液晶素子の透過率対印加電圧特性における１階調当たりの透過率の一定変化幅に対応する印加電圧の変化幅（Δ（ｐ）；ｐ＝１，２…，Ｎ）に応じた値に設定される。
【００１６】
（３）請求項３の発明では、前記（１）において、一連のストリング抵抗器の隣接する端子、タップ間に存在する個々のストリング抵抗器の個数（ｍ₁ ，ｍ₂,…，ｍ_n ）は、前記液晶素子の透過率対印加電圧特性曲線の傾斜に応じて一連のストリング抵抗器の両端側で少なく、中間で多く設定される。
【００１７】
【発明の実施の形態】
Ｎ（例えばＮ＝６４）階調表示を行うものとして、図２に示す液晶のＴ−Ｖ特性より、階調に対応する一定ピッチ幅の透過率Ｔ（０），Ｔ（１）…，Ｔ（Ｎ）に対応する印加電圧Ｅ（０），Ｅ（１），…，Ｅ（Ｎ）を求める。従来と同様にｎ＋１個（例えばｎ＝８）の端子（タップ）５₀ 〜５_n を設けるものとする。隣接端子、タップ間の電位差ΔＶ_j をｊに関係なくΔＶに等しくする。即ち、
ΔＶ_j （ｊ＝１〜ｎ）＝ΔＶ …………（８）
また、隣接タップ間の抵抗値Ｒｔ_i を全てほぼ等しくする。即ち、
Ｒｔ_j （ｊ＝１〜ｎ）≒Ｒｔ（一定） …………（９）
このとき、隣接端子、タップ間のストリング抵抗器を流れる電流Ｉ_i は
Ｉ_j ＝ΔＶ_j ／Ｒｔ_j ＝ΔＶ／Ｒｔ …………（10）
即ち、Ｉ_j はｊに依らず一定値となる。従って、γ電圧補正回路４_j に流れ込むΔＩ_j は、
ΔＩ_j ＝Ｉ_j+1 −Ｉ_j ＝０ （ｊ＝１〜ｎ−１） …………（11）
となり、低消費電力化が図られる。
【００１８】
透過率Ｔ（ｊ）（ｊ＝０〜Ｎ）に対応する印加電圧Ｅ（ｐ）（ｐ＝０〜Ｎ）より１階調増すごとのステップ電圧Δ（ｐ）（図２）を求める。
Δ（１）＝Ｅ（１）−Ｅ（０）
Δ（２）＝Ｅ（２）−Ｅ（１），
・・・
Δ（ｐ）＝Ｅ（ｐ）−Ｅ（ｐ−１），
・・・
Δ（Ｎ）＝Ｅ（Ｎ）−Ｅ（Ｎ−１）， …………（12）
隣接端子、タップ間のストリング抵抗の合計Ｒｔ_j （ｊ＝１〜ｎ）を全てほぼ等しくするために、この発明では、各ストリング抵抗器Ｒ（ｐ）の値を、比例定数をＫとして、下式のように設定する。
【００１９】
Ｒ（ｐ）＝ＫΔ（ｐ）（ｐ＝１〜Ｎ） …………（13）
即ち、各ストリング抵抗値Ｒ（ｐ）はＬＣＤのＴーＶ特性における１階調当たりの透過率Ｔの一定変化幅に対応する印加電圧の変化幅Δ（ｐ）に応じた値とする。
隣接の端子、タップ間の電圧ΔＶを下式より求める。
【００２０】
ΔＶ＝｛Ｅ（Ｎ）−Ｅ（０）｝／ｎ …………（14）
従って、γ補正電圧発生回路４_j （ｊ＝０〜ｎ）より各端子、タップ５_j に印加する電圧Ｖ_j （ｊ＝０〜ｎ）を次のように設定する。
Ｖ₀ ＝Ｅ（０）
Ｖ₁ ＝Ｅ（０）＋ΔＶ，
Ｖ₂ ＝Ｅ（０）＋２ΔＶ，
・・・
Ｖ_j ＝Ｅ（０）＋ｊΔＶ，
・・・
Ｖ_n-1 ＝Ｅ（０）＋（ｎ−１）ΔＶ，
Ｖ_n ＝Ｅ（０）＋ｎΔＶ＝Ｅ（Ｎ）， …………（15）
Ｎ−１（例えば６３）個の階調電圧系列Ｅ（１），Ｅ（２）…，Ｅ（Ｎ−１）の中から、ｎ−１（例えば７）個のタップ電圧系列Ｖ₁ ，Ｖ₂ …，Ｖ_n-1 にそれぞれ最も近い階調電圧Ｅ（ｘ）を抽出する。即ち、
Ｅ（０）＝Ｖ₀ ，
Ｅ（ｍ₁ ）≒Ｖ₁ ，
Ｅ（ｍ₁ ＋ｍ₂ ）≒Ｖ₂ ，
・・・
Ｅ（ｍ₁ ＋…＋ｍ_j ）≒Ｖ_j ，
・・・
Ｅ（ｍ₁ ＋…＋ｍ_n-1 ）≒Ｖ_n-1 ，
Ｅ（ｍ₁ ＋…＋ｍ_n ）＝Ｖ_n …………（16）
ｍ_j はタップ５_j-1 〜５_j 間の階調数、つまりストリング抵抗器Ｒ（ｐ）の個数であり、ｍ₁ 〜ｍ_n の総和はＮに等しい。即ち、
Σｍ_n ＝ｍ₁ ＋ｍ₂ ＋…＋ｍ_n ＝Ｎ …………（17）
