在嵌入式系統依賴持續供電的電信、工業和汽車應用中，數據丟失是引人關切的。供電的突然中斷會導致正在對硬盤驅動器和閃存器進行讀寫操作時的數據受損。通常，嵌入式系統僅需 10ms 至50ms的時間對易失性數據進行備份以防止發生丟失。盡管如此，不可靠的電源使之難以(yi)實現。長的(de)電(dian)源線(xian)、電(dian)量耗盡的(de)電(dian)池、未調整的(de)AC 適配器、拋(pao)載(zai)和大(da)功率電(dian)動機的(de)開關操作都會導致電(dian)源的(de)不(bu)可(ke)靠。
假(jia)如沒有某種(zhong)形(xing)式的(de)(de)(de)安全(quan)網 (即(ji)某種(zhong)短期電(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)(yuan)保持系(xi)統，其可存(cun)儲(chu)充足的(de)(de)(de)電(dian)(dian)(dian)(dian)能(neng)以(yi)提供(gong)備用電(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)(yuan)，直到可以(yi)更換(huan)電(dian)(dian)(dian)(dian)池或可把數據存(cun)儲(chu)在永久性(xing)存(cun)儲(chu)器(qi)中為止)，則電(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)(yuan)的(de)(de)(de)不可靠會導致存(cun)儲(chu)在易失性(xing)存(cun)儲(chu)器(qi)中的(de)(de)(de)重要數據必定丟(diu)失。而應(ying)對(dui)此類情(qing)況最(zui)常用的(de)(de)(de)方法是使用不間斷(duan)電(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)(yuan)以(yi)覆蓋這些短暫的(de)(de)(de)故(gu)障停機時間，從而確保高可靠性(xing)的(de)(de)(de)系(xi)統持續(xu)運作，或者提供(gong)某種(zhong)替代形(xing)式電(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)(yuan)，以(yi)針對(dui)主電(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)(yuan)未接入(ru) (主電(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)(yuan)發生供(gong)電(dian)(dian)(dian)(dian)中斷(duan))場合的(de)(de)(de)備份計劃。
電源儲存的基本介質——電容&電池特性分析
既(ji)然(ran)嵌入式(shi)系統需要備用(yong)電(dian)源(yuan)，那么(me)該用(yong)什么(me)作(zuo)為這種電(dian)源(yuan)的(de)能量儲存介質呢(ni)? 傳統上，人們一直(zhi)選擇的(de)是電(dian)容(rong)和電(dian)池
電(dian)(dian)(dian)(dian)容(rong)技術在電(dian)(dian)(dian)(dian)力(li)傳輸和配送應用(yong)中發揮作用(yong)已(yi)有數(shu)(shu)十年之(zhi)久。而(er)隨著近十年的(de)(de)(de)(de)研(yan)究和開發，電(dian)(dian)(dian)(dian)容(rong)的(de)(de)(de)(de)設(she)計和容(rong)量(liang)有了更加顯著的(de)(de)(de)(de)進步(bu)。這(zhe)些新型電(dian)(dian)(dian)(dian)容(rong)被稱為超(chao)級(ji)電(dian)(dian)(dian)(dian)容(rong)，非常適合(he)用(yong)于電(dian)(dian)(dian)(dian)池儲能(neng)(neng)和備(bei)用(yong)電(dian)(dian)(dian)(dian)源系統。其能(neng)(neng)量(liang)密度非常高(gao)，同時具有快速釋(shi)放高(gao)能(neng)(neng)量(liang)并快速充(chong)電(dian)(dian)(dian)(dian)的(de)(de)(de)(de)能(neng)(neng)力(li)。與電(dian)(dian)(dian)(dian)池相比，超(chao)級(ji)電(dian)(dian)(dian)(dian)容(rong)以(yi)更小的(de)(de)(de)(de)尺寸提(ti)供更高(gao)的(de)(de)(de)(de)突發峰值功率，并且充(chong)電(dian)(dian)(dian)(dian)循環次數(shu)(shu)更多，工(gong)作溫(wen)度范(fan)圍更寬。而(er)電(dian)(dian)(dian)(dian)池提(ti)供的(de)(de)(de)(de)能(neng)(neng)量(liang)比超(chao)級(ji)電(dian)(dian)(dian)(dian)容(rong)器多得多，因此(ci)對于需要后備(bei)電(dian)(dian)(dian)(dian)源以(yi)延長(chang)工(gong)作時間的(de)(de)(de)(de)應用(yong)而(er)言(yan)，電(dian)(dian)(dian)(dian)池則更有優勢。
