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兩個開關穩壓器的USB電池充電器

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,采用恒(heng)流(liu)/恒(heng)壓(ya)(ya)(ya)算法,可以從USB電(dian)(dian)源(yuan)汲取功率(lv)(lv),也(ye)可以利用墻上(shang)AC適配器對(dui)單節鋰離子/聚合物電(dian)(dian)池充電(dian)(dian),可編程(cheng)最(zui)大充電(dian)(dian)電(dian)(dian)流(liu)達950mA。LTC3558的(de)降(jiang)壓(ya)(ya)(ya)開關穩(wen)壓(ya)(ya)(ya)器和降(jiang)壓(ya)(ya)(ya)-升壓(ya)(ya)(ya)開關穩(wen)壓(ya)(ya)(ya)器,在2.25MHz的(de)固(gu)定頻率(lv)(lv)上(shang)操作,輸出電(dian)(dian)壓(ya)(ya)(ya)可編程(cheng),效率(lv)(lv)高于90%,能(neng)分別提供(gong)400mA的(de)電(dian)(dian)流(liu),為微控制器內核、微控制器I/O、存(cun)儲器、磁盤(pan)驅動器或其它(ta)邏(luo)輯(ji)電(dian)(dian)路加電(dian)(dian)。

  2、LTC3558封裝、內部結構及引腳功(gong)能

  LTC3558采用3mm×3mm×0.75mm的(de)21引腳(jiao)QFN封(feng)裝,引腳(jiao)排列如圖1所示。

圖1  LTC3558引腳(jiao)排列

  LTC3558含有一個線性USB電(dian)池充電(dian)器、一個電(dian)流模式(shi)降(jiang)壓(ya)(ya)(buck)開關穩壓(ya)(ya)器和一個電(dian)壓(ya)(ya)模式(shi)降(jiang)壓(ya)(ya)-升壓(ya)(ya)(buck-boost)開關穩壓(ya)(ya)器,其(qi)內部電(dian)路組成框圖如圖2所(suo)示(shi)。

圖2  LTC3558內部(bu)電路

  LTC3558各(ge)個引(yin)腳功能簡述如表1所示。

表1  LTC3558引腳(jiao)功能

  3、LTC3558功能與工作原理(li)

  3.1  電池充電器(qi) 

  3.1.1  欠(qian)電(dian)壓鎖定(ding)(UVLO)

  IC(LTC3558)的UVLO電(dian)(dian)(dian)路監視(shi)輸入電(dian)(dian)(dian)壓Vcc,其值范圍為4.3~5.5V。在Vcc升高到VUVLO(4V)以上(shang)之(zhi)(zhi)前,充電(dian)(dian)(dian)器(qi)失能;當Vcc降落到電(dian)(dian)(dian)池電(dian)(dian)(dian)壓VBAT的50mV之(zhi)(zhi)內時,充電(dian)(dian)(dian)器(qi)也將失效。

  3.1.2  暫(zan)停模式

  通過(guo)將IC引腳(jiao)SUSP上拉(la)到1.2V以上,充(chong)電器同樣會(hui)失效。在暫(zan)停模(mo)式,電池僅吸(xi)收(shou)1.5μA的電流(liu),輸入電流(liu)Icc減小到8. 5μA。

  3.1.3  編(bian)程充電電流

  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(man)標度電流(C)的20%(即0.2C)。

  3.1.4  充電(dian)時序(xu)

  在(zai)充(chong)(chong)(chong)電(dian)(dian)(dian)周(zhou)期開(kai)始時,充(chong)(chong)(chong)電(dian)(dian)(dian)器首(shou)先確(que)定電(dian)(dian)(dian)池是否深(shen)度放電(dian)(dian)(dian)。如果電(dian)(dian)(dian)池電(dian)(dian)(dian)壓低于2.9V,充(chong)(chong)(chong)電(dian)(dian)(dian)器則自動進(jin)行涓流充(chong)(chong)(chong)電(dian)(dian)(dian),充(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流為滿(man)標度電(dian)(dian)(dian)流的10%(即0.1C)。如果電(dian)(dian)(dian)池電(dian)(dian)(dian)壓在(zai)2.9V以(yi)下持續時間達0.5h以(yi)上,充(chong)(chong)(chong)電(dian)(dian)(dian)器則終止充(chong)(chong)(chong)電(dian)(dian)(dian)。