図１から明らかなように、タップ５₁ はストリング抵抗器Ｒ（ｍ₁)とＲ（ｍ₁ ＋１）との接続点から出せばよく、タップ５₂ はストリング抵抗器Ｒ（ｍ₁ ＋ｍ₂ ）とＲ（ｍ₁ ＋ｍ₂ ＋１）との接続点から出せばよく、以下同様である。
【００２１】
このようにして一連のストリング抵抗器の途中からタップ５_j （ｊ＝１〜ｎ−１）を導出すると共に、各ストリング抵抗器Ｒ（ｐ）（ｐ＝１〜Ｎ）の値をＴ−Ｖ特性における１階調ごとのステップ電圧Δ（ｐ）に応じた値に設定したとき、確かに隣接端子、タップ５_j ，５_j-1 間の抵抗値Ｒｔ_j （ｊ＝１〜ｎ）がｊに依らず、全てほぼ同じ値になることを検証してみる。
【００２２】
Ｒｔ_j ＝Ｒ（１＋Σｍ_j-1)＋Ｒ（２＋Σｍ_j-1)＋…＋Ｒ（ｍ_j ＋Σｍ_j-1)…………（18）
（13）式を代入して、
＝Ｋ｛Δ(1＋Σｍ_j-1)＋Δ(2＋Σｍ_j-1)＋…＋Δ（ｍ_j ＋Σｍ_j-1)｝
（12）式を代入して
＝Ｋ｛Ｅ(1＋Σｍ_j-1)−Ｅ（Σｍ_j-1)
＋Ｅ(2＋Σｍ_j-1)−Ｅ(1＋Σｍ_j-1)
・・・
＋Ｅ（ｍ_j ＋Σｍ_j-1)−Ｅ（ｍ_j-1 ＋Σｍ_j-1)｝
＝Ｋ｛Ｅ（Σｍ_j ）−Ｅ（Σｍ_j-1)｝
（16）式を代入して
≒Ｋ｛Ｖ_j −Ｖ_j-1 ｝ …………（19）
（15）式を代入して
＝Ｋ｛Ｅ（０）＋ｊΔＶ−Ｅ（０）−（ｊ−１）ΔＶ｝
＝ＫΔＶ …………（20）
以上により、各隣接端子、タップ間の抵抗値Ｒｔ_j （ｊ＝１〜ｎ）はｊの値に依らずほぼＫΔＶの一定値となっていることが検証できた。
【００２３】
実際にγ補正電圧発生回路４_j （ｊ＝０〜ｎ）よりタップに供給する電圧は（15）式より求めた電圧Ｖ₀ ，Ｖ₁ …Ｖ_n であるので、隣接タップ間の電位差は全てΔＶであり、両端の４₀ と４_n を除く中間のγ補正電圧発生回路４₁ 〜４_n-1 の出力端子を出入りする不平衡電流（従来技術で述べたΔＩ_j ）はほぼゼロとなり、省電力化が図られる。この場合、中間の補正電圧発生回路４₁ 〜４_n-1 は液晶ドライバ１の出力抵抗を下げる機能を発揮していることは勿論である。不平衡電流ΔＩ_j はストリング抵抗器を流れる。この発明ではΔＩ_j ＝０であるので、ストリング抵抗器での消費電力もそれだけ少なくなる。
【００２４】
各γ補正電圧発生回路４_j の印加電圧Ｖ₁ ，Ｖ₂ …，Ｖ_n-1 は、図２のＴ−Ｖ特性より求めたＥ（ｍ₁ ），Ｅ（ｍ₁ ＋ｍ₂ ）…，Ｅ（Σｍ_n-1 ）とそれぞれ僅かの差をもつので、出力端子ＯＵＴよりＬＣＤに供給する階調電圧は両端のＥ（０），Ｅ（Ｎ）を除いて僅かな誤差が存在する。しかし、この誤差は僅かであり、また階調数が例えばＮ＝６４と多いので何ら問題になることはない。
【００２５】
図２からも分かるように、この発明では隣接端子、タップ間のストリング抵抗器の数、即ち階調数は、Ｔ−Ｖ特性の傾斜に応じて設定され、Ｔ（０）（ゼロ階調）側及びＴ（Ｎ−１）（Ｎ−１階調）側では、従来の例えばｍ＝８に比べて、例えばｍ₁ ＝５，ｍ_n ＝５と少なく、中間のタップ間の階調数は例えばｍ_j ＝２０と多くなる。
【００２６】
隣接端子、タップ間のストリング抵抗Ｒｔの消費電力は
Ｐ＝（ΔＶ）² ／Ｒｔ …………（21）
であるので、ストリング抵抗は液晶の画像表示に影響の出ない範囲で、できるだけ大きくするのが望ましい。
【００２７】
【発明の効果】
この発明では、γ補正電圧Ｖ_j （ｊ＝０〜ｎ）を隣接端子、タップ間の電位差ΔＶ_j が全て相等しくなるように印加すると共に、それら隣接端子、タップ間のストリング抵抗Ｒｔ_j を全てほぼ等しく設定したので、中間のタップ５₁ 〜５_n-1 からγ補正電圧発生回路４₁ 〜４_n-1 側に流れる不平衡電流ΔＩ_j はほぼゼロとなり、それだけ電源回路４１及びストリング抵抗器２の消費電力を低減できる。
