JP4590828B2 - Light emitting element driving device - Google Patents

Description

より、
Ｉｐｄ＝Ｉ１／Ｇ
となる。すなわち、光電流Ｉｄｐは、消灯時の受光器１２の出力電流Ｉｌｅａｋを基準とし、増幅器１３の誤差電流ΔＩｇを補正しながら、光量制御時に設定した基準電流I１に基づいて設定される。
【００６６】
上述したように、第１実施形態に係る発光素子駆動装置では、補正回路１５において、発光素子１１の消灯時の増幅器１３の出力を基準として基準値の出力を補正し、発光素子１１の点灯時の増幅器１３の出力を検知して光量制御を実施することにより、例えば面発光レーザを光源とする露光装置に適用した場合であっても、面発光レーザに起因する受光器１２のリーク電流による誤差（オフセット）と増幅器１３の誤差を確実に補正することができるため高精度の光量制御を実現できる。
【００６７】
特に、補正回路１５が、コンデンサＣ１１と、発光素子１１の消灯時に増幅器１３の出力に応じた電圧、即ち負荷１８の端子電圧をコンデンサＣ１１に充電する経路を形成する回路系（スイッチＳＷ１１，ＳＷ１２が端子ａ側に切り替わったときの回路系）と、発光素子１１の点灯時に増幅器１３の出力に応じた電圧からコンデンサＣ１１の充電電圧を差し引く経路を形成する回路系（スイッチＳＷ１１，ＳＷ１２が端子ｂ側に切り替わったときの回路系）とを有する回路構成となっていることで、回路系を切り替えるだけで発光素子１１の消灯時に増幅器１３の出力を基準として基準値の出力を補正して、発光素子１１の点灯時の増幅器１３の出力を検知して光量制御を実施することができるため、簡単な回路構成によって受光器１２のリーク電流による誤差と増幅器１３の誤差を補正することができる。
【００６８】
［第２実施形態］
図５は、本発明の第２実施形態に係る発光素子駆動装置の構成を示す回路図であり、図中、図１と同等部分には同一符号を付して示している。
【００６９】
本実施形態に係る発光素子駆動装置では、光電流Ｉｐｄが流れる負荷１８−１側の増幅器１３−１と相似の構成の増幅器１３−２を負荷１８−２側にも有するとともに、基準電流供給回路１４Ｂおよび補正回路１５Ｂが、第１実施形態に係る発光素子駆動装置の基準電流供給回路１４Ａおよび補正回路１５Ａと構成が若干相違している。また、差分検出回路１６のサンプルホールド回路１６２において、スイッチＳＷ１′が２つの端子ａ，ｂを有し、コンデンサＣ１が一方の端子ａとＧＮＤとの間に接続され、他方の端子ｂが差動アンプ１６１の反転入力端子に接続されている。そして、スイッチＳＷ１′は、光量制御時に端子ａ側に切り替わり、それ以外では端子ｂ側に切り替わることになる。
【００７０】
図６に、増幅器１３−１，１３−２の具体的な構成の一例を示す。増幅器１３−１は図１の増幅器１３に相当し、したがって図２と同じ構成となっている。増幅器１３−２は、上述したように、増幅器１３−１と相似の構成となっている。すなわち、増幅器１３−２は、インピーダンス変換回路２２１、バイアス電流源２２、折り返し回路２２３および電流源２２４を有する構成となっている。
【００７１】
増幅器１３−２において、インピーダンス変換回路２２１は、ゲートとドレインが共通に接続され、ソースが接地されたＮｃｈＭＯＳトランジスタＱ１５と、このＭＯＳトランジスタＱ１５のゲート・ドレインにゲートが共通に接続され、ソースが接地されたＮｃｈＭＯＳトランジスタＱ１６とからなるカレントミラー回路によって構成されている。
【００７２】
バイアス電流源２２２は、電源ＶＤＤとトランジスタＱ１５のドレイン・ゲート共通接続点との間に接続されている。折り返し回路２２３は、ソースが電源ＶＤＤに接続され、ゲートおよびドレインがトランジスタＱ１６のドレインに共通に接続されたＰｃｈＭＯＳトランジスタＱ１７と、このＭＯＳトランジスタＱ１７のゲート・ドレインにゲートが共通に接続され、ソースが電源ＶＤＤに接続されたＰｃｈＭＯＳトランジスタＱ１８とからなるカレントミラー回路によって構成されている。
【００７３】
電流源２２４は、ＭＯＳトランジスタＱ１８のドレインとグランドＧＮＤとの間には接続され、折り返し回路２２３の出力電流から、増幅器１３−２の入力段でバイアス電流源２２２によって重畳されたバイアス電流Ｉｂｉａｓと等しい電流値の電流を差し引いて、残りの電流を増幅器１３−２の出力電流とする。
【００７４】
基準電流供給回路１４Ｂは、基準電流Ｉ１を出力する電流源１４１に加えて、基準電流Ｉ１と等しい電流値の疑似電流Ｉ２を出力する電流源１４２と、スイッチＳＷ２に連動し、当該スイッチＳＷ２がオフ状態のときオン状態となるスイッチＳＷ３とを有する構成となっている。この基準電流供給回路１４Ｂにおいて、スイッチＳＷ３がオン状態になることで、光量制御時以外のとき、即ち受光器１２から光信号Ｉｐｄが出力されないときに、目標光量に対応した電流源１４１の基準電流Ｉ１が増幅器１３−１に供給される。また、電流源１４２の疑似電流Ｉ２が常時増幅器１３−２に供給される。
【００７５】
光量制御時において、発光素子１１が目標光量で発光しているときは、受光器１２のリーク電流を無視するものとすれば、受光器１２から出力され、増幅器１３−１を介して負荷１８−１に流れ込む光電流Ｉｐｄの電流値は当然のことながら基準電流Ｉ１の電流値と一致している。したがって、負荷１８−１には、基準電流Ｉ１の電流値に応じた端子電圧が光量検出電圧Ｖｄｅｔとして発生する。また、電流源１４２からは、基準電流Ｉ１と電流値が等しい疑似電流Ｉ２が増幅器１３−２を介して負荷１８−２に流れ込む。したがって、負荷１８−２にも、基準電流Ｉ１の電流値に応じた端子電圧が基準電圧Ｖｒｅｆとして発生する。その結果、基準電圧Ｖｒｅｆに対する光量検出電圧Ｖｄｅｔの誤差は０となる。
【００７６】
発光素子１１の光量が目標光量からずれると、そのずれに応じた変動分ΔＩｐｄだけ光電流Ｉｐｄが変動し、Ｉｐｄ±ΔＩｐｄの電流が増幅器１３−１を介して負荷１８−１に流れ込む。すると、負荷１８−１の端子電圧、即ち光量検出電圧Ｖｄｅｔが、基準電圧Ｖｒｅｆに対して変動分ΔＩｐｄに応じてΔＶｄｅｔだけ変動し、この変動分ΔＶｄｅｔが差動アンプ１６１で誤差電圧として検出される。そして、この誤差電圧が０になるように、即ち発光素子１１の光量が目標光量にし収束するように、駆動回路１７によって発光素子１１の駆動制御が行われることになる。
【００７７】
また、光量制御時以外のとき、即ち受光器１２から光信号Ｉｐｄが出力されないときには、電流源１４１の基準電流Ｉ１を増幅器１３−１に供給するようにすることにより、光量制御によって発光素子１１の光量が目標光量に収束し、その光量制御が終了して増幅器１３−１に光信号Ｉｐｄが供給されなくなっても、基準電流Ｉ１が引き続き増幅器１３−１に供給され続けられることになる。これにより、光量制御開始時に、負荷１８−１の端子電圧が大きく変動することはないため、発光素子１１の光量を目標光量に速やかに収束させることができる。
【００７８】
次に、第２実施形態に係る発光素子駆動装置における補正回路１５Ｂの具体的な回路例およびその誤差補正のための回路動作について説明する。
【００７９】
補正回路１５Ｂは、スイッチＳＷ１１，ＳＷ１２，ＳＷ１３およびコンデンサＣ１１に加えて、スイッチ１４を有する構成となっている。スイッチＳＷ１１，ＳＷ１２，ＳＷ１３およびコンデンサＣ１１の接続関係は、基本的に、第１実施形態に係る発光素子駆動装置の補正回路１５Ａのそれと同じである。ただし、スイッチＳＷ１２の他方の端子ａは、差動アンプ１６１の反転入力端子に接続されている。スイッチＳＷ１４は、負荷１８−１の端子と差動アンプ１６１の反転入力端子との間に接続されている。
【００８０】
次に、上記構成の補正回路１５Ｂにおける誤差補正のための回路動作について説明する。
【００８１】
まず、補正電圧を検知してコンデンサＣ１１に充電するときには、発光素子１１の消灯状態でスイッチＳＷ１３がオン状態になり、基準電流Ｉ１と電流値が等しい疑似電流Ｉ２が増幅器１３−２を介して負荷１８−２に流れる。これにより、負荷１８−２には基準電流Ｉ１に対応した端子電圧が発生し、差動アンプ１６１の非反転入力端子に入力される。また、差動アンプ１６１の出力端子に接続されているスイッチＳＷ１は端子ｂ側に切り替わっているので、差動アンプ１６１は出力が反転入力端子に入力されるボルテージフォロアを構成している。このときの差動アンプ１６１の反転入力端子と非反転入力端子間の電圧差が入力端子間オフセット電圧（誤差電圧）である。
【００８２】
さらに、スイッチＳＷ１１は端子ａ側、即ち増幅器１３−１側に切り替わり、スイッチＳＷ１２もａ側、即ち差動アンプ１６１の反転入力端子側に切り替わり、スイッチＳＷ１４はオフ状態となる。これにより、負荷１８−１→スイッチＳＷ１１→コンデンサＣ１１→スイッチＳＷ１２→差動アンプ１６１の反転入力端子の充電経路が形成され、負荷１８−１の端子電圧と差動アンプ１６１の反転入力端子の電圧差によってコンデンサＣ１１が充電される。
【００８３】
このとき、コンデンサＣ１１には充電される電圧値は、受光器１２の消灯時逆バイアスリーク電流による誤差と差動アンプ１６１の誤差（オフセット）の電圧を差し引きした電圧となる。このことについて、以下に具体的に説明する。
【００８４】
ここで、差動アンプ１６１の反転入力端子電圧をＶｎ、非反転入力端子電圧をＶｐ、入力端子間オフセット電圧をΔＶｏｐ、受光器１２のリーク電流をＩｌｅａｋ、負荷１８−１，１８−２の抵抗値をＲ１、基準電流をＩ１、増幅器１３−１，１３−２のゲインをＧ、増幅器１３−１，１３−２の出力誤差電流をΔＩｇと定義する。
【００８５】
Ｖｐ＝（Ｉ１×Ｇ＋ΔＩｇ）×Ｒ１
Ｖｐ＋ΔＶｏｐ＝Ｖｎ
なので、コンデンサＣ１１に充電されるオフセット電圧ΔＶｏｃは、

となる。
【００８６】
次に、誤差補正を行う光量制御時には、スイッチＳＷ１１は端子ｂ側、即ち差動アンプ１６１の非反転入力端子側に接続され、スイッチＳＷ１２も端子ｂ側、即ち負荷１８−２側に接続され、スイッチＳＷ１３はオフ状態、スイッチＳＷ１４はオン状態になる。これにより、負荷１８−２の端子にコンデンサＣ１１の負側（ＧＮＤ側）端子が接続され、コンデンサＣ１１の正側端子が差動アンプ１６１の非反転入力端子に接続された経路が形成される。
【００８７】
そして、上記（１）式で表される検知時のコンデンサＣ１１の充電電圧ΔＶｏｐを基準電流Ｉ１に対応する電圧から差し引きして光量制御を行うことで誤差の補正が実施される。このことについて、式を用いて具体的に説明すると、
