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(xi)統設(she)(she)計中扮演著重(zhong)要(yao)的(de)角色。然而，設(she)(she)計師們還在使用(yong)著從較(jiao)老(lao)并且相對粗糙和低成本的(de)充(chong)電(dian)器件(jian)、到較(jiao)新的(de)更復雜(za)芯片(pian)等(deng)各種各樣的(de)器件(jian)，而后者集成了越(yue)來(lai)越(yue)復雜(za)的(de)智能，可以延長電(dian)池(chi)壽命，保護被(bei)充(chong)電(dian)系(xi)統不受損害(hai)。

　　安全第一

　　便攜式系統設計師(shi)面(mian)臨的(de)挑戰(zhan)是：如何快速高效(xiao)地(di)對系統進(jin)行充(chong)(chong)電(dian)(dian)(dian)，同時(shi)不(bu)影響用(yong)(yong)(yong)戶安(an)全(quan)或損壞電(dian)(dian)(dian)池(chi)，并且要(yao)占用(yong)(yong)(yong)最(zui)少的(de)電(dian)(dian)(dian)路板面(mian)積。鋰離子(zi)充(chong)(chong)電(dian)(dian)(dian)器通常采用(yong)(yong)(yong)三種充(chong)(chong)電(dian)(dian)(dian)模式來最(zui)大化充(chong)(chong)電(dian)(dian)(dian)效(xiao)率并確保用(yong)(yong)(yong)戶安(an)全(quan)性。深度放電(dian)(dian)(dian)的(de)電(dian)(dian)(dian)池(chi)必須(xu)先用(yong)(yong)(yong)較小的(de)電(dian)(dian)(dian)流(liu)進(jin)行逐步預充(chong)(chong)電(dian)(dian)(dian)，將電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓慢(man)慢(man)提升到安(an)全(quan)閥(fa)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說，它(ta)通常會測量被(bei)充電(dian)池(chi)(chi)的電(dian)壓，當電(dian)池(chi)(chi)電(dian)壓低(di)于3V時(shi)(shi)，它(ta)將進入預充或“慢充”模式。慢充模式可以在電(dian)池(chi)(chi)內阻(zu)很(hen)低(di)時(shi)(shi)防止對電(dian)池(chi)(chi)高速充電(dian)，從而盡量減少發熱(re)。一般預充期間的充電(dian)速率(lv)約(yue)是全速充電(dian)時(shi)(shi)的10%。

　　圖1：Microchip科技公司(si)的MCP73837雙(shuang)輸入鋰離子/聚合物充電器具有電源選(xuan)擇功能，可以自動選(xuan)擇從(cong)交流或USB源進行充電。

　　一(yi)旦電(dian)(dian)池(chi)(chi)達到(dao)(dao)最小電(dian)(dian)壓(ya)值，充電(dian)(dian)器(qi)就(jiu)切換到(dao)(dao)恒流或快充模(mo)式(shi)，此(ci)時將對電(dian)(dian)池(chi)(chi)全速(su)充電(dian)(dian)，直到(dao)(dao)電(dian)(dian)池(chi)(chi)達到(dao)(dao)接近滿充電(dian)(dian)壓(ya)的第二個閥值。隨后充電(dian)(dian)器(qi)將進入恒壓(ya)模(mo)式(shi)，此(ci)時電(dian)(dian)池(chi)(chi)電(dian)(dian)壓(ya)保持不變，從(cong)而讓充電(dian)(dian)電(dian)(dian)流逐漸停(ting)止(zhi)。當充電(dian)(dian)電(dian)(dian)流小于預設的終止(zhi)閥值時，恒壓(ya)充電(dian)(dian)模(mo)式(shi)也就(jiu)結束了。

