1、概述

　　凌力爾特（Linear Technology）推出的LTC3558型USB電池充電器，帶有兩個開關穩壓器，適合于在手持式設備中應用。LTC3558的充電器，采(cai)用(yong)恒流/恒壓(ya)(ya)(ya)(ya)算法，可以(yi)從USB電(dian)(dian)源汲(ji)取功率，也可以(yi)利用(yong)墻(qiang)上AC適配器(qi)對(dui)單節鋰離子/聚合物電(dian)(dian)池充(chong)電(dian)(dian)，可編程(cheng)最大充(chong)電(dian)(dian)電(dian)(dian)流達950mA。LTC3558的降(jiang)壓(ya)(ya)(ya)(ya)開(kai)關(guan)穩壓(ya)(ya)(ya)(ya)器(qi)和降(jiang)壓(ya)(ya)(ya)(ya)-升(sheng)壓(ya)(ya)(ya)(ya)開(kai)關(guan)穩壓(ya)(ya)(ya)(ya)器(qi)，在2.25MHz的固定頻率上操作(zuo)，輸出電(dian)(dian)壓(ya)(ya)(ya)(ya)可編程(cheng)，效率高(gao)于90%，能分別提(ti)供400mA的電(dian)(dian)流，為微控制器(qi)內核、微控制器(qi)I/O、存儲器(qi)、磁(ci)盤驅動器(qi)或其它(ta)邏輯電(dian)(dian)路加(jia)電(dian)(dian)。

　　2、LTC3558封(feng)裝、內部結(jie)構及引腳功能(neng)

　　LTC3558采用3mm×3mm×0.75mm的(de)21引腳(jiao)QFN封裝，引腳(jiao)排列如圖1所(suo)示。

圖1  LTC3558引腳排列(lie)

　　LTC3558含有一(yi)個線性USB電(dian)池(chi)充電(dian)器(qi)(qi)、一(yi)個電(dian)流模(mo)式(shi)降(jiang)壓（buck）開(kai)關(guan)穩壓器(qi)(qi)和一(yi)個電(dian)壓模(mo)式(shi)降(jiang)壓-升壓（buck-boost）開(kai)關(guan)穩壓器(qi)(qi)，其內部電(dian)路組成框(kuang)圖(tu)如圖(tu)２所示(shi)。

圖2  LTC3558內部電(dian)路

　　LTC3558各個引腳功能簡述如表1所示。

表1  LTC3558引腳(jiao)功能

　　3、LTC3558功(gong)能(neng)與工(gong)作原理

　　3.1  電(dian)池充(chong)電(dian)器(qi) 

　　3.1.1  欠(qian)電壓鎖定（UVLO）

　　IC（LTC3558）的UVLO電路監(jian)視輸(shu)入電壓(ya)Vcc，其值范圍(wei)為4.3~5.5V。在Vcc升高到VUVLO（4V）以上之前(qian)，充電器(qi)失能；當Vcc降落到電池電壓(ya)VBAT的50mV之內時，充電器(qi)也(ye)將(jiang)失效。

　　3.1.2  暫停模式

　　通過將IC引腳SUSP上拉到(dao)1.2V以(yi)上，充電(dian)器同(tong)樣(yang)會失效。在暫(zan)停模(mo)式(shi)，電(dian)池僅(jin)吸(xi)收(shou)1.5μA的電(dian)流(liu)，輸入電(dian)流(liu)Icc減小到(dao)8. 5μA。

　　3.1.3  編(bian)程充(chong)電電流(liu)

　　連接在IC引腳PROG上與地之間的電阻RPROG可以編程滿標度（full-scale）充電電流（即在恒流模式充電電流的100%），計算公式為ICHG=800V/RPROG。
在恒流模式，若將IC引腳HPWR拉至高電平，引腳PROG上的電壓VPROG=1V，對應于用滿標度電流充電；若引腳HPWR為低電平，VPROG則為0.2V，相應的充電電流為滿標度(du)電流（C）的20%（即0.2C）。