Ｖｐ＝（Ｉ１×Ｇ＋ΔＩｇ）×Ｒ１−ΔＶｏｃ
Ｖｎ＝（Ｉｐｄ×Ｇ＋Ｉｌｅａｋ×Ｇ＋ΔＩｇ）×Ｒ１
Ｖｐ＋ΔＶｏｐ＝Ｖｎ
より、代入して相殺すると（ΔＶｏｃは（１）式より）、結局、Ｉ１＝Ｉｐｄとなる。すなわち、光電流Ｉｄｐは、消灯時の受光器１２の出力電流Ｉｌｅａｋを基準とし、増幅器１３の誤差電流ΔＩｇ、差動アンプ１６１の誤差電圧（オフセット電圧）ΔＶｏｐを補正しながら、光量制御時に設定した基準電流I１に基づいて設定される。
【００８８】
上述したように、第２実施形態に係る発光素子駆動装置では、差動アンプ１６１の非反転入力側の負荷１８−２に基準電流Ｉ１と電流値が等しい疑似電流Ｉ２を流すとともに、受光器１２から光電流Ｉｐｄが出力されないときは、基準電流Ｉ１を差動アンプ１６１の反転入力側の負荷１８−１に流すことで、光量制御開始時に負荷１８−１の端子電圧が大きく変動して収束時間が長くなるのを防止する構成を採る場合において、補正回路１５Ｂを用いることにより、面発光レーザに起因する受光器１２のリーク電流による誤差（オフセット）と増幅器１３の誤差を補正することにできることに加えて、特に駆動回路１７をＩＣ化し、ＣＭＯＳで回路を構成した際に問題となる差動アンプ１６１のオフセットについても確実に補正することができるため、より高精度の光量制御を実現できる。
【００８９】
（第２実施形態の変形例）
【００９０】
図７は、第２実施形態の変形例に係る発光素子駆動装置の構成を示す回路図であり、図中、図５と同等部分には同一符号を付して示している。
【００９１】
本変形例に係る発光素子駆動装置では、補正回路１５Ｃの構成、特にスイッチＳＷ１１′，ＳＷ１２′の構成が第２実施形態の補正回路１５Ｂのそれと相違しているのみであり、それ以外の構成は第２実施形態に係る発光素子駆動装置と同じである。
【００９２】
スイッチＳＷ１１′，ＳＷ１２′は共に３つの端子ａ，ｂ，ｃを有している。
スイッチＳＷ１１′，ＳＷ１２′の端子ａ，ｂが、第２実施形態のスイッチＳＷ１１，ＳＷ１２の端子ａ，ｂに対応している。第３の端子ｃはオープン状態になっている。そして、スイッチＳＷ１１′，ＳＷ１２′は共に、光量制御時およびコンデンサに補正電圧を充電するとき以外は第３の端子ｃに切り替わることで、可動端子ｄ間に接続されているコンデンサＣ１１の両端をオープン状態にする。
【００９３】
次に、上記構成の補正回路１５Ｃにおける誤差補正のための回路動作について説明する。
【００９４】
光量制御終了後は、スイッチＳＷ１１′，ＳＷ１２′は共に第３の端子ｃに切り替わることで、コンデンサＣ１１の両端をどの端子からも切り離し、オープン状態にする。そして、次回コンデンサＣ１１の充電時には、第２実施形態において説明したように、スイッチＳＷ１１′は増幅器１３−１側に切り替わり、スイッチＳＷ１２′は差動アンプ１６１の反転入力端子側に切り替わり、再度充電を開始する。
【００９５】
これは、スイッチＳＷ１１′，ＳＷ１２′が光量制御終了後、端子ａあるいは端子ｂに接続されていると、負荷１８−１，１８−２や他のスイッチを介してコンデンサＣ１１の充電電圧がリークして変動しやすくなることから、それを防止するためである。コンデンサＣ１１の両端をオープン状態にしておくことで、次回オフセット補正時までの充電電圧のリークによる変動を最小限に抑えることができる。
【００９６】
プリンターなどの露光装置で走査毎に光量制御とオフセット補正をセットで行う場合、誤差の値は走査毎には大きく変動しない。このように、オフセット補正毎の期間のコンデンサＣ１１の充電電圧の変動を抑制するのは、次回のコンデンサＣ１１の充電時に充電値が収束値近くから充電を開始することで、充電電圧の変動を小さく抑えることができ、その結果収束時間を短縮できるからである。
【００９７】
コンデンサＣ１１の充電開始時には、第２実施形態で述べたように、スイッチＳＷ１３をオンにするとともに、スイッチＳＷ１′を端子ｂ側に切り替えることによって差動アンプ１６１の出力端子を反転入力端子に接続し、かつスイッチ１２を端子ａ側切り替えることによって差動アンプ１６１の出力端子をコンデンサＣ１１に接続して充電を開始する。
【００９８】
このとき、スイッチＳＷ１３をオン、スイッチＳＷ１′を端子ｂ側に切り替えた直後は、差動アンプ１６１の出力電圧はまだ収束値に対する差が大きい。したがって、このときの差動アンプ１６１の出力電圧を、スイッチＳＷ１２を介してコンデンサＣ１１に充電しようとすると、充電値が一度大きく変動してから収束値に向かう。しかも、差動アンプ１６１の出力端子に容量負荷が接続され、当該差動アンプ１６１の出力電圧、即ち充電電圧の収束に時間がかかってしまう。
【００９９】
そこで、スイッチＳＷ１２を端子ａ側に切り替えて、差動アンプ１６１の出力端子をコンデンサＣ１１に接続するタイミングを、スイッチＳＷ１３をオン、スイッチＳＷ１′を端子ｂ側に切り替えてから一定時間が経過し、差動アンプ１６１の出力電圧が収束値近くなるタイミングに設定する。すなわち、補正時に差動アンプ１６１の出力電圧によってコンデンサＣ１１を充電する場合において、差動アンプ１６１の出力電圧が収束するまでの一定時間が経過した後に差動アンプ１６１の出力電圧をコンデンサＣ１１に供給する経路を形成することで、上記の場合と同様に、コンデンサＣ１１の充電時に充電電圧の変動を小さく抑えることができるため収束時間を短縮できる。
【０１００】
図８は、補正回路１５Ｃの具体的な回路構成の一例を示す回路図である。本回路例においては、図７のスイッチＳＷ１１′，ＳＷ１２′，スイッチＳＷ１３，スイッチＳＷ１４が、ＮｃｈＭＯＳトランジスタＱ２１〜Ｑ２６によって構成されている。ここで、図７との関係において、ＭＯＳトランジスタＱ２１がスイッチＳＷ１１′のｂ−ｄ間、ＭＯＳトランジスタＱ２２がスイッチＳＷ１１′のａ−ｄ間、ＭＯＳトランジスタＱ２３がスイッチＳＷ１２′のｂ−ｄ間、ＭＯＳトランジスタＱ２４がスイッチＳＷ１２′のａ−ｄ間にそれぞれ対応している。
【０１０１】
ＭＯＳトランジスタＱ２１〜Ｑ２４をオン状態にするには各ゲートに高レベルの電圧を、オフ状態にするには各ゲートに低レベルの電圧をそれぞれ印加すれば良い。コンデンサＣ１１の両端をオープン状態にするには、コンデンサＣ１１の両端子に接続されている４つのＮｃｈＭＯＳトランジスタＱ２１〜Ｑ２４の各ゲート電圧をすべて低レベルにして、トランジスタＱ２１〜Ｑ２４の全てをオフ状態にすれば良い。
【０１０２】
ここで、ＮｃｈＭＯＳトランジスタＱ２１〜Ｑ２４がオン状態だと、コンデンサＣ１１に充電された電荷がＭＯＳトランジスタＱ２１〜Ｑ２４のサブストレート電位（通常、ＮｃｈＭＯＳトランジスタの場合はグランド、ＰｃｈＭＯＳトランジスタの場合は電源電圧）に対してリークして充電電圧が変動してしまう。したがって、コンデンサＣ１１の両端をどの端子からも切り離してオープン状態にすることにより、コンデンサＣ１１の充電電圧の変動を抑えることができる。
【０１０３】
なお、上記各実施形態では、単一の発光素子を駆動する駆動装置の場合を例に挙げて説明したが、これに限られるものではなく、上記各実施形態に係る発光素子駆動装置は光量制御時の収束時間を短縮できるため、発光素子として面発光レーザを用い、各ビーム毎に順次繰り返して光量制御を行う構成のマルチレーザビームの駆動装置に適用すると、その効果をより一層発揮できることになる。
【図面の簡単な説明】
【図１】本発明の第１実施形態に係る発光素子駆動装置の構成を示す回路図である。
【図２】第１実施形態に係る発光素子駆動装置における増幅器の構成の一例を示す回路図である。
【図３】負荷の具体的な構成の一例を示す回路図である。
【図４】第１実施形態に係る発光素子駆動装置の具体的な回路例を示す回路図である。
【図５】本発明の第２実施形態に係る発光素子駆動装置の構成を示す回路図である。
【図６】第２実施形態に係る発光素子駆動装置における増幅器の構成の一例を示す回路図である。
【図７】第２実施形態の変形例に係る発光素子駆動装置の構成を示す回路図である。
【図８】第２実施形態の変形例における補正回路の具体的な回路構成の一例を示す回路図である。
【符号の説明】
１１…発光素子、１２…受光器、１３Ｍ１３−１，１３−２…増幅器、１４Ａ，１４Ｂ…基準電流供給回路、１５Ａ，１５Ｂ，１５Ｃ…補正回路、１６…差分検出回路、１７…駆動回路、１６１…差動アンプ、１６２…サンプルホールド回路[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a light emitting element driving apparatus for driving a light emitting element that emits light by passing a direct current, and more particularly to a light emitting element driving apparatus suitable for driving a laser element used as a light source in laser xerography.
[0002]
[Prior art]
In the field of laser xerography using a laser element as a light source, there is an increasing demand for higher resolution and higher speed. There is a limit to the speed (hereinafter referred to as modulation speed) at which on / off control of the driving of the laser element is performed according to the input image data. In the case where the number of laser beams is one, if the resolution in the sub-scanning direction is increased as well as the resolution in the main scanning direction, the modulation speed must be sacrificed. Therefore, the only way to increase the resolution in the sub-scanning direction without increasing the modulation speed is to increase the number of laser beams. For example, when the number of laser light beams is four, the resolution in the main scanning / sub-scanning direction can be doubled on the assumption that the modulation speed is the same as in the case of one.
