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)(pian)在便攜(xie)式系統設計(ji)中扮演著重要的(de)角色。然而，設計(ji)師們還在使用(yong)著從較(jiao)(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)(jiao)新(xin)的(de)更復雜芯(xin)片(pian)(pian)等各(ge)種各(ge)樣的(de)器件(jian)，而后者(zhe)集(ji)成了越(yue)來越(yue)復雜的(de)智能，可以延長(chang)電(dian)池壽命，保護被充(chong)電(dian)系統不受損害。

　　安全第一

　　便攜式(shi)系(xi)統設計師面臨(lin)的挑(tiao)戰是：如何快速高效(xiao)地(di)對系(xi)統進(jin)行充(chong)電(dian)(dian)，同時不影響用(yong)戶安(an)全或(huo)損壞電(dian)(dian)池，并且要占用(yong)最少的電(dian)(dian)路板面積。鋰離子(zi)充(chong)電(dian)(dian)器通常采用(yong)三種充(chong)電(dian)(dian)模式(shi)來最大(da)化充(chong)電(dian)(dian)效(xiao)率并確保用(yong)戶安(an)全性。深度放(fang)電(dian)(dian)的電(dian)(dian)池必須先用(yong)較小的電(dian)(dian)流(liu)進(jin)行逐步(bu)預充(chong)電(dian)(dian)，將電(dian)(dian)池電(dian)(dian)壓慢慢提升(sheng)到安(an)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說，它(ta)通常會測量被充(chong)電池的電壓(ya)，當電池電壓(ya)低于3V時，它(ta)將進入預充(chong)或“慢充(chong)”模(mo)式。慢充(chong)模(mo)式可以(yi)在電池內阻很低時防(fang)止對電池高速(su)充(chong)電，從而盡量減少發熱。一般預充(chong)期間的充(chong)電速(su)率約(yue)是全速(su)充(chong)電時的10%。

　　圖1：Microchip科(ke)技公司的(de)MCP73837雙(shuang)輸入鋰離子/聚合物(wu)充(chong)電(dian)器(qi)具有電(dian)源選擇功能，可(ke)以自(zi)動選擇從交流或USB源進行充(chong)電(dian)。

　　一旦(dan)電(dian)(dian)(dian)池(chi)達(da)到最小電(dian)(dian)(dian)壓(ya)值，充(chong)電(dian)(dian)(dian)器就(jiu)(jiu)切換到恒流(liu)或快充(chong)模式(shi)，此(ci)時(shi)將(jiang)(jiang)對電(dian)(dian)(dian)池(chi)全速充(chong)電(dian)(dian)(dian)，直到電(dian)(dian)(dian)池(chi)達(da)到接(jie)近(jin)滿(man)充(chong)電(dian)(dian)(dian)壓(ya)的第二個閥(fa)(fa)值。隨后充(chong)電(dian)(dian)(dian)器將(jiang)(jiang)進入恒壓(ya)模式(shi)，此(ci)時(shi)電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓(ya)保持不變，從而讓充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)逐漸(jian)停止。當(dang)充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)小于預設(she)的終止閥(fa)(fa)值時(shi)，恒壓(ya)充(chong)電(dian)(dian)(dian)模式(shi)也就(jiu)(jiu)結束了(le)。

　　熱(re)管(guan)理(li)是電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器設計(ji)師面臨的(de)(de)(de)(de)另(ling)一大(da)(da)挑戰。每個充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器芯片(pian)都會在充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)過(guo)程中(zhong)經歷由于散熱(re)引起的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)壓下降。為了(le)避免電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)損壞或系(xi)統(tong)關斷(duan)，大(da)(da)多數充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器都集成了(le)某種(zhong)形式(shi)的(de)(de)(de)(de)控制(zhi)機(ji)制(zhi)來(lai)管(guan)理(li)熱(re)量累積。較(jiao)老(lao)的(de)(de)(de)(de)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器芯片(pian)常采(cai)用(yong)“非(fei)彼即(ji)此”的(de)(de)(de)(de)方法來(lai)解決過(guo)熱(re)或過(guo)流問題――當(dang)熱(re)量達到(dao)預設的(de)(de)(de)(de)門限時(shi)它們(men)將簡單(dan)地(di)中(zhong)斷(duan)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)過(guo)程。較(jiao)新的(de)(de)(de)(de)器件(jian)則采(cai)用(yong)更復雜的(de)(de)(de)(de)反(fan)饋技術連續(xu)(xu)監視裸(luo)片(pian)溫度，并(bing)以(yi)正比于環境溫度變化(hua)的(de)(de)(de)(de)速率動態或通過(guo)計(ji)算調整充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流。這種(zhong)內(nei)置智能(neng)(neng)允許當(dang)前的(de)(de)(de)(de)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器芯片(pian)逐漸減(jian)少充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流，直到(dao)達到(dao)熱(re)平衡，裸(luo)片(pian)溫度停止上升。這種(zhong)技術能(neng)(neng)讓充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器以(yi)最大(da)(da)可能(neng)(neng)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流對電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)進行連續(xu)(xu)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)，同時(shi)不會導致系(xi)統(tong)關斷(duan)，從而縮短電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)時(shi)間。

