JP2004212866A - El display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an EL display device in which a pulse number modulation system is introduced to constitution for driving a dot matrix type EL display, in which an inorganic EL element is used as a pixel, by a field inversion driving system. <P>SOLUTION: A control IC of the EL display device alternately reverses polarity of composite voltage of scanning voltage and data voltage applied to both ends of the EL element and further performs gradation display control making a set of even number of successive screens a gradation display unit. Thereby pulse number modulation is performed to field inversion driving. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

また、上記した「符号」は、図１に示すテーブルの「印加電圧」の欄に示されている符号に対応する。
【００３４】
尚、無機ＥＬ素子は交流電圧を印加することで発光し、一方の極性の電圧を連続して印加しても２回目以降は発光しない。その理由は、例えば、一旦正電圧を印加すると、無機ＥＬ素子にはその電界方向に内部分極電圧が発生するためである。従って、その状態で同極性の電圧を印加しても、ＥＬ素子の発光層には電界が印加されず発光しない。
【００３５】
次に、図１に示すテーブルの「相対移動電荷量」は、発光層を流れる伝導電流、即ち輝度に相当するもので、相対値「０．５」，「１．０」，「２．０」で示している。但し、後述するように「−ｖｔ」から「＋」に変化した場合の移動電荷量を相対値の基準「１．０」としている。この相対値が大きいほど、ＥＬ素子の発光輝度は高くなる。
【００３６】
ここで、図２を参照する。図２は、ＥＬ素子の所謂ＱＶ特性であり、ＥＬ素子に印加する合成電圧Ｖと、ＥＬ素子に蓄積された電荷Ｑとの関係を示す。
まず、ＥＬ素子に対して電圧が全く印加されていない状態を原点▲１▼とする。この状態から発光電圧２５５Ｖを印加すると、蓄積電荷量Ｑは▲２▼を経て状態▲３▼に至る。状態▲１▼−▲２▼間は、ＥＬ素子の発光層に伝導電流は流れず発光しない。そして、ＥＬ素子は状態▲２▼でクランプし（ＥＬ素子が容量的な特性から抵抗体的な特性に変化する）、状態▲２▼−▲３▼間は、ＥＬ素子の発光層に伝導電流が流れて発光する。この場合の伝導電流の大きさは、クランプ後の電荷量（移動電荷量）で大凡決まる。そこで、状態▲２▼−▲３▼間の移動電荷量の大きさを「１」とする。
【００３７】
次に、状態▲３▼から電圧を０Ｖにすると状態▲４▼に遷移する。そして、伝導電流が流れたことで分極電荷が発光層の両端に発生しており、ＥＬ素子は発光層が絶縁層に挟み込まれた二重絶縁膜構造のキャパシタンスをなしているから、発生した分極電荷は消滅せず、０Ｖに戻っても正電荷が残留している。
【００３８】
次に、負電圧（−２５５Ｖ）を印加すると、残留分極電荷の影響により状態▲５▼でクランプし、状態▲６▼を経て状態▲７▼に至る。状態▲４▼−▲５▼間では、ＥＬ素子の発光層に伝導電流が流れず発光しないが、状態▲５▼−▲７▼間では伝導電流が流れて発光する。この時の伝導電流の大きさは残留分極電荷の分だけ大きくなり、移動電荷量は「２」となる。
【００３９】
次に、状態▲７▼から電圧を０Ｖにすると状態▲８▼に遷移する。この場合は、状態▲４▼と大きさが等しい負電荷が残留する。そして、状態▲８▼から再び正の発光電圧（２５５Ｖ）を印加すると、残留分極電荷の影響により状態▲９▼でクランプし、状態▲２▼を経て状態▲３▼に至る。この時の伝導電流の大きさも「２」となる。
以降は、上記の状態▲３▼より開始されるプロセスを繰り返す。このように、無機ＥＬ素子は、電圧が印加されると分極電荷が発生し、その影響によってＱＶ特性が平行四辺形を描くように変化する。従って、同一極性の電圧を印加し続けても発光しない。
【００４０】
即ち、ＱＶ特性が図２のようになることで、合成電圧ｖｔ，−ｖｔを▲２▼，▲６▼の電位に設定してＥＬ素子に印加する合成電圧を変化させると、蓄積電荷Ｑの相対値は以下のように変化する。

尚、上記の符号の極性を反転させた場合も同様である。
【００４１】
ここで、再び、図１を参照する。図１に示すテーブルでは、１階調表示単位を構成する１４枚の画面のトータルでＥＬ素子の発光層に生じる相対移動電荷量で７レベルの階調表示を行なっている。フィールド反転駆動であるから、１フレーム毎に印加電圧極性が反転することが前提である。例えば、階調レベル０では、階調表示単位中では全フレームにわたってｖｔ，−ｖｔを交互に繰り返すだけであり、トータルでの移動電荷量は０である。
【００４２】
そして、階調レベル１では、

とすることで、トータルの移動電荷量、即ち相対輝度は「４」となる。
【００４３】
また、階調レベル２では、

とすることで、トータルの電荷量は「８」となる。
【００４４】
以上のようにして、階調レベルがアップする毎に相対輝度は４ずつ上昇するようになり、階調レベル７では全フレームにわたって＋，−を交互に繰り返すことで、トータルでの電荷量は「２８」となっている。
【００４５】
また、図１に示す印加電圧パターンには、以下のような規則性がある。７レベルの階調制御を３ビットのデータで行うようになっているが。印加電圧パターンを縦の関係（階調レベルの大小の関係）で見た場合に、例えば、フレーム１３は、「ｖｔ」をデータＬ，「＋」をデータＨに対応させると、ビット０のデータ値の変化パターンに一致している。そして、フレーム１４は、合成電圧の極性を反転させた同じパターンである。
【００４６】
同様に、フレーム９及び１１は、ビット１のデータ値の変化パターンに一致しており、フレーム１０及び１２は、合成電圧の極性を反転させた同じパターンである。更に、フレーム１，３，５，７は、ビット２のデータ値の変化パターンに一致しており、フレーム２，４，６，８は、合成電圧の極性を反転させた同じパターンである。
【００４７】
即ち、３ビットデータにより７レベル階調を実現する場合に、各ビットのデータ変化パターンに印加電圧パターンを一致させて、各パターン数をビット（０，１，２）に応じて（１，２，４）とし（それらのトータルで「７」）、極性を交互に変化させることで夫々を２倍すれば全部で「１４」となり、階調表示単位を構成する画面数ｍに一致するようになっている。
【００４８】
以上のように本実施例によれば、ＥＬディスプレイ表示装置の制御ＩＣ８は、ＥＬ素子の両端に印加される走査電圧とデータ電圧との合成電圧の極性を交互に反転させると共に、連続する偶数個の画面の集合を１つの階調表示単位として階調表示制御を行うことで、フィールド反転駆動に対してパルス数変調を行なうようにした。従って、階調表示単位内で連続する画面において、ＥＬ素子に印加される発光閾値を超える合成電圧のパルス数に応じて、線形性の良好な階調制御を行うことができる。
【００４９】
また、制御ＩＣ８は、階調表示単位によって表示可能な階調レベルが「７」である場合、その階調レベルを制御するための３（＝ｉ）ビットの各データ線で示される二値データに応じて、ＥＬ素子に印加する合成電圧を非発光電圧（ｖｔ，−ｖｔ）と発光電圧（＋，−）とに変化させ、階調表示単位を構成する画面数ｍを２×（２^３−１）＝１４とした。従って、３ビットデータによって線形性の良好な７段階の階調レベルを、データビットパターンに応じて簡単に設定し制御することができる。
【００５０】
（第２実施例）
図４及び図５は本発明の第２実施例を示すものであり、第１実施例と同一部分には同一符号を付して説明を省略し、以下異なる部分についてのみ説明する。第２実施例では、基本的な構成は第１実施例と同様であり、階調表示単位内における電圧の印加パターンが異なっている。
【００５１】
また、合成電圧ｖｔは、（１）式に基づいて設定されている。
｜クランプ電圧｜×２−｜走査電圧＋データ電圧｜
≦｜ｖｔ｜＜｜クランプ電圧｜ ・・・（１）
ここで、クランプ電圧は、ＥＬ素子に伝導電流が流れ始める電圧であり、図２の▲２▼に相当する電圧である。そして、合成電圧ｖｔは、（１）式を満たした上で図２において▲９▼，▲２▼の中間に相当する電圧”１０”（２１０Ｖ）に設定する。
【００５２】
斯様に合成電圧ｖｔを定めることで、電圧ｖｔにかかる移動電荷量は以下のようになる。

