作為控制鋰離子充電狀態的功率器件，電池充電器芯片(pian)在便攜式系統(tong)設計中(zhong)扮(ban)演著(zhu)重要的角色。然而(er)，設計師們還在使(shi)用著(zhu)從較(jiao)老并且相對粗(cu)糙(cao)和低成(cheng)本的充電器件、到較(jiao)新的更復(fu)雜芯片(pian)等(deng)各種(zhong)各樣的器件，而(er)后(hou)者集成(cheng)了越(yue)來越(yue)復(fu)雜的智能，可以(yi)延長電池壽命，保護被(bei)充電系統(tong)不受損害(hai)。

　　安全第一

　　便攜式系(xi)統設計師(shi)面臨的挑戰是：如何快(kuai)速高效(xiao)地對系(xi)統進(jin)行充(chong)電(dian)(dian)(dian)，同時不影響用(yong)(yong)戶安全或損壞電(dian)(dian)(dian)池，并且要(yao)占用(yong)(yong)最少的電(dian)(dian)(dian)路板面積。鋰離子充(chong)電(dian)(dian)(dian)器通(tong)常采用(yong)(yong)三種充(chong)電(dian)(dian)(dian)模式來(lai)最大化充(chong)電(dian)(dian)(dian)效(xiao)率(lv)并確保用(yong)(yong)戶安全性。深度放電(dian)(dian)(dian)的電(dian)(dian)(dian)池必須先用(yong)(yong)較小的電(dian)(dian)(dian)流進(jin)行逐步預充(chong)電(dian)(dian)(dian)，將電(dian)(dian)(dian)池電(dian)(dian)(dian)壓慢慢提升(sheng)到安全閥值之上。

　　就拿工作電壓范圍是2.8V到4.2V的單節鋰離子電池充電器來(lai)說，它通常會(hui)測量被(bei)充電池的電壓(ya)，當電池電壓(ya)低(di)于3V時，它將進(jin)入預充或“慢充”模式(shi)。慢充模式(shi)可以(yi)在電池內阻很低(di)時防(fang)止(zhi)對電池高(gao)速(su)充電，從(cong)而盡量減少(shao)發熱。一(yi)般預充期間(jian)的充電速(su)率約是全速(su)充電時的10%。

　　圖1：Microchip科技公司的(de)MCP73837雙輸(shu)入鋰離子/聚(ju)合物充電器具(ju)有電源選擇功(gong)能，可以(yi)自動選擇從交流或USB源進(jin)行充電。

　　一(yi)旦(dan)電(dian)(dian)(dian)(dian)(dian)池達到最小(xiao)電(dian)(dian)(dian)(dian)(dian)壓(ya)值(zhi)，充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器就切換到恒(heng)流(liu)(liu)(liu)或快充(chong)(chong)模式(shi)，此時將對電(dian)(dian)(dian)(dian)(dian)池全速充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)，直(zhi)到電(dian)(dian)(dian)(dian)(dian)池達到接近滿充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)壓(ya)的第二(er)個閥值(zhi)。隨后充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器將進入恒(heng)壓(ya)模式(shi)，此時電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓(ya)保持不(bu)變，從而讓(rang)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)逐漸停止。當充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)小(xiao)于(yu)預設的終止閥值(zhi)時，恒(heng)壓(ya)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)模式(shi)也就結束了。

　　熱(re)管理是電(dian)(dian)(dian)池充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)設計師面(mian)臨的(de)(de)(de)另一大挑(tiao)戰(zhan)。每個充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)芯(xin)(xin)片都(dou)(dou)會在充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)過(guo)程(cheng)中經歷由于(yu)散(san)熱(re)引起的(de)(de)(de)電(dian)(dian)(dian)壓(ya)下降。為了避免電(dian)(dian)(dian)池損壞(huai)或系統關斷(duan)，大多(duo)數充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)都(dou)(dou)集成(cheng)了某種形式的(de)(de)(de)控制機制來(lai)管理熱(re)量(liang)累積。較(jiao)老的(de)(de)(de)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)芯(xin)(xin)片常(chang)采用(yong)“非彼即此”的(de)(de)(de)方(fang)法來(lai)解決過(guo)熱(re)或過(guo)流(liu)(liu)(liu)問題――當熱(re)量(liang)達到(dao)預設的(de)(de)(de)門限(xian)時它們將簡(jian)單地中斷(duan)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)過(guo)程(cheng)。較(jiao)新的(de)(de)(de)器(qi)件則采用(yong)更復(fu)雜(za)的(de)(de)(de)反饋(kui)技術連續監視裸片溫度，并(bing)以正比于(yu)環境溫度變化的(de)(de)(de)速率動態或通(tong)過(guo)計算調整充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)(liu)(liu)。這種內(nei)置智能(neng)允許當前的(de)(de)(de)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)芯(xin)(xin)片逐(zhu)漸減少充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)(liu)(liu)，直到(dao)達到(dao)熱(re)平衡(heng)，裸片溫度停(ting)止上升。這種技術能(neng)讓充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)以最大可能(neng)的(de)(de)(de)電(dian)(dian)(dian)流(liu)(liu)(liu)對電(dian)(dian)(dian)池進行連續充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)，同時不會導(dao)致系統關斷(duan)，從而(er)縮短電(dian)(dian)(dian)池充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)時間。

