當今許多靠鋰離子電池供電的手持設備都內建有一個電池充電器，目前通常使用一種線(xian)性(xing)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器給這種電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)，但這種充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器盡管操(cao)作更(geng)簡(jian)便和(he)成本(ben)更(geng)低(di)，但它有一個主要缺點：能耗太高。當輸(shu)入電(dian)(dian)(dian)(dian)壓為高和(he)電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓為低(di)(已放電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池(chi))時，線(xian)性(xing)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器產生的熱(re)量足以損壞自(zi)身或其它鄰近元(yuan)件。在本(ben)文中，凌(ling)特技術公司提出了一種替代方案(an)，它可解(jie)決所有的過熱(re)問題而(er)且充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)快速(su)。

　　當(dang)今(jin)許(xu)多靠(kao)電(dian)(dian)(dian)(dian)池(chi)供電(dian)(dian)(dian)(dian)的(de)(de)手持(chi)設(she)(she)備(bei)都內建有(you)一(yi)個電(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)器(qi)(qi)。這(zhe)(zhe)些(xie)電(dian)(dian)(dian)(dian)池(chi)通(tong)常都為(wei)鋰離子電(dian)(dian)(dian)(dian)池(chi)，并且(qie)其(qi)功率密度可(ke)在范圍400mA小(xiao)時(shi)到(dao)1.5A小(xiao)時(shi)變化。你(ni)可(ke)以使用一(yi)種(zhong)線(xian)性充電(dian)(dian)(dian)(dian)器(qi)(qi)給這(zhe)(zhe)種(zhong)電(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)，而(er)且(qie)這(zhe)(zhe)種(zhong)充電(dian)(dian)(dian)(dian)器(qi)(qi)通(tong)常比基于轉換器(qi)(qi)的(de)(de)方(fang)案在外形上明顯更小(xiao)，操(cao)作更簡便，成本更低(di)，但它有(you)一(yi)個主要(yao)缺點(dian)：能耗太高。當(dang)輸入電(dian)(dian)(dian)(dian)壓(ya)為(wei)高和電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓(ya)為(wei)低(di)(已放(fang)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池(chi))時(shi)，線(xian)性充電(dian)(dian)(dian)(dian)器(qi)(qi)產生的(de)(de)熱量足以損壞(huai)自身(shen)或(huo)其(qi)它鄰近元(yuan)件。典型地，這(zhe)(zhe)種(zhong)狀(zhuang)態(即電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓(ya)隨(sui)充電(dian)(dian)(dian)(dian)的(de)(de)進(jin)行(xing)而(er)升高)只是(shi)(shi)暫時(shi)現象，但正是(shi)(shi)這(zhe)(zhe)些(xie)最糟糕的(de)(de)狀(zhuang)態需要(yao)設(she)(she)計師在決定充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流與IC溫度的(de)(de)最大容許(xu)值(zhi)時(shi)必須充分地加(jia)以考(kao)慮。當(dang)然，一(yi)種(zhong)解(jie)決這(zhe)(zhe)種(zhong)過熱問題(ti)的(de)(de)簡單方(fang)法是(shi)(shi)降低(di)充電(dian)(dian)(dian)(dian)過程中整(zheng)個恒(heng)流段的(de)(de)充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流，但這(zhe)(zhe)種(zhong)方(fang)法伴(ban)隨(sui)的(de)(de)問題(ti)是(shi)(shi)充電(dian)(dian)(dian)(dian)時(shi)間(jian)的(de)(de)相應延長。

　　不過，凌特技術公司現在提出了一種替代方案，即LTC1733鋰離子單電池線性充電器，它解決了所有的過熱問題且充電快速。這種新穎的IC采用內部反饋來調節充電電流和限制裸片溫度。這意味著更快的充電時間，因為設計師可以通過編程設定一個很高充電電流，而且不會冒任何損壞IC或任何其它元件的風險。這一性能直接帶來的另一好處是毋需設計過熱保護電路。為了進一步改進熱傳遞性，LTC1733采用了熱增強型10針MSOP封裝。該IC只需要3個外部元件便可提供一個完整的鋰離子充電器解決方案。

