一次性電池不能夠充電，完全放電后的電池即為報廢電池。二次電池可以多次充電，但不同的化學特性對充電的要求也不同，特別是“快充”控制。目前，最常用的可充電電池是Li+、鎳氫(NiMH)、鎳鎘(Nicd)和鉛酸電池，其它類型的電池還很多，但沒有大規模投入使用。因為本文僅討論利用交流適配器供電的充電器設計，沒有考慮汽車電源供電的SLI鉛酸電池或更大容量電池的充電器。     

    不同(tong)化學類型的電池對充電器的要求   

    通常(chang)快速充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)比涓流(liu)(liu)(liu)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)更受(shou)歡迎，但快速充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)對(dui)電(dian)(dian)(dian)(dian)路的(de)(de)(de)要求也(ye)(ye)復雜，鎳(nie)氫或鎳(nie)鎘電(dian)(dian)(dian)(dian)池(chi)(chi)的(de)(de)(de)快速充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)技(ji)術(shu)與其它電(dian)(dian)(dian)(dian)池(chi)(chi)也(ye)(ye)是不(bu)(bu)同的(de)(de)(de)。鎳(nie)鎘和(he)(he)鎳(nie)氫電(dian)(dian)(dian)(dian)池(chi)(chi)通常(chang)在(zai)(zai)(zai)恒(heng)流(liu)(liu)(liu)狀態(tai)下(xia)快充(chong)(chong)(chong)(chong)(chong)，而(er)不(bu)(bu)是恒(heng)壓(ya)(ya)狀態(tai)下(xia)。快充(chong)(chong)(chong)(chong)(chong)終(zhong)止(zhi)檢(jian)測基于(yu)電(dian)(dian)(dian)(dian)壓(ya)(ya)和(he)(he)溫度(du)的(de)(de)(de)變(bian)化率，充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)還應(ying)具備(bei)過壓(ya)(ya)保護(hu)和(he)(he)過熱保護(hu)功能。Li+和(he)(he)鉛酸電(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)必(bi)須保證(zheng)作用到(dao)電(dian)(dian)(dian)(dian)池(chi)(chi)上(shang)的(de)(de)(de)電(dian)(dian)(dian)(dian)壓(ya)(ya)和(he)(he)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)最(zui)大(da)值在(zai)(zai)(zai)規定(ding)的(de)(de)(de)范圍內，當電(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)(ya)維持在(zai)(zai)(zai)最(zui)大(da)單節電(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)(ya)，充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)下(xia)降到(dao)一個較低值時(shi)，充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)終(zhong)止(zhi)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)。Li+和(he)(he)鉛酸電(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)的(de)(de)(de)區(qu)別在(zai)(zai)(zai)于(yu)所(suo)允許(xu)的(de)(de)(de)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)、最(zui)大(da)電(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)(ya)，另外鉛酸電(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)一般具有(you)浮(fu)(fu)充(chong)(chong)(chong)(chong)(chong)能力(浮(fu)(fu)充(chong)(chong)(chong)(chong)(chong)是為(wei)了補(bu)充(chong)(chong)(chong)(chong)(chong)鉛酸電(dian)(dian)(dian)(dian)池(chi)(chi)的(de)(de)(de)自放電(dian)(dian)(dian)(dian)，而(er)對(dui)一個已經(jing)完全充(chong)(chong)(chong)(chong)(chong)滿的(de)(de)(de)電(dian)(dian)(dian)(dian)池(chi)(chi)繼(ji)續輸入充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)的(de)(de)(de)技(ji)術(shu))，Li+電(dian)(dian)(dian)(dian)池(chi)(chi)的(de)(de)(de)自放電(dian)(dian)(dian)(dian)非常(chang)低，所(suo)以不(bu)(bu)需要浮(fu)(fu)充(chong)(chong)(chong)(chong)(chong)。    

