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。     

    不同化學類型的電池對充電器的要(yao)求   

    通(tong)常快(kuai)速(su)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)比(bi)涓流(liu)(liu)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)更受歡迎，但快(kuai)速(su)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)對電(dian)(dian)(dian)(dian)路的(de)(de)(de)(de)要求也復雜，鎳(nie)氫或鎳(nie)鎘電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的(de)(de)(de)(de)快(kuai)速(su)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)技(ji)術與(yu)其它電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)也是(shi)(shi)不同(tong)的(de)(de)(de)(de)。鎳(nie)鎘和(he)鎳(nie)氫電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)通(tong)常在恒流(liu)(liu)狀態(tai)(tai)下快(kuai)充(chong)(chong)(chong)(chong)(chong)(chong)，而不是(shi)(shi)恒壓(ya)(ya)狀態(tai)(tai)下。快(kuai)充(chong)(chong)(chong)(chong)(chong)(chong)終止檢測基于電(dian)(dian)(dian)(dian)壓(ya)(ya)和(he)溫度的(de)(de)(de)(de)變化率(lv)，充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器還應(ying)具備過壓(ya)(ya)保護和(he)過熱(re)保護功能。Li+和(he)鉛(qian)(qian)酸(suan)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器必須(xu)保證作用(yong)到電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)上(shang)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)壓(ya)(ya)和(he)電(dian)(dian)(dian)(dian)流(liu)(liu)最(zui)大值在規定(ding)的(de)(de)(de)(de)范圍內(nei)，當電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)(ya)維持在最(zui)大單節電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)(ya)，充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)下降到一(yi)個(ge)(ge)較低值時，充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器終止充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)。Li+和(he)鉛(qian)(qian)酸(suan)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器的(de)(de)(de)(de)區別(bie)在于所(suo)允(yun)許的(de)(de)(de)(de)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)、最(zui)大電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)(ya)，另外鉛(qian)(qian)酸(suan)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器一(yi)般具有(you)浮充(chong)(chong)(chong)(chong)(chong)(chong)能力(浮充(chong)(chong)(chong)(chong)(chong)(chong)是(shi)(shi)為了補充(chong)(chong)(chong)(chong)(chong)(chong)鉛(qian)(qian)酸(suan)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的(de)(de)(de)(de)自放電(dian)(dian)(dian)(dian)，而對一(yi)個(ge)(ge)已經完全(quan)充(chong)(chong)(chong)(chong)(chong)(chong)滿的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)繼續輸入充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)的(de)(de)(de)(de)技(ji)術)，Li+電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的(de)(de)(de)(de)自放電(dian)(dian)(dian)(dian)非常低，所(suo)以不需要浮充(chong)(chong)(chong)(chong)(chong)(chong)。    

    Li+電池    

    為(wei)便(bian)攜式產(chan)品選擇(ze)電(dian)(dian)池時(shi)，最(zui)重要的(de)是高能量密度、尺寸、重量和(he)價格。迄今為(wei)止(zhi)，Li+電(dian)(dian)池在最(zui)為(wei)流行的(de)電(dian)(dian)池中能夠提供最(zui)高的(de)體積和(he)能量密度，但其每(mei)瓦(wa)時(shi)的(de)成本也較高。鉛酸電(dian)(dian)池的(de)每(mei)瓦(wa)時(shi)成本最(zui)低，但比較笨(ben)重。本文以(yi)Li+電(dian)(dian)池為(wei)例討論(lun)充電(dian)(dian)器的(de)設(she)計(ji)，它在便(bian)攜式產(chan)品中已經被普遍使用。我們選擇(ze)780mA的(de)充電(dian)(dian)電(dian)(dian)流和(he)4.2V的(de)終止(zhi)電(dian)(dian)壓(ya)。 

 通(tong)用輸入交流電壓的離(li)線式Li+電池充電器。    

    所示充電器無需微處理器干預，輸入電源(yuan)可(ke)以采用全(quan)球范圍的(de)任(ren)何交(jiao)流電源(yuan)，省去了(le)(le)大(da)體積(ji)60Hz變(bian)壓(ya)器，而且可(ke)提(ti)供單節Li+電池所要求的(de)精確(que)的(de)充電電壓(ya)和電流。充電器IC(U4)集成了(le)(le)全(quan)面的(de)保護功能(電壓(ya)、電流和自(zi)保護)，確(que)保Li+電池的(de)安(an)全(quan)充電。    

