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,采用恒流/恒壓(ya)算法,可(ke)以從USB電(dian)(dian)源(yuan)汲取功率,也可(ke)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(ke)編(bian)程(cheng)最(zui)大充電(dian)(dian)電(dian)(dian)流達950mA。LTC3558的降(jiang)壓(ya)開(kai)關穩(wen)壓(ya)器和降(jiang)壓(ya)-升壓(ya)開(kai)關穩(wen)壓(ya)器,在2.25MHz的固(gu)定頻率上(shang)操作,輸出(chu)電(dian)(dian)壓(ya)可(ke)編(bian)程(cheng),效率高于90%,能分別提供400mA的電(dian)(dian)流,為微(wei)控(kong)制(zhi)器內核、微(wei)控(kong)制(zhi)器I/O、存(cun)儲(chu)器、磁盤驅(qu)動(dong)器或(huo)其它邏輯電(dian)(dian)路加電(dian)(dian)。
2、LTC3558封裝、內部結(jie)構及引腳功能
LTC3558采用(yong)3mm×3mm×0.75mm的(de)21引腳(jiao)QFN封裝,引腳(jiao)排列(lie)如圖1所(suo)示。
圖1 LTC3558引腳(jiao)排列
LTC3558含(han)有一(yi)個(ge)線性USB電(dian)(dian)(dian)池充電(dian)(dian)(dian)器、一(yi)個(ge)電(dian)(dian)(dian)流模(mo)式降(jiang)壓(ya)(buck)開關(guan)穩壓(ya)器和一(yi)個(ge)電(dian)(dian)(dian)壓(ya)模(mo)式降(jiang)壓(ya)-升壓(ya)(buck-boost)開關(guan)穩壓(ya)器,其內部電(dian)(dian)(dian)路組成框(kuang)圖如圖2所示。
圖2 LTC3558內部電路
LTC3558各個(ge)引腳功能簡述如表1所示。
表1 LTC3558引腳功(gong)能
3、LTC3558功能與工作原理(li)
3.1 電池(chi)充(chong)電器
3.1.1 欠(qian)電(dian)壓鎖(suo)定(UVLO)
IC(LTC3558)的UVLO電(dian)路監(jian)視輸入(ru)電(dian)壓Vcc,其值(zhi)范圍為4.3~5.5V。在(zai)Vcc升高(gao)到(dao)VUVLO(4V)以(yi)上之前,充(chong)電(dian)器(qi)失(shi)能;當Vcc降落(luo)到(dao)電(dian)池電(dian)壓VBAT的50mV之內時,充(chong)電(dian)器(qi)也將(jiang)失(shi)效。
3.1.2 暫(zan)停模式
通(tong)過將IC引腳SUSP上拉到1.2V以上,充(chong)電器同樣(yang)會(hui)失效。在暫停(ting)模式,電池僅吸收1.5μA的電流(liu)(liu),輸入(ru)電流(liu)(liu)Icc減小到8. 5μA。
3.1.3 編(bian)程(cheng)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標度電(dian)流(liu)(C)的20%(即0.2C)。
3.1.4 充電時序
在充(chong)電(dian)(dian)(dian)周期(qi)開始(shi)時,充(chong)電(dian)(dian)(dian)器(qi)首(shou)先確定電(dian)(dian)(dian)池是否深度放電(dian)(dian)(dian)。如果(guo)電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya)低于2.9V,充(chong)電(dian)(dian)(dian)器(qi)則自(zi)動進行涓流充(chong)電(dian)(dian)(dian),充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流為滿標度電(dian)(dian)(dian)流的10%(即(ji)0.1C)。如果(guo)電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya)在2.9V以下持續時間達0.5h以上(shang),充(chong)電(dian)(dian)(dian)器(qi)則終(zhong)止充(chong)電(dian)(dian)(dian)。
