作為控制鋰離子充電狀態的功率器件，電池充電器芯(xin)片(pian)在(zai)便(bian)攜式(shi)系(xi)統設計(ji)中(zhong)扮演著重要的角色(se)。然而(er)，設計(ji)師(shi)們還在(zai)使用著從較老并(bing)且相(xiang)對粗(cu)糙和低成本的充(chong)(chong)電器件、到較新(xin)的更復雜芯(xin)片(pian)等各種各樣的器件，而(er)后(hou)者集成了越來越復雜的智能，可以延長電池壽命(ming)，保(bao)護被充(chong)(chong)電系(xi)統不(bu)受損害。

　　安全第一

　　便攜式系統設(she)計師面臨的挑戰是：如何(he)快速高效(xiao)地(di)對系統進行充電(dian)(dian)(dian)(dian)(dian)，同時(shi)不影響用戶安(an)全(quan)或(huo)損壞電(dian)(dian)(dian)(dian)(dian)池(chi)，并且(qie)要占用最少的電(dian)(dian)(dian)(dian)(dian)路板(ban)面積。鋰離子充電(dian)(dian)(dian)(dian)(dian)器通(tong)常采用三種充電(dian)(dian)(dian)(dian)(dian)模式來(lai)最大(da)化充電(dian)(dian)(dian)(dian)(dian)效(xiao)率并確保(bao)用戶安(an)全(quan)性。深度放電(dian)(dian)(dian)(dian)(dian)的電(dian)(dian)(dian)(dian)(dian)池(chi)必(bi)須先用較小的電(dian)(dian)(dian)(dian)(dian)流進行逐步預(yu)充電(dian)(dian)(dian)(dian)(dian)，將電(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)壓慢慢提升到安(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(hui)測(ce)量被(bei)充(chong)電(dian)(dian)(dian)池的電(dian)(dian)(dian)壓，當電(dian)(dian)(dian)池電(dian)(dian)(dian)壓低于(yu)3V時，它(ta)將進入預充(chong)或“慢(man)充(chong)”模(mo)(mo)式。慢(man)充(chong)模(mo)(mo)式可以在(zai)電(dian)(dian)(dian)池內阻(zu)很低時防止對電(dian)(dian)(dian)池高速充(chong)電(dian)(dian)(dian)，從而(er)盡量減少發(fa)熱(re)。一般預充(chong)期間的充(chong)電(dian)(dian)(dian)速率約是全速充(chong)電(dian)(dian)(dian)時的10%。

　　圖1：Microchip科技公司的MCP73837雙輸入鋰離子/聚(ju)合物充(chong)電(dian)(dian)器(qi)具有電(dian)(dian)源選擇功能，可以自(zi)動選擇從交(jiao)流或(huo)USB源進行(xing)充(chong)電(dian)(dian)。

　　一(yi)旦電(dian)(dian)(dian)(dian)(dian)池達到(dao)(dao)最小電(dian)(dian)(dian)(dian)(dian)壓(ya)值(zhi)，充(chong)電(dian)(dian)(dian)(dian)(dian)器就切換到(dao)(dao)恒(heng)流或快充(chong)模式，此時將對電(dian)(dian)(dian)(dian)(dian)池全速充(chong)電(dian)(dian)(dian)(dian)(dian)，直到(dao)(dao)電(dian)(dian)(dian)(dian)(dian)池達到(dao)(dao)接近滿(man)充(chong)電(dian)(dian)(dian)(dian)(dian)壓(ya)的(de)第二個(ge)閥(fa)值(zhi)。隨后充(chong)電(dian)(dian)(dian)(dian)(dian)器將進(jin)入恒(heng)壓(ya)模式，此時電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓(ya)保持不變，從而(er)讓(rang)充(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流逐漸停止。當(dang)充(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流小于預設的(de)終止閥(fa)值(zhi)時，恒(heng)壓(ya)充(chong)電(dian)(dian)(dian)(dian)(dian)模式也就結束了。