[0003]
A semiconductor laser used as a light source for laser xerography includes an edge-emitting laser element (hereinafter referred to as an edge-emitting laser) having a structure in which laser light is extracted in a direction parallel to the active layer, and the laser light is perpendicular to the active layer. It is roughly classified into surface emitting laser elements (hereinafter, referred to as surface emitting lasers) having a structure that can be extracted in any direction. Conventionally, in laser xerography, an edge emitting laser is generally used as a laser light source.
[0004]
However, from the viewpoint of increasing the number of laser light beams, edge-emitting lasers are technically difficult, and structurally, surface-emitting lasers increase the number of laser light beams more than edge-emitting lasers. It is advantageous. For these reasons, in recent years, in order to meet the demand for higher resolution and higher speed in the field of laser xerography, an apparatus using a surface emitting laser capable of emitting a large number of laser light beams as a laser light source. Development is underway.
[0005]
By the way, in a semiconductor laser driving device, an automatic light amount control circuit is used that detects the light amount of the semiconductor laser with a light receiver and automatically controls the light amount based on the detected light amount. When controlling the amount of light, in the case of a surface emitting laser, a part of the emitted light is separated by an optical system including a half mirror because of the structural restriction that the laser light is emitted in a direction perpendicular to the active layer. A configuration is adopted in which the amount of light of the surface emitting laser is detected by causing light to enter the light receiver as monitor light.
[0006]
As described above, when the elements of the surface emitting laser, the optical system, and the light receiver are assembled, the positional accuracy between the elements is poor. Therefore, the light receiving area of the light receiver is required to reliably receive the monitor light. It gets bigger. Moreover, the output current (photocurrent) of the photoreceiver itself is very small due to the fact that an optical system including a half mirror is interposed between the surface emitting laser and the photoreceiver, and the photocurrent of the edge emitting laser is about 100 μA. On the other hand, it is a minute current of about several μA.
[0007]
As described above, in an exposure apparatus for an image forming apparatus using a surface emitting laser as a light source, the photocurrent is very small due to optical system limitations, and the light receiving area of the light receiving device is large. The reverse bias leak current becomes a magnitude that cannot be ignored with respect to the photocurrent, and the offset caused by the leak current becomes a problem in performing light quantity control. Therefore, it is necessary to cancel the offset caused by the leakage current when detecting the error of the light emission amount of the surface emitting laser with respect to the reference light amount in the control circuit that performs the light amount control.
[0008]
Conventionally, as an offset cancel circuit, two differential amplifier circuits are connected in cascade, and a pair of coupling capacitors and these capacitors are charged between the differential amplifier circuit of the first stage and the differential amplifier circuit of the next stage. A differential amplifier circuit is provided by providing an offset charging circuit comprising a pair of switches for short-circuiting, shorting the input of the first-stage differential amplifier circuit, and charging a pair of coupling capacitors with each of two output voltage differences due to the offset There is known a configuration in which the offset is corrected (see, for example, Patent Document 1).
[0009]
In the offset cancel circuit having such a configuration, the input of the first stage differential amplifier circuit is short-circuited, and at the same time, the two outputs are short-circuited so that the differential voltage between the pair of coupling capacitors becomes zero. Capacitor charging time is shortened.