　　一(yi)個內部(bu)功率MOSFET允許編程設定的(de)(de)最大充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)可(ke)(ke)(ke)達1.5A，精度(du)為7%，以(yi)確保充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)快速和完全(quan)。這(zhe)(zhe)種內部(bu)MOSFET還(huan)可(ke)(ke)(ke)省掉一(yi)個外部(bu)電(dian)(dian)(dian)(dian)流(liu)敏(min)(min)感電(dian)(dian)(dian)(dian)阻或(huo)遏流(liu)二極(ji)管，最終的(de)(de)無負載(zai)電(dian)(dian)(dian)(dian)壓是引(yin)腳可(ke)(ke)(ke)選(xuan)的(de)(de)，或(huo)為4.1V或(huo)為 4.2V，精度(du)是1%，這(zhe)(zhe)可(ke)(ke)(ke)防止過充(chong)(chong)(chong)帶(dai)來的(de)(de)危險或(huo)欠充(chong)(chong)(chong)造成的(de)(de)電(dian)(dian)(dian)(dian)池(chi)(chi)容量不足(zu)。遵從電(dian)(dian)(dian)(dian)池(chi)(chi)制(zhi)造商的(de)(de)使用(yong)要求，LTC1733包(bao)(bao)含了一(yi)個可(ke)(ke)(ke)編程充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)終止定時器(qi)和熱敏(min)(min)電(dian)(dian)(dian)(dian)阻輸入，以(yi)保證充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)全(quan)過程的(de)(de)溫度(du)符合要求。狀態輸出包(bao)(bao)括：用(yong)以(yi)指(zhi)示充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)后期狀態的(de)(de)C/10充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)檢(jian)(jian)測、決定充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)是否(fou)可(ke)(ke)(ke)以(yi)進行下(xia)去的(de)(de)壁式(shi)電(dian)(dian)(dian)(dian)源適配器(qi)狀態檢(jian)(jian)測、充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)監測、以(yi)及用(yong)以(yi)識別壞電(dian)(dian)(dian)(dian)池(chi)(chi)的(de)(de)錯誤(wu)檢(jian)(jian)測。低電(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)壓充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)調(diao)理電(dian)(dian)(dian)(dian)路(lu)(滴充(chong)(chong)(chong))可(ke)(ke)(ke)安(an)全(quan)地為過放電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)，并自動續充(chong)(chong)(chong)以(yi)確保電(dian)(dian)(dian)(dian)池(chi)(chi)總能(neng)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)完全(quan)。當沒有壁式(shi)電(dian)(dian)(dian)(dian)源適配器(qi)或(huo)該部(bu)件(jian)關閉(bi)的(de)(de)時候，為保持電(dian)(dian)(dian)(dian)池(chi)(chi)能(neng)量， LTC1733的(de)(de)電(dian)(dian)(dian)(dian)池(chi)(chi)漏電(dian)(dian)(dian)(dian)流(liu)降(jiang)到5(A以(yi)下(xia)。

　　給電池充電
　　給單電(dian)(dian)池鋰(li)離子電(dian)(dian)池系統充(chong)電(dian)(dian)時，用(yong)(yong)戶(hu)須在VCC腳處施加至少4.5V的輸入(ru)電(dian)(dian)壓。ACPR腳隨(sui)后降(jiang)為(wei)(wei)低電(dian)(dian)平(pull low)，指(zhi)示(shi)輸入(ru)電(dian)(dian)壓條件已滿足(見(jian)圖2)。此外，必須用(yong)(yong)一個1%的電(dian)(dian)阻(zu)器把(ba)PROG與GND相(xiang)連(lian)，使額定(ding)充(chong)電(dian)(dian)電(dian)(dian)流設定(ding)為(wei)(wei)100V/RPROG。而后CHPG腳為(wei)(wei)低電(dian)(dian)平，指(zhi)示(shi)充(chong)電(dian)(dian)循環開始。TIMER和GND之(zhi)間(jian)用(yong)(yong)電(dian)(dian)容(rong)器連(lian)接，充(chong)電(dian)(dian)終(zhong)止時間(jian)設定(ding)為(wei)(wei)3小時/100nF。