　　熱管(guan)理是電(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)器(qi)(qi)設計師面臨的(de)另(ling)一大(da)挑戰。每個充(chong)電(dian)(dian)(dian)器(qi)(qi)芯片(pian)都會(hui)在充(chong)電(dian)(dian)(dian)過(guo)(guo)程中(zhong)經(jing)歷由于(yu)(yu)散熱引(yin)起的(de)電(dian)(dian)(dian)壓下降。為了避免(mian)電(dian)(dian)(dian)池(chi)損(sun)壞或系統關斷，大(da)多數充(chong)電(dian)(dian)(dian)器(qi)(qi)都集成了某種(zhong)(zhong)形式的(de)控制機制來(lai)管(guan)理熱量(liang)累積。較老的(de)充(chong)電(dian)(dian)(dian)器(qi)(qi)芯片(pian)常采(cai)用“非彼(bi)即此”的(de)方法來(lai)解(jie)決(jue)過(guo)(guo)熱或過(guo)(guo)流(liu)問題――當熱量(liang)達到(dao)預設的(de)門限時(shi)它們將(jiang)簡(jian)單地中(zhong)斷充(chong)電(dian)(dian)(dian)過(guo)(guo)程。較新的(de)器(qi)(qi)件則(ze)采(cai)用更復雜的(de)反饋技(ji)術連續監(jian)視裸片(pian)溫(wen)度，并(bing)以(yi)(yi)正比(bi)于(yu)(yu)環境溫(wen)度變化的(de)速率(lv)動態或通過(guo)(guo)計算調整充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)。這種(zhong)(zhong)內置智能允許當前的(de)充(chong)電(dian)(dian)(dian)器(qi)(qi)芯片(pian)逐漸(jian)減少充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)，直到(dao)達到(dao)熱平衡，裸片(pian)溫(wen)度停止上升。這種(zhong)(zhong)技(ji)術能讓充(chong)電(dian)(dian)(dian)器(qi)(qi)以(yi)(yi)最大(da)可能的(de)電(dian)(dian)(dian)流(liu)對電(dian)(dian)(dian)池(chi)進(jin)行連續充(chong)電(dian)(dian)(dian)，同(tong)時(shi)不會(hui)導(dao)致系統關斷，從(cong)而縮短電(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)時(shi)間。

　　MAX8804是美信集(ji)成產品公司去(qu)年7月份推出(chu)的(de)(de)(de)充(chong)電器(qi)芯片，它采(cai)用(yong)了專門的(de)(de)(de)熱(re)調整(zheng)電路，可(ke)以在快速(su)充(chong)電階段或系統處于高(gao)溫環境中限制裸片的(de)(de)(de)溫度。該充(chong)電器(qi)耐受(shou)30V的(de)(de)(de)直流輸入(ru)電壓(ya)，只占用(yong)6平方毫米(mi)的(de)(de)(de)電路板面積。另(ling)外，像TI公司提供的(de)(de)(de)1A bq24060充(chong)電器(qi)可(ke)提供熱(re)過(guo)載保護(hu)功能，允許器(qi)件(jian)在環境溫度很高(gao)的(de)(de)(de)惡劣環境中連續運行，例如夏天的(de)(de)(de)汽車或不小心(xin)連接到具有(you)較(jiao)高(gao)輸入(ru)電壓(ya)的(de)(de)(de)其它適(shi)配器(qi)。目前大多數較(jiao)新的(de)(de)(de)器(qi)件(jian)一般還會(hui)增加過(guo)壓(ya)保護(hu)機(ji)制。

　　多種電壓源

　　有個越來越常(chang)見的要求是(shi)從(cong)不同電壓(ya)源給電池充電，要點是(shi)在(zai)不使(shi)電壓(ya)源過載的情(qing)況下提供這個功(gong)能。可(ke)以通過在(zai)輸入條件改變時動態調整充電電流實現(xian)這一功(gong)能。

　　舉例來(lai)說，移(yi)動用(yong)戶經常(chang)沒有時間去找交(jiao)(jiao)流電(dian)(dian)(dian)插座來(lai)給他們(men)的(de)(de)設(she)備(bei)充(chong)電(dian)(dian)(dian)，相反(fan)，他們(men)希望通過大(da)多(duo)(duo)數電(dian)(dian)(dian)子設(she)備(bei)上都有的(de)(de)眾多(duo)(duo)USB端(duan)口(kou)(kou)來(lai)給他們(men)的(de)(de)電(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)，包(bao)括其它電(dian)(dian)(dian)池供電(dian)(dian)(dian)的(de)(de)設(she)備(bei)，如筆記本電(dian)(dian)(dian)腦。因(yin)此(ci)，目前市場(chang)上的(de)(de)許多(duo)(duo)充(chong)電(dian)(dian)(dian)器(qi)芯片支(zhi)持(chi)從交(jiao)(jiao)流和USB輸入(ru)口(kou)(kou)給單節鋰電(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)。此(ci)時的(de)(de)挑戰是補償USB端(duan)口(kou)(kou)提供的(de)(de)不斷(duan)變(bian)(bian)化的(de)(de)功率電(dian)(dian)(dian)平。隨著負載的(de)(de)變(bian)(bian)化，系統必(bi)須控制恒定的(de)(de)充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流值以確保USB端(duan)口(kou)(kou)的(de)(de)正(zheng)常(chang)操作(zuo)。