如(ru)果VBAT>2.9V,充電(dian)(dian)(dian)(dian)(dian)器則(ze)在(zai)恒流模式開始充電(dian)(dian)(dian)(dian)(dian)。當VBAT>4.105V時(shi),充電(dian)(dian)(dian)(dian)(dian)器的充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流將大大減小(xiao),則(ze)進入(ru)恒壓(ya)充電(dian)(dian)(dian)(dian)(dian)模式。如(ru)果IC內部4h的安全定時(shi)器期滿,電(dian)(dian)(dian)(dian)(dian)池充電(dian)(dian)(dian)(dian)(dian)將停止。如(ru)果電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓(ya)VBAT降至4.105V以下,充電(dian)(dian)(dian)(dian)(dian)器將自動開始重新充電(dian)(dian)(dian)(dian)(dian)。
在充(chong)電(dian)過程中(zhong),如果芯(xin)片溫度(du)約達115℃,充(chong)電(dian)器將減小已編(bian)程的充(chong)電(dian)電(dian)流(liu),以(yi)防止IC及其周圍功(gong)率元件因(yin)過熱而損壞。
3.1.5 充電(dian)狀(zhuang)態指示
IC引腳 通過(guo)微處理器或(huo)人工接口可以指示(shi)正在(zai)充電(dian)、不(bu)充電(dian)、電(dian)池無反(fan)應和電(dian)池溫(wen)度超(chao)出范圍四種可能狀態中的一(yi)種。比較(jiao)簡(jian)單的方法是利用LED作為指示(shi)器,將其串聯(lian)一(yi)個限(xian)流電(dian)阻連接在(zai)IC引腳 與Vcc之(zhi)間(jian)。
當(dang)充(chong)電開始后,IC引腳 為低電平,LED導通而(er)被點(dian)亮,無閃(shan)爍(shuo)現象發生(sheng)。
當充電電流(liu)(liu)降至(zhi)滿標度電流(liu)(liu)的10%(即0.1C)以下時(shi),引腳 釋(shi)放,呈現高阻抗,LED熄滅。
如果IC引(yin)腳NTC外部的NTC熱(re)敏(min)電阻出現故障,使電池溫(wen)度超出范圍,LED將出現閃爍(shuo),頻率為1.5Hz,人眼很容易識(shi)別。
如果電(dian)(dian)池已(yi)壞或不良(電(dian)(dian)池電(dian)(dian)壓在2.9V以(yi)下超過(guo)0.5h),LED將在6.1Hz的頻率上閃爍,頻閃速率比NTC熱(re)敏電(dian)(dian)阻故障(zhang)要快得多。
3.1.6 用NTC熱敏電阻測量電池溫(wen)度
電(dian)池充(chong)電(dian)時的(de)溫度可利(li)用NTC熱(re)敏電(dian)阻來測量,典型電(dian)路如圖(tu)3所示(shi)。
圖3 典型NTC熱敏(min)電(dian)阻電(dian)路(lu)