　　如果在充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)循(xun)(xun)環(huan)開始時BAT腳(jiao)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)低于2.48V，那么，充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流將(jiang)(jiang)為(wei)設(she)定值的(de)1/10，以(yi)(yi)使(shi)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)足夠高，安全實現(xian)(xian)全充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(見圖(tu)3)。如果電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)損壞(huai)(huai)，且在1/4的(de)設(she)定終止(zhi)時間內電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)都升不到(dao)2.48V以(yi)(yi)上，則(ze)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)循(xun)(xun)環(huan)終止(zhi)，且FAULT狀態輸出將(jiang)(jiang)鎖存(cun)為(wei)低電(dian)(dian)(dian)(dian)(dian)(dian)平，指示電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)已壞(huai)(huai)。ACPR、CHRG和FAULT三個狀態輸出腳(jiao)全都產生足夠的(de)電(dian)(dian)(dian)(dian)(dian)(dian)流衰(shuai)弱(ruo)量，使(shi)LED亮燈。一旦電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)升到(dao)2.48V以(yi)(yi)上(充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)循(xun)(xun)環(huan)開始后不久會(hui)明顯表現(xian)(xian)出來)，LTC1733將(jiang)(jiang)為(wei)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)提供由RPROG設(she)定的(de)恒定電(dian)(dian)(dian)(dian)(dian)(dian)流。LTC1733將(jiang)(jiang)一直保持恒流模(mo)(mo)式(shi)，直到(dao)BAT腳(jiao)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)接(jie)近所選擇的(de)最終浮置電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(SEL=0V時為(wei) 4.1V ，SEL=VCC時為(wei)4.2V)為(wei)止(zhi)。此后，器(qi)件進入恒壓(ya)(ya)模(mo)(mo)式(shi)。

　　在恒壓模式下，LTC1733將(jiang)開始(shi)降低充電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)以保持BAT腳的(de)恒壓而非恒流(liu)輸出。當電(dian)(dian)(dian)流(liu)降至最大(da)設(she)定充電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)的(de)10%時，內部(bu)比較器會斷開CHRG腳，并將(jiang)一(yi)弱(ruo)電(dian)(dian)(dian)流(liu)源(yuan)(約25(A)接(jie)地，以顯(xian)示接(jie)近充電(dian)(dian)(dian)結(jie)束(shu)(C/10)狀態。

　　與電流達到C/10時便結束充電過程的電池充電器不同，LTC1733在(zai)到達C/10點后(hou)，只要終止(zhi)定時(shi)器(qi)時(shi)間(jian)不到，會繼續給電(dian)(dian)池充(chong)(chong)電(dian)(dian)，以保(bao)證電(dian)(dian)池完全充(chong)(chong)滿(man)。在(zai)C/10處終止(zhi)充(chong)(chong)電(dian)(dian)過程使(shi)電(dian)(dian)池只能(neng)充(chong)(chong)到其容(rong)量的(de)90%到95%，而在(zai)C/10后(hou)繼續充(chong)(chong)并按時(shi)間(jian)要求(qiu)結束可(ke)使(shi)電(dian)(dian)池的(de)容(rong)量充(chong)(chong)到100%。一旦充(chong)(chong)電(dian)(dian)結束，CHRG腳便呈現(xian)高阻(zu)抗狀(zhuang)態。

    電池的再充電
    假定在第一次充電(dian)(dian)(dian)(dian)時(shi)，電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)已充到(dao)了3.95V(SEL = 0V時(shi)) 或(huo)(huo)4.05V (SEL = VCC時(shi))以上，那么LTC1733可對電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)進行再充電(dian)(dian)(dian)(dian)。一旦超出這些閾值，如果(guo)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)因電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)負載(zai)的存在或(huo)(huo)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的自放電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流而掉到(dao)3.9V (SEL = 0V)或(huo)(huo)4.0V (SEL = VCC)以下時(shi)，則新(xin)一輪的充電(dian)(dian)(dian)(dian)循(xun)環開始了。再充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)路集結BAT腳電(dian)(dian)(dian)(dian)壓(ya)數毫(hao)秒，以防(fang)止(zhi)充電(dian)(dian)(dian)(dian)循(xun)環重(zhong)啟造成(cheng)的瞬態沖擊。