    Li+電(dian)池    

    為便攜(xie)式產品選擇電(dian)(dian)(dian)池(chi)(chi)時，最重(zhong)要的(de)是(shi)高(gao)能(neng)量密度、尺寸、重(zhong)量和(he)(he)價格。迄今為止(zhi)，Li+電(dian)(dian)(dian)池(chi)(chi)在最為流(liu)行(xing)的(de)電(dian)(dian)(dian)池(chi)(chi)中能(neng)夠提供最高(gao)的(de)體積和(he)(he)能(neng)量密度，但其每(mei)瓦(wa)時的(de)成(cheng)本(ben)也較高(gao)。鉛(qian)酸電(dian)(dian)(dian)池(chi)(chi)的(de)每(mei)瓦(wa)時成(cheng)本(ben)最低，但比較笨重(zhong)。本(ben)文以Li+電(dian)(dian)(dian)池(chi)(chi)為例討論(lun)充(chong)電(dian)(dian)(dian)器的(de)設計(ji)，它在便攜(xie)式產品中已經被普遍使用。我們選擇780mA的(de)充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)和(he)(he)4.2V的(de)終止(zhi)電(dian)(dian)(dian)壓(ya)。 

 通用(yong)輸入(ru)交流電壓的離線式(shi)Li+電池充電器。    

    所示充電(dian)(dian)(dian)(dian)(dian)器(qi)無(wu)需微處理器(qi)干預，輸入電(dian)(dian)(dian)(dian)(dian)源可以(yi)采(cai)用全球范圍的任何交流電(dian)(dian)(dian)(dian)(dian)源，省去了(le)大體積60Hz變壓器(qi)，而且可提供單節Li+電(dian)(dian)(dian)(dian)(dian)池所要(yao)求(qiu)的精確的充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)壓和電(dian)(dian)(dian)(dian)(dian)流。充電(dian)(dian)(dian)(dian)(dian)器(qi)IC(U4)集(ji)成了(le)全面的保(bao)(bao)護功能(電(dian)(dian)(dian)(dian)(dian)壓、電(dian)(dian)(dian)(dian)(dian)流和自(zi)保(bao)(bao)護)，確保(bao)(bao)Li+電(dian)(dian)(dian)(dian)(dian)池的安全充電(dian)(dian)(dian)(dian)(dian)。    

    假設(she)Li+電(dian)池(chi)帶(dai)有過(guo)流(liu)、過(guo)壓(ya)和過(guo)熱保(bao)(bao)護，這些保(bao)(bao)護特性在(zai)電(dian)池(chi)包里是必需的，因為充電(dian)期間一旦(dan)電(dian)路出現(xian)故障，Li+電(dian)池(chi)將存在(zai)很(hen)大(da)的危險性，過(guo)流(liu)、過(guo)壓(ya)或溫度過(guo)高都(dou)有可能造成Li+電(dian)池(chi)爆炸。    

    電路描述     

    包括(kuo)兩顆主(zhu)器(qi)(qi)(qi)件(U1和(he)U4)、精密基(ji)準(zhun)(U3)和(he)光(guang)耦(ou)(U2)。U1是離線式開(kai)關控制器(qi)(qi)(qi)，配合隔(ge)離變壓器(qi)(qi)(qi)、光(guang)耦(ou)和(he)基(ji)準(zhun)產(chan)生隔(ge)離的(de)(de)穩壓直流輸(shu)出。直流電(dian)(dian)壓供給電(dian)(dian)池(chi)充電(dian)(dian)器(qi)(qi)(qi)(U4)，U4提(ti)供電(dian)(dian)流限制、電(dian)(dian)壓調節、電(dian)(dian)池(chi)充電(dian)(dian)和(he)充電(dian)(dian)指示等功能。本(ben)方(fang)案(an)中，U4是一款簡單的(de)(de)線性(xing)充電(dian)(dian)器(qi)(qi)(qi)，內置調整管，結合其熱控制環路提(ti)供快速充電(dian)(dian)。    