　　MAX8804是美(mei)信集(ji)成產品公(gong)司去年7月(yue)份推(tui)出的(de)(de)充電(dian)器芯片(pian)，它采用(yong)了(le)專(zhuan)門(men)的(de)(de)熱(re)(re)調整電(dian)路，可以在快(kuai)速充電(dian)階段或(huo)系統處于高(gao)溫(wen)環(huan)境中限制裸片(pian)的(de)(de)溫(wen)度(du)。該充電(dian)器耐受30V的(de)(de)直(zhi)流(liu)輸入(ru)電(dian)壓，只(zhi)占(zhan)用(yong)6平方毫米的(de)(de)電(dian)路板(ban)面積(ji)。另外(wai)，像TI公(gong)司提供的(de)(de)1A bq24060充電(dian)器可提供熱(re)(re)過載保(bao)護(hu)(hu)功能，允許器件在環(huan)境溫(wen)度(du)很高(gao)的(de)(de)惡劣(lie)環(huan)境中連續運行，例如(ru)夏(xia)天的(de)(de)汽車或(huo)不小心(xin)連接到(dao)具(ju)有較高(gao)輸入(ru)電(dian)壓的(de)(de)其它適配器。目(mu)前大多數較新(xin)的(de)(de)器件一般還會增加過壓保(bao)護(hu)(hu)機制。

　　多種電壓源

　　有個越(yue)來越(yue)常見(jian)的(de)要求是(shi)從不同(tong)電(dian)(dian)壓(ya)源給電(dian)(dian)池充(chong)(chong)電(dian)(dian)，要點是(shi)在(zai)(zai)不使電(dian)(dian)壓(ya)源過(guo)載的(de)情(qing)況下提供這個功能(neng)(neng)。可以(yi)通過(guo)在(zai)(zai)輸入條件(jian)改(gai)變時動態(tai)調整充(chong)(chong)電(dian)(dian)電(dian)(dian)流實現這一功能(neng)(neng)。

　　舉例(li)來說，移動用戶經常沒有時(shi)間去找交流電(dian)(dian)插座來給他(ta)們(men)的(de)設(she)備(bei)充(chong)電(dian)(dian)，相反，他(ta)們(men)希望通(tong)過大多數電(dian)(dian)子(zi)設(she)備(bei)上(shang)都有的(de)眾多USB端口(kou)來給他(ta)們(men)的(de)電(dian)(dian)池充(chong)電(dian)(dian)，包括其它電(dian)(dian)池供電(dian)(dian)的(de)設(she)備(bei)，如(ru)筆記本電(dian)(dian)腦。因此，目前市(shi)場上(shang)的(de)許多充(chong)電(dian)(dian)器(qi)芯片支持從交流和USB輸(shu)入(ru)口(kou)給單節鋰電(dian)(dian)池充(chong)電(dian)(dian)。此時(shi)的(de)挑戰是補償USB端口(kou)提供的(de)不斷變化(hua)的(de)功率電(dian)(dian)平。隨(sui)著負載的(de)變化(hua)，系統(tong)必須控制恒定的(de)充(chong)電(dian)(dian)電(dian)(dian)流值以確保(bao)USB端口(kou)的(de)正常操作。