表1總(zong)結了超級電(dian)容、普通電(dian)容和電(dian)池的優缺點。
因此，基于使(shi)用場景的(de)不同(tong)，超(chao)(chao)級(ji)電(dian)(dian)容(rong)、電(dian)(dian)池(chi)或二(er)者的(de)組合幾乎可(ke)以(yi)用作所有(you)電(dian)(dian)子系統的(de)備(bei)(bei)用電(dian)(dian)源(yuan)(yuan)。儲存介質選(xuan)定(ding)了，該(gai)(gai)怎么用在(zai)具體(ti)系統中呢？其電(dian)(dian)路設計有(you)啥差異？ADI公(gong)司就為(wei)備(bei)(bei)用電(dian)(dian)源(yuan)(yuan)系統推(tui)出(chu)了眾多專為(wei)滿足(zu)該(gai)(gai)應(ying)用需求而(er)特別設計的(de)IC，本文給出(chu)了該(gai)(gai)公(gong)司針對(dui)電(dian)(dian)池(chi)及超(chao)(chao)級(ji)電(dian)(dian)容(rong)及電(dian)(dian)解電(dian)(dian)容(rong)的(de)備(bei)(bei)用電(dian)(dian)源(yuan)(yuan)管理(li)電(dian)(dian)路，對(dui)比分(fen)析可(ke)以(yi)為(wei)相關(guan)應(ying)用找對(dui)最佳(jia)的(de)備(bei)(bei)用電(dian)(dian)源(yuan)(yuan)方(fang)案。
基于電池的電源保持解決方案
LTC4040是一(yi)(yi)款面(mian)向 3.5V 至 5V 電(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)軌的完整鋰電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)后(hou)備(bei)電(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)管(guan)理系(xi)(xi)統。其(qi)用(yong)(yong)一(yi)(yi)個內(nei)置雙向同步(bu)轉(zhuan)換器(qi)(qi)提(ti)供(gong)高效率電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)以及(ji)大電(dian)(dian)(dian)(dian)(dian)(dian)流、高效率后(hou)備(bei)電(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)。當外部電(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)可用(yong)(yong)時，該(gai)器(qi)(qi)件作(zuo)為降壓(ya)型電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)工作(zuo)，用(yong)(yong)于單節鋰離(li)子或LiFePO4電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)，同時優(you)先為系(xi)(xi)統負載供(gong)電(dian)(dian)(dian)(dian)(dian)(dian)。當輸入電(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)降至低于可調電(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)故障輸入(PFI)門限時，LTC4040就作(zuo)為升壓(ya)型穩(wen)壓(ya)器(qi)(qi)工作(zuo)，能夠從后(hou)備(bei)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)向系(xi)(xi)統輸出提(ti)供(gong)高達(da) 2.5A 電(dian)(dian)(dian)(dian)(dian)(dian)流。
在發生電(dian)(dian)源故障時，該器件的電(dian)(dian)源通(tong)路 (PowerPath) 控制在輸入電(dian)(dian)源和后備(bei)(bei)電(dian)(dian)源之間提(ti)供(gong)反向隔離和無縫(feng)切(qie)換。LTC4040 的典型(xing)應用包(bao)括汽車(che) GPS 數(shu)據記錄儀、車(che)載多媒體系(xi)統(tong)、收費系(xi)統(tong)、安防系(xi)統(tong)、通(tong)信系(xi)統(tong)、工業(ye)備(bei)(bei)份和USB供(gong)電(dian)(dian)型(xing)設備(bei)(bei)。