  如果VBAT>2.9V,充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)則(ze)在恒流(liu)模式(shi)開(kai)始(shi)(shi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)。當(dang)VBAT>4.105V時,充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)的充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)將(jiang)大(da)大(da)減小,則(ze)進入恒壓充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)模式(shi)。如果IC內部(bu)4h的安全定時器(qi)期滿,電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)將(jiang)停止(zhi)。如果電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓VBAT降至4.105V以下(xia),充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)將(jiang)自動開(kai)始(shi)(shi)重新充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)。

  在充(chong)(chong)電過程中(zhong),如果芯片溫度約達115℃,充(chong)(chong)電器(qi)將減小已(yi)編程的充(chong)(chong)電電流,以防止IC及其周圍功率(lv)元件因過熱而損(sun)壞(huai)。

  3.1.5  充(chong)電狀(zhuang)態指示

  IC引腳(jiao) 通過微(wei)處理(li)器或人工接口可(ke)以指(zhi)示正在充電、不充電、電池(chi)無反應和電池(chi)溫度超出范圍四種(zhong)可(ke)能狀態(tai)中的(de)一種(zhong)。比較簡單的(de)方法是利用LED作為指(zhi)示器,將其串聯一個限流電阻連(lian)接在IC引腳(jiao) 與Vcc之間。

  當充(chong)電開(kai)始后,IC引腳 為(wei)低電平,LED導通而被點亮,無(wu)閃爍現象發生。

  當充電(dian)電(dian)流(liu)降至(zhi)滿標度(du)電(dian)流(liu)的10%(即0.1C)以(yi)下時,引腳 釋放,呈現(xian)高(gao)阻抗,LED熄滅(mie)。

  如(ru)果(guo)IC引腳NTC外部的NTC熱敏電阻出現故障,使電池溫度超(chao)出范圍,LED將出現閃爍,頻率為1.5Hz,人眼(yan)很(hen)容易(yi)識別。

  如(ru)果(guo)電池(chi)已(yi)壞或不(bu)良(電池(chi)電壓在(zai)2.9V以下超過0.5h),LED將在(zai)6.1Hz的(de)頻率(lv)上閃(shan)爍,頻閃(shan)速率(lv)比(bi)NTC熱敏電阻故障要快得多(duo)。

  3.1.6  用NTC熱敏電(dian)阻(zu)測量電(dian)池(chi)溫度(du)

  電池充電時的溫度可利用NTC熱敏電阻來測量,典型電路如(ru)圖3所示。

 

圖(tu)3  典型NTC熱敏電阻電路

  圖(tu)中,RNTC為(wei)NTC熱敏電阻(zu),應(ying)將其(qi)緊(jin)貼(tie)電池盒。RNTC在(zai)25℃時(shi)的電阻(zu)值(zhi)R25=100kΩ,與偏(pian)置電阻(zu)RNOM值(zhi)相等。IC引(yin)腳NTC內部(bu)較低溫度(冷(leng)態(tai))門(men)(men)限(xian)為(wei)0.765Vcc,較高溫度(熱態(tai))門(men)(men)限(xian)為(wei)0.349Vcc(此(ci)時(shi)阻(zu)值(zhi)降低),使能門(men)(men)限(xian)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條件下,rHOT=0.536,rCOLD=3.25。對于(yu)Vishay公司的(de)NTHS0603N011-N1003F來(lai)說,當溫(wen)(wen)度(du)(du)升高(gao)其電(dian)(dian)阻(zu)值降至R25(100kΩ)的(de)0.536倍(約54kΩ)時,相應的(de)溫(wen)(wen)度(du)(du)約為40℃,充(chong)電(dian)(dian)器(qi)將暫停(ting)充(chong)電(dian)(dian);當RNTC電(dian)(dian)阻(zu)值增加到R25的(de)3.25倍(即325kΩ)時,對應的(de)溫(wen)(wen)度(du)(du)是0℃,充(chong)電(dian)(dian)器(qi)也會中止充(chong)電(dian)(dian)。很顯然(ran),只有當電(dian)(dian)池溫(wen)(wen)度(du)(du)高(gao)于(yu)0℃而低于(yu)40℃時,充(chong)電(dian)(dian)器(qi)才會充(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:
 