【図面の簡単な説明】
【図１】この発明の実施例を示す回路図。
【図２】この発明における階調電圧Ｅ（ｐ）（ｐ＝０，１…，Ｎ），透過率Ｔ（ｐ）及びγ補正電圧Ｖ_j （ｊ＝０〜ｎ）の対応を説明するためのＬＣＤの透過率対印加電圧特性を示すグラフ。
【図３】従来の液晶ドライバを電源回路と共に示す回路図。
【図４】従来の階調電圧Ｅ（ｐ）（ｐ＝０〜Ｎ），透過率Ｔ（ｐ）及びγ補正電圧Ｖ_i （ｉ＝０〜ｎ）の対応を説明するためのＬＣＤの透過率対印加電圧特性を示すグラフ。
【図５】図３の要部の電圧、電流を示すための回路図。
【図６】図３の液晶ドライバ１の出力抵抗を説明するための回路図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a multi-tone liquid crystal driver that drives each signal bus of a TFT matrix liquid crystal display element, for example, and more particularly to reduction of power consumption of the driver and a power supply circuit that supplies operation power to the driver.
[0002]
[Prior art]
In recent years, with the development of multimedia, the number of display colors of computers has increased dramatically. Along with this, TFT matrix liquid crystal display elements (LCD) mounted on notebook personal computers have also increased the number of display colors. In order to increase the number of display colors, the number of gradations of the driver for the signal bus (source bus) used in the LCD is increased. This can be solved by installing a DAC (digital / analog converter) in the driver. On the other hand, it is inevitable that the circuit scale increases in proportion to the increase in the number of colors and the power consumption of the driver increases. Since it is desirable that the LCD has as low power consumption as possible in view of its use, driver manufacturers adopt various methods and make efforts to reduce power consumption.