    MAX5022的特性     

    U1(MAX5022)在寬輸(shu)入(ru)范圍(wei)的(de)(de)(de)(de)應(ying)用中很容易設計，本(ben)文所提供的(de)(de)(de)(de)方案(an)要求能夠滿(man)足(zu)全球各地的(de)(de)(de)(de)交流電(dian)源規(gui)格，這里假設通用的(de)(de)(de)(de)輸(shu)入(ru)電(dian)源電(dian)壓范圍(wei)為(wei)85VAC至(zhi)265VAC。MAX5022的(de)(de)(de)(de)輸(shu)入(ru)范圍(wei)指標(biao)以(yi)及(ji)評估板(ban)電(dian)路的(de)(de)(de)(de)輸(shu)入(ru)范圍(wei)恰好滿(man)足(zu)這一需(xu)求。對于所示(shi)充電(dian)器的(de)(de)(de)(de)電(dian)源端MAX5022評估板(ban)不需(xu)要作(zuo)任何修改，隔離端只需(xu)對評估板(ban)的(de)(de)(de)(de)電(dian)阻值稍作(zuo)修改。    

    對于(yu)更大功率(lv)的(de)(de)應用，只(zhi)需選(xuan)取功率(lv)較大的(de)(de)開關(guan)管和能(neng)夠(gou)處理更大功率(lv)的(de)(de)變(bian)(bian)壓(ya)(ya)器(qi)(qi)即(ji)可。圖(tu)1所(suo)(suo)(suo)示電(dian)(dian)路(lu)(lu)要求(qiu)充電(dian)(dian)電(dian)(dian)流為780mA，最大電(dian)(dian)池電(dian)(dian)壓(ya)(ya)為4.2V。對充電(dian)(dian)電(dian)(dian)源的(de)(de)要求(qiu)與MAX5022評(ping)(ping)估(gu)板(ban)所(suo)(suo)(suo)提(ti)供的(de)(de)輸出相吻(wen)合：5V@1A，因此，不需要更換(huan)評(ping)(ping)估(gu)板(ban)的(de)(de)晶(jing)體管和變(bian)(bian)壓(ya)(ya)器(qi)(qi)。U4采用SOT23封裝，所(suo)(suo)(suo)以，充電(dian)(dian)器(qi)(qi)的(de)(de)尺寸在(zai)很(hen)大程度上取決于(yu)應用對充電(dian)(dian)電(dian)(dian)源的(de)(de)需求(qiu)。變(bian)(bian)壓(ya)(ya)器(qi)(qi)是電(dian)(dian)路(lu)(lu)中尺寸最大的(de)(de)元件，由(you)于(yu)開關(guan)控制器(qi)(qi)的(de)(de)工作頻(pin)率(lv)大于(yu)250kHz，允許系統選(xuan)用小(xiao)尺寸的(de)(de)變(bian)(bian)壓(ya)(ya)器(qi)(qi)。     

    上電(dian)(dian)(dian)(dian)時(shi)，在(zai)開關控制器工(gong)作之(zhi)前(qian)，U1調(diao)節器直接通(tong)過(guo)泄漏電(dian)(dian)(dian)(dian)阻(zu)R1、R2從整(zheng)流(liu)通(tong)路吸(xi)取少量的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu)，兩個(ge)(ge)電(dian)(dian)(dian)(dian)阻(zu)阻(zu)值(zhi)(zhi)保證每(mei)個(ge)(ge)電(dian)(dian)(dian)(dian)阻(zu)上的(de)(de)(de)(de)壓(ya)(ya)(ya)降不會超出每(mei)個(ge)(ge)電(dian)(dian)(dian)(dian)阻(zu)的(de)(de)(de)(de)容(rong)限，Vin引腳電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)不會超出其28V額(e)定值(zhi)(zhi)(30V為(wei)絕(jue)對最大(da)值(zhi)(zhi))，U1在(zai)輸(shu)入電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)達到(dao)24V時(shi)開始開關操作，并吸(xi)取更大(da)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu)(高(gao)于R1、R2所能(neng)提(ti)供(gong)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu))，在(zai)變壓(ya)(ya)(ya)器副(fu)邊線圈能(neng)夠(gou)(gou)提(ti)供(gong)足夠(gou)(gou)的(de)(de)(de)(de)能(neng)量之(zhi)前(qian)，額(e)外的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu)由(you)Vin引腳的(de)(de)(de)(de)旁路電(dian)(dian)(dian)(dian)容(rong)供(gong)給。這(zhe)種(zhong)處理方式能(neng)夠(gou)(gou)適(shi)合較寬的(de)(de)(de)(de)輸(shu)入電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)范圍，對效率(lv)的(de)(de)(de)(de)影響也很(hen)小(xiao)，這(zhe)是因為(wei)在(zai)啟動過(guo)程(cheng)中只需極小(xiao)的(de)(de)(de)(de)供(gong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(典型值(zhi)(zhi)為(wei)50uA)。   