    假設Li+電(dian)(dian)池(chi)(chi)帶(dai)有過(guo)(guo)(guo)流、過(guo)(guo)(guo)壓和過(guo)(guo)(guo)熱保護，這些(xie)保護特性(xing)在(zai)電(dian)(dian)池(chi)(chi)包里(li)是必需的，因為充電(dian)(dian)期間一旦電(dian)(dian)路出(chu)現故障，Li+電(dian)(dian)池(chi)(chi)將存在(zai)很大的危險性(xing)，過(guo)(guo)(guo)流、過(guo)(guo)(guo)壓或(huo)溫度過(guo)(guo)(guo)高都有可能造成(cheng)Li+電(dian)(dian)池(chi)(chi)爆炸。    

    電路(lu)描述     

    包括兩顆(ke)主器(qi)件(U1和U4)、精密基準(U3)和光(guang)耦(ou)(U2)。U1是(shi)離線式(shi)開關(guan)控(kong)制(zhi)器(qi)，配合隔(ge)離變壓器(qi)、光(guang)耦(ou)和基準產生隔(ge)離的穩(wen)壓直(zhi)流(liu)(liu)輸出。直(zhi)流(liu)(liu)電(dian)(dian)(dian)壓供給(gei)電(dian)(dian)(dian)池充電(dian)(dian)(dian)器(qi)(U4)，U4提(ti)供電(dian)(dian)(dian)流(liu)(liu)限制(zhi)、電(dian)(dian)(dian)壓調(diao)節、電(dian)(dian)(dian)池充電(dian)(dian)(dian)和充電(dian)(dian)(dian)指(zhi)示等(deng)功能。本方(fang)案中(zhong)，U4是(shi)一款(kuan)簡單(dan)的線性充電(dian)(dian)(dian)器(qi)，內置調(diao)整管，結(jie)合其熱控(kong)制(zhi)環路提(ti)供快速充電(dian)(dian)(dian)。    

    MAX5022的(de)特性(xing)     

    U1(MAX5022)在寬(kuan)輸入(ru)范(fan)(fan)圍(wei)的(de)應用(yong)中很(hen)容易設計(ji)，本文所提供(gong)的(de)方(fang)案要求(qiu)能夠滿足全(quan)球各地的(de)交流電(dian)(dian)源規格，這(zhe)里假設通用(yong)的(de)輸入(ru)電(dian)(dian)源電(dian)(dian)壓范(fan)(fan)圍(wei)為85VAC至265VAC。MAX5022的(de)輸入(ru)范(fan)(fan)圍(wei)指(zhi)標以(yi)及評(ping)估(gu)板(ban)電(dian)(dian)路的(de)輸入(ru)范(fan)(fan)圍(wei)恰好滿足這(zhe)一需求(qiu)。對于所示(shi)充電(dian)(dian)器(qi)的(de)電(dian)(dian)源端(duan)MAX5022評(ping)估(gu)板(ban)不需要作任何(he)修(xiu)改，隔離端(duan)只需對評(ping)估(gu)板(ban)的(de)電(dian)(dian)阻值稍作修(xiu)改。    

    對(dui)于更(geng)大(da)功率的應用，只(zhi)需選(xuan)(xuan)取(qu)功率較大(da)的開(kai)(kai)關管和(he)能(neng)夠處理更(geng)大(da)功率的變壓(ya)(ya)器(qi)即可。圖1所示電(dian)路要(yao)求(qiu)(qiu)充電(dian)電(dian)流為780mA，最(zui)大(da)電(dian)池電(dian)壓(ya)(ya)為4.2V。對(dui)充電(dian)電(dian)源(yuan)的要(yao)求(qiu)(qiu)與MAX5022評(ping)(ping)估(gu)板所提供的輸出相(xiang)吻合：5V@1A，因(yin)此(ci)，不需要(yao)更(geng)換評(ping)(ping)估(gu)板的晶體(ti)管和(he)變壓(ya)(ya)器(qi)。U4采用SOT23封裝，所以，充電(dian)器(qi)的尺寸在很(hen)大(da)程度上(shang)取(qu)決于應用對(dui)充電(dian)電(dian)源(yuan)的需求(qiu)(qiu)。變壓(ya)(ya)器(qi)是電(dian)路中尺寸最(zui)大(da)的元件，由于開(kai)(kai)關控制器(qi)的工作(zuo)頻率大(da)于250kHz，允許系統(tong)選(xuan)(xuan)用小尺寸的變壓(ya)(ya)器(qi)。     

