作為控制鋰離子充電狀態的功率器件，電池充電器芯片在便攜(xie)式系統設計中扮演著(zhu)重要的(de)(de)(de)角色。然(ran)而，設計師(shi)們還在使(shi)用著(zhu)從較老并且(qie)相對粗糙(cao)和(he)低(di)成本的(de)(de)(de)充(chong)電器(qi)件、到較新的(de)(de)(de)更復(fu)雜(za)芯片等各(ge)種各(ge)樣的(de)(de)(de)器(qi)件，而后者集(ji)成了越來越復(fu)雜(za)的(de)(de)(de)智能，可以延長(chang)電池(chi)壽命(ming)，保護(hu)被充(chong)電系統不(bu)受損害。

　　安全第一

　　便(bian)攜式(shi)系統(tong)設(she)計師(shi)面臨(lin)的挑戰(zhan)是：如(ru)何(he)快速高(gao)效地(di)對(dui)系統(tong)進行充電(dian)(dian)(dian)，同時不影響用戶安全(quan)或損壞(huai)電(dian)(dian)(dian)池(chi)，并(bing)(bing)且要占用最(zui)少(shao)的電(dian)(dian)(dian)路板面積。鋰離(li)子充電(dian)(dian)(dian)器通常采(cai)用三(san)種充電(dian)(dian)(dian)模式(shi)來最(zui)大化充電(dian)(dian)(dian)效率并(bing)(bing)確保用戶安全(quan)性。深(shen)度放電(dian)(dian)(dian)的電(dian)(dian)(dian)池(chi)必須先用較(jiao)小的電(dian)(dian)(dian)流進行逐步預充電(dian)(dian)(dian)，將電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓慢慢提升到安全(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(chang)會測量被充電(dian)池的電(dian)壓，當電(dian)池電(dian)壓低于3V時(shi)，它(ta)將進入預(yu)充或“慢(man)充”模式。慢(man)充模式可以在電(dian)池內阻很(hen)低時(shi)防止(zhi)對電(dian)池高速(su)充電(dian)，從而盡量減(jian)少發熱。一般預(yu)充期(qi)間的充電(dian)速(su)率約是全速(su)充電(dian)時(shi)的10%。

　　圖1：Microchip科技公司的(de)MCP73837雙輸入鋰離子(zi)/聚合(he)物充電(dian)器具(ju)有電(dian)源選(xuan)擇功能，可以自動選(xuan)擇從交流或(huo)USB源進行充電(dian)。

　　一旦電(dian)(dian)(dian)(dian)池達(da)到(dao)最小(xiao)電(dian)(dian)(dian)(dian)壓(ya)值，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器就切換(huan)到(dao)恒流或(huo)快充(chong)(chong)(chong)(chong)模(mo)式，此時將對電(dian)(dian)(dian)(dian)池全速充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)，直(zhi)到(dao)電(dian)(dian)(dian)(dian)池達(da)到(dao)接近(jin)滿充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)壓(ya)的(de)第二個閥(fa)值。隨后充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器將進(jin)入恒壓(ya)模(mo)式，此時電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓(ya)保持不變，從而(er)讓(rang)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流逐漸停止(zhi)。當充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流小(xiao)于預設的(de)終止(zhi)閥(fa)值時，恒壓(ya)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)模(mo)式也(ye)就結束了(le)。