    為(wei)(wei)了防(fang)止U1的(de)(de)Vin引腳出(chu)現過壓，可(ke)以(yi)在Vin與(yu)地之間接一(yi)個26V至30V的(de)(de)齊納管，電(dian)壓值(zhi)(zhi)需(xu)要高于(yu)欠壓鎖存電(dian)壓的(de)(de)最(zui)大(da)值(zhi)(zhi)，低于(yu)該引腳所允(yun)許的(de)(de)最(zui)大(da)絕對電(dian)壓。如果器件沒有啟動正常(chang)的(de)(de)開關操作，則可(ke)認為(wei)(wei)電(dian)路發生了某些問(wen)題，所以(yi)，這一(yi)保護措施(shi)對于(yu)U1來說并不是必需(xu)的(de)(de)。     

    U1是一款電(dian)流(liu)模式PWM控制器，具有(you)逐周期限(xian)(xian)流(liu)電(dian)路(lu)，必要(yao)時其占空(kong)比可以提(ti)高到75%，非常適(shi)合(he)圖1所示的反激拓撲(pu)。原(yuan)邊(bian)開關(guan)電(dian)流(liu)檢(jian)測采用尺寸為1206的1.78Ω電(dian)阻(zu)(R7)，該電(dian)阻(zu)限(xian)(xian)制了變壓器原(yuan)邊(bian)的最(zui)(zui)大(da)電(dian)流(liu)。固定開關(guan)頻率和最(zui)(zui)大(da)占空(kong)比限(xian)(xian)制使得最(zui)(zui)大(da)限(xian)(xian)流(liu)具有(you)輸出短路(lu)保護功能。   

    一旦(dan)短路輸出(chu)造(zao)成電路達到(dao)了上述(shu)限(xian)制條(tiao)件，第三組線圈(quan)將無法提供10V以上的電壓(UVLO的低(di)電壓門限(xian))，U1將再次進入(ru)啟動狀(zhuang)態，停止開關操作。短路故障解除后系統(tong)會自(zi)動恢復(fu)正(zheng)常的工作狀(zhuang)態。   

    電壓調節   

    變壓(ya)(ya)器(qi)副邊的(de)電(dian)(dian)(dian)壓(ya)(ya)經過整流(liu)、濾波，然后由R4、R5分壓(ya)(ya)在(zai)并聯型穩壓(ya)(ya)器(qi)U3的(de)調(diao)節點產(chan)生1.24V(對應于4.5V輸(shu)出)的(de)反(fan)(fan)饋電(dian)(dian)(dian)壓(ya)(ya)。為保證(zheng)電(dian)(dian)(dian)池充電(dian)(dian)(dian)器(qi)正常(chang)工作，4.5V是(shi)最低電(dian)(dian)(dian)壓(ya)(ya)(包括(kuo)容差(cha))。因為充電(dian)(dian)(dian)器(qi)采(cai)用的(de)是(shi)線性(xing)結構，充電(dian)(dian)(dian)效(xiao)率(lv)與輸(shu)入、輸(shu)出電(dian)(dian)(dian)壓(ya)(ya)的(de)差(cha)值成反(fan)(fan)比，用最低電(dian)(dian)(dian)壓(ya)(ya)為充電(dian)(dian)(dian)器(qi)供電(dian)(dian)(dian)有助于降低功率(lv)損耗。   