[0003]
One DAC system for signal bus drivers is an R-STRING system having a series of resistors connected in series (referred to as STRING resistors). A circuit block diagram of this system is shown in FIG. 3 together with a power supply circuit. For example, if the code 1 is a driver of N = 64 gradations, 64 string resistors 2 are connected in series, and only one voltage generated by the resistance division is selected by 64 switches SW. Thus, a gradation voltage is generated.
[0004]
In FIG. 3, the resistance values of the string resistors are all R, the resistance values between adjacent terminals and taps 5 _{i to} 5 _i-1 (i = _{1 to} n) are Rt (= m × R), Let the on-resistance be r.
The power consumption of the R-STRING type liquid crystal driver 1 and the power supply circuit 4 is divided into the following four types.
[0005]
(1) Power consumption by applying voltage from the power supply circuit 4 to m × n = N STRING resistors R × m × n (2) Switch for selecting N = 64 voltages according to the 6-bit data taken Power consumption of decoder part controlling SW (3) Power used to charge / discharge source bus (signal bus) via output terminal OUT (4) Logic circuit (not shown) in liquid crystal driver 1 Next, the structural features of the liquid crystal driver 1 shown in FIG. 3 are as follows.
[0006]
B) N = m × n = 64 resistors R are all equal.
B) A tap 5 _i (i = 1 to n−1) for γ correction is provided for every m = 8 of N = 64 string resistors 2.
Since the LCD generally has a transmittance-voltage characteristic (TV characteristic) as shown in FIG. 4, the applied voltage V must be changed for each gradation so that the gradation can be visually recognized as a smooth change. Don't be. This is so-called γ correction.
[0007]
A conventional R-STRING type driver applies the γ correction voltage V _i to the taps 5 _i (i = 1 to n−1) and the terminals 5 ₀ and 5 _n at both ends of the LCD 5). It has been tailored to the characteristics. Therefore, the potential difference ΔV _i (i = 1 to n) between the adjacent terminals and the taps 5 _{i to} 5 _i−1 is different. Since resistance values m × R = Rt between adjacent terminals and taps are equal, the current I _i flowing through the combined resistor Rt is apparent from FIG. 5 by applying different voltages V _i to the respective terminals 5 _i . ,
I _i = ΔV _i / Rt (1)
The current ΔI _i expressed by the equation (2) flows through the intermediate tap 5 _i (i = 1 to n−1).
[0008]

The current ΔI _i expressed by the equation (2) causes the power consumption P _i in the γ correction voltage generation circuit 4 _i (i = 1 to n−1) connected to the intermediate tap.
[0009]
P _i = V _i × ΔI _i = V _i (ΔV _{i + 1} −ΔV _i ) / Rt
(I = 1 ..., n-1) (3)
The circuit 4 _n and power consumed respectively 4 within _0, P if the supply voltage and _{V cc n = (V cc -V} n) I n
P ₀ = V ₀ I ₁ (4)
There is also power consumed by the R-STRING 2 itself in the driver 1. Although it is clear that the power consumption of the R-STRING 2 itself decreases as the value of the R-STRING 2 increases, the combined time constant of the output impedance of the dry 1 and the capacity of the LCD panel determines the time for charging and discharging the LCD panel. If the value is not sufficiently small, an appropriate voltage cannot be applied to the LCD. This lowers the display quality as contrast decreases and crosstalk increases. Therefore, connecting a voltage source to the terminal and tap 5 _i (i = 0 to n) is not only an input for γ correction, but also increases the resistance value of R-STRING and increases the output impedance of the driver. There is an aspect of lowering.
[0010]
The output impedance of the driver 1 varies depending on the position of the selected switch SW. Assuming that the division ratio of Rt divided by the switch SW is k, the output impedance Rout is the sum of the parallel resistance value of the resistors (1-k) Rt and kRt and the on-resistance r, and is expressed by the following equation (5). The

Then k = 1/2 is obtained. At this time, Rout becomes maximum, and its value is given by equation (7).