    熱調節
　　LTC1733的(de)另一個特(te)性(xing)是具(ju)有內部熱(re)調節回路。如果高功率(lv)操作(zuo)或(huo)高的(de)環境溫度導致LTC1733的(de)溫度接(jie)近(jin)105(C，那么充(chong)電電流(liu)會自動降低(di)以將溫度保(bao)持在105(C左(zuo)右 (板溫度通常低(di)于85(C)，這便是所謂(wei)的(de)恒溫模式。這一特(te)性(xing)允許用(yong)戶基于具(ju)體的(de)工作(zuo)條件設定充(chong)電電流(liu)，并(bing)去掉許多(duo)線性(xing)應用(yong)必(bi)需(xu)的(de)復(fu)雜過(guo)熱(re)保(bao)護(hu)設計。

　　LTC1733會(hui)自動(dong)監(jian)視最糟狀態，這一特性(xing)除可(ke)保(bao)護LTC1733外，還能去除板上的(de)“熱(re)點”區，從而保(bao)護周邊元(yuan)件。其(qi)它的(de)電(dian)(dian)池充電(dian)(dian)器，其(qi)熱(re)關閉(bi)特性(xing)只是(shi)簡單地在極高溫(wen)(wen)度下(xia)(一般是(shi)150(C)關閉(bi)充電(dian)(dian)器而已(yi)。這種基(ji)于溫(wen)(wen)度的(de)關閉(bi)特性(xing)允許電(dian)(dian)池充電(dian)(dian)器和(he)周邊PCB板溫(wen)(wen)度升至非常高，盡管具備關閉(bi)“保(bao)護”功能，但必須對應(ying)用進行(xing)仔(zi)細設計(ji)以避免(mian)到(dao)達熱(re)關閉(bi)溫(wen)(wen)度。LTC1733通過自動(dong)均衡(heng)充電(dian)(dian)電(dian)(dian)流、散熱(re)和(he)工作溫(wen)(wen)度，可(ke)使(shi)設計(ji)得到(dao)簡化(hua)。

　　為進一步改進LTC1733的(de)熱(re)(re)(re)調節性(xing)能，它采用(yong)了熱(re)(re)(re)增強型MSOP封裝。其占用(yong)板面積可做(zuo)到(dao)小于72 mm2，室溫功耗2W。這(zhe)相當于用(yong)5V輸(shu)入電源(yuan)實現最大充(chong)(chong)電電流(liu)1.5A。這(zhe)里(li)假定鋰(li)離子電池在充(chong)(chong)電大部分時間(jian)里(li)處(chu)于(3.7V狀態(tai)。實際(ji)上，由于典型的(de)鋰(li)離子電池在充(chong)(chong)電開始幾分鐘(zhong)內便升到(dao)3.8V以上，因此這(zhe)一假定有些保守(shou)。LTC1733的(de)這(zhe)一獨特(te)熱(re)(re)(re)特(te)性(xing)和7%的(de)設定充(chong)(chong)電電流(liu)精(jing)(jing)度，使單(dan)電池鋰(li)離子電池系(xi)統(tong)的(de)充(chong)(chong)電過程非常迅速(su)和精(jing)(jing)確。

　　PROG電流監控器
　　在恒(heng)(heng)流(liu)模式下，PROG腳(jiao)(jiao)的電壓總為1.5V，指示設定的充電電流(liu)從BAT腳(jiao)(jiao)流(liu)出。在恒(heng)(heng)溫或恒(heng)(heng)壓模式下，BAT腳(jiao)(jiao)的電流(liu)有所下降。PROG腳(jiao)(jiao)與三個開漏狀態輸出(ACPR、CHRG和 FAULT)時刻通告用戶(hu)LTC1733到底在做什么。

　　NTC熱敏電阻
　　除了(le)可編(bian)程定(ding)時(shi)(shi)器和低的電(dian)池充(chong)電(dian)限定(ding)條件外，LTC1733在推(tui)薦給電(dian)池制造商的安全性(xing)特性(xing)列表中(zhong)還增(zeng)加了(le)充(chong)電(dian)時(shi)(shi)的溫(wen)度限定(ding)。電(dian)池溫(wen)度通過緊貼電(dian)池組放置一(yi)只負溫(wen)度系數(NTC)熱敏電(dian)阻(zu)來測定(ding)。借(jie)助IC中(zhong)的內部溫(wen)度限制電(dian)路(lu)，LTC1733在電(dian)池溫(wen)度降到0(C以(yi)下或升到50(C以(yi)上時(shi)(shi)，可臨時(shi)(shi)停(ting)住(zhu)內部定(ding)時(shi)(shi)器并停(ting)止充(chong)電(dian)。實(shi)施這(zhe)一(yi)功能，選用(yong)RHOT的電(dian)阻(zu)值應與50(C下所用(yong)熱敏電(dian)阻(zu)的阻(zu)值相同(tong)。這(zhe)樣將可以(yi)確保內部比較(jiao)器的1/2VCC摔(shuai)點(trip point)與NTC的50(C溫(wen)度相對(dui)應。