【００５３】
図４において、階調レベル０，７のパターンは第１実施例と同じである。そして、階調レベル２〜６については、階調表示単位を７フレームずつ前半と後半とに分けると、夫々においてＥＬ素子が同じタイミングに同じ期間だけ、同じ輝度で発光するように電圧印加パターンが設定されている。
【００５４】
即ち、階調レベル１では、フレーム３，４において電荷量が「１．５」，「０．５」に変化し、フレーム１０，１１においては、極性が反転した合成電圧によって同じ電荷量の変化が生じている。同様に、階調レベル２では、フレーム４，５，６において電荷量が「０．５」，「２．０」，「０．５」に変化し、フレーム１１，１２，１３において、極性が反転した合成電圧により同じ電荷量の変化が生じている。
【００５５】
ここで、図２を参照して説明すると、階調レベル２のフレーム１２では「−」（−２５５Ｖ）を印加することで状態▲７▼にあるが、フレーム１３で「ｖｔ」を印加すると状態▲９▼でクランプして状態”１０”に至る。この間に伝導電流が流れて相対移動電荷量は「０．５」となる。次のフレーム１４では「−ｖｔ」を印加するが、状態”１１”に遷移するだけでクランプせず、移動電荷量は「０」である。
【００５６】
その後、フレーム１〜３では「ｖｔ」，「−ｖｔ」間の変化で状態”１０”，”１１”間を移動するだけであるから移動電荷量は「０」である。そして、フレーム４において「−」（−２５５Ｖ）を印加することで、状態”１０”，”１１”を経て状態”１２”でクランプし、状態▲７▼に至る。この時、移動電荷量は「０．５」となる。続くフレーム５では「＋」（２５５Ｖ）を印加することで、状態▲７▼から▲９▼でクランプして状態▲３▼に至り、移動電荷量は「２．０」となる。
【００５７】
図５は、走査電圧，データ電圧の印加波形の一例を示すものである。階調表示単位のフレーム３〜６，１０，１１において、ＥＬディスプレイの１行目について階調レベル１，２で表示する場合と、２行目について階調レベル７，０で表示する場合とを示す。
【００５８】
１行目，２行目の走査電圧としては、走査タイミングをずらして＋２５５Ｖ，−２１０Ｖが交互に出力されている（（ａ），（ｂ）参照）。（ｃ）は、１行目について階調レベル１，２行目について階調レベル７で表示する場合のデータ電圧パターンである。例えば、フレーム３〜６については以下のようになっている。

この時、１，２行目のＥＬ素子に印加される合成電圧は（ｅ），（ｇ）となる。
【００５９】

【００６０】
（ｄ）は、１行目について階調レベル２，２行目について階調レベル０で表示する場合のデータ電圧パターンである。例えば、フレーム３〜６については以下のようになっている。

この時、１，２行目のＥＬ素子に印加される合成電圧は（ｆ），（ｈ）となる。
【００６１】

【００６２】
また、図４に示す印加電圧パターンには、以下のような規則性がある。３ビットデータにより７レベル階調を実現する場合に、各ビットのデータ変化パターンに印加電圧パターンを一致させ、各パターン数をビット（０，１，２）に応じて（１，２，４）とし、夫々を２倍することで全部で「１４」としていることは第１実施例と同様である。しかし、第２実施例では、ビット（０，１，２）に応じた印加電圧パターンの配列が第１実施例とは異なっている。
【００６３】
即ち、第１実施例において、階調表示単位を構成するフレームと、各ビットに対応する電圧印加パターンとの関係は、以下のようになっている。