圖(tu)中,RNTC為(wei)NTC熱敏電(dian)阻(zu),應(ying)將其(qi)緊貼電(dian)池盒。RNTC在25℃時(shi)的電(dian)阻(zu)值R25=100kΩ,與偏(pian)置電(dian)阻(zu)RNOM值相等。IC引(yin)腳NTC內部較低(di)溫度(冷態(tai))門限為(wei)0.765Vcc,較高(gao)溫度(熱態(tai))門限為(wei)0.349Vcc(此時(shi)阻(zu)值降低(di)),使能門限為(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條件下,rHOT=0.536,rCOLD=3.25。對于(yu)Vishay公司的NTHS0603N011-N1003F來說(shuo),當溫度(du)升(sheng)高其(qi)電阻值降至R25(100kΩ)的0.536倍(約54kΩ)時(shi)(shi),相(xiang)應的溫度(du)約為(wei)40℃,充(chong)(chong)電器將暫停充(chong)(chong)電;當RNTC電阻值增(zeng)加到R25的3.25倍(即325kΩ)時(shi)(shi),對應的溫度(du)是0℃,充(chong)(chong)電器也會中止充(chong)(chong)電。很顯然,只有當電池(chi)溫度(du)高于(yu)0℃而低于(yu)40℃時(shi)(shi),充(chong)(chong)電器才會充(chong)(chong)電。
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(wu)差最接近(jin)1%的電阻值(zhi)是105kΩ。
附加(jia)電(dian)阻(zu)R1值為:
R1=0.536RNOM-rHOT?R25=0.536×105kΩ-0.436×100kΩ=12.6kΩ
誤差(cha)最接近(jin)1%的電阻值是12.7kΩ。
RNOM和R1按以上計算結果選(xuan)擇,當電池溫度升至45℃或降為0℃時(shi),充(chong)電器都(dou)將(jiang)暫停充(chong)電。
圖4 帶附加偏置電阻的NTC
3.2 降(jiang)壓開關穩壓器(qi)
LTC3558的電(dian)流模式降壓(ya)開關穩壓(ya)器主(zhu)要由(you)PWM控制器、主(zhu)開關P溝道MOSFET(MP)、N溝道MOSFET(MN)同步整流器及誤差放(fang)大器等組成(cheng),如圖(tu)5所(suo)示。
圖5 降壓(ya)開關穩壓(ya)器(qi)電路(lu)
LTC3558還含有(you)軟啟(qi)動、短(duan)路保護和減小輻射(she)EM1的開關節點擺幅(fu)限制電路。
降(jiang)壓(ya)(ya)穩壓(ya)(ya)器可以(yi)提供直到400mA的電流(liu),最(zui)低輸出電壓(ya)(ya)為0.8V。降(jiang)壓(ya)(ya)穩壓(ya)(ya)器輸出電壓(ya)(ya)可通過(guo)IC引(yin)腳FB1上的R1/R2來(lai)編程,其值為VOUT=0.8V(1+R1+R2)。R1取值范圍為40kΩ~1MΩ,CFB容(rong)量在2PF與(yu)22PF之間。
通(tong)過IC引腳MODE,可(ke)以設(she)置開關穩壓器的突發操作模式(shi)或脈沖跳(tiao)越(yue)模式(shi)。
3.2.1 脈沖(chong)跳越(yue)模式(Pulse Skip Mode)
在脈(mo)沖跳(tiao)(tiao)越模(mo)式,誤差放大器(qi)(qi)(qi)調節峰值(zhi)電(dian)(dian)(dian)感(gan)器(qi)(qi)(qi)電(dian)(dian)(dian)流(liu)(liu),以(yi)交付所需(xu)要的輸出(chu)功率。在輕載時(shi),在每(mei)個脈(mo)沖上電(dian)(dian)(dian)感(gan)電(dian)(dian)(dian)流(liu)(liu)可(ke)以(yi)達到零(ling)(關(guan)斷(duan)同步整流(liu)(liu)器(qi)(qi)(qi)MIN),在不連(lian)續模(mo)式操(cao)作。在非常輕的負載下,降壓(ya)開關(guan)穩壓(ya)器(qi)(qi)(qi)將會自動跳(tiao)(tiao)過脈(mo)沖,以(yi)維(wei)持輸出(chu)電(dian)(dian)(dian)壓(ya)調節。在高占(zhan)空因數(VOUT>PVIN1/2)時(shi),電(dian)(dian)(dian)感(gan)電(dian)(dian)(dian)流(liu)(liu)可(ke)能會反向,致(zhi)使降壓(ya)變換(huan)器(qi)(qi)(qi)開關(guan)連(lian)續。欲(yu)利用脈(mo)沖跳(tiao)(tiao)越模(mo)式,需(xu)使IC引腳MODE保持低電(dian)(dian)(dian)平。