[0010]
[Patent Document 1]
JP-A-6-85562
[0011]
[Problems to be solved by the invention]
In the offset cancel circuit according to the above prior art, the input of the first stage differential amplifier circuit is short-circuited, and at the same time, the terminal of the next stage differential amplifier circuit of the coupling capacitor is grounded, so that the first stage differential amplifier circuit Output is started to converge from a voltage value different from the convergence value, so that when performing offset correction repeatedly, both outputs of the first-stage differential amplifier circuit are short-circuited regardless of whether they are short-circuited or not.・ Each time charging, the capacitor charging voltage will fluctuate from the convergence value and converge.
[0012]
Therefore, particularly in the light amount control of a surface emitting laser (multi-beam laser) capable of emitting a large number of laser light beams, it takes time to converge each time offset correction is performed in the light amount control of each laser light beam. Therefore, the effect of shortening the charging time cannot be obtained sufficiently. Moreover, basically only the offset of the differential amplifier circuit for controlling the light quantity is corrected, so even if the offset cancellation circuit is used for the light quantity control circuit of the surface emitting laser, it is caused by the leak current of the light receiver. The offset cannot be corrected.
[0013]
The present invention has been made in view of the above problems, and an object of the present invention is to correct an error (offset) caused by a leak current of a light receiver and an error of an amplifier in a light quantity control circuit system. An object of the present invention is to provide a light emitting element driving device capable of controlling light quantity with high accuracy.
[0014]
[Means for Solving the Problems]
  The invention according to claim 1 is a photoreceiver that receives light emitted from a light emitting element and outputs a photocurrent corresponding to the amount of the light, and a first amplifier that amplifies the photocurrent output from the photoreceiver A first current source for supplying a reference current corresponding to a target light amount of the light emitting element to the first amplifier within a first period in which the light emitting element is turned off, a differential amplifier, a capacitor And, within the first period, a voltage corresponding to the current value of the reference current is input to the non-inverting input terminal of the differential amplifier, and the voltage output from the differential amplifier according to the input is A first circuit system that is input to an inverting input terminal of a differential amplifier and charges the capacitor by a voltage difference between a voltage corresponding to an output of the first amplifier and a voltage of the inverting input terminal; Within the second period that is lit, the reference A second circuit that inputs a voltage obtained by subtracting the voltage of the capacitor from a voltage corresponding to a current value of the current to the non-inverting input terminal, and inputs a voltage corresponding to the output of the first amplifier to the inverting input terminal. A light emitting element driving apparatus comprising: a system; and a drive circuit that controls a light amount of the light emitting element using a voltage output from the differential amplifier within the second period.
[0019]
  According to a second aspect of the present invention, in the light-emitting element driving device according to the first aspect, the second circuit system opens both ends of the capacitor within a third period following the second period. It is characterized by doing.
[0020]
  According to a third aspect of the present invention, in the light emitting element driving device according to the first aspect, the first circuit system supplies a voltage corresponding to a current value of the reference current within the first period. Input to the non-inverting input terminal of the amplifier, and the voltage output from the differential amplifier in accordance with this input is input to the inverting input terminal of the differential amplifier, and the output voltage of the differential amplifier is constant until convergence. A path for supplying the output voltage of the differential amplifier to the capacitor is formed after a lapse of time.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In each embodiment described below, for example, a semiconductor laser, particularly a GaN blue laser or a single mode surface emitting laser having a large internal resistance is used as a light emitting element to be driven.
[0022]
[First Embodiment]
FIG. 1 is a circuit diagram showing a configuration of a light emitting element driving apparatus according to the first embodiment of the present invention. As is clear from FIG. 1, the light emitting element driving apparatus according to the present embodiment controls the light receiving element 11, the light receiver 12, the amplifier 13, the reference current supply circuit 14A, the correction circuit 15A, and the difference detection circuit 16. And it has the structure which has the drive circuit 17 at least. Here, the configuration of the circuit system of the light amount control circuit that automatically controls the light emission amount of the light emitting element 11 to be the target light amount is shown.
[0023]
The light receiver 12 is made of, for example, a photodiode, receives a light beam emitted from the light emitting element 11, and outputs a photocurrent Ipd corresponding to the light amount. Here, when the light emitting element 11 is a surface emitting laser, as described above, the light receiving area of the light receiver 12 is increased due to the restrictions of the optical system, so that the parasitic capacitance of the light receiver 12 (mainly the depletion layer of the photodiode). Capacity) Co becomes very large.
[0024]
Thus, when the parasitic capacitance Co of the light receiver 12 is large, the convergence time of the light amount control becomes long. When current-voltage conversion is performed with a low resistance value in order to shorten the convergence time of the light amount control, a sufficient light amount detection voltage cannot be obtained because the photocurrent Ipd output from the light receiver 12 is very small, and the light amount control is performed. Can't ensure enough gain for. The amplifier 13 is provided to compensate for this insufficient gain, and amplifies the photocurrent Ipd output from the light receiver 12 and flows it to the load 18-1.
[0025]
Here, as one of the effective methods for solving the above shortage of gain, there is a method of performing current-voltage conversion with a resistance value that can obtain a sufficient output voltage after impedance conversion by a current mirror circuit to achieve high impedance. Can be mentioned. According to this method, even when the photocurrent Ipd is amplified, the amplification degree can be set stably at the ratio of the current mirror. However, the current mirror circuit has a problem that the response is deteriorated in the case of a minute current.
[0026]
Although it is possible to increase the speed by supplying a bias current to the current mirror circuit, it is necessary to add a bias current and then subtract the bias current to obtain an amplified current value. However, by subtracting the bias current, the output potential is determined in relation to the subsequent stage, so that the sink current is deviated. The larger the bias current, the larger the error, and there is a concern that the accuracy of the light amount control is deteriorated. .
[0027]
From this point of view, it can be said that the amplifier 13 is an effective means for eliminating the gain shortage than the current mirror circuit in the light amount control in the light emitting element driving apparatus using, for example, a surface emitting laser as the light emitting element 11. An example of a specific configuration of the amplifier 13 is shown in FIG.
[0028]
The amplifier 13 has an impedance conversion circuit 131 for converting a low input side impedance into a higher output side impedance at its input stage. The impedance conversion circuit 131 has an NchMOS transistor Q11 whose gate and drain are commonly connected and whose source is grounded, and an NchMOS transistor Q12 whose gate is commonly connected to the gate and drain of the MOS transistor Q11 and whose source is grounded. It is comprised by the current mirror circuit which consists of.
[0029]
  In the impedance conversion circuit 131 having this current mirror circuit configuration, a current corresponding to the photocurrent Ipd flowing into the drain of the transistor Q11 flows through the drain of the transistor Q12. Amplifier section according to this circuit example13Further includes a bias current source 132 connected between the power supply VDD and the common drain / gate connection point of the transistor Q11. The bias current source 132 superimposes the bias current Ibias set by the bias voltage V2 on the photocurrent Ipd flowing into the drain of the transistor Q11.
[0030]
A folding circuit 133 is provided at the subsequent stage of the impedance conversion circuit 131. This folding circuit 133 has a source connected to the power supply VDD, a gate and drain connected in common to the drain of the transistor Q12, a gate connected to the gate and drain of the MOS transistor Q13, and a gate connected to the source. Is formed of a current mirror circuit including a PchMOS transistor Q14 connected to the power supply VDD.
[0031]
A current source 134 that sucks a current value equal to the bias current Ibias of the bias current source 132 is connected between the drain of the MOS transistor Q14 and the ground GND. The current source 134 subtracts a current having a current value equal to the bias current Ibias superimposed by the bias current source 132 at the input stage of the amplifier 13 from the output current of the folding circuit 133, and uses the remaining current as the output current of the amplifier 13. And
[0032]
The reference current supply circuit 14A has a current source 141 that supplies a reference current I1 having a current value corresponding to the DC voltage V1 set corresponding to the target light amount (specified light amount) of the light emitting element 11, and this current source The reference current I1 generated in 141 is supplied to the load 18-2. The correction circuit 15 </ b> A corrects an error (offset) due to a reverse bias leakage current when the light receiver 12 is turned off and an error of the amplifier 13. A specific circuit configuration of the correction circuit 15 will be described later.
[0033]
Here, an example of a specific configuration of the load 18 (18-1, 18-2) will be described with reference to FIG.
[0034]
  The load 18 has, for example, two resistors Ra and Rb having different resistance values. One end of each of the resistors Ra and Rb is commonly connected and grounded (GND). The other end of each of the resistors Ra and Rb has a switchSWa, SWb are connected at one end.SWa, SWb are connected in common and connected to the inverting input terminal / non-inverting input terminal of the differential amplifier 161. switchSWa, SWb is turned off when one is turned on.