これに対して、第２実施例では以下のようになっている。

つまり、第２実施例では、フレーム１〜７，８〜１４が同じ配列であり、フレーム３を、フレーム１〜７内においてビット０に対応するパターンを配置する基準位置とすると、その右隣にビット１に対応するパターンを２個配置し、その右隣にビット２に対応するパターンを４個配置している。但し、フレーム７を超えた部分はフレーム１に戻って、フレーム１，２に配置されている。そして、フレーム８〜１４については、上記に「７」を加えただけである。
【００６４】
この様な配列パターンを選択すると、階調レベル７を除き、階調表示単位の前半と後半とにおいて、ＥＬ素子が同じタイミングに同じ期間だけ同じ輝度で発光するように電圧印加パターンを設定することができる。そして、この場合、実質的な発光周波数は階調表示単位が連続する周波数の２倍となる。従って、画面数ｎ＝７に基づく発光周波数を、人間の視覚にフリッカが認識されないレベルに設定すれば良い。
【００６５】
以上のように第２実施例によれば、制御ＩＣ８は、階調表示単位内においてＥＬ素子の発光層に伝導電流が流れるように合成電圧を印加する期間を、最大階調レベル以外は２つの等しい期間に分割し、階調表示単位内ではその合成電圧の極性が正となる期間と負となる期間とが等しくなるようにした。
【００６６】
より具体的には、階調表示単位において、３ビットの各データパターンに対応する電圧変化パターンの配列順を、ビット０に対応する電圧変化パターンを２つの基準位置となる３（＝ｊ）番目，７番目に配置し、ビット１に対応する電圧変化パターンを、前記２つの基準位置の次に夫々２個ずつ配置し、ビット２に対応する電圧変化パターンを、ビット１に対応する電圧変化パターンの次に夫々４個ずつ配置し、それらの配置位置の番号数が７（＝ｎ）または１４を超える場合は７を減じた番号数に配置した。
【００６７】
従って、１階調表示単位内において、ＥＬ素子を等しい時間と等しい輝度とで２回発光させることができ、フリッカ対策を有効に行なうことができる。逆に、フリッカが発生し難くなるため、その分だけ階調表示単位を構成する画面数を増やして階調レベルを増やすこともできる。
【００６８】
（第３実施例）
図６は本発明の第３実施例を示すものであり、第２実施例と同一部分には同一符号を付して説明を省略し、以下異なる部分についてのみ説明する。第３実施例では、階調表示単位内における電圧の印加パターンは第２実施例と同様だが、合成電圧ｖｔの設定が異なっている。
【００６９】
即ち、合成電圧ｖｔは、（２）式に基づいて設定される。
｜ｖｔ｜≦｜クランプ電圧｜×２−｜走査電圧＋データ電圧｜ ・・・（２）
（２）式に基づくと、合成電圧ｖｔは図２の状態▲９▼の電圧以下に設定される。
【００７０】
この場合、第２実施例の図４におけるパターンにおいて移動電荷量が「１．５」であったものは「２．０」になり、移動電荷量が「０．５」であったものは「０．０」になっている。その結果、階調レベル１，２，階調レベル３，４，階調レベル５，６の相対輝度が等しくなっており、階調度は４段階に低下している。
【００７１】
（第４実施例）
図７及び図８は本発明の第４実施例を示すものであり、第２実施例と同一部分には同一符号を付して説明を省略し、以下異なる部分についてのみ説明する。第４実施例も階調表示単位内における電圧の印加パターンは第２実施例と同様だが、合成電圧ｖｔの設定が異なっている。
【００７２】
即ち、合成電圧ｖｔは、（３）式に基づいて設定される。
｜ｖｔ｜≒｜クランプ電圧｜ ・・・（３）
この（３）式に基づくと、合成電圧ｖｔは、例えば図２の状態▲２▼の電圧付近に設定される。この場合、第２実施例の図４におけるパターンにおいて移動電荷量が「１．５」，「０．５」であったものは何れも「１．０」になっている。
【００７３】
ここで、図４に示す第２実施例の印加電圧パターンでは、階調レベル０から１の相対輝度差が「４」、レベル１から２への相対輝度差が「２」、レベル２から３への相対輝度差が「６」、レベル３から４への相対輝度差が「２」、と階調レベルの線形性が、図１に示す第１実施例のパターンに比較して低下していた。
【００７４】
そこで、発明者らが、第２実施例の印加電圧パターンを維持したまま、階調レベルの線形性を良好にすることを検討した結果、非発光電圧｜ｖｔ｜と発光電圧（＋，−）との間における移動電荷量を、互いに逆極性の発光電圧（＋，−）間における移動電荷量の約１／２にすると階調の線形性が良好となることが判明した。そのためには、非発光電圧｜ｖｔ｜をＥＬ素子のクランプ電圧付近に設定すると良いことが判った。
【００７５】
斯様に電圧｜ｖｔ｜を設定することで、図７に示す印加電圧パターンにより図２に示すＱＶ特性上の状態を遷移させると分極電荷が良好に打ち消されるようになり、その結果として階調レベルの線形性が良好になると考えられる。
【００７６】
また、図８は一般的な無機ＥＬ素子のＱＶ特性を示すものである。ＥＬ素子の発光開始電圧は、通常クランプ電圧よりも低くなっている。これは、ＥＬ素子の発光原理が伝導電流に基づくものに限らず、その他トンネル効果などの現象によってクランプ電圧よりも低い電圧で伝導電子が移動するためと考えられる。従って、（３）式に基づいて合成電圧ｖｔを設定すると、階調レベル０における輝度が完全な「０」にならない可能性がある。
【００７７】
しかしながら、例えば、車両において昼間に表示を行う場合には周囲が明るいため、階調レベル０における輝度が完全な「０」でなくとも、表示品位的には全く気にならない。従って、そのような用途に用いるには十分である。
以上のように第４実施例によれば、合成電圧ｖｔをＥＬ素子のクランプ電圧付近に設定したので、各階調レベル間の輝度差における線形性を一層向上させることができる。
【００７８】
（第５実施例）
図９は本発明の第５実施例を示すものであり、第１実施例と同一部分には同一符号を付して説明を省略し、以下異なる部分についてのみ説明する。図９は、第１実施例の図３におけるトランス１，スイッチングレギュレータＩＣ２，スイッチング回路３，平滑回路４及び走査電圧調整回路６に相当する部分の電気的構成を、より詳細に示すものである。
【００７９】
トランス１の一次側巻線２１にはバッテリ２２接続されていると共に、ＮＭＯＳＦＥＴで構成されるスイッチング回路３が直列に接続されている。スイッチング回路３は、スイッチングレギュレータＩＣ２によってスイッチング制御される。トランス１の二次側巻線２３ａ，２３ｂ，２３ｃには，ダイオードとコンデンサで構成される平滑回路４Ａ，４Ｂ，４Ｃが夫々接続されている。
【００８０】
二次側巻線２３ａは、４５Ｖのデータ電圧を生成する。尚、このデータ電圧は、レギュレータＩＣ２によってモニタリングされ、フィードバック制御されるようになっている。また、二次側巻線２３ｂは、図９では図示しない３端子レギュレータ５に接続されており、Ｆ５Ｖが生成される。
【００８１】
また、二次側巻線２３ｃには、走査電圧調整回路（走査側駆動回路）２４が接続されている。走査電圧調整回路２４は、ダーリントン接続された２個のトランジスタ２５，２６とそれらの出力側に接続される抵抗２７及び基準電圧設定用のツェナーダイオード２８の直列回路と、それらの共通接続点とトランジスタ２５のベースとにエミッタ，コレクタが夫々接続されるトランジスタ２９と、中段に可変抵抗３０ｂを有する３直列抵抗回路３０（ａ〜ｃ）で構成されている。抵抗３０ｂ，３０ｃの共通接続点は、トランジスタ２９のベースに接続されている。
【００８２】
そして、走査電圧調整回路２４は、約Ｆ２１０Ｖの走査電圧を生成して走査電圧駆動ＩＣ（走査側駆動回路）１０に出力するが、第５実施例では、ユーザが可変抵抗３０ｂを操作することで走査電圧を調整できるようになっている。即ち、可変抵抗３０ｂの抵抗値を高めると、トランジスタ２９に流れるベース電流が増加してコレクタ電流も増加する。すると、ダーリントン接続トランジスタ２５，２６に流れるベース電流は減少するので、抵抗２７の端子電圧は減少し、走査電圧は低くなる。逆に、可変抵抗３０ｂの抵抗値が低くなるように設定すると走査電圧は高くなる。
【００８３】
以上のように構成された第５実施例によれば、走査電圧調整回路２４を、走査電圧を調整するためにユーザによって操作可能な可変抵抗器３０ｂを備えて構成したので、経時変化によりＥＬ素子のクランプ電圧が変化した場合でも、階調レベル変化の線形性を維持するための調整をユーザが自ら行なうことができる。
【００８４】
本発明は上記し且つ図面に記載した実施例にのみ限定されるものではなく、以下のような変形または拡張が可能である。
上記各実施例では、同じフレームでは同じ極性の合成電圧を印加するようにしているが、フリッカを防止するための周知の方式である、１行ごとに極性を反転させるライン反転駆動方式を採用しても良い。
３ビットによって７レベルの階調制御を行うものに限らず、４ビット以上で１５レベル以上の階調制御を行っても良い。尚、４ビットの場合、階調表示単位を構成する画面数ｍは「３０」となる。
車両に搭載されるＥＬディスプレイに適用するものに限らず、無機ＥＬ素子を画素に用いたドットマトリクス型のディスプレイであれば、広く適用することができる。
【図面の簡単な説明】
【図１】本発明を車両に搭載されるＥＬディスプレイに適用した場合の第１実施例であり、表示装置の制御ＩＣがＥＬディスプレイをフィールド反転駆動すると共に、パルス数変調によって７階調表示を行う場合の電圧印加パターンを示す図
【図２】ＥＬ素子に印加する合成電圧Ｖと、ＥＬ素子に蓄積された電荷Ｑとの関係を示す図
【図３】ＥＬディスプレイ表示装置の電気的構成を示す機能ブロック図
【図４】本発明の第２実施例を示す図１相当図
【図５】走査電圧，データ電圧の印加波形の一例を示す図
【図６】本発明の第３実施例を示す図１相当図
【図７】本発明の第４実施例を示す図１相当図
【図８】無機ＥＬ素子の発光開始電圧と、クランプ電圧との関係を示す図
【図９】本発明の第５実施例であり、走査側駆動回路の電気的構成を示す図
【符号の説明】
８は制御ＩＣ（表示制御回路）、１０は走査電圧駆動ＩＣ（走査側駆動回路）、２４は走査電圧調整回路（走査側駆動回路）を示す。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an EL display device that drives an EL display having a dot matrix configuration using inorganic EL elements as pixels by a field inversion driving method.
[0002]
[Prior art]
An EL display having a simple dot matrix configuration using an inorganic EL element is, for example, a night vision (infrared night vision device) mounted on a vehicle, a video display device for monitoring surroundings, or a so-called turn-by-turn, which is a simple navigation device. It is used for devices that display information about vehicles ahead using radar. There are three methods of voltage control, pulse number (or frequency) modulation, and pulse width modulation to perform gradation control when driving such an EL display by line-sequential scanning.
[0003]
For example, Patent Documents 1 and 2 disclose examples of a voltage modulation method and a pulse width modulation method, respectively. In these methods, a data voltage of a potential corresponding to the grayscale data or a data voltage having a pulse width corresponding to the grayscale data is applied by the data electrode during a period in which the scanning electrode is selected (corresponding to a horizontal synchronization period). is there. Therefore, gradation display can be performed in one scanning period, the frame frequency can be reduced to just before 50 to 60 Hz at which flicker occurs, and the number of scanning lines can be increased. It is being developed for use.
Patent Document 3 discloses a technique for improving the linearity of gradation control in a pulse width modulation method of a multi-color EL display.
[0004]
However, in the pulse width modulation method, a dedicated column driver IC is required to output data voltages having different pulse widths, so that the cost is increased and the luminance voltage characteristic (LV characteristic) of the EL element is changed over time. There are issues such as necessity. Also, a special circuit is required to ensure the linearity of the luminance with respect to the degree of gradation.
These problems also exist in the voltage modulation method, and there is a demand for a gradation method in which the degree of deterioration with time is small and the cost does not increase.
[0005]
Further, for example, Patent Documents 4 and 5 disclose techniques using a pulse number modulation method. In these methods, the gradation level is set by the number of times of light emission in a plurality of scanning periods, and the data voltage applied in one scanning period is simply the light emission of the EL element as in the case where the gradation control is not performed. This is the voltage that determines non-light emission. In this method, the linearity of gradation control is good, and the cost does not increase because it is not necessary to use a dedicated column driver IC.
[0006]
However, in this method, since a plurality of scanning periods are combined to form one gradation display unit, the display frequency decreases in proportion to the gradation level, and flicker occurs when applied to a large-screen display. Resulting in. Therefore, the advantages of a relatively small display having a small number of scanning electrodes can be fully enjoyed.
[0007]
[Patent Document 1]
JP-A-2-149889
[0008]
[Patent Document 2]
JP-A-4-229895
[0009]
[Patent Document 3]
JP-A-10-39835
[0010]
[Patent Document 4]
JP-A-62-160273
[0011]
[Patent Document 5]
JP-A-62-172396
[0012]
[Problems to be solved by the invention]
Incidentally, it is a principle that the inorganic EL element is driven by an AC voltage. In a simple matrix type display, line-sequential scanning is performed. The scanning method includes field refresh driving and field inversion (frame inversion) driving. In the field refresh driving method, a scanning voltage of a negative voltage is applied by one-screen line-sequential scanning, and then a positive voltage called a refresh pulse is simultaneously applied to all the EL elements. For this reason, when a refresh pulse is applied, a large current flows instantaneously, and there is a problem that a driving circuit becomes large, luminance is reduced, and luminance unevenness occurs due to a dull driving waveform. Therefore, the field refresh driving method has been widely adopted in a relatively early stage of a product using an inorganic EL element, but the field inversion driving method is currently in use.
[0013]
The field inversion driving method repeats an operation in which line-sequential scanning for one screen is performed with a positive voltage, and scanning for the next one screen is performed with a negative voltage. Therefore, unlike the field refresh driving method, there are advantages in that waveform rounding is small, luminance is high, and luminance unevenness is small.
[0014]
The techniques disclosed in Patent Literatures 4 and 5 employ a field refresh driving method. However, a technique in which a pulse number modulation method is introduced to the field inversion driving method has not yet been put to practical use. The reason is considered as follows. That is, in the field refresh driving method, since the driving voltage of the negative polarity and the refresh voltage of the positive polarity are alternately applied, the data voltage for controlling the gradation may be superimposed on the scanning voltage of the negative polarity.
[0015]