  誤差最接近1%的電阻值是(shi)105kΩ。

  附加電阻R1值(zhi)為:

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

  誤差最(zui)接(jie)近(jin)1%的(de)電(dian)阻值是12.7kΩ。

  RNOM和R1按以上計算結果選擇(ze),當電池(chi)溫度(du)升至45℃或(huo)降為0℃時,充(chong)(chong)電器都將暫停充(chong)(chong)電。

圖(tu)4  帶附加偏置(zhi)電阻的NTC

3.2  降壓開(kai)關穩(wen)壓器(qi)

  LTC3558的電流模式降(jiang)壓(ya)(ya)開(kai)關穩壓(ya)(ya)器(qi)主(zhu)要由PWM控(kong)制器(qi)、主(zhu)開(kai)關P溝道(dao)MOSFET(MP)、N溝道(dao)MOSFET(MN)同(tong)步整流器(qi)及誤差(cha)放大器(qi)等組成,如(ru)圖5所(suo)示。

 

圖(tu)5  降壓開(kai)關穩壓器電路

  LTC3558還含(han)有軟啟動、短路保(bao)護和減小輻射EM1的開關節(jie)點擺幅限制(zhi)電路。

  降(jiang)壓穩壓器(qi)可以提供直(zhi)到(dao)400mA的電(dian)流,最低輸(shu)出電(dian)壓為0.8V。降(jiang)壓穩壓器(qi)輸(shu)出電(dian)壓可通過IC引腳FB1上的R1/R2來(lai)編程,其值(zhi)(zhi)為VOUT=0.8V(1+R1+R2)。R1取(qu)值(zhi)(zhi)范圍為40kΩ~1MΩ,CFB容(rong)量在(zai)2PF與(yu)22PF之(zhi)間。

  通過(guo)IC引(yin)腳MODE,可以設置開關穩壓器的(de)突(tu)發操作模(mo)式或(huo)脈沖跳越模(mo)式。

  3.2.1  脈沖跳越模(mo)式(Pulse Skip Mode)

  在(zai)脈(mo)沖跳(tiao)越模式,誤差放大器(qi)調節(jie)峰值電(dian)感(gan)器(qi)電(dian)流,以(yi)交付所需要的輸出功(gong)率。在(zai)輕載時(shi),在(zai)每(mei)個脈(mo)沖上電(dian)感(gan)電(dian)流可以(yi)達到零(關斷(duan)同步整流器(qi)MIN),在(zai)不連續模式操作。在(zai)非(fei)常輕的負載下(xia),降壓(ya)開關穩壓(ya)器(qi)將會(hui)自動跳(tiao)過脈(mo)沖,以(yi)維持輸出電(dian)壓(ya)調節(jie)。在(zai)高(gao)占(zhan)空因數(VOUT>PVIN1/2)時(shi),電(dian)感(gan)電(dian)流可能會(hui)反(fan)向,致使(shi)(shi)降壓(ya)變換器(qi)開關連續。欲利用脈(mo)沖跳(tiao)越模式,需使(shi)(shi)IC引(yin)腳MODE保(bao)持低電(dian)平。

  3.2.2  突發(fa)模式(Burst Mode)

