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(shi)系統(tong)設計中(zhong)扮演著重要的(de)角色。然而，設計師們還在使用(yong)著從(cong)較老并且相對粗糙和(he)低成本的(de)充電器(qi)件(jian)(jian)、到(dao)較新的(de)更復雜(za)芯片(pian)等各種各樣的(de)器(qi)件(jian)(jian)，而后者集成了(le)越來越復雜(za)的(de)智能，可以延長電池(chi)壽命(ming)，保護被充電系統(tong)不受損(sun)害。

　　安全第一

　　便攜式系統(tong)設計師(shi)面臨(lin)的挑(tiao)戰是(shi)：如何(he)快速(su)高效地(di)對系統(tong)進行充(chong)(chong)電(dian)(dian)(dian)，同時不影響用(yong)(yong)戶安(an)全或損壞電(dian)(dian)(dian)池(chi)，并(bing)且(qie)要占用(yong)(yong)最少(shao)的電(dian)(dian)(dian)路板面積(ji)。鋰離子充(chong)(chong)電(dian)(dian)(dian)器通常采用(yong)(yong)三種充(chong)(chong)電(dian)(dian)(dian)模(mo)式來最大化充(chong)(chong)電(dian)(dian)(dian)效率并(bing)確保用(yong)(yong)戶安(an)全性。深度放(fang)電(dian)(dian)(dian)的電(dian)(dian)(dian)池(chi)必須先用(yong)(yong)較小的電(dian)(dian)(dian)流進行逐步預充(chong)(chong)電(dian)(dian)(dian)，將電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓(ya)慢(man)慢(man)提升到安(an)全閥(fa)值(zhi)之(zhi)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，它通常會(hui)測量被充(chong)電(dian)(dian)池(chi)的電(dian)(dian)壓，當電(dian)(dian)池(chi)電(dian)(dian)壓低于3V時，它將進入預(yu)充(chong)或“慢(man)充(chong)”模式。慢(man)充(chong)模式可以在電(dian)(dian)池(chi)內阻很低時防止對(dui)電(dian)(dian)池(chi)高速充(chong)電(dian)(dian)，從而盡量減少發(fa)熱(re)。一般預(yu)充(chong)期間的充(chong)電(dian)(dian)速率約是全(quan)速充(chong)電(dian)(dian)時的10%。

　　圖1：Microchip科技(ji)公(gong)司的MCP73837雙輸(shu)入鋰離子(zi)/聚(ju)合(he)物充電(dian)器具有電(dian)源選擇(ze)(ze)功(gong)能，可(ke)以自動(dong)選擇(ze)(ze)從交流或USB源進行充電(dian)。

　　一旦電(dian)(dian)(dian)池達(da)到(dao)(dao)(dao)最小(xiao)電(dian)(dian)(dian)壓(ya)(ya)值(zhi)，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器就切換(huan)到(dao)(dao)(dao)恒流或(huo)快充(chong)(chong)(chong)(chong)模式，此(ci)(ci)時將(jiang)對電(dian)(dian)(dian)池全(quan)速充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)，直(zhi)到(dao)(dao)(dao)電(dian)(dian)(dian)池達(da)到(dao)(dao)(dao)接近(jin)滿(man)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)壓(ya)(ya)的第二個閥(fa)(fa)值(zhi)。隨后充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器將(jiang)進入(ru)恒壓(ya)(ya)模式，此(ci)(ci)時電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya)(ya)保持不變(bian)，從而讓充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流逐漸停止(zhi)。當充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流小(xiao)于(yu)預設的終止(zhi)閥(fa)(fa)值(zhi)時，恒壓(ya)(ya)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)模式也就結束了。