On the other hand, in the field inversion driving method, since the polarity of the scanning voltage alternates between positive and negative, the EL element does not emit light even when the data voltage for gradation control is superimposed only on the scanning voltage of positive polarity. Therefore, it is considered that there was a problem that the voltage application method had to be devised.
[0016]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a configuration in which a dot-matrix EL display having inorganic EL elements as pixels is driven by a field inversion driving method, and an EL device employing a pulse number modulation method. An object of the present invention is to provide a display device.
[0017]
[Means for Solving the Problems]
According to the EL display device of the first aspect, the display control circuit alternately inverts the polarity of the combined voltage of the scanning voltage and the data voltage applied to both ends of the EL element, and continuously displays an even number of screens. Is set as one gradation display unit, and gradation display control is performed.
[0018]
That is, in order to introduce the pulse number modulation system into the configuration for performing the field inversion drive, two continuous screens for alternately inverting the polarity of the composite voltage applied to both ends of the EL element are paired (basic voltage application unit). , A set of multiples thereof may be used as one gradation display unit. Then, in a continuous screen in the gradation display unit, the amount of conduction current flowing through the light emitting layer of the EL element can be changed in accordance with the number of pulses of the composite voltage applied to the EL element, and good linearity can be obtained. It is possible to control the gradation.
[0019]
According to the EL display device of the second aspect, the display control circuit sets the period of applying the combined voltage so that the conduction current flows to the light emitting layer of the EL element in the gradation display unit except for the maximum gradation level. The period is divided into two equal periods, and control is performed such that a period in which the polarity of the composite voltage is positive and a period in which the polarity of the composite voltage are negative are equal in the gradation display unit.
[0020]
With such a configuration, the period during which the EL element emits light in the gradation display unit is divided into two (excluding the maximum gradation level), so that the substantial emission frequency is such that the gradation display unit is continuous. It is twice the frequency. Therefore, flicker can be made less likely to occur, or the number of screens constituting the gradation display unit can be increased by that amount, thereby increasing the gradation level.
[0021]
According to the EL display device of the third aspect, the display control circuit determines that the displayable gradation level is (2) in the gradation display unit. ⁱ -1) (the luminance 0 is excluded from the gradation level), the composite voltage applied to the EL element according to the binary data indicated by each i-bit data line for controlling the gradation level Is changed to a non-light emitting voltage and a light emitting voltage. Then, the number m of screens constituting the gradation display unit is calculated as 2 × (2 ⁱ -1). That is, if an emission voltage and a non-emission voltage are assigned to the binary data (high, low) of each bit, (2) ⁱ -1) It is possible to perform gradation control with good linearity according to the data change pattern at the stage. Accordingly, data setting for gradation control is facilitated.
[0022]
According to the EL display device of the fourth aspect, in the gradation display unit, the arrangement order of the voltage change pattern corresponding to each i-bit data pattern is determined by setting the voltage change pattern corresponding to bit 0 to two reference positions. J-th, j + (2 ⁱ -1) and two voltage change patterns corresponding to bit 1 are arranged next to each of the two reference positions. Then, the voltage change pattern corresponding to the bit (i-1) is set next to the voltage change pattern group corresponding to the bit (i-2). ^i-1 / 2 at a time. With this configuration, in one gradation display unit, the EL element emits light twice at the same time and with the same luminance except for the maximum gradation level, so that flicker can be effectively prevented.
[0023]
According to the EL display device of the fifth aspect, the non-emission voltage is set near the clamp voltage of the EL element. In other words, according to the inventors, when the amount of mobile charge between the non-emission voltage and the emission voltage is set to about の of the amount of mobile charge between the emission voltages of opposite polarities, the linearity of gradation is improved. There was found. To this end, it has been found that the non-emission voltage should be set near the clamp voltage of the EL element. Therefore, by setting the non-emission voltage as in claim 5, it is possible to further improve the linearity in the luminance difference between each gradation level.
[0024]
According to the EL display device of the sixth aspect, the scanning-side driving circuit includes a variable resistor operable by a user to adjust a scanning voltage. Therefore, even when the clamp voltage of the EL element changes due to a change over time, the user can adjust the scanning voltage in accordance with the change, and can make adjustments so that the degree of change in gradation is maintained well.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
(First embodiment)
Hereinafter, a first embodiment in which the present invention is applied to an EL display mounted on a vehicle will be described with reference to FIGS. FIG. 3 is a functional block diagram showing an electrical configuration of the EL display device. Note that the configuration shown in FIG. 3 is basically well-known as a simple matrix type inorganic EL display that performs field inversion driving by line-sequential scanning. A similar configuration is described in, for example, Japanese Patent No. 291234. Are also disclosed. A characteristic part of the present invention lies in a method of applying a scanning voltage and a data voltage by a control IC.
[0026]
Hereinafter, FIG. 3 will be schematically described. The 12V power supplied from the vehicle battery is supplied to the primary side of the transformer 1 and the switching regulator IC2. The IC 2 controls the switching by the switching circuit 3 to boost the voltage output to the secondary side of the transformer 1.
[0027]
The voltage output to the secondary side of the transformer 1 is divided into three systems after passing through the smoothing circuit 4. One of them is output as F (floating, here, not using the ground GND on the primary side as a potential reference) 5 V via the three-terminal regulator 5. The other one is output as a data voltage of 45V. The data voltage is also provided to IC2. The other one is output as F210V through the scanning voltage adjustment circuit 6.
[0028]
The I / F (interface) circuit 7 is connected to a system bus of an external CPU, receives display data of an EL display generated and output by the CPU, and outputs it to a control IC (display control circuit) 8. The I / F circuit 7 is supplied with a control power supply of 5 V separately generated from 12 V of the battery power supply.
[0029]
The control IC 8 refers to the built-in gradation display memory 9 according to the display data of the EL display output from the CPU, and controls the row side (scanning side) and column side (data side) control signals. Are output to the scanning voltage driving IC 10 and the data voltage driving IC 11, respectively. Further, the control IC 8 outputs a switching control signal to the power supply switching circuit 12. Then, the control IC 8 changes the display of the EL display to seven gradations (excluding a state in which the EL element is not emitting light (gray level 0)) according to the 3-bit gradation display data output by the CPU. Control.
[0030]
The level converter 13 is configured by a photocoupler or the like, and insulates F210V and F5V provided from the scanning voltage adjustment circuit 5 and outputs the F210V and F5V to the scanning voltage driving IC 10. The power supply switching circuit 12 switches the reference potential of the floating voltage to F210V, 45V (and GND) and outputs the same to the scanning voltage driving IC 10.
[0031]
The scanning voltage driving IC 10 outputs +255 V and −210 V scanning voltages to the scanning electrodes of a simple matrix type EL display (not shown) according to the reference potential supplied from the power supply switching circuit 12. Note that F5V is supplied as a power supply for operating a logic circuit inside the scanning voltage driving IC 10. Further, the data voltage driving IC 11 outputs a data voltage of 45 V to the data electrodes of the EL display.
[0032]
Next, the operation of the present embodiment will be described with reference to FIGS. FIG. 1 shows a voltage application pattern when the control IC 8 performs field inversion driving of an EL display and performs seven gradation display by pulse number modulation. This table is stored in the memory 9 of the control IC 8. In this embodiment, in order to display seven gradations, a continuous screen of m = 7 × 2 = 14 (sheets) is set as a gradation display unit. Here, the reason for multiplying by “2” is to alternately reverse the polarity of the combined voltage applied to both ends of the EL element. These 14 screens are referred to as frame 1, frame 2, ..., frame 14.
[0033]
The scanning voltage changes to + 255V and -210V, and the data voltage changes to 0V and 45V. “Applied voltage” is the difference between the voltages applied to both ends of the EL element. The light emission start voltage of the EL element is about 220 V. Therefore, when the EL element emits light or does not emit light, the voltage is output in the following combination.