　　熱管理是電(dian)(dian)池充(chong)電(dian)(dian)器(qi)設計師(shi)面臨的(de)另一大挑戰。每個充(chong)電(dian)(dian)器(qi)芯片(pian)都會在充(chong)電(dian)(dian)過程(cheng)中經歷由于散熱引(yin)起的(de)電(dian)(dian)壓下降(jiang)。為了(le)避免(mian)電(dian)(dian)池損壞(huai)或(huo)系統關斷(duan)，大多數充(chong)電(dian)(dian)器(qi)都集成了(le)某(mou)種(zhong)(zhong)形式的(de)控制機制來管理熱量累積。較老的(de)充(chong)電(dian)(dian)器(qi)芯片(pian)常采用“非彼即此(ci)”的(de)方法來解(jie)決過熱或(huo)過流問題――當熱量達(da)到預設的(de)門限時(shi)它(ta)們將簡單地中斷(duan)充(chong)電(dian)(dian)過程(cheng)。較新的(de)器(qi)件則(ze)采用更(geng)復雜的(de)反饋技術連續(xu)監視(shi)裸片(pian)溫(wen)度，并以(yi)正比于環境溫(wen)度變化的(de)速率(lv)動態或(huo)通過計算調整充(chong)電(dian)(dian)電(dian)(dian)流。這種(zhong)(zhong)內置(zhi)智能允(yun)許當前(qian)的(de)充(chong)電(dian)(dian)器(qi)芯片(pian)逐(zhu)漸減少(shao)充(chong)電(dian)(dian)電(dian)(dian)流，直到達(da)到熱平衡(heng)，裸片(pian)溫(wen)度停(ting)止(zhi)上升。這種(zhong)(zhong)技術能讓充(chong)電(dian)(dian)器(qi)以(yi)最大可能的(de)電(dian)(dian)流對電(dian)(dian)池進行連續(xu)充(chong)電(dian)(dian)，同(tong)時(shi)不會導致系統關斷(duan)，從(cong)而縮(suo)短電(dian)(dian)池充(chong)電(dian)(dian)時(shi)間(jian)。

　　MAX8804是美信集成產品公(gong)司去年(nian)7月(yue)份推出(chu)的(de)(de)充(chong)電(dian)(dian)器(qi)芯片(pian)(pian)，它采用了專(zhuan)門(men)的(de)(de)熱調整電(dian)(dian)路，可(ke)以(yi)在快速充(chong)電(dian)(dian)階段或(huo)(huo)系統處于高溫(wen)環境(jing)中(zhong)限制(zhi)裸片(pian)(pian)的(de)(de)溫(wen)度。該充(chong)電(dian)(dian)器(qi)耐(nai)受30V的(de)(de)直流輸(shu)入(ru)電(dian)(dian)壓，只占用6平方毫米的(de)(de)電(dian)(dian)路板面(mian)積。另外(wai)，像TI公(gong)司提供的(de)(de)1A bq24060充(chong)電(dian)(dian)器(qi)可(ke)提供熱過載保護功能，允許器(qi)件(jian)在環境(jing)溫(wen)度很(hen)高的(de)(de)惡(e)劣環境(jing)中(zhong)連續(xu)運行，例如夏天的(de)(de)汽車或(huo)(huo)不(bu)小心連接到(dao)具有較(jiao)高輸(shu)入(ru)電(dian)(dian)壓的(de)(de)其它適配器(qi)。目(mu)前大多數較(jiao)新的(de)(de)器(qi)件(jian)一般還(huan)會(hui)增加過壓保護機制(zhi)。

　　多種電壓源

　　有個越(yue)來(lai)越(yue)常(chang)見的(de)要求是(shi)從不同電壓源(yuan)給(gei)電池(chi)充電，要點是(shi)在不使電壓源(yuan)過(guo)載(zai)的(de)情況下提(ti)供這個功能。可以(yi)通過(guo)在輸入條(tiao)件改變時動(dong)態(tai)調整(zheng)充電電流實現這一(yi)功能。

　　舉例來(lai)說，移動用戶經常(chang)沒(mei)有時間去找交(jiao)流電(dian)(dian)插(cha)座來(lai)給(gei)他們的(de)(de)(de)(de)(de)設(she)備(bei)充(chong)(chong)電(dian)(dian)，相反，他們希(xi)望通(tong)過大多(duo)(duo)數電(dian)(dian)子設(she)備(bei)上(shang)(shang)都(dou)有的(de)(de)(de)(de)(de)眾(zhong)多(duo)(duo)USB端口來(lai)給(gei)他們的(de)(de)(de)(de)(de)電(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)，包括其它電(dian)(dian)池(chi)供(gong)電(dian)(dian)的(de)(de)(de)(de)(de)設(she)備(bei)，如筆記本電(dian)(dian)腦。因此(ci)(ci)，目前市場上(shang)(shang)的(de)(de)(de)(de)(de)許多(duo)(duo)充(chong)(chong)電(dian)(dian)器芯片(pian)支持從交(jiao)流和USB輸入口給(gei)單節鋰電(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)。此(ci)(ci)時的(de)(de)(de)(de)(de)挑戰(zhan)是(shi)補償USB端口提供(gong)的(de)(de)(de)(de)(de)不(bu)斷變(bian)化(hua)的(de)(de)(de)(de)(de)功率電(dian)(dian)平。隨(sui)著負載的(de)(de)(de)(de)(de)變(bian)化(hua)，系統(tong)必須控制恒定的(de)(de)(de)(de)(de)充(chong)(chong)電(dian)(dian)電(dian)(dian)流值以確保USB端口的(de)(de)(de)(de)(de)正常(chang)操(cao)作(zuo)。

