某電(dian)腦顯(xian)(xian)示器(qi)產品，在正常(chang)顯(xian)(xian)示畫面時，對金屬端子進行±4KV接觸放電(dian)時出現(xian)自動待機現(xian)象(xiang)，反復驗證多次問題(ti)現(xian)象(xiang)確實存在，排除概率性、隨機性問題(ti)的可能(neng)性。

出(chu)現自動待(dai)機現象(xiang)時，重新(xin)按開(kai)關按鍵可以(yi)正常開(kai)機，電(dian)(dian)腦顯示器(qi)電(dian)(dian)路原(yuan)理框圖(tu)如下所(suo)示：

根據電(dian)腦顯示器電(dian)路(lu)原理框圖可知，出現(xian)(xian)自動(dong)待(dai)機現(xian)(xian)象(xiang)主(zhu)要與復位(wei)電(dian)路(lu)、DC-DC電(dian)源、按鍵控制(zhi)電(dian)路(lu)、顯示控制(zhi)芯片電(dian)路(lu)相關。引(yin)起系統出現(xian)(xian)自動(dong)待(dai)機現(xian)(xian)象(xiang)的條件(jian)如下：

12V轉5V DC-DC工(gong)作狀態異(yi)(yi)常(chang)，出現(xian)輸出電(dian)壓(ya)(ya)跌落，導致后端LD0、顯(xian)示控(kong)制芯片(pian)因工(gong)作電(dian)壓(ya)(ya)低于(yu)額定工(gong)作電(dian)壓(ya)(ya)出現(xian)異(yi)(yi)常(chang)，系統出現(xian)自(zi)動(dong)待機。顯(xian)示控(kong)制芯片(pian)的(de)(de)復位(wei)電(dian)路(lu)受到靜電(dian)放電(dian)噪(zao)聲干(gan)擾，導致復位(wei)電(dian)路(lu)的(de)(de)誤動(dong)作，引發系統錯誤的(de)(de)復位(wei)操作，系統出現(xian)自(zi)動(dong)待機。

按鍵(jian)控制電路(lu)受到(dao)靜電放電干(gan)擾(rao)，導(dao)(dao)致(zhi)按鍵(jian)誤觸發，發出錯誤的關機指令(ling)，系統出現自(zi)動待(dai)機。顯示控制芯(xin)片本身受到(dao)靜電放電干(gan)擾(rao)，導(dao)(dao)致(zhi)其工作(zuo)狀態出現異常，出現系統自(zi)動進入(ru)待(dai)機模式。

靜電(dian)(dian)放(fang)電(dian)(dian)過程中(zhong)使(shi)用示波(bo)器測(ce)量12V轉5V DC-DC輸出端5V電(dian)(dian)壓(ya)，發現出現瞬間電(dian)(dian)壓(ya)跌(die)(die)落(luo)(luo)到(dao)2.96V，反復測(ce)量多次確認DC-DC輸出端確實出現電(dian)(dian)壓(ya)跌(die)(die)落(luo)(luo)現象，12V轉5V DC-DC電(dian)(dian)路如下圖所示：

為確認是(shi)否因5V DC-DC輸出電(dian)(dian)(dian)壓跌(die)落(luo)引起系(xi)統自動待機問題，將DC-DC電(dian)(dian)(dian)源斷開(kai)使用穩壓源提供系(xi)統5V供電(dian)(dian)(dian)，靜電(dian)(dian)(dian)放(fang)電(dian)(dian)(dian)系(xi)統自動待機現象消失，確認是(shi)DC-DC輸出端電(dian)(dian)(dian)壓跌(die)落(luo)引起后(hou)端系(xi)統工作狀態異常，DC-DC輸出電(dian)(dian)(dian)壓在靜電(dian)(dian)(dian)放(fang)電(dian)(dian)(dian)過程(cheng)中測量波形如下圖所示：

DC-DC輸(shu)出(chu)端電(dian)(dian)(dian)(dian)壓(ya)跌落的原因主要有：EN腳(jiao)開啟(qi)電(dian)(dian)(dian)(dian)壓(ya)瞬間低(di)于額定(ding)電(dian)(dian)(dian)(dian)壓(ya)，DC-DC出(chu)現瞬間關斷輸(shu)出(chu)電(dian)(dian)(dian)(dian)壓(ya)跌落，后端電(dian)(dian)(dian)(dian)解電(dian)(dian)(dian)(dian)容(rong)(rong)(rong)容(rong)(rong)(rong)量較小，電(dian)(dian)(dian)(dian)壓(ya)補償來(lai)不(bu)及(ji)；DC-DC反(fan)饋環路受到靜(jing)電(dian)(dian)(dian)(dian)放(fang)電(dian)(dian)(dian)(dian)干擾(rao)，導(dao)致反(fan)饋信息出(chu)錯，DC-DC芯片(pian)做出(chu)誤調整引起輸(shu)出(chu)電(dian)(dian)(dian)(dian)壓(ya)瞬間跌落，后端電(dian)(dian)(dian)(dian)解電(dian)(dian)(dian)(dian)容(rong)(rong)(rong)容(rong)(rong)(rong)量較小，電(dian)(dian)(dian)(dian)壓(ya)補償來(lai)不(bu)及(ji)。

根據(ju)DC-DC電(dian)(dian)路設計可知，DC-DC芯片EN腳采用(yong)12V電(dian)(dian)壓輸入被RD7/RD4電(dian)(dian)阻(zu)分壓，其中RD4電(dian)(dian)阻(zu)參數(shu)為(wei)(wei)20Kohm,RD7電(dian)(dian)阻(zu)為(wei)(wei)100Kohm，通(tong)過(guo)計算得出(chu)EN腳的(de)供電(dian)(dian)電(dian)(dian)壓為(wei)(wei)12V*（20/100+20）=2V。