    上電(dian)(dian)(dian)(dian)(dian)時(shi)，在開關控制器(qi)(qi)工作之前，U1調節器(qi)(qi)直接(jie)通過泄漏電(dian)(dian)(dian)(dian)(dian)阻R1、R2從(cong)整流通路吸(xi)取少量(liang)的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流，兩個電(dian)(dian)(dian)(dian)(dian)阻阻值保證每(mei)個電(dian)(dian)(dian)(dian)(dian)阻上的(de)(de)(de)壓(ya)降不(bu)會(hui)超出每(mei)個電(dian)(dian)(dian)(dian)(dian)阻的(de)(de)(de)容限，Vin引(yin)腳(jiao)電(dian)(dian)(dian)(dian)(dian)壓(ya)不(bu)會(hui)超出其(qi)28V額定值(30V為絕對最大值)，U1在輸入電(dian)(dian)(dian)(dian)(dian)壓(ya)達到24V時(shi)開始開關操作，并(bing)吸(xi)取更大的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(高于R1、R2所能(neng)提(ti)供(gong)的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流)，在變(bian)壓(ya)器(qi)(qi)副邊線圈能(neng)夠(gou)(gou)提(ti)供(gong)足夠(gou)(gou)的(de)(de)(de)能(neng)量(liang)之前，額外(wai)的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流由Vin引(yin)腳(jiao)的(de)(de)(de)旁路電(dian)(dian)(dian)(dian)(dian)容供(gong)給(gei)。這種處理方式能(neng)夠(gou)(gou)適合(he)較寬的(de)(de)(de)輸入電(dian)(dian)(dian)(dian)(dian)壓(ya)范圍，對效率的(de)(de)(de)影(ying)響也(ye)很小(xiao)，這是因(yin)為在啟動過程中只需極(ji)小(xiao)的(de)(de)(de)供(gong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(典型值為50uA)。   

    為了(le)(le)防止U1的Vin引(yin)腳(jiao)出現過壓(ya)(ya)(ya)，可(ke)以在(zai)Vin與地之間(jian)接(jie)一(yi)(yi)個26V至30V的齊納(na)管，電(dian)壓(ya)(ya)(ya)值(zhi)需要高于欠壓(ya)(ya)(ya)鎖存電(dian)壓(ya)(ya)(ya)的最大(da)值(zhi)，低(di)于該(gai)引(yin)腳(jiao)所允許的最大(da)絕對電(dian)壓(ya)(ya)(ya)。如果器件沒有啟動正(zheng)常的開關操(cao)作(zuo)，則可(ke)認為電(dian)路發(fa)生了(le)(le)某些問題(ti)，所以，這一(yi)(yi)保護措施對于U1來(lai)說(shuo)并不是(shi)必(bi)需的。     

    U1是一款(kuan)電流(liu)(liu)模式PWM控制器，具(ju)(ju)有逐周期限流(liu)(liu)電路，必要時其占空比可(ke)以提高到75%，非(fei)常適(shi)合圖1所(suo)示(shi)的(de)反激拓(tuo)撲(pu)。原邊(bian)開(kai)關電流(liu)(liu)檢測采用尺(chi)寸為1206的(de)1.78Ω電阻(zu)(R7)，該電阻(zu)限制了變壓器原邊(bian)的(de)最大(da)電流(liu)(liu)。固定(ding)開(kai)關頻(pin)率和最大(da)占空比限制使(shi)得最大(da)限流(liu)(liu)具(ju)(ju)有輸出(chu)短路保護功能。   