　　一(yi)(yi)些(xie)充(chong)電(dian)(dian)器(qi)(qi)(qi)芯片(pian)利用(yong)簡單的雙重方法支持(chi)USB操作，該方法使用(yong)兩個預設的充(chong)電(dian)(dian)電(dian)(dian)流值(通常(chang)(chang)是500mA和100mA)來支持(chi)USB端口(kou)允許(xu)的最(zui)大電(dian)(dian)流或(huo)必要時的最(zui)小(xiao)電(dian)(dian)流以(yi)確保端口(kou)的完(wan)整性(xing)。通常(chang)(chang)這要求(qiu)接口(kou)邏輯(ji)嵌(qian)入(ru)在(zai)充(chong)電(dian)(dian)器(qi)(qi)(qi)芯片(pian)中，以(yi)便系統微(wei)控制器(qi)(qi)(qi)能夠連續讀取(qu)USB端口(kou)的狀態，并指示充(chong)電(dian)(dian)器(qi)(qi)(qi)從(cong)一(yi)(yi)個充(chong)電(dian)(dian)電(dian)(dian)流切換到另一(yi)(yi)個充(chong)電(dian)(dian)電(dian)(dian)流。

　　最近，有(you)些充(chong)(chong)電(dian)(dian)(dian)器(qi)芯(xin)(xin)片(pian)制造(zao)商已經開發出能(neng)夠自(zi)動檢(jian)測USB端口的(de)可用(yong)功率、并據此調(diao)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(xiao)率的(de)芯(xin)(xin)片(pian)。另外，許(xu)多(duo)充(chong)(chong)電(dian)(dian)(dian)器(qi)芯(xin)(xin)片(pian)不(bu)需要(yao)用(yong)戶干(gan)涉，就能(neng)夠自(zi)動檢(jian)測電(dian)(dian)(dian)源(yuan)類型并調(diao)整它(ta)們(men)的(de)充(chong)(chong)電(dian)(dian)(dian)過程。

　　當凌力爾(er)特(te)公司(si)的LTC4075充電(dian)器芯(xin)片(pian)檢測到輸入端電(dian)源時，它能(neng)自(zi)動選擇(ze)合適的電(dian)源進行充電(dian)，同時不需要(yao)外接MOSFET、檢測電(dian)阻(zu)或阻(zu)流二(er)極管。

　　Microchip Technology公(gong)司(si)提(ti)供(gong)的(de)MCP73837也能提(ti)供(gong)類似自動的(de)電源選擇功能。

　　采用雙節電池

　　隨著便攜(xie)式系(xi)統(tong)(tong)(tong)設(she)(she)計復雜性的提(ti)高，它們使(shi)用的子(zi)系(xi)統(tong)(tong)(tong)、顯示(shi)器和處(chu)理器的數(shu)量與類型也越來越多，單節鋰離(li)子(zi)電(dian)(dian)池已(yi)經無法(fa)滿(man)足系(xi)統(tong)(tong)(tong)需要。因此，一(yi)(yi)些(xie)便攜(xie)式媒(mei)體(ti)播放器、高性能SLR型數(shu)碼相機和GPS導航系(xi)統(tong)(tong)(tong)都開(kai)始采用串聯的兩節鋰離(li)子(zi)電(dian)(dian)池供電(dian)(dian)。這(zhe)些(xie)便攜(xie)式產品一(yi)(yi)般要求(qiu)(qiu)輸入在8.4V和8.8V之間進行充電(dian)(dian)。設(she)(she)計師以(yi)前都是(shi)使(shi)用分立器件實現(xian)這(zhe)種(zhong)設(she)(she)計，但(dan)支持這(zhe)個要求(qiu)(qiu)的新一(yi)(yi)代充電(dian)(dian)器芯片已(yi)經開(kai)始上(shang)市。

　　具(ju)有代表性(xing)(xing)的(de)例子是AT3663，這(zhe)是Advanced Analogic Technologies公司(AnalogicTech)提供的(de)1A線性(xing)(xing)電池充(chong)(chong)電器系列芯(xin)片(pian)的(de)第一個(ge)產品，可以用(yong)來給最多兩節串聯(lian)的(de)鋰(li)離(li)(li)子電池充(chong)(chong)電。AT3663采(cai)用(yong)該制(zhi)造(zao)商(shang)的(de)專有模(mo)塊(kuai)化BCD工(gong)藝技術制(zhi)造(zao)。這(zhe)種工(gong)藝集(ji)成了(le)完全(quan)隔離(li)(li)的(de)CMOS和高(gao)速互(hu)補雙極晶體管以及(ji)30V DMOS功率器件(jian)，沒有采(cai)用(yong)復雜而且昂貴的(de)外延附生(sheng)或高(gao)溫(wen)擴散(san)技術。

　　上述新工藝允許(xu)AAT3663支持從4V到13.2V的輸入電(dian)壓(ya)，因此設計師(shi)可以自(zi)由使用較(jiao)低成本的非(fei)穩(wen)壓(ya)適配器。