　　MAX8804是美信集成產品公司去年7月份推(tui)出的(de)(de)充(chong)電(dian)器(qi)(qi)芯片，它采用了專(zhuan)門(men)的(de)(de)熱(re)調整電(dian)路，可以(yi)在(zai)快速充(chong)電(dian)階(jie)段或(huo)系統(tong)處于高(gao)(gao)溫環境中(zhong)(zhong)限制(zhi)裸(luo)片的(de)(de)溫度。該充(chong)電(dian)器(qi)(qi)耐受30V的(de)(de)直流輸入電(dian)壓，只占用6平(ping)方毫米的(de)(de)電(dian)路板面積。另外(wai)，像TI公司提供的(de)(de)1A bq24060充(chong)電(dian)器(qi)(qi)可提供熱(re)過(guo)載保(bao)護功能，允許器(qi)(qi)件在(zai)環境溫度很高(gao)(gao)的(de)(de)惡劣環境中(zhong)(zhong)連續(xu)運(yun)行，例如夏天的(de)(de)汽車或(huo)不小心(xin)連接到具有較高(gao)(gao)輸入電(dian)壓的(de)(de)其它適配器(qi)(qi)。目前大多數較新的(de)(de)器(qi)(qi)件一(yi)般還會增(zeng)加(jia)過(guo)壓保(bao)護機制(zhi)。

　　多種電壓源

　　有個越來越常(chang)見的(de)要求(qiu)是從不(bu)同電(dian)(dian)壓源給電(dian)(dian)池充(chong)(chong)電(dian)(dian)，要點是在不(bu)使電(dian)(dian)壓源過載的(de)情(qing)況(kuang)下(xia)提供這(zhe)個功(gong)能。可以通過在輸入條(tiao)件改變時動態調整充(chong)(chong)電(dian)(dian)電(dian)(dian)流實現這(zhe)一功(gong)能。

　　舉(ju)例來(lai)說(shuo)，移(yi)動用戶經常沒有(you)時間去找(zhao)交流電(dian)(dian)(dian)(dian)(dian)(dian)插座來(lai)給(gei)他們(men)的(de)(de)(de)(de)設備充電(dian)(dian)(dian)(dian)(dian)(dian)，相反，他們(men)希望(wang)通(tong)過大多數(shu)電(dian)(dian)(dian)(dian)(dian)(dian)子設備上(shang)都(dou)有(you)的(de)(de)(de)(de)眾(zhong)多USB端(duan)口(kou)來(lai)給(gei)他們(men)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)(dian)(dian)，包括其它(ta)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)供電(dian)(dian)(dian)(dian)(dian)(dian)的(de)(de)(de)(de)設備，如筆記(ji)本電(dian)(dian)(dian)(dian)(dian)(dian)腦。因(yin)此，目前市場上(shang)的(de)(de)(de)(de)許多充電(dian)(dian)(dian)(dian)(dian)(dian)器芯片支持從交流和USB輸入口(kou)給(gei)單(dan)節鋰電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)(dian)(dian)。此時的(de)(de)(de)(de)挑戰是(shi)補償(chang)USB端(duan)口(kou)提(ti)供的(de)(de)(de)(de)不斷變(bian)化的(de)(de)(de)(de)功率電(dian)(dian)(dian)(dian)(dian)(dian)平。隨著負載的(de)(de)(de)(de)變(bian)化，系統必(bi)須控(kong)制恒定的(de)(de)(de)(de)充電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流值以確保USB端(duan)口(kou)的(de)(de)(de)(de)正常操作(zuo)。