[0011]
[Rout] max = (Rt / 4) + r (7)
In general, in the R-STRING driver, the time constant between the maximum value of output impedance and the capacity of the LCD is set to be sufficiently small with respect to one horizontal time H.
[0012]
[Problems to be solved by the invention]
In the conventional R-STRING type driver, since N = m × n = 64 resistors all have the same value (R), a voltage is applied from the outside to the terminal for γ correction, tap 5 _i , The gamma characteristic was corrected. To flow to the current [Delta] I _i to each intermediate tap 5 _i represented by the formula (2) for the γ correction γ correction voltage generation circuit _{4 i (i = 1~n-1} ), the current [Delta] I _i X The output voltage V _i = P _i is the power consumption in the circuit 4 _i . Also, since this unbalanced current ΔI _i flows through the string resistor, the power consumption of the string resistor increases accordingly.
[0013]
The present invention reduces the power consumption of the string resistor of the driver 1 and the power supply circuit 4 by making the current ΔI _i sucked into the γ correction voltage generation circuit 4 _i (i = 1 to n−1) substantially zero. It is an object.
[0014]
[Means for Solving the Problems]
(1) The invention of claim 1 divides a DC voltage by N string resistors (N is an integer of 3 or more) connected in series to obtain N grayscale voltages, and based on input digital data One of the gradation voltages is selected and output to the liquid crystal element, and n−1 (1 ≦ n−1 <N−1) connection points are extracted from the N−1 connection points of the string resistor. Then, taps are derived from the extracted connection points, and γ correction voltages (5 ₀ , 5 _n ) are applied to the taps (5 _{1 to} 5 _n-1 ) and the terminals (5 ₀ , 5 _n ) of the series string resistors. V ₀ , V ₁ ..., V _n ).
[0015]
In particular, the γ correction voltage is applied so that the potential differences (ΔV _j ) between the adjacent terminals and taps are all equal, and the resistance value of the string resistor between the adjacent terminals and taps ( Rt _j ) are all set approximately equal.
(2) In the invention of claim 2, in (1), the resistance value R (p) of each of the N string resistors is the transmittance per one gradation in the transmittance versus applied voltage characteristics of the liquid crystal element. A value corresponding to the change width (Δ (p); p = 1, 2,..., N) of the applied voltage corresponding to the constant change width is set.
[0016]
(3) In the invention of claim 3, in (1), the number of individual string resistors (m ₁ , m ₂ ,..., M _n ) existing between adjacent terminals and taps of a series of string resistors. Is reduced at both ends of a series of string resistors according to the slope of the transmittance versus applied voltage characteristic curve of the liquid crystal element, and is set at a large value in the middle.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Assuming that N (for example, N = 64) gradation display is performed, transmittances T (0), T (1),... Applied voltages E (0), E (1),..., E (N) corresponding to (N) are obtained. As in the conventional case, n + 1 (for example, n = 8) terminals (tap) 5 _{0 to} 5 _n are provided. The potential difference ΔV _j between adjacent terminals and taps is made equal to ΔV regardless of j. That is,
ΔV _j (j = 1 to n) = ΔV (8)
Further, the resistance values Rt _i between adjacent taps are all made substantially equal. That is,
Rt _j (j = 1 to n) ≈Rt (constant) (9)
At this time, the current I _i flowing through the string resistor between the adjacent terminal and the tap is I _j = ΔV _j / Rt _j = ΔV / Rt (10)
That is, I _j is a constant value regardless of j. Thus, [Delta] I _j flowing into γ voltage correction circuit 4 _j is
ΔI _j = I _{j + 1} −I _j = 0 (j = 1 to n−1) (11)
Thus, low power consumption is achieved.
[0018]
A step voltage Δ (p) (FIG. 2) is obtained for each gradation increase from the applied voltage E (p) (p = 0 to N) corresponding to the transmittance T (j) (j = 0 to N).
Δ (1) = E (1) −E (0)
Δ (2) = E (2) −E (1),
...
Δ (p) = E (p) −E (p−1),
...
Δ (N) = E (N) −E (N−1), (12)
In order to make all of the string resistances Rt _j (j = 1 to n) between adjacent terminals and taps substantially equal, in the present invention, the value of each string resistor R (p) is Set as an expression.