　　3.1.4  充電時序

　　在充(chong)(chong)電(dian)(dian)(dian)(dian)周期開始時，充(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)首先確定電(dian)(dian)(dian)(dian)池(chi)是否(fou)深度放電(dian)(dian)(dian)(dian)。如(ru)(ru)果電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓(ya)低(di)于2.9V，充(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)則自動進行涓流充(chong)(chong)電(dian)(dian)(dian)(dian)，充(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流為滿標(biao)度電(dian)(dian)(dian)(dian)流的10%（即0.1C）。如(ru)(ru)果電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓(ya)在2.9V以(yi)下持續時間(jian)達0.5h以(yi)上，充(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)則終止充(chong)(chong)電(dian)(dian)(dian)(dian)。

　　如(ru)果VBAT>2.9V，充(chong)電(dian)(dian)(dian)器則在恒(heng)流(liu)模式開始充(chong)電(dian)(dian)(dian)。當VBAT>4.105V時，充(chong)電(dian)(dian)(dian)器的(de)充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)將大大減(jian)小，則進入恒(heng)壓充(chong)電(dian)(dian)(dian)模式。如(ru)果IC內部(bu)4h的(de)安全定時器期滿，電(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)將停止。如(ru)果電(dian)(dian)(dian)池電(dian)(dian)(dian)壓VBAT降至4.105V以下，充(chong)電(dian)(dian)(dian)器將自動開始重新充(chong)電(dian)(dian)(dian)。

　　在充(chong)電過(guo)程中，如果芯片溫度約達(da)115℃，充(chong)電器將減小已(yi)編程的充(chong)電電流，以防止IC及其(qi)周圍功率元件(jian)因(yin)過(guo)熱(re)而損壞。

　　3.1.5  充電狀態指示(shi)

　　IC引(yin)腳 通過微處理(li)器(qi)或人工接口可以指(zhi)示正在(zai)充電(dian)(dian)(dian)、不充電(dian)(dian)(dian)、電(dian)(dian)(dian)池無(wu)反應和電(dian)(dian)(dian)池溫度超出范圍四種(zhong)可能狀態中的(de)一種(zhong)。比較簡單的(de)方法是(shi)利用(yong)LED作為指(zhi)示器(qi)，將其串聯一個限流電(dian)(dian)(dian)阻連接在(zai)IC引(yin)腳 與Vcc之間。

　　當充(chong)電(dian)開始后，IC引(yin)腳(jiao) 為低(di)電(dian)平，LED導通而被點亮(liang)，無閃爍現象發生。

　　當充(chong)電電流(liu)降至(zhi)滿標度電流(liu)的10%（即0.1C）以下時，引腳 釋(shi)放，呈現(xian)高阻抗(kang)，LED熄滅。

　　如果IC引腳NTC外部(bu)的NTC熱敏電阻出現故障，使(shi)電池溫(wen)度超(chao)出范圍，LED將出現閃爍(shuo)，頻率為1.5Hz，人眼很容易識別(bie)。

　　如果電(dian)池已壞或不良（電(dian)池電(dian)壓在(zai)2.9V以下(xia)超過(guo)0.5h），LED將在(zai)6.1Hz的頻率上(shang)閃爍，頻閃速率比NTC熱敏電(dian)阻故(gu)障(zhang)要快得多(duo)。

　　3.1.6  用NTC熱(re)敏電(dian)阻(zu)測量電(dian)池溫度

　　電池充電時的(de)溫(wen)度(du)可利用NTC熱敏電阻來測量，典(dian)型電路如圖3所示。

 

圖3  典型NTC熱敏電(dian)阻電(dian)路(lu)

　　圖中，RNTC為(wei)NTC熱敏(min)電(dian)(dian)阻，應(ying)將其(qi)緊貼(tie)電(dian)(dian)池盒。RNTC在(zai)25℃時的電(dian)(dian)阻值(zhi)(zhi)R25=100kΩ，與(yu)偏(pian)置(zhi)電(dian)(dian)阻RNOM值(zhi)(zhi)相等。IC引腳NTC內部較(jiao)(jiao)低(di)溫度（冷態(tai)）門(men)限為(wei)0.765Vcc，較(jiao)(jiao)高溫度（熱態(tai)）門(men)限為(wei)0.349Vcc（此時阻值(zhi)(zhi)降低(di)），使能門(men)限為(wei)0.017Vcc。

　　設RNTC熱解扣點和冷解扣點時的電阻值分別是RHOT和RCOLD，RHOT和RCOLD與R25之比率分別是rHOT和rCOLD，根據圖3可得：
　　由式（1）和式（2）得：
　　RHOT=0.536RNOM            （3）
　　RCOLD=3.25RNOM             （4）