Further, the above “code” corresponds to the code shown in the column of “applied voltage” in the table shown in FIG.
[0034]
The inorganic EL element emits light when an AC voltage is applied, and does not emit light from the second time onward even if a voltage of one polarity is continuously applied. This is because, for example, once a positive voltage is applied, an internal polarization voltage is generated in the direction of the electric field in the inorganic EL element. Therefore, even if a voltage of the same polarity is applied in this state, no electric field is applied to the light emitting layer of the EL element, and no light is emitted.
[0035]
Next, the “relative moving charge amount” in the table shown in FIG. 1 corresponds to the conduction current flowing through the light emitting layer, that is, the luminance, and the relative values “0.5”, “1.0”, “2.0” ". However, as will be described later, the moving charge amount when the value changes from “−vt” to “+” is set as a relative value reference “1.0”. The larger the relative value is, the higher the emission luminance of the EL element is.
[0036]
Here, reference is made to FIG. FIG. 2 shows a so-called QV characteristic of an EL element, and shows a relationship between a combined voltage V applied to the EL element and a charge Q stored in the EL element.
First, a state where no voltage is applied to the EL element is defined as an origin {circle around (1)}. When a light emitting voltage of 255 V is applied from this state, the accumulated charge amount Q reaches state (3) via (2). During states (1) and (2), no conduction current flows through the light emitting layer of the EL element, and no light is emitted. Then, the EL element is clamped in the state (2) (the EL element changes from a capacitive characteristic to a resistive characteristic). During the state (2)-(3), the conduction current is applied to the light emitting layer of the EL element. Flows to emit light. In this case, the magnitude of the conduction current is roughly determined by the amount of charge (moving charge) after clamping. Therefore, the magnitude of the amount of mobile charge between states (2) and (3) is set to "1".
[0037]
Next, when the voltage is set to 0 V from the state (3), the state transits to the state (4). Polarized charges are generated at both ends of the light emitting layer due to the conduction current, and the EL element has a double insulating film structure in which the light emitting layer is sandwiched between insulating layers. The charge does not disappear, and a positive charge remains even when the voltage returns to 0V.
[0038]
Next, when a negative voltage (−255 V) is applied, the state is clamped in the state (5) due to the influence of the residual polarization charge, and the state reaches the state (7) through the state (6). Between states (4) and (5), no conduction current flows through the light emitting layer of the EL element and no light is emitted. However, between states (5) and (7), conduction current flows to emit light. At this time, the magnitude of the conduction current increases by the amount of the residual polarization charge, and the amount of the moving charge becomes “2”.
[0039]
Next, when the voltage is changed to 0 V from the state (7), the state changes to the state (8). In this case, a negative charge having the same magnitude as the state (4) remains. Then, when a positive emission voltage (255 V) is applied again from the state (8), the state is clamped in the state (9) due to the influence of the residual polarization charge, and the state reaches the state (3) through the state (2). The magnitude of the conduction current at this time is also “2”.
Thereafter, the process started from the above state (3) is repeated. As described above, when a voltage is applied to the inorganic EL element, a polarization charge is generated, and the QV characteristic changes so as to draw a parallelogram due to the influence of the polarization charge. Therefore, no light emission occurs even when voltages of the same polarity are continuously applied.
[0040]
That is, when the combined voltage vt and -vt are set to the potentials of (2) and (6) and the combined voltage applied to the EL element is changed by the QV characteristic as shown in FIG. The relative value changes as follows.