3.2.2 突(tu)發模式(Burst Mode)
將(jiang)IC引腳MODE拉至(zhi)高電(dian)平,降壓(ya)變(bian)換器將(jiang)在突發(fa)模(mo)式操(cao)作,自(zi)動在固定頻率(lv)PWM操(cao)作與(yu)滯(zhi)后(hou)控制之間轉換。當(dang)輸出電(dian)容的充(chong)電(dian)電(dian)壓(ya)稍高于(yu)調節點時,降壓(ya)變(bian)換器則(ze)自(zi)動進入睡眠模(mo)式。在睡眠模(mo)式,開關穩壓(ya)器的大(da)多(duo)數電(dian)路將(jiang)切斷電(dian)源,因此(ci)在輕載(zai)時有(you)著(zhu)比脈沖跳躍模(mo)式更高的效率(lv)。
3.3 降壓(ya)-升壓(ya)開關(guan)穩壓(ya)器
LTC3558的(de)降壓-升壓開關穩壓器(qi)在電(dian)壓模式(shi)和2.25MHz的(de)固定頻率上(shang)操作,能提(ti)供(gong)直到400mA的(de)輸(shu)出負載電(dian)流,輸(shu)出電(dian)壓可在2.75V與5.45V之間來編(bian)程,其值為VOUT2=0.8V(1+R1+R2)。R1與R2為IC引腳FB2上(shang)的(de)分壓器(qi)電(dian)阻,如圖6所示(shi)。
當輸入電(dian)壓(ya)(ya)(ya)(ya)遠高于輸出電(dian)壓(ya)(ya)(ya)(ya)時(shi)(shi),穩壓(ya)(ya)(ya)(ya)器工作在(zai)(zai)降壓(ya)(ya)(ya)(ya)模(mo)式(shi);當輸入電(dian)壓(ya)(ya)(ya)(ya)遠低于輸出電(dian)壓(ya)(ya)(ya)(ya)時(shi)(shi),穩壓(ya)(ya)(ya)(ya)器則(ze)在(zai)(zai)升壓(ya)(ya)(ya)(ya)模(mo)式(shi)操作。
3.3.1 PWM操(cao)作模式與突發模式操(cao)作
通過(guo)將IC引腳保持低電平,降(jiang)壓-升(sheng)壓變(bian)換器將進(jin)入PWM操作模式。在適度的重(zhong)載條件下(xia),在定頻(pin)PWM操作有著最(zui)小的開關噪聲。
若將IC引腳(jiao)MODE拉至高(gao)電(dian)(dian)(dian)平,穩(wen)壓器(qi)將進入突(tu)發(fa)模(mo)式。在較輕的負載下,在突(tu)發(fa)模(mo)式操作,輸出電(dian)(dian)(dian)流被(bei)限制在50mA。當輸出電(dian)(dian)(dian)容上的充電(dian)(dian)(dian)電(dian)(dian)(dian)壓稍高(gao)于調節點時,穩(wen)壓器(qi)則進入睡眠模(mo)式。在突(tu)發(fa)模(mo)式操作,能使(shi)開關損耗大大降低,易于滿足(zu)輕載效率標準要求。
3.3.2 環路補償
降壓(ya)-升壓(ya)開關穩壓(ya)器(qi)可以采用簡單(dan)的I型補償(見圖7)
IC引腳FB2是誤(wu)差(cha)放大(da)器反相輸(shu)入端,引腳VC2是誤(wu)差(cha)放大(da)器輸(shu)出端。誤(wu)差(cha)放大(da)器單位增益(yi)頻率(lv)fUG可表示為:
圖8所示為誤(wu)差放大(da)器環(huan)路的III型補償網絡,其傳輸函數(shu)為:
圖8 III型(xing)補(bu)償(chang)電(dian)路
式中,s為傳(chuan)輸系數。
III型補償網絡(luo)可獲得較大的(de)頻帶寬度,改善(shan)瞬態響應,減小(xiao)啟動時(shi)VOUT2產生(sheng)的(de)過沖。對(dui)于3.3V的(de)輸出,補償元件推薦值(zhi)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、實際應用電(dian)路