[0035]
  The load 18 having the above configuration, for example, the load 18-1, obtains the light amount detection voltage Vdet by converting the voltage of the photocurrent Ipd whose impedance has been converted by the impedance conversion circuit 131. During this voltage conversion, the switchSWa, SWb, the gain can be adjusted by changing the resistance value of the load 18 by switching the resistors Ra, Rb. As described above, by making the resistance value of the load 18 variable, when the appropriate gain changes depending on the light emission efficiency of the light emitting elements 11-1 and 11-2 and the ratio of the amount of light incident on the light receiver 12, the load By adjusting the resistance value of 18 (18-1, 18-2), it is possible to select an optimum gain that can achieve both the accuracy of light quantity control and convergence.
[0036]
The difference detection circuit 16 includes a differential amplifier 161 and a sample hold circuit 162. The differential amplifier 161 receives a reference voltage Vref generated according to the reference current I1 at the terminal of the load 18-2 as a non-inverted (+) input, and detects the amount of light generated at the terminal of the load 18-1 according to the photocurrent Ipd. The voltage Vdet is inverted (−) input, and an error voltage corresponding to the difference of the light amount detection voltage Vdet with respect to the reference voltage Vref is output as a light amount control voltage. Here, when the light emitting element 11 emits light with the target light amount, a photocurrent Ipd having a current value equal to the reference current I1 set corresponding to the target light amount is output from the light receiver 12, and the light amount detection voltage is output. Since Vdet matches the reference voltage Vref, the error voltage becomes zero.
[0037]
The sample hold circuit 162 includes a switch SW1 having one end connected to the output terminal of the differential amplifier 161, and a capacitor C1 connected between the other end of the switch SW1 and the ground. In the sample and hold circuit 162, when the light emitting element 11 is driven, the switch SW1 is turned on (closed), whereby the error voltage detected by the differential amplifier 161 is held in the capacitor C1. The drive circuit 17 drives the light emitting element 11 according to the hold voltage of the capacitor C1. Between the drive circuit 17 and the light emitting element 11, a switch SW <b> 2 that is turned on at the timing when the light amount control of the light emitting element 11 is performed is provided.
[0038]
FIG. 4 is a circuit diagram illustrating a specific circuit example of the light emitting element driving apparatus according to the first embodiment. Here, it is assumed that the light emitting element 11 is a semiconductor laser LD and the light receiver 12 is a photodiode PD. In the correspondence relationship between FIG. 1 and FIG. 4, the drive circuit 17 in FIG. 1 corresponds to the drive current control circuit 17A and the drive voltage control circuit 17B in FIG.
[0039]
The drive current control circuit 17A includes an analog inverter 111, a limit voltage generation circuit 112, a comparator 113, a current source 114, a capacitor C51, and switches SW51, SW52, and SW53. In the drive current control circuit 17A, the drive current of the semiconductor laser LD is controlled so that the light emission amount of the semiconductor laser LD becomes a prescribed light amount (target light amount). The drive current control circuit 17A is supplied with the error voltage detected by the difference detection circuit 16 as the light amount control voltage Vcont via the drive voltage control circuit 17B.
[0040]
  This light control voltage Vcont isAIt is inverted by the analog inverter 111 and given to the input side terminal of the switch SW51. The capacitor C51 is connected between the power supply VDD and the terminal on the output side of the switch SW51 to constitute a sample hold circuit together with the switch SW51. This sample and hold circuit samples and holds the light amount control voltage Vcont that is given by being inverted by the analog inverter 111.
[0041]
  The limit voltage generation circuit 112, the comparator 113, and the switch SW52 constitute a current limiting circuit (limiter) for limiting the drive current flowing through the semiconductor laser LD. The limit voltage generation circuit 112 generates a limit voltage Vlim corresponding to the limit current that limits the drive current flowing through the semiconductor laser LD. The comparator 113 uses the limit voltage Vlim generated by the limit voltage generation circuit 112 as an inverting (−) input, and the analog inverter 111 and the switchSW51The light quantity control voltage Vcont applied through the non-inverted (+) input is used, and the comparison output is inverted when the light quantity control voltage Vcont exceeds the limit voltage Vlim.
[0042]
The switch SW52 has the light amount control voltage Vcont supplied through the analog inverter 111 and the switch SW51 as one input, and the limit voltage Vlim generated by the limit voltage generation circuit 112 as the other input. During normal light amount control, the light amount control voltage When Vcont is selected and the comparison output of the comparator 113 is inverted, the limit voltage Vlim is selected instead of the light amount control voltage Vcont in response to the inversion of the comparison output.
[0043]
The light amount control voltage Vcont or limit voltage Vlim selected by the switch SW52 is supplied to the current source 114 as its control voltage. One end of the current source 114 is connected to the power supply VDD. The switch SW53 has one end connected to the other end of the current source 114 and the other end connected to the anode (drive end) of the semiconductor laser LD.
[0044]
In the drive current control circuit 17A having such a configuration, the light amount control voltage Vcont is inverted by the inverter 111 and then sample-held by the sample-hold circuit including the switch SW51 and the capacitor C51. The sampled and held light amount control voltage Vcont is selected by the switch SW52 and is given as a control voltage to the current source 114, and the drive current supplied from the current source 114 to the semiconductor laser LD through the switch SW53 is controlled. .
[0045]
Thereby, in the drive current control circuit 17A, the drive current of the semiconductor laser LD is controlled so that the light quantity of the semiconductor laser LD becomes a prescribed light quantity (target light quantity) determined by the reference voltage Vref of the difference detection circuit 16. This is APC (Automatic Power Control) for controlling the laser power of the semiconductor laser LD so as to be the power specified by the reference voltage Vref.
[0046]
The light amount control voltage Vcont after the sample hold is further compared with the limit voltage Vlim generated by the limit voltage generation circuit 112 in the comparator 113. When the light amount control voltage Vcont exceeds the limit voltage Vlim, the comparator 113 inverts the comparison output and controls the switch SW52. As a result, the switch SW52 selects the limit voltage Vlim instead of the light amount control voltage Vcont that has been selected so far, and applies it to the semiconductor laser LD as its control voltage. As a result, in the drive current control circuit 17A, current limitation is performed so that the drive current flowing through the semiconductor laser LD becomes a constant limit current corresponding to the limit voltage Vlim.
[0047]
  The drive voltage control circuit 17B includes a differential amplifier 121, four switches SW54 to SW57, and a capacitor C52. In this drive voltage control circuit 17B, a drive current control circuit17ABased on the terminal voltage of the semiconductor laser LD when the light quantity is controlled to a prescribed light amount, the drive voltage applied to the semiconductor laser LD during lighting is controlled.
[0048]
The differential amplifier 121 uses the output voltage of the difference detection circuit 16 as a non-inverting input, and operates as a buffer when the inverting input terminal and the output terminal are short-circuited by the switch SW57. One end of the switch SW 54 is connected to the output terminal of the differential amplifier 121. The capacitor C52 is connected between the other end of the switch SW54 and the ground to constitute a sample and hold circuit together with the switch SW54.
[0049]
The switch SW55 has one end connected to the other end of the switch SW54 and the other end connected to the anode of the semiconductor laser LD. The switch SW56 has one end connected to the other end of the switch SW55 and the anode of the semiconductor laser LD, and the other end connected to the inverting input terminal of the differential amplifier 121.
[0050]
  Driving voltage control circuit having such a configuration17BWhen the drive current of the semiconductor laser LD is controlled, that is, during APC (light quantity control), the switches SW54, SW55, and SW57 are all in the OFF state.SW56Turns on. Then, the error voltage detected by the difference detection circuit 16 is applied to the semiconductor laser LD via the differential amplifier 121 and the switch SW56, thereby forming a feedback loop.
[0051]
Further, at the time of modulating the semiconductor laser LD after the light amount control, both the switches SW54 and SW57 are turned on, and both the switches SW55 and SW56 are turned off. Then, the output voltage of the differential amplifier 121, that is, the terminal voltage of the semiconductor laser LD at the end of the light amount control is held in the capacitor C52. The hold voltage of the capacitor C52 is applied as a drive voltage to the drive end (anode) of the semiconductor laser LD via the switch SW55 when the switch SW55 is turned on when the semiconductor laser LD is turned on.
[0052]
In the drive circuit 17 including the drive current control circuit 17A and the drive voltage control circuit 17B described above, the switches SW53, SW55, and SW56 correspond to the switch SW2 in FIG.