　　RNOM=R25=100kΩ條(tiao)件下，rHOT=0.536，rCOLD=3.25。對(dui)于(yu)Vishay公(gong)司的(de)NTHS0603N011-N1003F來說，當溫(wen)(wen)度(du)升高其電(dian)(dian)阻值降至R25（100kΩ）的(de)0.536倍（約(yue)54kΩ）時，相(xiang)應的(de)溫(wen)(wen)度(du)約(yue)為40℃，充(chong)電(dian)(dian)器(qi)(qi)將暫(zan)停(ting)充(chong)電(dian)(dian)；當RNTC電(dian)(dian)阻值增加到R25的(de)3.25倍（即325kΩ）時，對(dui)應的(de)溫(wen)(wen)度(du)是0℃，充(chong)電(dian)(dian)器(qi)(qi)也(ye)會(hui)中止充(chong)電(dian)(dian)。很顯然，只有當電(dian)(dian)池溫(wen)(wen)度(du)高于(yu)0℃而低于(yu)40℃時，充(chong)電(dian)(dian)器(qi)(qi)才會(hui)充(chong)電(dian)(dian)。

　　如圖4所示，如果RNTC再加進一個偏置電阻R1，則可以將解扣點溫度改變。若冷和熱解扣點設置在0℃和45℃，根據Vishay公司提供的數據，rCOLD=3.266，rHOT=0.4368，偏置電阻RNOM值為：
 
　　誤差最接(jie)近1%的電阻值(zhi)是105kΩ。

　　附加電(dian)阻R1值為：

　　R1=0.536RNOM-rHOT?R25=0.536×105kΩ-0.436×100kΩ=12.6kΩ

　　誤(wu)差(cha)最接近(jin)1%的電阻(zu)值(zhi)是(shi)12.7kΩ。

　　RNOM和R1按以上計算(suan)結果選擇，當(dang)電池溫度(du)升(sheng)至45℃或降(jiang)為(wei)0℃時，充(chong)電器都將暫停充(chong)電。

圖(tu)4  帶附(fu)加(jia)偏(pian)置(zhi)電(dian)阻的NTC

3.2  降壓開關(guan)穩壓器

　　LTC3558的電(dian)流(liu)模式降壓(ya)(ya)開關穩壓(ya)(ya)器(qi)主要由PWM控制器(qi)、主開關P溝(gou)道(dao)(dao)MOSFET（MP）、N溝(gou)道(dao)(dao)MOSFET（MN）同步(bu)整流(liu)器(qi)及誤(wu)差放大器(qi)等組成(cheng)，如圖(tu)5所(suo)示。

 

圖5  降(jiang)壓(ya)(ya)開關穩壓(ya)(ya)器(qi)電路(lu)

　　LTC3558還含有軟(ruan)啟(qi)動、短路(lu)(lu)保護和(he)減小(xiao)輻射EM1的(de)開(kai)關節點擺幅限制電路(lu)(lu)。

　　降壓(ya)穩壓(ya)器可(ke)(ke)以提(ti)供直到400mA的電(dian)流(liu)，最低輸出電(dian)壓(ya)為0.8V。降壓(ya)穩壓(ya)器輸出電(dian)壓(ya)可(ke)(ke)通(tong)過IC引(yin)腳FB1上的R1/R2來編程(cheng)，其值為VOUT=0.8V（1+R1+R2）。R1取(qu)值范圍(wei)為40kΩ~1MΩ，CFB容量(liang)在2PF與22PF之(zhi)間。