    一(yi)旦短路(lu)輸出造成(cheng)電(dian)路(lu)達到了上(shang)(shang)述限制(zhi)條(tiao)件，第(di)三組線圈將(jiang)無法提(ti)供10V以上(shang)(shang)的電(dian)壓(ya)(UVLO的低電(dian)壓(ya)門限)，U1將(jiang)再次進入啟動狀(zhuang)態(tai)，停止開關操(cao)作(zuo)。短路(lu)故障解除后系(xi)統(tong)會自動恢復正常的工作(zuo)狀(zhuang)態(tai)。   

    電壓調節   

    變壓(ya)器(qi)副(fu)邊的(de)電(dian)(dian)壓(ya)經(jing)過(guo)整流、濾波，然后由R4、R5分壓(ya)在(zai)并聯型穩壓(ya)器(qi)U3的(de)調節(jie)點產生1.24V(對(dui)應(ying)于4.5V輸出)的(de)反饋電(dian)(dian)壓(ya)。為(wei)保證電(dian)(dian)池充(chong)電(dian)(dian)器(qi)正常工(gong)作，4.5V是最低(di)電(dian)(dian)壓(ya)(包括容差)。因為(wei)充(chong)電(dian)(dian)器(qi)采用(yong)的(de)是線性(xing)結構，充(chong)電(dian)(dian)效率(lv)與(yu)輸入、輸出電(dian)(dian)壓(ya)的(de)差值成反比，用(yong)最低(di)電(dian)(dian)壓(ya)為(wei)充(chong)電(dian)(dian)器(qi)供電(dian)(dian)有助于降(jiang)低(di)功率(lv)損耗(hao)。   

    當調節(jie)點電(dian)(dian)壓高于1.24V時(shi)，并聯型穩壓器將(jiang)吸入電(dian)(dian)流，導通(tong)(tong)(tong)光(guang)耦LED，原(yuan)邊的(de)(de)光(guang)敏(min)(min)晶體(ti)管(guan)(guan)將(jiang)處于發光(guang)狀態。U1的(de)(de)OPTO輸(shu)入引腳(jiao)用于直接連接光(guang)敏(min)(min)晶體(ti)管(guan)(guan)，以極其簡單的(de)(de)方式構成(cheng)閉合環路。光(guang)敏(min)(min)晶體(ti)管(guan)(guan)導通(tong)(tong)(tong)時(shi)，OPTO引腳(jiao)被拉低，內部比較器在(zai)它的(de)(de)一個輸(shu)入端檢(jian)測(ce)到較低的(de)(de)電(dian)(dian)壓，比較器的(de)(de)另(ling)一輸(shu)入電(dian)(dian)流檢(jian)測(ce)引腳(jiao)(CS)的(de)(de)峰值(zhi)電(dian)(dian)壓，OPTO引腳(jiao)的(de)(de)低電(dian)(dian)壓將(jiang)在(zai)每個周期(qi)直接限(xian)制(zhi)功率MOSFETN1的(de)(de)峰值(zhi)電(dian)(dian)流。由于U1是一個PWM控(kong)制(zhi)器，電(dian)(dian)流限(xian)制(zhi)使(shi)得電(dian)(dian)壓調節(jie)受作用在(zai)N1上的(de)(de)占空比(導通(tong)(tong)(tong)時(shi)間(jian)與斷開時(shi)間(jian)的(de)(de)比值(zhi))的(de)(de)影(ying)響，環路穩定(ding)(ding)性(xing)通(tong)(tong)(tong)過調節(jie)R11和(he)(he)(he)C12來保證。R10和(he)(he)(he)C5(連接在(zai)U3的(de)(de)第3和(he)(he)(he)第4引腳(jiao))可消除穩定(ding)(ding)工作情況下(xia)的(de)(de)噪聲干擾。    