[0053]
  As in the case of FIG. 1, the difference detection circuit 16 is configured by a differential amplifier 161 and a sample hold circuit 162, and detects a difference (error voltage) of the light amount detection voltage Vdet with respect to the reference voltage Vref. However, the differential amplifier161A switch SW58 and a capacitor C53 are connected in series between the inverting input terminal and the output terminal. Switch SW59 and capacitor constituting sample hold circuit 162C52Corresponds to the switch SW1 and the capacitor C1 in FIG.
[0054]
In the differential detection circuit 16 having such a configuration, the differential amplifier 161 compares the light amount detection voltage Vdet corresponding to the light amount of the semiconductor laser LD with a reference voltage Vref set corresponding to the target light amount of the semiconductor laser LD. The difference, that is, the error voltage is output. The output voltage (error voltage) of the differential amplifier 161 is supplied to the sample hold circuit 162, and is held by the capacitor 54 when the switch SW59 is turned on during light quantity control.
[0055]
Next, the circuit operation of the light emitting element driving apparatus according to the first embodiment having the above-described configuration will be described.
[0056]
First, at the time of error correction by the correction circuit 15A, both the switches SW1 and SW2 are turned off (open), and the light emitting element 11 is turned off. The current output from the light receiver 12 when the light emitting element 11 is turned off is a reverse bias leakage current when the light emitting element 11 is turned off. This reverse bias leakage current during turn-off is amplified by the amplifier 13, converted into a voltage by the load 18-1, and supplied to the correction circuit 15A. The correction circuit 15A detects a voltage corresponding to the reverse bias leakage current when the light is turned off, and corrects an error caused by the leakage current based on the voltage detected during the light amount control. Specific operations relating to this error correction will be described later.
[0057]
During the light amount control, both the switches SW1 and SW2 are turned on. From the light receiver 12, a photocurrent Ipd corresponding to the light amount of the light emitting element 11 is output. This photocurrent Ipd is amplified by the amplifier 13 and then flows into the load 18-1. Then, a light amount detection voltage Vdet corresponding to the photocurrent Ipd is generated in the load 18-1, and becomes an inverting input of the differential amplifier 161. Further, the reference current I1 output from the reference current supply circuit 14A flows into the load 18-2. As a result, a reference voltage Vref corresponding to the reference current I1 is generated in the load 18-2 and becomes a non-inverting input of the differential amplifier 161 via the correction circuit 15.
[0058]
From the differential amplifier 161, an error voltage of the light amount detection voltage Vdet with respect to the reference voltage Vref is output as a light amount control voltage. This light amount control voltage is held in the capacitor C1 at the timing when the switch SW1 is turned on. The drive circuit 17 drives the light emitting element 11 based on the hold voltage of the capacitor C1 so that the light amount of the light emitting element 11 becomes the target light amount. By this series of light quantity control, the light quantity of the light emitting element 11 converges to the target light quantity.
[0059]
Next, a specific circuit example of the correction circuit 15A in the light emitting element driving device according to the first embodiment and a circuit operation for correcting the error will be described.
[0060]
The correction circuit 15A includes three switches SW11, SW12, SW13 and a capacitor C11. The switches SW11 and SW12 have two fixed terminals a and b. One terminal a of the switch SW11 is connected to the terminal of the load 18-1, and the other terminal b is connected to the non-inverting input terminal of the differential amplifier 161. One terminal a of the switch SW12 is grounded, and the other terminal b is connected to the terminal of the load 18-2. The switch SW13 is connected between the load 18-2 and the non-inverting input terminal of the differential amplifier 161. The capacitor C11 is connected between the movable terminals c of the switches SW11 and SW12.
[0061]
Next, a circuit operation for error correction in the correction circuit 15A having the above configuration will be described. Here, the reference current is I1, the resistance values of the loads 18-1 and 18-2 are R1, the gain of the amplifier 13 is G, the output error current of the amplifier 13 is ΔIg, and the reverse bias leakage current when the light receiver 12 is turned off is Ileak, the photocurrent output from the light receiver 12 will be described as Ipd.
[0062]
First, when detecting the amplified voltage of the output of the light receiver 12 when the light emitting element 11 is turned off, the switch SW11 is switched to the terminal a side, that is, the amplifier 13 side, and the switch SW12 is also switched to the a side, that is, the GND side. The switch SW13 is turned on. As a result, a charging path of load 18-1 → switch SW11 → capacitor C11 → switch SW12 → GND is formed, and the capacitor C11 is charged by the terminal voltage of the load 18-1.
[0063]
At this time, ΔIg flows into the load 18-1 from the current obtained by amplifying the leakage current Ileak and the output error current of the amplifier 13, so that the terminal voltage Va of the load 18-1 is
Va = (Ileak × G + ΔIg) × R1
It becomes. Then, the capacitor C11 is charged by the voltage Va of the load 18-1 at this time.
[0064]
At the time of light amount control for error correction, the switch SW11 is connected to the terminal b side, that is, the non-inverting input terminal side of the differential amplifier 161, the switch SW12 is also connected to the terminal b side, that is, the load 18-2 side, and the switch SW13 is Turns off. As a result, a path is formed in which the negative side (GND side) terminal of the capacitor C11 is connected to the terminal of the load 18-2, and the positive side terminal of the capacitor C11 is connected to the non-inverting input terminal of the differential amplifier 161.
[0065]
At this time, if the input error of the differential amplifier 161 is 0, that is, the error of the light amount detection voltage Vdet with respect to the reference voltage Vref is 0,

Than,
Ipd = I1 / G
It becomes. That is, the photocurrent Idp is set based on the reference current I1 set during the light amount control while correcting the error current ΔIg of the amplifier 13 based on the output current Ileak of the light receiver 12 when the light is extinguished.
[0066]
As described above, in the light emitting element driving apparatus according to the first embodiment, the correction circuit 15 corrects the output of the reference value based on the output of the amplifier 13 when the light emitting element 11 is turned off, and the light emitting element 11 is turned on. By detecting the output of the amplifier 13 and controlling the amount of light, for example, even when applied to an exposure apparatus using a surface emitting laser as a light source, an error due to a leak current of the light receiver 12 caused by the surface emitting laser (Offset) and the error of the amplifier 13 can be reliably corrected, so that the light amount control with high accuracy can be realized.
[0067]
In particular, the correction circuit 15 forms a circuit system (switches SW11 and SW12 are used for charging the capacitor C11 and the voltage corresponding to the output of the amplifier 13, that is, the terminal voltage of the load 18 to the capacitor C11 when the light emitting element 11 is turned off. A circuit system when switched to the terminal a side) and a circuit system that forms a path for subtracting the charging voltage of the capacitor C11 from the voltage corresponding to the output of the amplifier 13 when the light emitting element 11 is lit (the switches SW11 and SW12 are on the terminal b side). When the light emitting element 11 is turned off, the output of the reference value is corrected with reference to the output of the amplifier 13 only by switching the circuit system. 11 can detect the output of the amplifier 13 when it is turned on and control the light quantity. It is possible to correct an error of the error and amplifier 13 due to leakage current.
[0068]
[Second Embodiment]
FIG. 5 is a circuit diagram showing a configuration of the light emitting element driving apparatus according to the second embodiment of the present invention. In FIG. 5, the same parts as those in FIG.
[0069]
In the light emitting element driving device according to the present embodiment, the load 13-2 has an amplifier 13-2 having a configuration similar to that of the amplifier 13-1 on the load 18-1 through which the photocurrent Ipd flows, and a reference current supply circuit. 14B and the correction circuit 15B are slightly different in configuration from the reference current supply circuit 14A and the correction circuit 15A of the light emitting element driving apparatus according to the first embodiment. In the sample hold circuit 162 of the difference detection circuit 16, the switch SW1 ′ has two terminals a and b, the capacitor C1 is connected between one terminal a and GND, and the other terminal b is differential. It is connected to the inverting input terminal of the amplifier 161. The switch SW1 ′ is switched to the terminal a side during the light amount control, and is switched to the terminal b side otherwise.
[0070]
FIG. 6 shows an example of a specific configuration of the amplifiers 13-1 and 13-2. The amplifier 13-1 corresponds to the amplifier 13 of FIG. 1, and therefore has the same configuration as that of FIG. As described above, the amplifier 13-2 has a configuration similar to that of the amplifier 13-1. That is, the amplifier 13-2 includes an impedance conversion circuit 221, a bias current source 22, a folding circuit 223, and a current source 224.
[0071]
In the amplifier 13-2, the impedance conversion circuit 221 has an NchMOS transistor Q15 whose gate and drain are connected in common and whose source is grounded, a gate connected to the gate and drain of the MOS transistor Q15, and a source connected to ground. This is constituted by a current mirror circuit composed of the NchMOS transistor Q16.