LTC3558可以在沒有電池連接到引腳(jiao)BAT上的情況下操作,如圖9所(suo)示。
圖9 LTC3558可(ke)以在無電池連接時操作(zuo)
降(jiang)壓(ya)(ya)開(kai)關(guan)穩壓(ya)(ya)器(qi)(qi)輸出為1.8V(@400mA),降(jiang)壓(ya)(ya)-升壓(ya)(ya)開(kai)關(guan)穩壓(ya)(ya)器(qi)(qi)輸出為3.3V(@400mA)。在IC引腳SWAB2與SWCD2之間連接的(de)(de)(de)2.2μH的(de)(de)(de)電感器(qi)(qi)為降(jiang)壓(ya)(ya)-升壓(ya)(ya)穩壓(ya)(ya)器(qi)(qi)的(de)(de)(de)輸出電感器(qi)(qi)。NTC熱(re)敏電阻(zu)(zu)(R25=100kΩ)的(de)(de)(de)偏置電阻(zu)(zu)分別為110kΩ和28.7kΩ,故充電器(qi)(qi)的(de)(de)(de)熱(re)和冷解(jie)扣點分別設置在55℃和0℃。在IC引腳BAT上連接的(de)(de)(de)1Ω的(de)(de)(de)電阻(zu)(zu)和4.7μF的(de)(de)(de)電容,可(ke)保證充電器(qi)(qi)穩定。
從墻上AC適配器加電(dian)的LTC3558應(ying)用電(dian)路如圖10所示。
圖10 從墻上適(shi)配(pei)器加(jia)電(dian)的應用(yong)電(dian)路
其充電(dian)器(qi)的(de)充電(dian)電(dian)流(liu)可達950mA,降(jiang)壓開關穩壓器(qi)的(de)輸(shu)出(chu)為(wei)1.2V(@400mA),降(jiang)壓-升壓開關穩壓器(qi)的(de)輸(shu)出(chu)為(wei)3.3V(@400mA)。充電(dian)器(qi)通過NTC熱敏電(dian)阻將電(dian)池溫(wen)度解扣(kou)點設置在0℃和40℃,LED用作(zuo)充電(dian)狀態(tai)指示。
5、結束語
LTC3558是一種帶兩個高效開關穩壓器的USB充電器,它(ta)是為應用于小功率(lv)手持式設備而(er)專(zhuan)門(men)設計(ji)的。直達(da)950mA的可(ke)(ke)編程(cheng)(cheng)充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)可(ke)(ke)對單節(jie)鋰離(li)子或聚合物電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian),在充(chong)電(dian)(dian)(dian)(dian)過(guo)程(cheng)(cheng)中(zhong)電(dian)(dian)(dian)(dian)池(chi)狀態被監視(shi),4h的安全定時(shi)(shi)器(qi)期滿時(shi)(shi)控(kong)制充(chong)電(dian)(dian)(dian)(dian)器(qi)終止。利用NTC熱敏電(dian)(dian)(dian)(dian)阻來測量電(dian)(dian)(dian)(dian)池(chi)溫度并設置(zhi)電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)時(shi)(shi)的溫度范圍。LTC3558降(jiang)壓(ya)和降(jiang)壓(ya)-升壓(ya)開關穩壓(ya)器(qi)輸(shu)出電(dian)(dian)(dian)(dian)壓(ya)均可(ke)(ke)編程(cheng)(cheng),效(xiao)率(lv)高于90%,分別(bie)能(neng)提(ti)供直到400mA的輸(shu)出電(dian)(dian)(dian)(dian)流(liu),為微控(kong)制器(qi)內核及其I/O、存儲器(qi)、磁盤驅動器(qi)或其它(ta)邏(luo)輯電(dian)(dian)(dian)(dian)路供電(dian)(dian)(dian)(dian)。