發光二極體列印頭驅動電路之設計

Abstract

在電子顯像印表機中，採用發光二極體列印頭當作曝光機制，有縮小印表機體積、增加印表機穩定度及降低成本的好處，但這樣的技術一直把持在日本手中，因應國內市場的需求，本論文設計了一顆應用於發光二極體列印頭的驅動晶片，本晶片採用數位式脈衝寬度調變的技巧，在定電流下利用4-bit的數位訊號可產生16種線性的發光二極體發光能量，並提出一個動態時間脈衝寬度產生電路，使脈衝寬度有更多的變化。由於晶片在CMOS製程中，會產生晶片間的誤差，本論文對此設計了兩種補償電路，一個是脈衝起始限制電路，用來限制脈衝開始的時間，另一個是參考電流調整器，可線性調整電流鏡的參考電流，每階調整的幅度為0.1mA。另外對於發光二極體陣列每一顆發光二極體的不一致性，本晶片也提出補償的方法，採用一個輸出電流調整器，可針對個別的發光二極體驅動電流做些微的調整，每階調整的幅度為0.05mA。針對功率消耗的問題，也提出了兩種解決的方法，一個是多晶片分段曝光，另一個是單一晶片分段曝光，兩者皆可降低瞬間大電流的產生，並降低瞬間功率消耗。整個晶片是利用0.5um 2M1P CMOS製程所完成，經過量測後，本論文所設計的晶片可以確實的運作。

Using the LED Print Head for exposure structure inside the Photographic Printer has three benefits: First, it can make the volume of the printer smaller; second, it can make the printer more reliable; last, it can decrease the capital of the printer. But this kind of skill is always controlled by Japanese. To adapt the demand in internal market, this thesis designed a driver IC for LED Print Head. The driver IC adopts a skill of Pulse Width Modulation (PWM) which uses 4-bits digital signal to produce 16 kinds of pulse width under a fixed current. The thesis also designs a Dynamic Pulse Width circuit to make pulse width more variable. In CMOS process, the process will make each IC different. In order to solve this problem, the thesis provides two compensation circuits to compensate the difference between ICs. One is the Pulse Start-Limited circuit which is used to limit the start time of the pulse; the other is the Reference Current Control circuit which is used to adjust the reference current of the current mirror. The range of each adjustable reference current step is 0.1mA. As for the difference between each LED in the LED array itself, the thesis brings up a way to solve this problem. The way is adopting an Output Current Control circuit which can make some adjust in each LED drive current. The range of each adjustable output current step is 0.05mA. As to the problem of power consumption, the thesis also provides two solutions: One is Multi-Chip Segment Exposure, the other is Single-Chip Segment Exposure, both two can decrease the transient large current and the transient power consumption. The whole chip was completed by 0.5um 2M1P CMOS process. After testing, the drive chip can really work well.