　　一些(xie)充電(dian)器芯片利用(yong)簡單的(de)(de)雙重(zhong)方(fang)(fang)法支持USB操(cao)作，該方(fang)(fang)法使(shi)用(yong)兩(liang)個(ge)預設的(de)(de)充電(dian)電(dian)流(liu)值(zhi)(通(tong)常是500mA和100mA)來支持USB端口允許的(de)(de)最大電(dian)流(liu)或必要(yao)時的(de)(de)最小電(dian)流(liu)以確保端口的(de)(de)完整性。通(tong)常這要(yao)求接口邏輯(ji)嵌入在充電(dian)器芯片中，以便系統微(wei)控制器能夠連續讀取(qu)USB端口的(de)(de)狀態，并指示充電(dian)器從一個(ge)充電(dian)電(dian)流(liu)切換到另一個(ge)充電(dian)電(dian)流(liu)。

　　最近，有(you)些充(chong)(chong)電(dian)(dian)(dian)器(qi)(qi)芯(xin)片(pian)制(zhi)造商已經開發出能夠(gou)自動檢(jian)測(ce)USB端口的可用功率、并(bing)據此調整充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流來最大化充(chong)(chong)電(dian)(dian)(dian)效率的芯(xin)片(pian)。另外，許多充(chong)(chong)電(dian)(dian)(dian)器(qi)(qi)芯(xin)片(pian)不需要用戶(hu)干涉(she)，就能夠(gou)自動檢(jian)測(ce)電(dian)(dian)(dian)源類(lei)型并(bing)調整它(ta)們的充(chong)(chong)電(dian)(dian)(dian)過程。

　　當凌力爾特公司的LTC4075充電(dian)(dian)器芯片檢測(ce)到輸(shu)入端電(dian)(dian)源時，它能自(zi)動(dong)選擇合適的電(dian)(dian)源進(jin)行充電(dian)(dian)，同時不需要外接MOSFET、檢測(ce)電(dian)(dian)阻或阻流二極管(guan)。

　　Microchip Technology公司提(ti)(ti)供的MCP73837也能提(ti)(ti)供類(lei)似自動的電源選擇功能。

　　采用雙節電池

　　隨著便(bian)(bian)(bian)攜式(shi)系(xi)統設計復雜(za)性的提高，它(ta)們使用的子(zi)系(xi)統、顯示器(qi)(qi)和(he)處(chu)理器(qi)(qi)的數量(liang)與(yu)類型也越(yue)來越(yue)多，單節鋰(li)(li)離子(zi)電池已經無(wu)法(fa)滿(man)足(zu)系(xi)統需要(yao)(yao)。因此，一(yi)些(xie)便(bian)(bian)(bian)攜式(shi)媒體播放器(qi)(qi)、高性能SLR型數碼(ma)相機和(he)GPS導航系(xi)統都開(kai)始(shi)采用串聯的兩(liang)節鋰(li)(li)離子(zi)電池供電。這(zhe)些(xie)便(bian)(bian)(bian)攜式(shi)產品一(yi)般要(yao)(yao)求(qiu)輸入在8.4V和(he)8.8V之間進(jin)行充電。設計師以前都是(shi)使用分立器(qi)(qi)件實現這(zhe)種設計，但支(zhi)持(chi)這(zhe)個(ge)要(yao)(yao)求(qiu)的新一(yi)代充電器(qi)(qi)芯片已經開(kai)始(shi)上市。

　　具(ju)有代表性(xing)的(de)(de)(de)例(li)子是AT3663，這是Advanced Analogic Technologies公司(si)(AnalogicTech)提供的(de)(de)(de)1A線性(xing)電(dian)(dian)(dian)池充電(dian)(dian)(dian)器系列芯片的(de)(de)(de)第一個產(chan)品(pin)，可以用(yong)來給最多兩節(jie)串聯的(de)(de)(de)鋰離子電(dian)(dian)(dian)池充電(dian)(dian)(dian)。AT3663采(cai)用(yong)該制(zhi)造商的(de)(de)(de)專(zhuan)有模塊化BCD工藝技術制(zhi)造。這種(zhong)工藝集成了完(wan)全隔離的(de)(de)(de)CMOS和高速互補雙(shuang)極晶體(ti)管以及30V DMOS功(gong)率器件(jian)，沒(mei)有采(cai)用(yong)復雜而且昂貴(gui)的(de)(de)(de)外延附生或高溫擴散技術。

　　上述新工藝允許AAT3663支(zhi)持從4V到13.2V的輸入(ru)電(dian)壓(ya)，因(yin)此設計師(shi)可(ke)以自由使用較低成本的非(fei)穩(wen)壓(ya)適配器(qi)。