　　一些充電(dian)(dian)器芯片利用簡單的(de)雙重方法(fa)支持USB操作(zuo)，該(gai)方法(fa)使(shi)用兩個(ge)預設的(de)充電(dian)(dian)電(dian)(dian)流值(通常是500mA和100mA)來支持USB端(duan)口允許的(de)最大電(dian)(dian)流或必要時的(de)最小電(dian)(dian)流以確(que)保端(duan)口的(de)完整性。通常這要求(qiu)接口邏輯嵌(qian)入在充電(dian)(dian)器芯片中(zhong)，以便(bian)系統(tong)微控制器能夠連續讀取USB端(duan)口的(de)狀(zhuang)態(tai)，并指示充電(dian)(dian)器從一個(ge)充電(dian)(dian)電(dian)(dian)流切換到另一個(ge)充電(dian)(dian)電(dian)(dian)流。

　　最近，有些充電器芯片制造商已(yi)經開發出能(neng)夠自動(dong)檢測(ce)USB端(duan)口的(de)(de)可用功率、并(bing)據此調(diao)整(zheng)充電電流來最大(da)化(hua)充電效率的(de)(de)芯片。另外，許多充電器芯片不(bu)需要用戶干涉(she)，就能(neng)夠自動(dong)檢測(ce)電源類型并(bing)調(diao)整(zheng)它們的(de)(de)充電過程(cheng)。

　　當凌(ling)力爾(er)特公司的(de)LTC4075充電(dian)器芯片檢測到輸入(ru)端電(dian)源時(shi)，它能自動(dong)選(xuan)擇合適的(de)電(dian)源進(jin)行充電(dian)，同時(shi)不需要外接MOSFET、檢測電(dian)阻或阻流(liu)二極管(guan)。

　　Microchip Technology公司提供的MCP73837也能提供類似自動的電源選擇功(gong)能。

　　采用雙節電池

　　隨著便(bian)攜(xie)式系統設(she)計(ji)復雜性(xing)的(de)提高，它(ta)們(men)使用的(de)子(zi)系統、顯示(shi)器和處理器的(de)數(shu)量與類型也越(yue)來越(yue)多，單節鋰離(li)子(zi)電(dian)池(chi)已經(jing)無(wu)法(fa)滿足系統需要(yao)(yao)。因此，一(yi)些便(bian)攜(xie)式媒體(ti)播放器、高性(xing)能SLR型數(shu)碼(ma)相機和GPS導航系統都開始(shi)(shi)采用串(chuan)聯的(de)兩(liang)節鋰離(li)子(zi)電(dian)池(chi)供(gong)電(dian)。這些便(bian)攜(xie)式產品一(yi)般要(yao)(yao)求輸(shu)入在8.4V和8.8V之間進行充(chong)電(dian)。設(she)計(ji)師(shi)以前都是使用分立器件實現這種設(she)計(ji)，但支持這個(ge)要(yao)(yao)求的(de)新(xin)一(yi)代(dai)充(chong)電(dian)器芯片已經(jing)開始(shi)(shi)上市(shi)。

　　具(ju)有代表性(xing)的例子是(shi)AT3663，這(zhe)是(shi)Advanced Analogic Technologies公司(AnalogicTech)提(ti)供的1A線性(xing)電(dian)池(chi)充(chong)電(dian)器(qi)系列芯片的第(di)一個產品，可以用來給最多兩節串聯的鋰(li)離子電(dian)池(chi)充(chong)電(dian)。AT3663采(cai)用該制造(zao)商的專有模塊化BCD工藝技術制造(zao)。這(zhe)種工藝集成了(le)完(wan)全隔離的CMOS和高速互補雙極晶體管(guan)以及30V DMOS功率(lv)器(qi)件，沒有采(cai)用復雜而(er)且昂(ang)貴的外(wai)延附生或高溫擴散技術。

　　上(shang)述新工藝允(yun)許AAT3663支持從4V到(dao)13.2V的輸入電壓，因此(ci)設計師可以(yi)自由使(shi)用較低成本的非穩壓適配(pei)器。