[0019]
R (p) = KΔ (p) (p = 1 to N) (13)
That is, each string resistance value R (p) is a value corresponding to the change width Δ (p) of the applied voltage corresponding to the constant change width of the transmittance T per gradation in the TV-V characteristic of the LCD.
The voltage ΔV between adjacent terminals and taps is obtained from the following equation.
[0020]
ΔV = {E (N) −E (0)} / n (14)
Therefore, the voltage V _j (j = 0 to n) applied to each terminal and the tap 5 _j from the γ correction voltage generation circuit 4 _j (j = 0 to n) is set as follows.
V ₀ = E (0)
V ₁ = E (0) + ΔV,
V ₂ = E (0) + 2ΔV,
...
V _j = E (0) + jΔV,
...
V _n−1 = E (0) + (n−1) ΔV,
V _n = E (0) + nΔV = E (N), (15)
From N−1 (for example, 63) grayscale voltage series E (1), E (2)..., E (N−1), n−1 (for example, 7) tap voltage series V ₁ , V ₂ ..., the gradation voltage E (x) closest to V _n-1 is extracted. That is,
E (0) = V ₀ ,
E (m ₁ ) ≈V ₁ ,
E (m ₁ + m ₂ ) ≈V ₂ ,
...
E (m ₁ +... + M _j ) ≈V _j ,
...
E (m ₁ + ... + m _n-1 ) ≈V _n-1 ,
E (m ₁ + ... + m _n ) = V _n (16)
m _j is the number of gradations between the taps 5 _{j-1 to} 5 _j , that is, the number of string resistors R (p), and the sum of m _{1 to mn} is equal to N. That is,
Σm _n = m ₁ + m ₂ + ... + m _n = N (17)
As is apparent from FIG. 1, the tap 5 ₁ may be taken out from the connection point between the string resistors R (m ₁ ) and R (m ₁ +1), and the tap 5 ₂ is the string resistor R (m ₁ + m ₂ ). And R (m ₁ + m ₂ +1) may be taken out from the connection point, and so on.
[0021]
In this way, the tap 5 _j (j = 1 to n−1) is derived from the middle of a series of string resistors, and the value of each string resistor R (p) (p = 1 to N) is set to TV. When the characteristic is set to a value corresponding to the step voltage Δ (p) for each gradation, the resistance value Rt _j (j = 1 to n) between the adjacent terminals and the taps 5 _j and 5 _{j−1 is} certainly j. Let's verify that all values are almost the same regardless of.
[0022]
Rt _j = R (1 + Σm _j-1 ) + R (2 + Σm _j-1 ) +... + R (m _j + Σm _j-1 ) (18)
(13) Substituting the equation,
= K {Δ (1 + Σm _j-1 ) + Δ (2 + Σm _j-1 ) + ... + Δ (m _j + Σm _j-1 )}
Substituting equation (12) = K {E (1 + Σm _j-1 ) −E (Σm _j-1 )
+ E (2 + Σm _j-1 ) -E (1 + Σm _j-1 )
...
+ E (m _j + Σm _j−1 ) −E (m _j−1 + Σm _j−1 )}
= K {E (Σm _j ) -E (Σm _j-1 )}
Substituting equation (16) ≈ K {V _j −V _j−1 } (19)
Substituting equation (15) = K {E (0) + jΔV−E (0) − (j−1) ΔV}
= KΔV ............ (20)
From the above, it was verified that the resistance value Rt _j (j = 1 to n) between each adjacent terminal and the tap was a constant value of KΔV regardless of the value of j.
[0023]
Actually, the voltages supplied to the taps from the γ correction voltage generation circuit 4 _j (j = _{0 to n} ) are the voltages V ₀ , V ₁ ... V _n obtained from the equation (15). a [Delta] V, 4 ₀ an intermediate γ correction voltage generating circuit 4 ₁ ~4 _n-1 of the unbalanced current into and out of the output terminal, except for 4 _n at both ends ([Delta] I described in the prior art _j) becomes substantially zero, Power saving is achieved. In this case, it goes without saying that the intermediate correction voltage generating circuits 4 ₁ to 4 _n-1 exhibit a function of reducing the output resistance of the liquid crystal driver 1. The unbalanced current ΔI _j flows through the string resistor. Since ΔI _j = 0 in the present invention, the power consumption in the string resistor is reduced accordingly.