    U4的工作特(te)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)(dian)(dian)(dian)(dian)(dian)模式，內(nei)置(zhi)調(diao)整管和電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)檢測電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)。另外，它(ta)還集成了一個(ge)反向(xiang)(xiang)抑制二極(ji)管，在(zai)去掉輸入(ru)(ru)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源時(shi)(shi)可(ke)防(fang)止(zhi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池方(fang)向(xiang)(xiang)放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)。接(jie)入(ru)(ru)一個(ge)深度放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池時(shi)(shi)，U4能夠檢測到低電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(低于2.5V)，這樣，U4將首先啟(qi)(qi)動涓流(liu)充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)過程，在(zai)快充周(zhou)期的(de)1/10時(shi)(shi)間(jian)段對電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池進行涓流(liu)充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)，直到電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓上(shang)升到2.5V以上(shang)。在(zai)涓流(liu)充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)到快速充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)轉(zhuan)換時(shi)(shi)，軟啟(qi)(qi)動電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)迫(po)使充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)在(zai)3ms時(shi)(shi)間(jian)內(nei)緩慢(man)上(shang)升，避免較大的(de)浪涌電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)拉(la)低輸入(ru)(ru)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源。    

   上(shang)述功能的(de)(de)實施不需(xu)要微控制器的(de)(de)監管(guan)，而且，U4還具有(you)可(ke)編(bian)程的(de)(de)溫度限制功能，我們(men)會在下面就這一(yi)點進行詳細討論。用(yong)一(yi)個(ge)電(dian)(dian)阻設置最大充(chong)電(dian)(dian)電(dian)(dian)流(liu)，電(dian)(dian)阻兩端的(de)(de)電(dian)(dian)壓(ya)與充(chong)電(dian)(dian)電(dian)(dian)流(liu)成正比，通過監視該(gai)電(dian)(dian)壓(ya)即(ji)可(ke)連續跟(gen)蹤(zong)充(chong)電(dian)(dian)電(dian)(dian)流(liu)，更(geng)為復雜的(de)(de)系統(tong)也可(ke)以采用(yong)ADC跟(gen)蹤(zong)監視充(chong)電(dian)(dian)電(dian)(dian)流(liu)。    

    Maxim還提供有(you)另一款(kuan)與MAX1507非常相似的(de)充電(dian)(dian)器―MAX1555，這(zhe)款(kuan)IC能夠在沒有(you)交流電(dian)(dian)源的(de)情況下從(cong)USB主(zhu)機(ji)獲得充電(dian)(dian)電(dian)(dian)源。   

    電流調節 

    如果把Li+電(dian)(dian)池直接與一(yi)個低阻電(dian)(dian)壓源連(lian)接，充(chong)電(dian)(dian)電(dian)(dian)流(liu)將(jiang)不受任何限制(zhi)(zhi)(zhi)，這種狀況(kuang)會(hui)對電(dian)(dian)池及(ji)其周圍環境造(zao)成災難性(xing)的損(sun)害，從(cong)Li+電(dian)(dian)池的安全性(xing)考慮(lv)，必(bi)須(xu)將(jiang)其充(chong)電(dian)(dian)電(dian)(dian)流(liu)限制(zhi)(zhi)(zhi)在(zai)額, 定范圍以內。U4根據連(lian)接在(zai)ISET(第5引(yin)腳)的電(dian)(dian)阻限制(zhi)(zhi)(zhi)充(chong)電(dian)(dian)電(dian)(dian)流(liu)，圖1電(dian)(dian)路(lu)中選用了一(yi)個1.87kΩ的電(dian)(dian)阻，將(jiang)最大(da)電(dian)(dian)流(liu)設置在(zai)780mA: 

    Ifast=1461V/Rset=1461/1870≈780mA 

    電池電壓(ya)最(zui)小時，由(you)于(yu)輸(shu)入(ru)穩定在4.5V，U4輸(shu)入(ru)與輸(shu)出的(de)壓(ya)差最(zui)大(da)，如(ru)果此時電池是以最(zui)大(da)電流(liu)充電，U4功耗將(jiang)達到最(zui)大(da)值。例如(ru)，假(jia)設(she)電池電壓(ya)為2.5V，則對應的(de)功耗為：  

    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(wen)度(du)限(xian)制的(de)條件下(xia)將(jiang)以最大充電電流對電池充電，直到(dao)電池電壓(ya)達到(dao)內部設置的(de)4.2V