    當調節點(dian)電(dian)壓(ya)(ya)高于(yu)1.24V時，并(bing)聯型穩壓(ya)(ya)器(qi)將吸(xi)入電(dian)流，導(dao)通(tong)光耦(ou)LED，原(yuan)邊的(de)(de)(de)(de)光敏晶(jing)體管(guan)將處于(yu)發光狀(zhuang)態。U1的(de)(de)(de)(de)OPTO輸(shu)(shu)入引(yin)腳(jiao)用于(yu)直(zhi)接(jie)連(lian)接(jie)光敏晶(jing)體管(guan)，以極其(qi)簡(jian)單的(de)(de)(de)(de)方式構(gou)成閉合環路。光敏晶(jing)體管(guan)導(dao)通(tong)時，OPTO引(yin)腳(jiao)被拉低(di)(di)，內部比(bi)較(jiao)器(qi)在它的(de)(de)(de)(de)一個(ge)輸(shu)(shu)入端檢測(ce)到較(jiao)低(di)(di)的(de)(de)(de)(de)電(dian)壓(ya)(ya)，比(bi)較(jiao)器(qi)的(de)(de)(de)(de)另(ling)一輸(shu)(shu)入電(dian)流檢測(ce)引(yin)腳(jiao)(CS)的(de)(de)(de)(de)峰值(zhi)電(dian)壓(ya)(ya)，OPTO引(yin)腳(jiao)的(de)(de)(de)(de)低(di)(di)電(dian)壓(ya)(ya)將在每個(ge)周(zhou)期直(zhi)接(jie)限制(zhi)功率MOSFETN1的(de)(de)(de)(de)峰值(zhi)電(dian)流。由于(yu)U1是一個(ge)PWM控(kong)制(zhi)器(qi)，電(dian)流限制(zhi)使得電(dian)壓(ya)(ya)調節受作用在N1上的(de)(de)(de)(de)占空比(bi)(導(dao)通(tong)時間與(yu)斷開時間的(de)(de)(de)(de)比(bi)值(zhi))的(de)(de)(de)(de)影響(xiang)，環路穩定性通(tong)過調節R11和(he)C12來保證。R10和(he)C5(連(lian)接(jie)在U3的(de)(de)(de)(de)第3和(he)第4引(yin)腳(jiao))可(ke)消除穩定工作情況下的(de)(de)(de)(de)噪聲干擾。    

    U4的(de)工(gong)作特點    

    U4是一款用于單節Li+電池充電、功能完備的充電器，采用線性充電(dian)(dian)模(mo)式(shi)，內置(zhi)調整管和電(dian)(dian)流檢(jian)測(ce)電(dian)(dian)路。另外，它(ta)還(huan)集(ji)成了(le)一個反向(xiang)抑制二極管，在(zai)去掉輸入電(dian)(dian)源時可(ke)防止電(dian)(dian)池(chi)(chi)方向(xiang)放電(dian)(dian)。接入一個深度放電(dian)(dian)的(de)(de)電(dian)(dian)池(chi)(chi)時，U4能夠檢(jian)測(ce)到(dao)(dao)低(di)電(dian)(dian)池(chi)(chi)電(dian)(dian)壓(低(di)于2.5V)，這樣，U4將(jiang)首先啟動(dong)涓(juan)流充電(dian)(dian)過(guo)程，在(zai)快(kuai)充周期的(de)(de)1/10時間段對電(dian)(dian)池(chi)(chi)進行涓(juan)流充電(dian)(dian)，直(zhi)到(dao)(dao)電(dian)(dian)池(chi)(chi)電(dian)(dian)壓上升(sheng)到(dao)(dao)2.5V以上。在(zai)涓(juan)流充電(dian)(dian)到(dao)(dao)快(kuai)速充電(dian)(dian)轉換時，軟啟動(dong)電(dian)(dian)路迫(po)使(shi)充電(dian)(dian)電(dian)(dian)流在(zai)3ms時間內緩慢(man)上升(sheng)，避免(mian)較(jiao)大的(de)(de)浪(lang)涌電(dian)(dian)流拉(la)低(di)輸入電(dian)(dian)源。    