[0024]
The applied voltages V ₁ , V _2, ..., V _n-1 of each γ correction voltage generation circuit 4 _j are E (m ₁ ), E (m ₁ + m ₂ ),. Since there is a slight difference between (Σm _n-1 ) and the gradation voltage supplied to the LCD from the output terminal OUT, there is a slight error except for E (0) and E (N) at both ends. However, this error is slight, and there are no problems because the number of gradations is as large as N = 64, for example.
[0025]
As can be seen from FIG. 2, in the present invention, the number of string resistors between adjacent terminals and taps, that is, the number of gradations is set according to the slope of the TV characteristic, and T (0) (zero gradation). On the side and the T (N-1) (N-1 gradation) side, for example, m ₁ = 5 and _mn = 5 are smaller than, for example, m = 8, and the number of gradations between intermediate taps is For example, m _j = 20.
[0026]
The power consumption of the string resistor Rt between the adjacent terminal and the tap is P = (ΔV) ² / Rt (21)
Therefore, it is desirable to increase the string resistance as much as possible within a range that does not affect the liquid crystal image display.
[0027]
【The invention's effect】
In the present invention, the γ correction voltage V _j (j = 0 to n) is applied so that the potential differences ΔV _j between the adjacent terminals and taps are all equal, and the string resistances Rt _j between these adjacent terminals and taps are all applied. Since they are set to be approximately equal, the unbalanced current ΔI _j flowing from the intermediate taps 5 _{1 to} 5 _n-1 to the γ correction voltage generation circuit 4 ₁ to 4 _n-1 side becomes substantially zero, and the power supply circuit 41 and the string resistor are correspondingly increased. 2 power consumption can be reduced.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of the present invention.
FIG. 2 is a diagram for explaining the correspondence among gradation voltage E (p) (p = 0, 1,..., N), transmittance T (p), and γ correction voltage V _j (j = 0 to n) in the present invention; The graph which shows the transmittance | permeability with respect to the applied voltage of LCD of no.
FIG. 3 is a circuit diagram showing a conventional liquid crystal driver together with a power supply circuit.
FIG. 4 shows LCD transmission for explaining the correspondence between conventional gradation voltage E (p) (p = 0 to N), transmittance T (p), and γ correction voltage V _i (i = 0 to n). The graph which shows a rate versus applied voltage characteristic.
5 is a circuit diagram for showing voltages and currents in the main part of FIG. 3;
6 is a circuit diagram for explaining an output resistance of the liquid crystal driver 1 of FIG. 3;

Claims (3)

The DC voltage is divided by N (N is an integer of 3 or more) string resistors connected in series to obtain N grayscale voltages, and one of the grayscale voltages is selected based on the input digital data. Are output to the liquid crystal element, and n-1 (1 ≦ n-1 <N-1) connection points are extracted from N-1 connection points in the middle of the series of string resistors. Taps are derived from the extracted connection points, and a γ correction voltage is applied to the taps and terminals at both ends of the series of string resistors in order to match the gradation voltage to the transmittance versus applied voltage characteristics of the liquid crystal element. In the multi-tone liquid crystal driver thus configured, the γ correction voltage is applied so that the potential differences between the adjacent terminals and taps are all equal, and the resistance value of the string resistor between the adjacent terminals and taps. approximately equal all the sum of the Multi-gradation liquid crystal driver, characterized in that the set.   The resistance value of each string resistor is set to a value corresponding to the change width of the applied voltage corresponding to the constant change width of the transmittance per gradation in the transmittance versus applied voltage characteristic of the liquid crystal element. The multi-tone liquid crystal driver according to claim 1.   The number of individual string resistors existing between adjacent terminals and taps of the series of string resistors depends on the slope of the transmittance vs. applied voltage characteristic curve of the liquid crystal element. 2. The multi-tone liquid crystal driver according to claim 1, wherein the multi-tone liquid crystal driver is set to a small number and a large number in the middle.

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