The same applies to the case where the polarity of the above-mentioned code is reversed.
[0041]
Here, FIG. 1 is referred to again. In the table shown in FIG. 1, seven levels of gradation display are performed with the total amount of relative moving charges generated in the light emitting layer of the EL element in a total of 14 screens constituting one gradation display unit. Since the field inversion driving is performed, it is assumed that the applied voltage polarity is inverted every frame. For example, at gray level 0, vt and -vt are simply repeated alternately over the entire frame in the gray scale display unit, and the total amount of mobile charge is zero.
[0042]
And at gradation level 1,

As a result, the total amount of mobile charges, that is, the relative luminance becomes “4”.
[0043]
Also, at gradation level 2,

As a result, the total charge amount becomes “8”.
[0044]
As described above, the relative luminance increases by 4 each time the gradation level increases. At the gradation level 7, by repeating + and-alternately over the entire frame, the total charge amount becomes “ 28 ".
[0045]
The applied voltage pattern shown in FIG. 1 has the following regularity. Although 7-level gradation control is performed with 3-bit data. When the applied voltage pattern is viewed in a vertical relationship (a relationship between the levels of the gradation levels), for example, in the frame 13, if “vt” corresponds to data L and “+” corresponds to data H, the data of bit 0 It matches the value change pattern. The frame 14 has the same pattern in which the polarity of the composite voltage is inverted.
[0046]
Similarly, frames 9 and 11 match the change pattern of the data value of bit 1, and frames 10 and 12 are the same pattern in which the polarity of the composite voltage is inverted. Further, frames 1, 3, 5, and 7 match the change pattern of the data value of bit 2, and frames 2, 4, 6, and 8 are the same pattern in which the polarity of the composite voltage is inverted.
[0047]
That is, when 7-level gradation is realized by 3-bit data, the applied voltage pattern is made to match the data change pattern of each bit, and the number of each pattern is set to (1, 2, 1) according to the bit (0, 1, 2). , 4) (a total of "7"), and by alternately changing the polarity, doubling each of them gives a total of "14", which is equal to the number m of screens constituting the gradation display unit. Has become.
[0048]
As described above, according to the present embodiment, the control IC 8 of the EL display device alternately reverses the polarity of the composite voltage of the scanning voltage and the data voltage applied to both ends of the EL element, By performing gradation display control using a set of screens as one gradation display unit, pulse number modulation is performed for field inversion driving. Therefore, in a continuous screen in the gradation display unit, gradation control with good linearity can be performed according to the number of pulses of the combined voltage exceeding the light emission threshold applied to the EL element.
[0049]
When the gray level that can be displayed in the gray scale display unit is “7”, the control IC 8 controls the binary data represented by each data line of 3 (= i) bits for controlling the gray level. , The composite voltage applied to the EL element is changed to a non-light-emitting voltage (vt, -vt) and a light-emitting voltage (+,-), and the number m of screens constituting a gradation display unit is 2 × (2 ³ -1) = 14 Therefore, seven gradation levels with good linearity can be easily set and controlled according to the data bit pattern by the 3-bit data.
[0050]
(Second embodiment)
FIGS. 4 and 5 show a second embodiment of the present invention. The same parts as those in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted. Only different parts will be described below. In the second embodiment, the basic configuration is the same as that of the first embodiment, and a voltage application pattern in a gradation display unit is different.
[0051]
The composite voltage vt is set based on the equation (1).
| Clamp voltage | × 2- | Scan voltage + Data voltage |
≦ | vt | <| clamp voltage | (1)
Here, the clamp voltage is a voltage at which a conduction current starts to flow through the EL element, and is a voltage corresponding to (2) in FIG. Then, after satisfying the expression (1), the composite voltage vt is set to a voltage “10” (210 V) corresponding to the middle between (9) and (2) in FIG.
[0052]
By determining the combined voltage vt in this manner, the amount of mobile charges applied to the voltage vt is as follows.

[0053]
In FIG. 4, the patterns of the gradation levels 0 and 7 are the same as in the first embodiment. For the gradation levels 2 to 6, when the gradation display unit is divided into the first half and the second half by seven frames, the voltage application pattern is such that the EL element emits light with the same luminance at the same timing and for the same period in each. Is set.
[0054]
That is, at the gradation level 1, the charge amount changes to "1.5" and "0.5" in the frames 3 and 4, and in the frames 10 and 11, the same change in the charge amount occurs due to the composite voltage whose polarity is inverted. Has occurred. Similarly, at gradation level 2, the charge amount changes to “0.5”, “2.0”, “0.5” in frames 4, 5, and 6, and in frames 11, 12, and 13, the polarity changes. The same amount of charge changes due to the inverted combined voltage.
[0055]
Here, with reference to FIG. 2, in the frame 12 of the gradation level 2, the state is “7” by applying “−” (−255 V). However, when “vt” is applied in the frame 13, the state is changed. The state is clamped by (9) to reach the state "10". During this time, the conduction current flows, and the relative moving charge amount becomes “0.5”. In the next frame 14, “−vt” is applied, but only transition to the state “11” is not clamped, and the moving charge amount is “0”.
[0056]
After that, in frames 1 to 3, only the movement between the states “10” and “11” is performed by the change between “vt” and “−vt”, so the amount of the transferred charge is “0”. Then, by applying “−” (−255 V) in the frame 4, the state is clamped at the state “12” via the states “10” and “11” to reach the state (7). At this time, the moving charge amount is “0.5”. In the subsequent frame 5, by applying “+” (255 V), the state is clamped from state (7) to state (9) to state (3), and the amount of mobile charges becomes “2.0”.
[0057]
FIG. 5 shows an example of the applied waveform of the scanning voltage and the data voltage. In the frames 3 to 6, 10, and 11 of the gradation display unit, the case where the first row of the EL display is displayed at the gradation levels 1 and 2 and the case where the second row is displayed at the gradation levels 7 and 0 are described. Show.
[0058]
As the scanning voltages of the first and second rows, +255 V and −210 V are alternately output with the scanning timing shifted (see (a) and (b)). (C) is a data voltage pattern in the case where the first row is displayed at the gradation level 1 and the second row is displayed at the gradation level 7. For example, frames 3 to 6 are as follows.