  將(jiang)IC引(yin)腳(jiao)MODE拉至高電(dian)(dian)平,降壓(ya)變換器將(jiang)在(zai)突發模(mo)式(shi)操作,自(zi)動(dong)在(zai)固定頻(pin)率PWM操作與滯后控制之間轉換。當輸出電(dian)(dian)容的(de)充電(dian)(dian)電(dian)(dian)壓(ya)稍(shao)高于調節點時,降壓(ya)變換器則自(zi)動(dong)進入睡(shui)眠(mian)模(mo)式(shi)。在(zai)睡(shui)眠(mian)模(mo)式(shi),開關穩壓(ya)器的(de)大多數電(dian)(dian)路將(jiang)切(qie)斷電(dian)(dian)源,因此在(zai)輕載時有著比脈沖(chong)跳躍模(mo)式(shi)更高的(de)效率。

  3.3  降壓-升壓開關(guan)穩壓器(qi)

  LTC3558的(de)(de)降壓(ya)(ya)-升壓(ya)(ya)開關(guan)穩壓(ya)(ya)器(qi)在電(dian)(dian)壓(ya)(ya)模式(shi)和2.25MHz的(de)(de)固定(ding)頻(pin)率上操作,能(neng)提(ti)供直到400mA的(de)(de)輸(shu)出(chu)負載電(dian)(dian)流,輸(shu)出(chu)電(dian)(dian)壓(ya)(ya)可在2.75V與5.45V之間來編程,其值為VOUT2=0.8V(1+R1+R2)。R1與R2為IC引腳FB2上的(de)(de)分壓(ya)(ya)器(qi)電(dian)(dian)阻,如圖(tu)6所示。

  當(dang)輸入電(dian)(dian)(dian)壓遠高于(yu)輸出(chu)電(dian)(dian)(dian)壓時,穩(wen)壓器(qi)工作(zuo)在降壓模(mo)式;當(dang)輸入電(dian)(dian)(dian)壓遠低于(yu)輸出(chu)電(dian)(dian)(dian)壓時,穩(wen)壓器(qi)則在升壓模(mo)式操作(zuo)。

  3.3.1  PWM操(cao)作模(mo)式與突發模(mo)式操(cao)作

  通過將(jiang)IC引腳(jiao)保持低(di)電(dian)平,降壓-升壓變換器將(jiang)進入PWM操作模式。在適度(du)的(de)重載條件下(xia),在定頻(pin)PWM操作有著最小的(de)開關噪聲。

  若將IC引腳MODE拉至(zhi)高(gao)電(dian)(dian)平,穩壓器(qi)將進入(ru)突發(fa)模式(shi)。在(zai)較輕的(de)負載下,在(zai)突發(fa)模式(shi)操作(zuo),輸出電(dian)(dian)流(liu)被限制在(zai)50mA。當輸出電(dian)(dian)容上(shang)的(de)充電(dian)(dian)電(dian)(dian)壓稍高(gao)于調(diao)節(jie)點時,穩壓器(qi)則進入(ru)睡眠模式(shi)。在(zai)突發(fa)模式(shi)操作(zuo),能使開(kai)關損耗大大降低,易于滿足輕載效率標準要求。

  3.3.2  環路補償

  降(jiang)壓(ya)(ya)-升壓(ya)(ya)開關穩壓(ya)(ya)器可以采(cai)用(yong)簡單(dan)的I型補償(chang)(見圖7)

  IC引腳FB2是誤差放大器反相(xiang)輸(shu)入端,引腳VC2是誤差放大器輸(shu)出端。誤差放大器單位(wei)增(zeng)益頻率fUG可表示為(wei):


  圖8所示為誤差放大器環路的(de)III型補償網絡,其傳輸函數為:

圖8  III型補償電路

  式中,s為傳(chuan)輸系(xi)數。

  III型(xing)補償網絡可(ke)獲得較大的(de)頻帶寬(kuan)度(du),改善瞬態響應,減小啟動時VOUT2產生的(de)過沖(chong)。對于3.3V的(de)輸出(chu),補償元件推薦值(zhi)分(fen)別為:R1=324kΩ,RFB=105 kΩ,C1=10pF,R2=15 kΩ,C2=330pF,R3=121 kΩ,C3=33pF及(ji)COUT=22μF和LOUT=2. 2μH。