　　熱管理是電(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)器(qi)設計(ji)師(shi)面臨(lin)的(de)(de)(de)(de)(de)(de)(de)另一大(da)挑戰(zhan)。每個充(chong)(chong)電(dian)(dian)(dian)器(qi)芯片(pian)都(dou)會在充(chong)(chong)電(dian)(dian)(dian)過(guo)(guo)(guo)程中經歷由于散熱引起的(de)(de)(de)(de)(de)(de)(de)電(dian)(dian)(dian)壓下(xia)降(jiang)。為(wei)了避免電(dian)(dian)(dian)池(chi)損壞或系統(tong)(tong)關斷(duan)，大(da)多(duo)數(shu)充(chong)(chong)電(dian)(dian)(dian)器(qi)都(dou)集成了某(mou)種(zhong)形式的(de)(de)(de)(de)(de)(de)(de)控(kong)制機制來管理熱量(liang)累積。較老的(de)(de)(de)(de)(de)(de)(de)充(chong)(chong)電(dian)(dian)(dian)器(qi)芯片(pian)常采(cai)用“非彼(bi)即此”的(de)(de)(de)(de)(de)(de)(de)方法來解決過(guo)(guo)(guo)熱或過(guo)(guo)(guo)流問題(ti)――當熱量(liang)達(da)到預設的(de)(de)(de)(de)(de)(de)(de)門限時(shi)(shi)它們將簡單(dan)地中斷(duan)充(chong)(chong)電(dian)(dian)(dian)過(guo)(guo)(guo)程。較新的(de)(de)(de)(de)(de)(de)(de)器(qi)件則采(cai)用更(geng)復雜的(de)(de)(de)(de)(de)(de)(de)反饋技術(shu)連續監(jian)視裸(luo)片(pian)溫(wen)度(du)，并以正比于環境溫(wen)度(du)變化的(de)(de)(de)(de)(de)(de)(de)速率動態或通過(guo)(guo)(guo)計(ji)算(suan)調整(zheng)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流。這種(zhong)內置智能(neng)(neng)(neng)允許當前(qian)的(de)(de)(de)(de)(de)(de)(de)充(chong)(chong)電(dian)(dian)(dian)器(qi)芯片(pian)逐漸減少充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流，直到達(da)到熱平衡，裸(luo)片(pian)溫(wen)度(du)停止上(shang)升。這種(zhong)技術(shu)能(neng)(neng)(neng)讓充(chong)(chong)電(dian)(dian)(dian)器(qi)以最大(da)可能(neng)(neng)(neng)的(de)(de)(de)(de)(de)(de)(de)電(dian)(dian)(dian)流對電(dian)(dian)(dian)池(chi)進行連續充(chong)(chong)電(dian)(dian)(dian)，同時(shi)(shi)不會導致(zhi)系統(tong)(tong)關斷(duan)，從(cong)而(er)縮短電(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)時(shi)(shi)間。

　　MAX8804是(shi)美信集成產品公司(si)去年(nian)7月份(fen)推出的(de)(de)充電(dian)(dian)器芯片，它采用了專門的(de)(de)熱(re)調整(zheng)電(dian)(dian)路(lu)，可以(yi)在(zai)快速充電(dian)(dian)階段或(huo)系統(tong)處于(yu)高(gao)(gao)溫環境中限制(zhi)裸(luo)片的(de)(de)溫度。該充電(dian)(dian)器耐(nai)受30V的(de)(de)直流輸入(ru)電(dian)(dian)壓(ya)(ya)，只占用6平(ping)方毫米的(de)(de)電(dian)(dian)路(lu)板面積。另外，像(xiang)TI公司(si)提供(gong)的(de)(de)1A bq24060充電(dian)(dian)器可提供(gong)熱(re)過載保護功能，允(yun)許器件(jian)在(zai)環境溫度很高(gao)(gao)的(de)(de)惡劣環境中連(lian)續運行，例如(ru)夏天的(de)(de)汽車或(huo)不小心連(lian)接(jie)到(dao)具(ju)有較高(gao)(gao)輸入(ru)電(dian)(dian)壓(ya)(ya)的(de)(de)其(qi)它適配器。目前大多數較新的(de)(de)器件(jian)一般還會(hui)增加過壓(ya)(ya)保護機制(zhi)。

　　多種電壓源

　　有個越來越常見的要(yao)求(qiu)是從不(bu)同(tong)電(dian)(dian)(dian)壓源給電(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)，要(yao)點是在不(bu)使(shi)電(dian)(dian)(dian)壓源過(guo)(guo)載的情況下提(ti)供(gong)這(zhe)個功能。可以通過(guo)(guo)在輸入條(tiao)件改變時動態調整(zheng)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流實現這(zhe)一(yi)功能。

　　舉(ju)例來說，移動用(yong)戶經常沒有時間去(qu)找交流電(dian)(dian)(dian)(dian)插座來給他們的(de)(de)設(she)備(bei)充(chong)電(dian)(dian)(dian)(dian)，相反，他們希望通過大多(duo)數電(dian)(dian)(dian)(dian)子設(she)備(bei)上都有的(de)(de)眾多(duo)USB端(duan)口(kou)來給他們的(de)(de)電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)，包括其它電(dian)(dian)(dian)(dian)池(chi)供(gong)電(dian)(dian)(dian)(dian)的(de)(de)設(she)備(bei)，如筆記本電(dian)(dian)(dian)(dian)腦。因此，目(mu)前市場上的(de)(de)許(xu)多(duo)充(chong)電(dian)(dian)(dian)(dian)器芯片支持從交流和(he)USB輸(shu)入(ru)口(kou)給單節鋰電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)。此時的(de)(de)挑戰是補償(chang)USB端(duan)口(kou)提供(gong)的(de)(de)不(bu)斷變化的(de)(de)功率電(dian)(dian)(dian)(dian)平。隨著負載的(de)(de)變化，系統必須控制恒定(ding)的(de)(de)充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流值以(yi)確保USB端(duan)口(kou)的(de)(de)正(zheng)常操作。