　　通(tong)過IC引腳MODE，可(ke)以設置(zhi)開關穩壓器的突發操作模式或脈沖跳(tiao)越模式。

　　3.2.1  脈沖(chong)跳(tiao)越模式(shi)（Pulse Skip Mode）

　　在(zai)脈(mo)沖跳越(yue)模(mo)(mo)式，誤(wu)差放(fang)大器調(diao)節(jie)峰值電感(gan)器電流(liu)，以(yi)交(jiao)付所需(xu)要的輸(shu)出功率。在(zai)輕載時，在(zai)每個(ge)脈(mo)沖上電感(gan)電流(liu)可以(yi)達到零（關(guan)斷同(tong)步整流(liu)器MIN），在(zai)不連續模(mo)(mo)式操作。在(zai)非常(chang)輕的負載下，降壓(ya)(ya)開關(guan)穩壓(ya)(ya)器將會自動跳過(guo)脈(mo)沖，以(yi)維持輸(shu)出電壓(ya)(ya)調(diao)節(jie)。在(zai)高(gao)占空(kong)因數（VOUT>PVIN1/2）時，電感(gan)電流(liu)可能(neng)會反向，致(zhi)使降壓(ya)(ya)變換器開關(guan)連續。欲利(li)用脈(mo)沖跳越(yue)模(mo)(mo)式，需(xu)使IC引腳MODE保(bao)持低電平(ping)。

　　3.2.2  突發(fa)模式（Burst Mode）

　　將IC引(yin)腳MODE拉至高(gao)(gao)電(dian)平(ping)，降壓(ya)(ya)變換器(qi)將在(zai)突發模(mo)(mo)式操作，自動在(zai)固定頻(pin)率PWM操作與(yu)滯后控(kong)制(zhi)之間轉換。當輸出電(dian)容的充電(dian)電(dian)壓(ya)(ya)稍(shao)高(gao)(gao)于調(diao)節(jie)點(dian)時，降壓(ya)(ya)變換器(qi)則自動進入(ru)睡(shui)眠模(mo)(mo)式。在(zai)睡(shui)眠模(mo)(mo)式，開關穩壓(ya)(ya)器(qi)的大多數電(dian)路(lu)將切斷電(dian)源(yuan)，因此在(zai)輕載(zai)時有著比脈沖跳躍模(mo)(mo)式更高(gao)(gao)的效率。

　　3.3  降壓-升壓開關穩壓器

　　LTC3558的降壓(ya)-升(sheng)壓(ya)開關穩壓(ya)器在電(dian)(dian)壓(ya)模式和2.25MHz的固定(ding)頻率上操作，能提供直到400mA的輸出負載電(dian)(dian)流，輸出電(dian)(dian)壓(ya)可在2.75V與5.45V之間來(lai)編程(cheng)，其值為VOUT2=0.8V(1+R1+R2)。R1與R2為IC引(yin)腳FB2上的分壓(ya)器電(dian)(dian)阻，如圖6所示。

　　當輸(shu)(shu)(shu)入電(dian)壓遠高于輸(shu)(shu)(shu)出電(dian)壓時(shi)，穩壓器工(gong)作(zuo)在降(jiang)壓模式(shi)；當輸(shu)(shu)(shu)入電(dian)壓遠低于輸(shu)(shu)(shu)出電(dian)壓時(shi)，穩壓器則在升壓模式(shi)操作(zuo)。

　　3.3.1  PWM操作模式與突發模式操作

　　通過(guo)將IC引(yin)腳(jiao)保(bao)持(chi)低電平，降壓-升壓變(bian)換器將進入PWM操作模式。在(zai)適(shi)度的重載(zai)條(tiao)件下，在(zai)定頻PWM操作有著最小的開關噪聲(sheng)。

　　若將(jiang)IC引腳MODE拉(la)至高電平，穩壓器(qi)將(jiang)進入(ru)突發(fa)模(mo)式(shi)。在(zai)(zai)較輕的負載下(xia)，在(zai)(zai)突發(fa)模(mo)式(shi)操作(zuo)，輸出(chu)(chu)電流被限(xian)制在(zai)(zai)50mA。當(dang)輸出(chu)(chu)電容上的充電電壓稍高于(yu)調節點時，穩壓器(qi)則進入(ru)睡眠模(mo)式(shi)。在(zai)(zai)突發(fa)模(mo)式(shi)操作(zuo)，能使開(kai)關損耗大(da)大(da)降低，易于(yu)滿足輕載效(xiao)率(lv)標準要求(qiu)。

　　3.3.2  環路補償

　　降(jiang)壓(ya)-升壓(ya)開關(guan)穩壓(ya)器(qi)可以采用簡單的I型補(bu)償（見圖7）

　　IC引腳FB2是誤差放(fang)大器(qi)反相輸入端，引腳VC2是誤差放(fang)大器(qi)輸出端。誤差放(fang)大器(qi)單(dan)位(wei)增益頻率fUG可表(biao)示為(wei)：