[0072]
The bias current source 222 is connected between the power supply VDD and the drain / gate common connection point of the transistor Q15. The folding circuit 223 has a PchMOS transistor Q17 having a source connected to the power supply VDD, a gate and a drain connected in common to the drain of the transistor Q16, a gate connected to the gate and drain of the MOS transistor Q17, and a source connected to the source. The current mirror circuit includes a PchMOS transistor Q18 connected to the power supply VDD.
[0073]
  Current source224Is connected between the drain of the MOS transistor Q18 and the ground GND, and has a current value equal to the bias current Ibias superimposed by the bias current source 222 at the input stage of the amplifier 13-2 from the output current of the folding circuit 223. The current is subtracted and the remaining current is used as the output current of the amplifier 13-2.
[0074]
In addition to the current source 141 that outputs the reference current I1, the reference current supply circuit 14B is linked to the current source 142 that outputs the pseudo current I2 having a current value equal to the reference current I1 and the switch SW2, and the switch SW2 is turned off. The switch SW3 is turned on when in the state. In the reference current supply circuit 14B, when the switch SW3 is turned on, the reference current of the current source 141 corresponding to the target light amount is obtained when the light amount control is not performed, that is, when the optical signal Ipd is not output from the light receiver 12. I1 is supplied to the amplifier 13-1. Further, the pseudo current I2 of the current source 142 is always supplied to the amplifier 13-2.
[0075]
When the light emitting element 11 emits light with the target light amount at the time of light amount control, if the leakage current of the light receiver 12 is ignored, the light output from the light receiver 12 is output via the amplifier 13-1. As a matter of course, the current value of the photocurrent Ipd flowing into 1 matches the current value of the reference current I1. Therefore, a terminal voltage corresponding to the current value of the reference current I1 is generated in the load 18-1 as the light amount detection voltage Vdet. Further, from the current source 142, a pseudo current I2 having a current value equal to the reference current I1 flows into the load 18-2 via the amplifier 13-2. Therefore, a terminal voltage corresponding to the current value of the reference current I1 is also generated as the reference voltage Vref in the load 18-2. As a result, the error of the light amount detection voltage Vdet with respect to the reference voltage Vref becomes zero.
[0076]
When the light amount of the light emitting element 11 deviates from the target light amount, the photocurrent Ipd fluctuates by a variation ΔIpd corresponding to the deviation, and a current of Ipd ± ΔIpd flows into the load 18-1 via the amplifier 13-1. Then, the terminal voltage of the load 18-1, that is, the light amount detection voltage Vdet varies by ΔVdet according to the variation ΔIpd with respect to the reference voltage Vref, and this variation ΔVdet is detected as an error voltage by the differential amplifier 161. . Then, drive control of the light emitting element 11 is performed by the drive circuit 17 so that the error voltage becomes 0, that is, the light quantity of the light emitting element 11 converges to the target light quantity.
[0077]
Further, when the light signal is not controlled, that is, when the optical signal Ipd is not output from the light receiver 12, the reference current I1 of the current source 141 is supplied to the amplifier 13-1, so that the light emitting element 11 is controlled by the light amount control. Even if the light quantity converges to the target light quantity and the light quantity control is completed and the optical signal Ipd is no longer supplied to the amplifier 13-1, the reference current I1 continues to be supplied to the amplifier 13-1. Thereby, since the terminal voltage of the load 18-1 does not fluctuate greatly at the start of the light amount control, the light amount of the light emitting element 11 can be quickly converged to the target light amount.
[0078]
Next, a specific circuit example of the correction circuit 15B in the light emitting element driving device according to the second embodiment and a circuit operation for correcting the error will be described.
[0079]
  The correction circuit 15B includes a switch 14 in addition to the switches SW11, SW12, SW13 and the capacitor C11. The connection relationship between the switches SW11, SW12, SW13 and the capacitor C11 is basically the same as that of the correction circuit 15A of the light emitting element driving apparatus according to the first embodiment. However, the other terminal of the switch SW12aIs connected to the inverting input terminal of the differential amplifier 161. The switch SW14 is connected between the terminal of the load 18-1 and the inverting input terminal of the differential amplifier 161.
[0080]
Next, a circuit operation for error correction in the correction circuit 15B having the above configuration will be described.
[0081]
  First, when the correction voltage is detected and the capacitor C11 is charged, the switch SW13 is turned on when the light emitting element 11 is turned off, and a pseudo current I2 having a current value equal to the reference current I1 is loaded via the amplifier 13-2. It flows to 18-2. This will load18-2Generates a terminal voltage corresponding to the reference current I1 and is input to the non-inverting input terminal of the differential amplifier 161. Further, since the switch SW1 connected to the output terminal of the differential amplifier 161 is switched to the terminal b side, the differential amplifier 161 forms a voltage follower in which the output is input to the inverting input terminal. The voltage difference between the inverting input terminal and the non-inverting input terminal of the differential amplifier 161 at this time is an input terminal offset voltage (error voltage).
[0082]
Further, the switch SW11 is switched to the terminal a side, that is, the amplifier 13-1, the switch SW12 is also switched to the a side, that is, the inverting input terminal side of the differential amplifier 161, and the switch SW14 is turned off. As a result, a charging path of the load 18-1 → switch SW11 → capacitor C11 → switch SW12 → inverting input terminal of the differential amplifier 161 is formed, and the terminal voltage of the load 18-1 and the voltage of the inverting input terminal of the differential amplifier 161 are formed. The capacitor C11 is charged by the difference.
[0083]
At this time, the voltage value charged in the capacitor C11 is a voltage obtained by subtracting the error due to the reverse bias leakage current when the light receiver 12 is turned off and the error (offset) voltage of the differential amplifier 161. This will be specifically described below.
[0084]
Here, the inverting input terminal voltage of the differential amplifier 161 is Vn, the non-inverting input terminal voltage is Vp, the offset voltage between the input terminals is ΔVop, the leak current of the light receiver 12 is Ileak, and the resistances of the loads 18-1 and 18-2 A value is defined as R1, a reference current is defined as I1, gains of the amplifiers 13-1 and 13-2 are defined as G, and output error currents of the amplifiers 13-1 and 13-2 are defined as ΔIg.
[0085]
Vp = (I1 × G + ΔIg) × R1
Vp + ΔVop = Vn
Therefore, the offset voltage ΔVoc charged in the capacitor C11 is

It becomes.
[0086]
Next, at the time of light quantity control for error correction, the switch SW11 is connected to the terminal b side, that is, the non-inverting input terminal side of the differential amplifier 161, and the switch SW12 is also connected to the terminal b side, that is, the load 18-2 side. The switch SW13 is turned off and the switch SW14 is turned on. As a result, a path is formed in which the negative side (GND side) terminal of the capacitor C11 is connected to the terminal of the load 18-2, and the positive side terminal of the capacitor C11 is connected to the non-inverting input terminal of the differential amplifier 161.
[0087]
Then, the error correction is performed by subtracting the charging voltage ΔVop of the capacitor C11 at the time of detection represented by the above formula (1) from the voltage corresponding to the reference current I1 and performing light quantity control. This will be specifically explained using an equation.
Vp = (I1 × G + ΔIg) × R1−ΔVoc
Vn = (Ipd × G + Ileak × G + ΔIg) × R1
Vp + ΔVop = Vn
Thus, if it is substituted and canceled (ΔVoc is from equation (1)), I1 = Ipd is eventually obtained. That is, the photocurrent Idp was set at the time of light intensity control while correcting the error current ΔIg of the amplifier 13 and the error voltage (offset voltage) ΔVop of the differential amplifier 161 with reference to the output current Ileak of the light receiving device 12 at the time of extinction. It is set based on the reference current I1.
[0088]
As described above, in the light emitting element driving apparatus according to the second embodiment, the pseudo current I2 having the current value equal to the reference current I1 is supplied to the load 18-2 on the non-inverting input side of the differential amplifier 161, and the light receiver 12 is used. When the photocurrent Ipd is not output from the reference voltage Ipd, the reference current I1 is passed through the load 18-1 on the inverting input side of the differential amplifier 161, so that the terminal voltage of the load 18-1 fluctuates greatly at the start of light quantity control, and the convergence time is reached. In the case of adopting a configuration for preventing the length of the light from becoming long, the correction circuit 15B can be used to correct the error (offset) due to the leakage current of the light receiver 12 caused by the surface emitting laser and the error of the amplifier 13. In addition, it is possible to surely correct the offset of the differential amplifier 161, which is a problem particularly when the drive circuit 17 is integrated into an IC and the circuit is configured with CMOS. Because, it is possible to realize a light control of higher accuracy.