　　一些充電(dian)器芯片(pian)(pian)利用簡單(dan)的(de)(de)雙重方法支持USB操作，該方法使用兩個預設的(de)(de)充電(dian)電(dian)流(liu)值(通常(chang)是500mA和100mA)來支持USB端(duan)口(kou)(kou)允(yun)許的(de)(de)最大電(dian)流(liu)或必要(yao)時的(de)(de)最小(xiao)電(dian)流(liu)以確保(bao)端(duan)口(kou)(kou)的(de)(de)完整性(xing)。通常(chang)這(zhe)要(yao)求接口(kou)(kou)邏輯嵌入在充電(dian)器芯片(pian)(pian)中(zhong)，以便系(xi)統微(wei)控制器能(neng)夠連續讀取USB端(duan)口(kou)(kou)的(de)(de)狀態，并指(zhi)示充電(dian)器從一個充電(dian)電(dian)流(liu)切換到(dao)另一個充電(dian)電(dian)流(liu)。

　　最近，有(you)些充電器芯(xin)片制造商已(yi)經開(kai)發出能夠自動檢(jian)測USB端口的(de)可用(yong)功率、并(bing)據此調整(zheng)(zheng)充電電流(liu)來最大化充電效率的(de)芯(xin)片。另外，許多充電器芯(xin)片不需要用(yong)戶干涉，就能夠自動檢(jian)測電源類型并(bing)調整(zheng)(zheng)它們(men)的(de)充電過程。

　　當凌力爾特(te)公司的LTC4075充電器芯片檢測到輸入端電源時，它能自動(dong)選擇合適的電源進行充電，同(tong)時不需要(yao)外接MOSFET、檢測電阻或(huo)阻流二極管。

　　Microchip Technology公(gong)司(si)提供(gong)的(de)MCP73837也(ye)能(neng)提供(gong)類似自動(dong)的(de)電源選擇功能(neng)。

　　采用雙節電池

　　隨著便(bian)攜(xie)式系統(tong)(tong)(tong)設(she)計復(fu)雜(za)性的(de)提高，它們使用的(de)子系統(tong)(tong)(tong)、顯示器和(he)處(chu)理器的(de)數量(liang)與類型也越(yue)來越(yue)多，單節鋰(li)離子電池(chi)已經(jing)無法滿足系統(tong)(tong)(tong)需(xu)要。因此，一(yi)些(xie)便(bian)攜(xie)式媒(mei)體播放器、高性能(neng)SLR型數碼(ma)相機(ji)和(he)GPS導航系統(tong)(tong)(tong)都開(kai)始(shi)采用串(chuan)聯的(de)兩節鋰(li)離子電池(chi)供電。這些(xie)便(bian)攜(xie)式產品一(yi)般(ban)要求輸(shu)入在(zai)8.4V和(he)8.8V之間進行充(chong)電。設(she)計師以前都是使用分立器件實(shi)現這種設(she)計，但支(zhi)持這個要求的(de)新(xin)一(yi)代充(chong)電器芯(xin)片已經(jing)開(kai)始(shi)上市。

　　具(ju)有(you)代表性(xing)的(de)(de)例子是AT3663，這(zhe)是Advanced Analogic Technologies公司(AnalogicTech)提供的(de)(de)1A線性(xing)電(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)器(qi)系列(lie)芯片的(de)(de)第一個產品，可以用(yong)(yong)來給(gei)最多兩節串聯的(de)(de)鋰離子電(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)。AT3663采用(yong)(yong)該制(zhi)造(zao)商的(de)(de)專(zhuan)有(you)模塊化BCD工藝技術制(zhi)造(zao)。這(zhe)種工藝集成了完全隔離的(de)(de)CMOS和高(gao)速互補雙(shuang)極晶體管以及(ji)30V DMOS功率器(qi)件，沒(mei)有(you)采用(yong)(yong)復雜而且昂貴的(de)(de)外延附生或(huo)高(gao)溫(wen)擴(kuo)散技術。

　　上述新工藝允許AAT3663支持從(cong)4V到(dao)13.2V的輸入電壓(ya)，因此(ci)設計師可以自(zi)由使(shi)用較低成本的非穩壓(ya)適配器(qi)。