　　熱管理是電(dian)(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)(dian)器(qi)設計師面臨的(de)(de)(de)另(ling)一大挑戰(zhan)。每(mei)個充(chong)電(dian)(dian)(dian)(dian)器(qi)芯(xin)(xin)片(pian)(pian)都會在充(chong)電(dian)(dian)(dian)(dian)過(guo)(guo)程(cheng)中經(jing)歷由于散熱引(yin)起(qi)的(de)(de)(de)電(dian)(dian)(dian)(dian)壓下降。為了避免電(dian)(dian)(dian)(dian)池損(sun)壞(huai)或系統關斷，大多數(shu)充(chong)電(dian)(dian)(dian)(dian)器(qi)都集(ji)成(cheng)了某種形式的(de)(de)(de)控(kong)制機制來管理熱量累積。較(jiao)老的(de)(de)(de)充(chong)電(dian)(dian)(dian)(dian)器(qi)芯(xin)(xin)片(pian)(pian)常采用(yong)“非彼即(ji)此”的(de)(de)(de)方法來解決過(guo)(guo)熱或過(guo)(guo)流(liu)問題(ti)――當熱量達(da)到(dao)預設的(de)(de)(de)門限時(shi)它(ta)們將簡單地(di)中斷充(chong)電(dian)(dian)(dian)(dian)過(guo)(guo)程(cheng)。較(jiao)新的(de)(de)(de)器(qi)件則采用(yong)更復雜的(de)(de)(de)反饋技術(shu)連(lian)(lian)續監(jian)視裸片(pian)(pian)溫度，并以正比于環境溫度變(bian)化(hua)的(de)(de)(de)速率動(dong)態或通(tong)過(guo)(guo)計算調整(zheng)充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)。這(zhe)種內置智(zhi)能(neng)(neng)允許(xu)當前的(de)(de)(de)充(chong)電(dian)(dian)(dian)(dian)器(qi)芯(xin)(xin)片(pian)(pian)逐漸減少充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)，直到(dao)達(da)到(dao)熱平衡，裸片(pian)(pian)溫度停止上升(sheng)。這(zhe)種技術(shu)能(neng)(neng)讓充(chong)電(dian)(dian)(dian)(dian)器(qi)以最大可能(neng)(neng)的(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu)對電(dian)(dian)(dian)(dian)池進行連(lian)(lian)續充(chong)電(dian)(dian)(dian)(dian)，同(tong)時(shi)不會導致系統關斷，從而縮短電(dian)(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)(dian)時(shi)間。

　　MAX8804是美信集成產品公司(si)去年7月份推出的(de)充(chong)電(dian)器(qi)芯(xin)片，它采用了專門的(de)熱(re)調整電(dian)路，可(ke)以在快速充(chong)電(dian)階段或系統處(chu)于高溫(wen)環境(jing)中限(xian)制(zhi)裸片的(de)溫(wen)度。該充(chong)電(dian)器(qi)耐受30V的(de)直流輸(shu)入電(dian)壓，只占用6平方毫米的(de)電(dian)路板面(mian)積。另外，像TI公司(si)提供的(de)1A bq24060充(chong)電(dian)器(qi)可(ke)提供熱(re)過載保護(hu)功能，允(yun)許器(qi)件在環境(jing)溫(wen)度很高的(de)惡劣環境(jing)中連(lian)續運(yun)行(xing)，例如夏天(tian)的(de)汽(qi)車或不小心連(lian)接到具(ju)有較高輸(shu)入電(dian)壓的(de)其它適配器(qi)。目前大多數較新的(de)器(qi)件一般還會(hui)增(zeng)加過壓保護(hu)機(ji)制(zhi)。

　　多種電壓源

　　有個越來越常見的要求是從不同電(dian)壓(ya)源給電(dian)池充(chong)電(dian)，要點是在(zai)不使電(dian)壓(ya)源過(guo)(guo)載的情況下提供這個功能(neng)。可(ke)以通過(guo)(guo)在(zai)輸(shu)入條件改變時動態調整充(chong)電(dian)電(dian)流實現這一功能(neng)。

　　舉例來(lai)說，移動用戶經常沒(mei)有時間(jian)去找交(jiao)(jiao)流電(dian)插座(zuo)來(lai)給他們(men)的設(she)備(bei)充(chong)電(dian)，相反，他們(men)希望通過(guo)大多(duo)數電(dian)子設(she)備(bei)上都有的眾多(duo)USB端(duan)口(kou)(kou)來(lai)給他們(men)的電(dian)池充(chong)電(dian)，包括其它(ta)電(dian)池供電(dian)的設(she)備(bei)，如筆記本電(dian)腦。因(yin)此(ci)，目前市(shi)場上的許多(duo)充(chong)電(dian)器(qi)芯片支持(chi)從(cong)交(jiao)(jiao)流和USB輸入口(kou)(kou)給單節鋰電(dian)池充(chong)電(dian)。此(ci)時的挑戰是補償USB端(duan)口(kou)(kou)提供的不斷變(bian)化的功率電(dian)平。隨(sui)著負載的變(bian)化，系統必須控制恒(heng)定的充(chong)電(dian)電(dian)流值以確(que)保(bao)USB端(duan)口(kou)(kou)的正常操作(zuo)。