[0089]
(Modification of the second embodiment)
[0090]
FIG. 7 is a circuit diagram showing a configuration of a light emitting element driving apparatus according to a modification of the second embodiment. In FIG. 7, the same parts as those in FIG.
[0091]
In the light emitting element driving apparatus according to this modification, the configuration of the correction circuit 15C, particularly the configuration of the switches SW11 ′ and SW12 ′, is only different from that of the correction circuit 15B of the second embodiment, and the other configurations are the same. This is the same as the light emitting element driving apparatus according to the second embodiment.
[0092]
Both the switches SW11 'and SW12' have three terminals a, b and c.
The terminals a and b of the switches SW11 ′ and SW12 ′ correspond to the terminals a and b of the switches SW11 and SW12 of the second embodiment. The third terminal c is in an open state. Both the switches SW11 ′ and SW12 ′ open both ends of the capacitor C11 connected between the movable terminals d by switching to the third terminal c except when controlling the light quantity and charging the capacitor with the correction voltage. Put it in a state.
[0093]
Next, a circuit operation for error correction in the correction circuit 15C having the above configuration will be described.
[0094]
After the light amount control is completed, the switches SW11 ′ and SW12 ′ are both switched to the third terminal c, so that both ends of the capacitor C11 are disconnected from any terminal and are opened. When the capacitor C11 is charged next time, as described in the second embodiment, the switch SW11 ′ is switched to the amplifier 13-1, the switch SW12 ′ is switched to the inverting input terminal side of the differential amplifier 161, and charging is performed again. Start.
[0095]
This is because if the switches SW11 'and SW12' are connected to the terminal a or b after the light quantity control is completed, the charging voltage of the capacitor C11 leaks via the loads 18-1 and 18-2 and other switches. This is to prevent it from becoming more variable. By keeping both ends of the capacitor C11 open, fluctuation due to leakage of the charging voltage until the next offset correction can be minimized.
[0096]
When a light amount control and offset correction are performed as a set for each scan by an exposure device such as a printer, the error value does not vary greatly from scan to scan. As described above, the fluctuation of the charging voltage of the capacitor C11 during the period of each offset correction is suppressed by starting charging from the vicinity of the convergence value when the capacitor C11 is charged next time, thereby reducing the fluctuation of the charging voltage. This is because the convergence time can be shortened.
[0097]
At the start of charging of the capacitor C11, as described in the second embodiment, the switch SW13 is turned on and the switch SW1 ′ is switched to the terminal b side to connect the output terminal of the differential amplifier 161 to the inverting input terminal. In addition, by switching the switch 12 to the terminal a side, the output terminal of the differential amplifier 161 is connected to the capacitor C11 to start charging.
[0098]
At this time, immediately after the switch SW13 is turned on and the switch SW1 ′ is switched to the terminal b, the output voltage of the differential amplifier 161 is still largely different from the convergence value. Therefore, if the output voltage of the differential amplifier 161 at this time is to be charged into the capacitor C11 via the switch SW12, the charged value is once greatly changed and then the convergence value is reached. In addition, a capacitive load is connected to the output terminal of the differential amplifier 161, and it takes time to converge the output voltage of the differential amplifier 161, that is, the charging voltage.
[0099]
Therefore, when the switch SW12 is switched to the terminal a side and the output terminal of the differential amplifier 161 is connected to the capacitor C11, a certain time has elapsed since the switch SW13 is turned on and the switch SW1 ′ is switched to the terminal b side. The timing is set so that the output voltage of the differential amplifier 161 becomes close to the convergence value. That is, when the capacitor C11 is charged with the output voltage of the differential amplifier 161 at the time of correction, the output voltage of the differential amplifier 161 is supplied to the capacitor C11 after a certain time has elapsed until the output voltage of the differential amplifier 161 converges. By forming the path to perform, as in the case described above, the fluctuation of the charging voltage can be suppressed to a small level when the capacitor C11 is charged, so that the convergence time can be shortened.
[0100]
FIG. 8 is a circuit diagram showing an example of a specific circuit configuration of the correction circuit 15C. In this circuit example, the switches SW11 ′, SW12 ′, switch SW13, and switch SW14 of FIG. 7 are configured by NchMOS transistors Q21 to Q26. Here, in relation to FIG. 7, the MOS transistor Q21 is between b and d of the switch SW11 ', the MOS transistor Q22 is between ad and the switch SW11', the MOS transistor Q23 is between b and d of the switch SW12 ', and the MOS The transistor Q24 corresponds to between a and d of the switch SW12 ′.
[0101]
To turn on the MOS transistors Q21 to Q24, a high level voltage may be applied to each gate, and to turn off, a low level voltage may be applied to each gate. In order to open both ends of the capacitor C11, the gate voltages of the four NchMOS transistors Q21 to Q24 connected to both terminals of the capacitor C11 are all set to a low level, and all the transistors Q21 to Q24 are turned off. Just do it.
[0102]
Here, when the Nch MOS transistors Q21 to Q24 are in the ON state, the charge charged in the capacitor C11 is set to the substrate potential of the MOS transistors Q21 to Q24 (usually ground in the case of NchMOS transistors and power supply voltage in the case of PchMOS transistors). On the other hand, the charging voltage fluctuates due to leakage. Therefore, the fluctuation of the charging voltage of the capacitor C11 can be suppressed by disconnecting both ends of the capacitor C11 from any terminal and making it open.
[0103]
In each of the above embodiments, the case of a driving device that drives a single light emitting element has been described as an example. However, the present invention is not limited to this, and the light emitting element driving device according to each of the above embodiments has a light amount control. Since the convergence time can be shortened, the effect can be further exerted by applying a surface emitting laser as a light emitting element and applying it to a multi-laser beam driving device configured to repeatedly and repeatedly control the amount of light for each beam. .
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a configuration of a light emitting element driving apparatus according to a first embodiment of the present invention.
FIG. 2 is a circuit diagram showing an example of a configuration of an amplifier in the light emitting element driving apparatus according to the first embodiment.
FIG. 3 is a circuit diagram showing an example of a specific configuration of a load.
FIG. 4 is a circuit diagram showing a specific circuit example of the light emitting element driving device according to the first embodiment.
FIG. 5 is a circuit diagram showing a configuration of a light emitting element driving apparatus according to a second embodiment of the present invention.
FIG. 6 is a circuit diagram showing an example of a configuration of an amplifier in the light emitting element driving apparatus according to the second embodiment.
FIG. 7 is a circuit diagram showing a configuration of a light emitting element driving apparatus according to a modification of the second embodiment.
FIG. 8 is a circuit diagram illustrating an example of a specific circuit configuration of a correction circuit according to a modification of the second embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Light emitting element, 12 ... Light receiver, 13M13-1, 13-2 ... Amplifier, 14A, 14B ... Reference current supply circuit, 15A, 15B, 15C ... Correction circuit, 16 ... Difference detection circuit, 17 ... Drive circuit, 161 ... Differential amplifier, 162 ... Sample hold circuit

Claims (3)

A light receiver that receives light emitted from the light emitting element and outputs a photocurrent according to the amount of the light;
A first amplifier for amplifying the photocurrent output from the light receiver;
A first current source that supplies a reference current corresponding to a target light amount of the light emitting element to the first amplifier within a first period in which the light emitting element is turned off;
A differential amplifier;
A capacitor,
Within the first period, a voltage corresponding to the current value of the reference current is input to a non-inverting input terminal of the differential amplifier, and a voltage output from the differential amplifier according to the input is input to the differential A first circuit system that inputs to the inverting input terminal of the amplifier and charges the capacitor by a voltage difference between a voltage corresponding to the output of the first amplifier and a voltage of the inverting input terminal;
A voltage obtained by subtracting the voltage of the capacitor from the voltage corresponding to the current value of the reference current is input to the non-inverting input terminal within the second period in which the light emitting element is turned on, and the first amplifier A second circuit system for inputting a voltage corresponding to an output to the inverting input terminal;
A drive circuit for controlling a light amount of the light emitting element by using a voltage output from the differential amplifier within the second period;
A light-emitting element driving device comprising: 2. The light-emitting element driving device according to claim 1, wherein the second circuit system opens both ends of the capacitor within a third period following the second period. The first circuit system inputs a voltage corresponding to the current value of the reference current to the non-inverting input terminal of the differential amplifier within the first period, and from the differential amplifier according to the input A path for inputting the output voltage to the inverting input terminal of the differential amplifier and supplying the output voltage of the differential amplifier to the capacitor after a certain time has elapsed until the output voltage of the differential amplifier converges The light emitting element driving device according to claim 1, wherein:

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