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IC693MDL734

IC693MDL734

IC693MDL734

IC693MDL734

該智能轉(zhuǎn)換器必須解決兩個關鍵問題即如何從RS-232線上獲得電路和RS-485/RS-422接口驅(qū)動所需的功率和如何智能控制RS-485/RS-422的收發(fā)使能
3.1.電源方案
標準的RS-232 定義中有三個發(fā)送信號TXD RTS 和DTR 每根線上的典型輸出電流為8mA/ 12V 考慮到TXD為負電平處于停止發(fā)送或發(fā)送數(shù)字1 時的時間較多因而電源轉(zhuǎn)換決定采用負電源輸入以最大限度地增加電源輸入功率升壓至所需的工作電源從RTS和DTR上輸入功率=2*8*12mW=192mW,另外由于通訊為間歇工作方式所以輸入電源端的儲能電容和TXD 為負電平時能夠補充一定的功率假設我們設計一個效率為85% 輸出電壓為3V 的DC-DC 轉(zhuǎn)換器則輸出電流可達54.4mA
3.2.智能控制收發(fā)使能
RS-232通訊接口采用電平方式傳輸適用于點-點通訊無須專門的收發(fā)使能控制而對于RS-485/RS-422通訊接口則不同由于采用差分電平方式傳輸且允許在一條通訊總線上掛接多個節(jié)點必然要求各個節(jié)點能夠獨立地控制總線驅(qū)動器關斷或打開保證不會影響到其它節(jié)點的正常通訊為了簡化與轉(zhuǎn)換器RS-232 接口端相連的軟件工作更重要的是為了提高本轉(zhuǎn)換器的通用性和靈活性即插即用無須要求用戶更改任何相關軟件和硬件本轉(zhuǎn)換器內(nèi)置微處理器實現(xiàn)收發(fā)使能的智能控制具體方法微處理器在檢測到UART 的通信起始位后打開發(fā)送使能允許串行數(shù)據(jù)發(fā)送至RS-485/RS-422 通訊網(wǎng)絡微處理器根據(jù)所設定的波特率延時至UART 停止位發(fā)送一半時例如11位格式時延時10.5T,T=1/fBAUD ,開始檢測是否有下一個起始位到來在時間T內(nèi)若有下一個起始位到來則保持發(fā)送狀態(tài)否則將關閉發(fā)送使能結束數(shù)據(jù)發(fā)送

4. 硬件設計
由于本轉(zhuǎn)換器供電來自RS-232信號線其輸入功率受到限制因而在本設計中將盡可能地采用+3V供電的低功耗器件保證總電流小于54.4mA 主要包括4個部分DC-DC轉(zhuǎn)換器RS-232接口RS-485/RS-422接口和微處理器分別介紹如下
4.1. DC-DC轉(zhuǎn)換器
顯然還沒有一個DC-DC 轉(zhuǎn)換器能夠直接實現(xiàn)-12V 輸入+3V 輸出的IC 但是如果我們利用現(xiàn)有的IC 稍作改動即可實現(xiàn)該功能圖2 所示的DC-DC 轉(zhuǎn)換電路就是利用MAX761 實現(xiàn)的-12V 輸入+3V 輸出效率高于85%的升壓DC-DC 轉(zhuǎn)換器該轉(zhuǎn)換器實際輸入電壓范圍為-2.5V 至-13.5V 靜態(tài)工作電流僅I1=120 A 具有輸出電流大于54.4mA的能力如果前端輸入功率未受到限制則輸出電流可達300mA以上由于MAX761采用高效率的PFM 控制方式,而且在本電路中,開關損耗較小(因為開關電流小于負載電流),所以能夠達到比MAX761 典型應用更高的效率(MAX761 典型應用效率為86%) 輸出電壓由下列方程確定
VOUT=VREF*R1/R2+0.7(V) 其中VREF=1.5V
選取R2=100K 根據(jù)所需要的輸出電壓計算R1

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