　　一(yi)些充(chong)電器(qi)(qi)芯(xin)片利用(yong)簡單的(de)雙重(zhong)方法支持(chi)USB操作(zuo)，該方法使用(yong)兩(liang)個預設的(de)充(chong)電電流值(通常(chang)是(shi)500mA和100mA)來支持(chi)USB端口允許(xu)的(de)最大(da)電流或(huo)必要時(shi)的(de)最小電流以確(que)保端口的(de)完整性。通常(chang)這要求接口邏輯(ji)嵌(qian)入在充(chong)電器(qi)(qi)芯(xin)片中，以便系統微控制器(qi)(qi)能(neng)夠連續讀取USB端口的(de)狀態，并指示充(chong)電器(qi)(qi)從一(yi)個充(chong)電電流切換(huan)到另一(yi)個充(chong)電電流。

　　最近，有些充電(dian)(dian)器(qi)芯(xin)片制(zhi)造商(shang)已經開發出(chu)能夠(gou)自(zi)動(dong)(dong)檢測USB端口的可用功(gong)率、并據此調整(zheng)充電(dian)(dian)電(dian)(dian)流(liu)來(lai)最大化(hua)充電(dian)(dian)效(xiao)率的芯(xin)片。另外，許多充電(dian)(dian)器(qi)芯(xin)片不需要用戶干涉，就能夠(gou)自(zi)動(dong)(dong)檢測電(dian)(dian)源(yuan)類型并調整(zheng)它們的充電(dian)(dian)過(guo)程。

　　當(dang)凌力爾特公司的LTC4075充電(dian)器(qi)芯片(pian)檢(jian)測到輸(shu)入端電(dian)源時，它能自動選(xuan)擇合(he)適的電(dian)源進行充電(dian)，同時不需要外接MOSFET、檢(jian)測電(dian)阻(zu)或阻(zu)流二(er)極管(guan)。

　　Microchip Technology公司提供(gong)(gong)的MCP73837也(ye)能(neng)提供(gong)(gong)類似自動的電(dian)源(yuan)選擇功能(neng)。

　　采用雙節電池

　　隨(sui)著便攜(xie)式(shi)系(xi)統設(she)計復(fu)雜(za)性(xing)的(de)提(ti)高(gao)，它們使用的(de)子系(xi)統、顯(xian)示器和(he)(he)處(chu)理器的(de)數(shu)量與類型(xing)(xing)也越來越多，單節鋰(li)離(li)子電(dian)池(chi)已經無(wu)法(fa)滿足系(xi)統需要(yao)。因此，一(yi)些便攜(xie)式(shi)媒(mei)體播放(fang)器、高(gao)性(xing)能(neng)SLR型(xing)(xing)數(shu)碼(ma)相機和(he)(he)GPS導航系(xi)統都開(kai)始采用串(chuan)聯的(de)兩節鋰(li)離(li)子電(dian)池(chi)供電(dian)。這(zhe)些便攜(xie)式(shi)產(chan)品(pin)一(yi)般要(yao)求輸入在8.4V和(he)(he)8.8V之間進行(xing)充電(dian)。設(she)計師以(yi)前都是使用分立器件實現這(zhe)種(zhong)設(she)計，但支(zhi)持這(zhe)個要(yao)求的(de)新一(yi)代(dai)充電(dian)器芯片已經開(kai)始上市。

　　具有代表性(xing)的(de)例子是(shi)AT3663，這(zhe)是(shi)Advanced Analogic Technologies公司(AnalogicTech)提供的(de)1A線(xian)性(xing)電池充電器系列芯(xin)片的(de)第(di)一個產(chan)品，可(ke)以用(yong)來給最多兩節串聯的(de)鋰離子電池充電。AT3663采用(yong)該制造商的(de)專(zhuan)有模塊化(hua)BCD工藝(yi)技(ji)術制造。這(zhe)種工藝(yi)集成了完全隔離的(de)CMOS和高速互(hu)補雙極晶體(ti)管以及30V DMOS功率器件，沒有采用(yong)復(fu)雜而(er)且(qie)昂貴(gui)的(de)外延(yan)附生或高溫擴散技(ji)術。

　　上述新工藝(yi)允許AAT3663支持從4V到13.2V的輸入電壓，因此(ci)設計師可以自由使用較低(di)成(cheng)本(ben)的非穩壓適(shi)配器。