   上述功(gong)能的(de)實施不需要(yao)微控制器的(de)監管，而且，U4還(huan)具有可編程的(de)溫度限制功(gong)能，我們會在下面就這(zhe)一點進行詳(xiang)細討論(lun)。用(yong)一個電(dian)(dian)(dian)(dian)阻設置最大充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流，電(dian)(dian)(dian)(dian)阻兩端的(de)電(dian)(dian)(dian)(dian)壓(ya)與充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流成(cheng)正比，通(tong)過監視(shi)該電(dian)(dian)(dian)(dian)壓(ya)即可連續跟蹤(zong)充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流，更為復雜的(de)系(xi)統(tong)也可以采用(yong)ADC跟蹤(zong)監視(shi)充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流。    

    Maxim還提供有(you)另一款與MAX1507非常(chang)相(xiang)似的(de)充(chong)電(dian)器―MAX1555，這款IC能夠在沒有(you)交流電(dian)源(yuan)(yuan)的(de)情(qing)況下從USB主(zhu)機獲得充(chong)電(dian)電(dian)源(yuan)(yuan)。   

    電流調節 

    如果把(ba)Li+電(dian)池(chi)(chi)直接與一個(ge)低(di)阻電(dian)壓源連(lian)接，充電(dian)電(dian)流將(jiang)不受任何限(xian)制(zhi)，這種狀況會對(dui)電(dian)池(chi)(chi)及(ji)其(qi)周圍(wei)環(huan)境造(zao)成災難性(xing)的(de)損害，從Li+電(dian)池(chi)(chi)的(de)安全性(xing)考慮，必須將(jiang)其(qi)充電(dian)電(dian)流限(xian)制(zhi)在(zai)(zai)額, 定范圍(wei)以內(nei)。U4根據連(lian)接在(zai)(zai)ISET(第5引腳)的(de)電(dian)阻限(xian)制(zhi)充電(dian)電(dian)流，圖1電(dian)路中選用(yong)了一個(ge)1.87kΩ的(de)電(dian)阻，將(jiang)最大(da)電(dian)流設置(zhi)在(zai)(zai)780mA: 

    Ifast=1461V/Rset=1461/1870≈780mA 

    電(dian)(dian)池電(dian)(dian)壓(ya)(ya)最(zui)小時，由于輸(shu)入穩定在4.5V，U4輸(shu)入與輸(shu)出的壓(ya)(ya)差(cha)最(zui)大(da)，如(ru)果此時電(dian)(dian)池是以最(zui)大(da)電(dian)(dian)流充電(dian)(dian)，U4功(gong)耗將達到最(zui)大(da)值。例如(ru)，假設電(dian)(dian)池電(dian)(dian)壓(ya)(ya)為2.5V，則(ze)對應的功(gong)耗為：  

    P=IV=(0.78A)×(4.5V-2.5V)=1.56W 

    U4封裝(8引腳薄型DFN-EP，EP表示裸露焊盤)在溫度高達70℃時可以耗散的最大功率是1.951W，這樣，在整個電池電壓范圍內以780mA的電流充電應該沒有任何問題。但是，如果IC管芯由于較高的環境溫度或不良的散熱條件達到了70℃，U4內部的熱調節環路將自動降低充電電流，這一過程有別于其它充電IC中的“熱關斷”，“熱關斷”方式只是簡單地中止芯片中的所有電路工作，直到晶片溫度降至規定的數值。MAX1507所采用的方案能夠保證在芯片溫度不超出最大限制范圍的條件下提供最快的充電速率，隨著電池電壓的升高，充電電流在保持最高溫度的前提下逐漸升高，直到達到最大充電電流。在此之后，充電器在滿足芯片(pian)溫度限制的條件下(xia)將(jiang)以最大充電(dian)(dian)電(dian)(dian)流對電(dian)(dian)池充電(dian)(dian)，直到電(dian)(dian)池電(dian)(dian)壓達(da)到內部設置的4.2V