At this time, the combined voltages applied to the EL elements in the first and second rows are (e) and (g).
[0059]

[0060]
(D) is a data voltage pattern in the case where the first row is displayed at gradation level 2 and the second row is displayed at gradation level 0. For example, frames 3 to 6 are as follows.

At this time, the combined voltages applied to the EL elements in the first and second rows are (f) and (h).
[0061]

[0062]
The applied voltage pattern shown in FIG. 4 has the following regularity. When 7-level gradation is realized by 3-bit data, the applied voltage pattern is made to match the data change pattern of each bit, and the number of each pattern is set to (1, 2, 4) according to the bit (0, 1, 2). It is the same as in the first embodiment that each is doubled to make "14" in total. However, in the second embodiment, the arrangement of the applied voltage patterns according to the bits (0, 1, 2) is different from that in the first embodiment.
[0063]
That is, in the first embodiment, the relationship between the frame forming the gradation display unit and the voltage application pattern corresponding to each bit is as follows.

On the other hand, the second embodiment is as follows.

That is, in the second embodiment, the frames 1 to 7 and 8 to 14 have the same arrangement, and if the frame 3 is a reference position for arranging the pattern corresponding to the bit 0 in the frames 1 to 7, the frame 3 Two patterns corresponding to bit 1 are arranged, and four patterns corresponding to bit 2 are arranged on the right side thereof. However, the portion beyond the frame 7 returns to the frame 1 and is arranged in the frames 1 and 2. For frames 8 to 14, only "7" is added to the above.
[0064]
When such an arrangement pattern is selected, the voltage application pattern is set so that the EL element emits light at the same timing and at the same luminance in the first half and the second half of the gradation display unit except for the gradation level 7. Can be. In this case, the substantial emission frequency is twice the frequency at which the gradation display units are continuous. Therefore, the emission frequency based on the number of screens n = 7 may be set to a level at which flicker is not recognized by human eyes.
[0065]
As described above, according to the second embodiment, the control IC 8 sets the period during which the combined voltage is applied so that the conduction current flows through the light emitting layer of the EL element in the gradation display unit to two periods except for the maximum gradation level. The period is divided into equal periods, and the period during which the polarity of the composite voltage is positive and the period during which the polarity of the composite voltage is negative are equal in the gradation display unit.
[0066]
More specifically, in the gradation display unit, the arrangement order of the voltage change patterns corresponding to the respective 3-bit data patterns is determined by setting the voltage change pattern corresponding to the bit 0 to the third (= j) th position where the reference positions are two reference positions. , And seven voltage change patterns corresponding to bit 1 are arranged next to each of the two reference positions, and the voltage change pattern corresponding to bit 2 is changed to the voltage change pattern corresponding to bit 1. Are arranged next to each other, and when the number of the arrangement positions is 7 (= n) or exceeds 14, they are arranged at the number reduced by 7.
[0067]
Therefore, within one gradation display unit, the EL element can be caused to emit light twice at the same time and with the same luminance, so that flicker can be effectively prevented. Conversely, since flicker is less likely to occur, the number of screens constituting the gray scale display unit can be increased by that amount to increase the gray scale level.
[0068]
(Third embodiment)
FIG. 6 shows a third embodiment of the present invention, in which the same parts as those in the second embodiment are denoted by the same reference numerals, and description thereof will be omitted. Only different parts will be described below. In the third embodiment, the voltage application pattern in the gradation display unit is the same as that of the second embodiment, but the setting of the composite voltage vt is different.
[0069]
That is, the composite voltage vt is set based on the equation (2).
| Vt | ≦ | clamp voltage | × 2- | scan voltage + data voltage | (2)
Based on the equation (2), the composite voltage vt is set to be equal to or lower than the voltage in the state (9) in FIG.
[0070]
In this case, in the pattern in FIG. 4 of the second embodiment, the moving charge amount of “1.5” is “2.0”, and the moving charge amount of “0.5” is “0.5”. 0.0 ". As a result, the relative luminances of the gradation levels 1 and 2, the gradation levels 3 and 4, and the gradation levels 5 and 6 are equal, and the gradation is reduced to four levels.
[0071]
(Fourth embodiment)
FIGS. 7 and 8 show a fourth embodiment of the present invention. The same parts as those in the second embodiment are denoted by the same reference numerals, and description thereof will be omitted. Only different parts will be described below. In the fourth embodiment, the voltage application pattern in the gradation display unit is the same as that of the second embodiment, but the setting of the composite voltage vt is different.
[0072]
That is, the composite voltage vt is set based on the equation (3).
| Vt | ≒ | clamp voltage | (3)
Based on the equation (3), the composite voltage vt is set, for example, near the voltage in the state (2) in FIG. In this case, in the patterns in FIG. 4 of the second embodiment, the moving charge amounts of “1.5” and “0.5” are all “1.0”.
[0073]
Here, in the applied voltage pattern of the second embodiment shown in FIG. 4, the relative luminance difference from gradation levels 0 to 1 is “4”, the relative luminance difference from levels 1 to 2 is “2”, and levels 2 to 3 When the relative luminance difference between the levels 3 and 4 is "6" and the relative luminance difference between the levels 3 and 4 is "2", the linearity of the gradation level is lower than that of the pattern of the first embodiment shown in FIG. Was.
[0074]
Then, the inventors studied to improve the linearity of the gradation level while maintaining the applied voltage pattern of the second embodiment. As a result, the non-emission voltage | vt | and the emission voltage (+,-) It has been found that the linearity of gradation is improved when the amount of mobile charge between the two is approximately の of the amount of mobile charge between the light emitting voltages (+, −) of opposite polarities. To this end, it has been found that the non-light-emitting voltage | vt | should be set near the clamp voltage of the EL element.
[0075]
By setting the voltage | vt | in this way, when the state on the QV characteristic shown in FIG. 2 is transited by the applied voltage pattern shown in FIG. 7, the polarization charge is favorably canceled out. It is considered that the level linearity is improved.
[0076]
FIG. 8 shows QV characteristics of a general inorganic EL element. The light emission start voltage of the EL element is usually lower than the clamp voltage. This is presumably because the principle of light emission of the EL element is not limited to that based on conduction current, and conduction electrons move at a voltage lower than the clamp voltage due to other phenomena such as a tunnel effect. Therefore, if the composite voltage vt is set based on the expression (3), the luminance at the gray level 0 may not be completely “0”.
[0077]
However, for example, when the display is performed in the daytime on a vehicle, the surroundings are bright, so that even if the luminance at the gradation level 0 is not completely “0”, the display quality is completely unnoticeable. Therefore, it is sufficient for use in such an application.
As described above, according to the fourth embodiment, since the combined voltage vt is set near the clamp voltage of the EL element, it is possible to further improve the linearity in the luminance difference between each gradation level.
[0078]
(Fifth embodiment)
FIG. 9 shows a fifth embodiment of the present invention. The same parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof will be omitted. Only different parts will be described below. FIG. 9 shows in more detail the electrical configuration of a portion corresponding to the transformer 1, the switching regulator IC 2, the switching circuit 3, the smoothing circuit 4, and the scanning voltage adjusting circuit 6 in FIG. 3 of the first embodiment.
[0079]
A battery 22 is connected to the primary winding 21 of the transformer 1 and a switching circuit 3 composed of an NMOSFET is connected in series. The switching of the switching circuit 3 is controlled by a switching regulator IC2. Smoothing circuits 4A, 4B, 4C each composed of a diode and a capacitor are connected to the secondary windings 23a, 23b, 23c of the transformer 1, respectively.
[0080]
The secondary winding 23a generates a data voltage of 45V. This data voltage is monitored by the regulator IC 2 and is subjected to feedback control. Further, the secondary winding 23b is connected to a three-terminal regulator 5, not shown in FIG. 9, and generates F5V.
[0081]
Further, a scanning voltage adjusting circuit (scanning side driving circuit) 24 is connected to the secondary side winding 23c. The scanning voltage adjusting circuit 24 includes a series circuit of two Darlington-connected transistors 25 and 26, a resistor 27 connected to their output side, and a Zener diode 28 for setting a reference voltage, a common connection point between them, and a transistor. It comprises a transistor 29 having an emitter and a collector respectively connected to the base of 25, and a three-series resistance circuit 30 (a to c) having a variable resistor 30b in the middle stage. The common connection point of the resistors 30b and 30c is connected to the base of the transistor 29.
[0082]
Then, the scanning voltage adjusting circuit 24 generates a scanning voltage of about F210V and outputs it to the scanning voltage driving IC (scanning side driving circuit) 10, but in the fifth embodiment, the user operates the variable resistor 30b by the user. The scanning voltage can be adjusted. That is, when the resistance value of the variable resistor 30b is increased, the base current flowing through the transistor 29 increases, and the collector current also increases. Then, the base current flowing through the Darlington connection transistors 25 and 26 decreases, so that the terminal voltage of the resistor 27 decreases and the scanning voltage decreases. Conversely, if the resistance of the variable resistor 30b is set to be low, the scanning voltage will be high.
[0083]
According to the fifth embodiment configured as described above, the scanning voltage adjustment circuit 24 is configured to include the variable resistor 30b that can be operated by the user to adjust the scanning voltage. Can be adjusted by the user to maintain the linearity of the gradation level change even when the clamp voltage changes.
[0084]
The present invention is not limited to the embodiment described above and shown in the drawings, and the following modifications or extensions are possible.
In each of the above embodiments, a combined voltage of the same polarity is applied in the same frame. However, a line inversion driving method of inverting the polarity for each row, which is a well-known method for preventing flicker, is adopted. May be.
The gradation control is not limited to 7-level gradation control using 3 bits, and 15-level or more gradation control may be performed using 4 bits or more. In the case of 4 bits, the number m of screens constituting the gradation display unit is “30”.
The present invention is not limited to the one applied to an EL display mounted on a vehicle, and can be widely applied to any dot matrix type display using inorganic EL elements for pixels.
[Brief description of the drawings]
FIG. 1 is a first embodiment in which the present invention is applied to an EL display mounted on a vehicle, and a control IC of a display device performs field inversion driving of the EL display and performs seven gradation display by pulse number modulation. Diagram showing voltage application pattern when performing
FIG. 2 is a diagram illustrating a relationship between a combined voltage V applied to an EL element and a charge Q stored in the EL element.
FIG. 3 is a functional block diagram showing an electric configuration of the EL display device.
FIG. 4 is a view corresponding to FIG. 1, showing a second embodiment of the present invention;
FIG. 5 is a diagram showing an example of an applied waveform of a scanning voltage and a data voltage.
FIG. 6 is a view corresponding to FIG. 1, showing a third embodiment of the present invention.
FIG. 7 is a view corresponding to FIG. 1, showing a fourth embodiment of the present invention;
FIG. 8 is a diagram showing a relationship between a light emission start voltage of an inorganic EL element and a clamp voltage.
FIG. 9 is a diagram illustrating an electrical configuration of a scanning-side drive circuit according to a fifth embodiment of the present invention.
[Explanation of symbols]
Reference numeral 8 denotes a control IC (display control circuit), 10 denotes a scanning voltage driving IC (scanning side driving circuit), and 24 denotes a scanning voltage adjusting circuit (scanning side driving circuit).