　　一(yi)(yi)些充(chong)電(dian)(dian)器(qi)芯片利用簡單(dan)的(de)雙重方法支持USB操(cao)作(zuo)，該方法使(shi)用兩個預設的(de)充(chong)電(dian)(dian)電(dian)(dian)流(liu)值(通常是(shi)500mA和100mA)來支持USB端(duan)口(kou)允許的(de)最大電(dian)(dian)流(liu)或必(bi)要(yao)時的(de)最小電(dian)(dian)流(liu)以確保端(duan)口(kou)的(de)完整性。通常這要(yao)求接口(kou)邏輯嵌入在充(chong)電(dian)(dian)器(qi)芯片中，以便系統微(wei)控制器(qi)能夠連續讀取USB端(duan)口(kou)的(de)狀態，并指示(shi)充(chong)電(dian)(dian)器(qi)從一(yi)(yi)個充(chong)電(dian)(dian)電(dian)(dian)流(liu)切換到另一(yi)(yi)個充(chong)電(dian)(dian)電(dian)(dian)流(liu)。

　　最(zui)近，有(you)些充(chong)(chong)電(dian)(dian)器芯片制造商已(yi)經開發出能夠(gou)自動(dong)檢(jian)測USB端口(kou)的(de)可用(yong)功率、并據此調(diao)整(zheng)充(chong)(chong)電(dian)(dian)電(dian)(dian)流來最(zui)大化(hua)充(chong)(chong)電(dian)(dian)效率的(de)芯片。另外(wai)，許多充(chong)(chong)電(dian)(dian)器芯片不需要用(yong)戶(hu)干(gan)涉，就能夠(gou)自動(dong)檢(jian)測電(dian)(dian)源類型(xing)并調(diao)整(zheng)它們的(de)充(chong)(chong)電(dian)(dian)過程。

　　當凌力爾特公司的LTC4075充(chong)電器芯片檢(jian)測(ce)到輸入端電源時(shi)，它能(neng)自動選擇合適的電源進行充(chong)電，同時(shi)不需要外接MOSFET、檢(jian)測(ce)電阻或(huo)阻流二(er)極管(guan)。

　　Microchip Technology公(gong)司(si)提供的MCP73837也(ye)能提供類似自動的電源選擇功能。

　　采用雙節電池

　　隨(sui)著便(bian)攜(xie)(xie)(xie)式系(xi)統(tong)設(she)計復(fu)雜性(xing)的(de)(de)提高(gao)，它們使用的(de)(de)子(zi)系(xi)統(tong)、顯示器(qi)(qi)和處理器(qi)(qi)的(de)(de)數量與類(lei)型也越(yue)來越(yue)多，單節鋰離(li)子(zi)電池已經無(wu)法滿足系(xi)統(tong)需要。因此，一(yi)些便(bian)攜(xie)(xie)(xie)式媒體播(bo)放器(qi)(qi)、高(gao)性(xing)能SLR型數碼相機和GPS導航系(xi)統(tong)都開始采(cai)用串聯的(de)(de)兩節鋰離(li)子(zi)電池供電。這(zhe)(zhe)些便(bian)攜(xie)(xie)(xie)式產品一(yi)般要求(qiu)輸入在8.4V和8.8V之間進(jin)行充電。設(she)計師以(yi)前(qian)都是使用分立器(qi)(qi)件實現(xian)這(zhe)(zhe)種設(she)計，但支持這(zhe)(zhe)個要求(qiu)的(de)(de)新一(yi)代充電器(qi)(qi)芯片已經開始上市(shi)。

　　具有代表(biao)性的(de)例子是AT3663，這是Advanced Analogic Technologies公司(AnalogicTech)提供的(de)1A線性電池充(chong)(chong)電器系列芯(xin)片的(de)第一個產品，可(ke)以用來給最多(duo)兩(liang)節串聯的(de)鋰離子電池充(chong)(chong)電。AT3663采用該制造(zao)(zao)商的(de)專有模塊(kuai)化BCD工藝技(ji)術(shu)制造(zao)(zao)。這種工藝集成了完(wan)全隔離的(de)CMOS和高(gao)速互補雙極(ji)晶體管以及30V DMOS功率器件(jian)，沒有采用復雜而且(qie)昂貴(gui)的(de)外延(yan)附生或高(gao)溫(wen)擴散技(ji)術(shu)。

　　上述新工(gong)藝允許AAT3663支持(chi)從4V到13.2V的輸入電(dian)壓，因(yin)此設計師可以自由使用(yong)較(jiao)低成本的非穩壓適配器。