根據DC-DC芯(xin)片(pian)規(gui)格(ge)書上(shang)對(dui)EN腳描(miao)述說明可知，EN腳需(xu)要(yao)(yao)高(gao)電(dian)(dian)平(ping)(ping)開(kai)啟，建議設(she)(she)計(ji)是直接接100Kohm電(dian)(dian)阻上(shang)拉到輸出電(dian)(dian)源Vin，而實際上(shang)的電(dian)(dian)路(lu)設(she)(she)計(ji)不符合芯(xin)片(pian)規(gui)格(ge)書要(yao)(yao)求(qiu)。根據數(shu)字(zi)電(dian)(dian)路(lu)高(gao)、低(di)電(dian)(dian)平(ping)(ping)定(ding)(ding)義，此電(dian)(dian)平(ping)(ping)值處于不確定(ding)(ding)電(dian)(dian)平(ping)(ping)段，極易導致芯(xin)片(pian)誤判。（數(shu)字(zi)電(dian)(dian)路(lu)電(dian)(dian)平(ping)(ping)定(ding)(ding)義：≦0.7V為低(di)電(dian)(dian)平(ping)(ping)，≧2.8V為高(gao)電(dian)(dian)平(ping)(ping)，0.7V≦開(kai)啟電(dian)(dian)壓≦2.8V應(ying)避免）

仔(zi)細查(cha)閱芯片(pian)規(gui)格書可(ke)知(zhi)EN腳工作最(zui)低(di)電(dian)壓要求為1.5V，實際工作電(dian)壓設(she)置為2V，處于臨界(jie)值日，非常不安全，將EN腳下拉電(dian)阻RD4移除，電(dian)壓達到12V，靜電(dian)放電(dian)測試(shi)時自(zi)動待機現象消失。

12V轉(zhuan)5V DC-DC是系(xi)統主電源，由于(yu)EN腳分壓電阻參數選擇不當，實(shi)(shi)際工(gong)作電壓為2V，而EN腳工(gong)作最低電壓要求(qiu)為1.5V，實(shi)(shi)際工(gong)作電壓處于(yu)SPEC要求(qiu)的下限(xian)。

靜(jing)電(dian)(dian)(dian)放電(dian)(dian)(dian)電(dian)(dian)(dian)流流過(guo)(guo)參(can)考(kao)地平面(mian)時，引(yin)起參(can)考(kao)地電(dian)(dian)(dian)位波(bo)(bo)動，DC-DC EN腳參(can)考(kao)電(dian)(dian)(dian)壓(ya)(ya)(ya)跟隨地電(dian)(dian)(dian)位波(bo)(bo)動，波(bo)(bo)動范圍超過(guo)(guo)DC-DC芯(xin)(xin)片EN腳開啟電(dian)(dian)(dian)壓(ya)(ya)(ya)容(rong)限，出現DC-DC芯(xin)(xin)片被(bei)誤關閉引(yin)起輸出電(dian)(dian)(dian)壓(ya)(ya)(ya)跌(die)落，后端(duan)電(dian)(dian)(dian)解電(dian)(dian)(dian)容(rong)容(rong)量較(jiao)小，來不及補償(chang)輸出電(dian)(dian)(dian)壓(ya)(ya)(ya)，導致后端(duan)電(dian)(dian)(dian)路因(yin)輸出電(dian)(dian)(dian)壓(ya)(ya)(ya)低于額(e)定(ding)工作電(dian)(dian)(dian)壓(ya)(ya)(ya)，系統出現自動待機現象。

根據(ju)問題原(yuan)因分析，試(shi)驗驗證結(jie)果，結(jie)合DC-DC芯片規(gui)格書使用說明要求(qiu)，結(jie)合成本(ben)最優(you)原(yuan)則(ze)，問題解決方(fang)案如(ru)下(xia)：

問題解決方(fang)案（一）：將DC-DC芯片(pian)EN腳(jiao)對地分壓電(dian)阻參數由20K改(gai)為47K，工(gong)(gong)作電(dian)壓由2V調(diao)整到3.6V，使DC-DC EN腳(jiao)實際工(gong)(gong)作電(dian)壓在(zai)芯片(pian)規格書允(yun)許的安全范圍內。

問(wen)題解決(jue)(jue)方案(an)（二）：將DC-DC芯片(pian)EN腳(jiao)(jiao)對(dui)地分壓(ya)電(dian)阻移除(chu)，工作(zuo)電(dian)壓(ya)由(you)2V調整到6.67V，使DC-DC EN腳(jiao)(jiao)實際工作(zuo)電(dian)壓(ya)在芯片(pian)規格(ge)書允許的安全范圍內。（解決(jue)(jue)方案(an)二）綜(zong)合(he)評(ping)估(gu)采(cai)用(yong)解決(jue)(jue)方案(an)（一(yi)）

關于DC-DC芯片電(dian)路設(she)計應(ying)嚴格遵(zun)守芯片應(ying)用手(shou)冊，尤(you)其(qi)是EN腳(jiao)、反饋信號、輸(shu)(shu)(shu)出(chu)端濾波電(dian)感、電(dian)解電(dian)容(rong)關鍵器件選型。本案例DC-DC輸(shu)(shu)(shu)出(chu)端電(dian)解電(dian)容(rong)參數由(you)100uF改(gai)為470uF甚至(zhi)更大容(rong)量電(dian)容(rong)時輸(shu)(shu)(shu)出(chu)電(dian)壓跌落問題就不是非常明顯，則(ze)很容(rong)易忽略，很難被發現。