　　還有，選定的NTC熱敏電阻在0(C時的阻值應盡量接近50(C時阻值的7倍。7：1的冷熱NTC比率可確保內部比較器7/8VCC的摔點與NTC 0(C的溫度相對應。每個熱和冷比較器都有約2(C的滯后以防摔點的振蕩。另外，NTC可以不使用任何外部元件，只簡單地通過將NTC腳接地就可使其功能失效。
 
　　結論
　　LTC1733是一種全功能的獨立鋰離子電池充電器。其結構極為簡單，只需要三個外部元件，就可安全精確地將高容量電池快充至1.5A的充電電流。在體現安全性及狀態特性方面，可加上一只NTC熱敏電阻和幾個LED。目前許多用鋰離子電池作供電電源的設備可采用這種類型的充電器，如數字靜態照相機(DSC)、個人數字助理(PDA)、蜂窩電話、MP3播放機以及GPS系統等。
 
　　值得注意的是，對上述諸多產品中有許多產品，有一種需求越來越強烈。就是通過ＵＳＢ接口插入一個主設備時，電池正在充電。盡管LTC1733可用于這類充電應用，但USB端口的可獲電流量有限，或500mA 或 100mA，依連接外設的數量而定。而在這些應用中LTC1733存在的一個缺陷是關閉模式下的靜態電流為0.9mA。這是一個難點，因為USB規范定義了一種暫停(SUSPEND)模式，即在USB+5V的情況下，漏電電流必須低于500(A。不過，凌特技術公司還有LTC4053，一種與USB兼容的鋰離子電池充電器。它在關閉模式下源電流僅為25(A。所以，當外設在USB請求暫停模式時，將中LTC4053，以滿足所要求的漏電電流限制。
 
    凌特公司（Linear Technology）推出獨立的開關模式鋰離子電池充電器 LTC4002，工作電源電壓可高達 24V，從而不需要 9V 和更高電壓墻上適配器所需的輸入瞬變保護。另外，器件的 4.7V 低輸入電源以及低壓差工作特性保證了 4.2V 鋰離子電池可從 5V 墻上適配器持續充電。為了實現安全自主的充電控制，LTC4002 還包括自動關機、電池預查驗、電池溫度檢測、充電結束指示以及一個充電 3 小時終止定時器等特性。
 
　　與遲滯拓樸結構充電器相比，LTC4002 恒定的 500kHz 工作頻率和電流模式結構允許使用更小的電感器和電容器。由于 100mV 的低檢測電壓，所以很容易達到超過86%。這種獨立的充電器有兩種封裝，3mm×3mm DFN 和 8 引線 SO。LTC4002 可應用于手持式儀表和通信裝置、充電座、便攜式計算機以及電池備份系統。
 
　　LTC4002 是一種高效率電流模式 PWM 控制器，開關頻率為 500kHz。它驅動一個外部 P 溝道 MOSFET，以最高 87% 的效率提供高達 2A 的充電電流。這種集成電路的恒定開關頻率將噪聲和濾波需求減至最低。配合使用各類輸出電容器（包括低 ESR 陶瓷電容器），LTC4002 都是穩定的。一個內部比較器檢測充電結束電流，而集成的定時器設定總充電時間，并在三小時后終止充電。LTC4002 自動進入休眠模式，使電池耗用電流降至 10uA。而且，當電池電壓降至 4.05V 以下時，這種集成電路還能自動開始充電
。
　　LTC4002 的工作溫度定在 －40°C 到 85°C。以 1000 片為單位批量購買，每片起價為 1.95美元。
 
　　LTC4002 性能概要：