Claims (6)

Driving a dot matrix type EL display using inorganic EL elements as pixels,
A scanning-side driving circuit that applies a scanning voltage to one end of the EL element;
A data drive circuit for applying a data voltage to the other end of the EL element;
The polarity of the composite voltage of the scanning voltage and the data voltage applied to both ends of the EL element is alternately inverted, and gradation display control is performed using a set of a continuous even number of screens as one gradation display unit. An EL display device comprising: a display control circuit. The display control circuit,
In the gradation display unit, a period in which a combined voltage is applied so that a conduction current flows in the light emitting layer of the EL element is divided into two equal periods except for the maximum gradation level.
2. The EL display device according to claim 1, wherein control is performed such that a period during which the polarity of the composite voltage is positive is equal to a period during which the polarity of the composite voltage is negative within the gradation display unit. The display control circuit,
When the gradation level that can be displayed by the gradation display unit is (2 ⁱ −1) (i is an integer of 2 or more), two levels indicated by i-bit data lines for controlling the gradation level are used. Changing the combined voltage applied to the EL element according to the value data to a non-emission voltage at a level at which the EL element does not emit light and an emission voltage at a level at which the EL element emits light,
3. The EL display device according to claim 1, wherein the number m of screens constituting the gradation display unit is 2 × (2 ⁱ −1). 4. In the gradation display unit, when the arrangement order of the voltage change patterns corresponding to the i-bit data patterns is j, where j is an integer satisfying (1 ≦ j ≦ n, n = m / 2),
A voltage change pattern corresponding to bit 0 is arranged at j-th and j + (2 ⁱ -1) -th reference positions,
Two voltage change patterns each corresponding to bit 1 are arranged next to the two reference positions, respectively.
The voltage change pattern corresponding to the bit (i-1) is arranged 2 ^i-1 / 2 next to the voltage change pattern corresponding to the bit (i-2),
The EL display device according to claim 3, wherein when the number of the arrangement positions exceeds n or 2n, the arrangement positions are arranged with the numbers reduced by n and 2n, respectively. 5. The EL display device according to claim 3, wherein the non-light emitting voltage is set near a clamp voltage of the EL element. The EL display device according to claim 5, wherein the scanning-side driving circuit includes a variable resistor operable by a user to adjust the scanning voltage.

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