　　圖(tu)8所示為誤差(cha)放大器(qi)環(huan)路的III型補償網絡，其(qi)傳輸函數為：

圖8  III型補償電路(lu)

　　式中，s為傳輸系數。

　　III型補償(chang)網絡可(ke)獲(huo)得較大的(de)頻帶(dai)寬度(du)，改善(shan)瞬態響應，減(jian)小啟動時VOUT2產生的(de)過(guo)沖。對于3.3V的(de)輸出，補償(chang)元(yuan)件推薦值分別為：R1=324kΩ，RFB=105 kΩ，C1=10pF，R2=15 kΩ，C2=330pF，R3=121 kΩ，C3=33pF及COUT=22μF和LOUT=2. 2μH。

　　4、實(shi)際應用(yong)電路(lu)

　　LTC3558可以在沒有(you)電池連接到引腳(jiao)BAT上(shang)的(de)情況下操作，如圖9所示(shi)。

圖9  LTC3558可以在(zai)無電池連接時操(cao)作

　　降(jiang)壓(ya)(ya)(ya)開關(guan)穩壓(ya)(ya)(ya)器(qi)輸(shu)出(chu)為(wei)1.8V（@400mA），降(jiang)壓(ya)(ya)(ya)-升壓(ya)(ya)(ya)開關(guan)穩壓(ya)(ya)(ya)器(qi)輸(shu)出(chu)為(wei)3.3V（@400mA）。在(zai)IC引腳(jiao)SWAB2與SWCD2之(zhi)間連(lian)接的(de)2.2μH的(de)電(dian)(dian)感(gan)器(qi)為(wei)降(jiang)壓(ya)(ya)(ya)-升壓(ya)(ya)(ya)穩壓(ya)(ya)(ya)器(qi)的(de)輸(shu)出(chu)電(dian)(dian)感(gan)器(qi)。NTC熱(re)敏電(dian)(dian)阻（R25=100kΩ）的(de)偏(pian)置電(dian)(dian)阻分(fen)別為(wei)110kΩ和28.7kΩ，故充電(dian)(dian)器(qi)的(de)熱(re)和冷(leng)解(jie)扣點分(fen)別設置在(zai)55℃和0℃。在(zai)IC引腳(jiao)BAT上連(lian)接的(de)1Ω的(de)電(dian)(dian)阻和4.7μF的(de)電(dian)(dian)容，可保(bao)證充電(dian)(dian)器(qi)穩定。

　　從墻上(shang)AC適配(pei)器加電的LTC3558應用電路(lu)如圖(tu)10所示。

圖10  從墻上(shang)適配器(qi)加電的應用電路

　　其充電(dian)器(qi)的充電(dian)電(dian)流可達950mA，降壓開關穩壓器(qi)的輸出為1.2V（@400mA），降壓-升壓開關穩壓器(qi)的輸出為3.3V（@400mA）。充電(dian)器(qi)通過(guo)NTC熱敏電(dian)阻將(jiang)電(dian)池(chi)溫度解扣點設置在0℃和40℃，LED用作(zuo)充電(dian)狀態指(zhi)示。

　　5、結束語

　　LTC3558是一種帶兩個高效開關穩壓器的USB充電器，它是(shi)為應用(yong)于(yu)小功率手持式設(she)(she)備(bei)而專門設(she)(she)計的。直(zhi)達950mA的可(ke)編程(cheng)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流可(ke)對(dui)單節(jie)鋰離子(zi)或聚合物(wu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)，在充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)過程(cheng)中電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)狀(zhuang)態被監視(shi)，4h的安全定時(shi)(shi)器(qi)期滿(man)時(shi)(shi)控(kong)制充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)終止。利用(yong)NTC熱敏電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻來(lai)測量電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)溫(wen)度(du)并設(she)(she)置電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)時(shi)(shi)的溫(wen)度(du)范圍(wei)。LTC3558降壓(ya)(ya)和降壓(ya)(ya)-升(sheng)壓(ya)(ya)開關穩壓(ya)(ya)器(qi)輸(shu)(shu)出電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)均可(ke)編程(cheng)，效率高于(yu)90%，分(fen)別能提供直(zhi)到(dao)400mA的輸(shu)(shu)出電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流，為微控(kong)制器(qi)內核及(ji)其I/O、存儲(chu)器(qi)、磁盤驅動器(qi)或其它邏輯電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)供電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。