  4、實際應(ying)用電路

  LTC3558可以在沒有電池連接到(dao)引腳BAT上的情況下操作(zuo),如(ru)圖9所示。

圖9  LTC3558可以在無電池連接時操(cao)作(zuo)

  降(jiang)壓(ya)(ya)(ya)開(kai)關穩(wen)(wen)壓(ya)(ya)(ya)器輸(shu)出(chu)(chu)為(wei)1.8V(@400mA),降(jiang)壓(ya)(ya)(ya)-升(sheng)(sheng)壓(ya)(ya)(ya)開(kai)關穩(wen)(wen)壓(ya)(ya)(ya)器輸(shu)出(chu)(chu)為(wei)3.3V(@400mA)。在IC引(yin)腳SWAB2與SWCD2之(zhi)間連接的(de)2.2μH的(de)電(dian)感(gan)器為(wei)降(jiang)壓(ya)(ya)(ya)-升(sheng)(sheng)壓(ya)(ya)(ya)穩(wen)(wen)壓(ya)(ya)(ya)器的(de)輸(shu)出(chu)(chu)電(dian)感(gan)器。NTC熱(re)敏(min)電(dian)阻(R25=100kΩ)的(de)偏(pian)置電(dian)阻分(fen)(fen)別為(wei)110kΩ和28.7kΩ,故充電(dian)器的(de)熱(re)和冷解扣點分(fen)(fen)別設置在55℃和0℃。在IC引(yin)腳BAT上連接的(de)1Ω的(de)電(dian)阻和4.7μF的(de)電(dian)容,可保證充電(dian)器穩(wen)(wen)定。

  從墻上AC適配(pei)器(qi)加電的LTC3558應用(yong)電路如(ru)圖(tu)10所示。

圖10  從(cong)墻上適配器加電(dian)的應用電(dian)路(lu)

  其充(chong)電器(qi)的(de)充(chong)電電流(liu)可達(da)950mA,降壓(ya)(ya)開關穩壓(ya)(ya)器(qi)的(de)輸出(chu)為1.2V(@400mA),降壓(ya)(ya)-升壓(ya)(ya)開關穩壓(ya)(ya)器(qi)的(de)輸出(chu)為3.3V(@400mA)。充(chong)電器(qi)通過NTC熱敏電阻將電池(chi)溫度解(jie)扣點設置在0℃和(he)40℃,LED用(yong)作充(chong)電狀態指(zhi)示(shi)。

  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(wei)應用(yong)于小功率(lv)手持式設備而專門設計的(de)。直達950mA的(de)可(ke)(ke)編程(cheng)充電(dian)(dian)電(dian)(dian)流(liu)可(ke)(ke)對單節鋰離子或(huo)聚合(he)物電(dian)(dian)池充電(dian)(dian),在充電(dian)(dian)過(guo)程(cheng)中電(dian)(dian)池狀態被(bei)監視(shi),4h的(de)安全(quan)定時(shi)器(qi)(qi)期滿時(shi)控制充電(dian)(dian)器(qi)(qi)終(zhong)止。利用(yong)NTC熱敏電(dian)(dian)阻來測量電(dian)(dian)池溫度并設置(zhi)電(dian)(dian)池充電(dian)(dian)時(shi)的(de)溫度范圍。LTC3558降(jiang)壓(ya)和降(jiang)壓(ya)-升壓(ya)開(kai)關穩壓(ya)器(qi)(qi)輸出(chu)電(dian)(dian)壓(ya)均可(ke)(ke)編程(cheng),效(xiao)率(lv)高于90%,分(fen)別能提供(gong)直到400mA的(de)輸出(chu)電(dian)(dian)流(liu),為(wei)微控制器(qi)(qi)內核及其I/O、存儲器(qi)(qi)、磁盤驅動器(qi)(qi)或(huo)其它邏輯(ji)電(dian)(dian)路供(gong)電(dian)(dian)。

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