具4.22V PFI 門限(xian)的 4.5V后(hou)備電源(yuan)
基于超級電容的電源保持解決方案
LTC3110 是一款雙向(xiang)、輸入電(dian)流可編程的降壓(ya)(ya)-升(sheng)壓(ya)(ya)型(xing)超級電(dian)容(rong)器充(chong)(chong)電(dian)器，該(gai)器件具主動充(chong)(chong)電(dian)平衡功能，適(shi)合單(dan)節或兩節串聯超級電(dian)容(rong)器。其專有的低噪(zao)聲降壓(ya)(ya)-升(sheng)壓(ya)(ya)型(xing)拓撲使該(gai)器件相當于(yu)兩個單(dan)獨的開(kai)關(guan)穩壓(ya)(ya)器，從而減小了(le)尺(chi)寸和成(cheng)本，并降低了(le)復雜性。LTC3110 可于(yu)后備和充(chong)(chong)電(dian)兩種模式工作(zuo)。
在后備(bei)(bei)模(mo)式(shi)中，該器件由超(chao)級(ji)電(dian)(dian)容(rong)器儲存的能量供電(dian)(dian)，保持一個 1.71V 至 5.25V 的系(xi)(xi)統(tong)電(dian)(dian)壓 (VSYS)。而(er)在充電(dian)(dian)模(mo)式(shi)中，當主(zhu)電(dian)(dian)源系(xi)(xi)統(tong)有效時(shi)，LTC3110 可自主(zhu)地(di)或通(tong)過用戶命令將功率流動(dong)改為相反方向，利用穩定(ding)的系(xi)(xi)統(tong)電(dian)(dian)壓給超(chao)級(ji)電(dian)(dian)容(rong)器充電(dian)(dian)并(bing)做出平衡。另(ling)外，該器件還具備(bei)(bei)充電(dian)(dian)模(mo)式(shi)平均輸入電(dian)(dian)流限制，能夠以 ±2% 的準(zhun)確(que)度設定(ding)至高達(da) 2A，從而(er)防止系(xi)(xi)統(tong)電(dian)(dian)源過載，同時(shi)最(zui)大限度地(di)縮短(duan)電(dian)(dian)容(rong)器再(zai)充電(dian)(dian)時(shi)間(jian)。
基于超級電(dian)容器的 LTC3110 備(bei)份解決(jue)方(fang)案 (VIN 高達(da) 5.25V)
基于電解電容的電源(yuan)保(bao)持解決方案(an)
下圖中的(de)(de)解決(jue)方案則(ze)以 LTC3643 雙向后備(bei)電(dian)(dian)(dian)源為中心(xin)。當輸入電(dian)(dian)(dian)壓(ya)(ya)(ya)存在(zai)時，LTC3643 在(zai)升壓(ya)(ya)(ya)模式中給存儲(chu)(chu)電(dian)(dian)(dian)容器(qi) CSTORAGE 充電(dian)(dian)(dian) (至高達 40V)。當輸入電(dian)(dian)(dian)壓(ya)(ya)(ya)被中斷(duan)時，LTC3643 在(zai)降壓(ya)(ya)(ya)模式中把存儲(chu)(chu)電(dian)(dian)(dian)容器(qi)的(de)(de)電(dian)(dian)(dian)量釋放(fang)至負(fu)載，從而將負(fu)載上的(de)(de)標稱電(dian)(dian)(dian)壓(ya)(ya)(ya) (VSYS) 保持在(zai) 3V 至 17V 的(de)(de)范圍內。
后(hou)備(bei)存儲電(dian)(dian)源軌(gui)相對高(gao)的(de)電(dian)(dian)壓增加了該解(jie)決方案的(de)儲能(neng)(neng) (E = CV2/2)，并使(shi)得(de)可把電(dian)(dian)解(jie)電(dian)(dian)容器用(yong)作(zuo)一種后(hou)備(bei)儲能(neng)(neng)組件。電(dian)(dian)解(jie)電(dian)(dian)容器便宜且廣泛地(di)使(shi)用(yong)，因而顯著地(di)降低了后(hou)備(bei)解(jie)決方案的(de)成本(ben)。LTC3643 的(de)另一個(ge)優(you)點是(shi)(shi)其(qi)能(neng)(neng)夠(gou)支持 12V 系統，在許多汽車(che)和工業應(ying)用(yong)中(zhong)，12V 是(shi)(shi)默認(ren)的(de)標準電(dian)(dian)壓軌(gui)。
LTC3643 高電壓備份解決方案；VIN 高達(da) 17V
如圖所示，LTM 4607 μModule降壓(ya)-升(sheng)壓(ya)型轉換器(qi)(qi)充當(dang)前(qian)端穩(wen)(wen)壓(ya)器(qi)(qi)，從(cong)(cong)一個 5V 至(zhi) 36V 輸(shu)入(ru) (例如：汽(qi)車電(dian)(dian)池) 產生 12V (高達 5A電(dian)(dian)流) 輸(shu)出。只要輸(shu)入(ru)電(dian)(dian)壓(ya)維持在(zai)規定的(de)范(fan)(fan)圍之內，該降壓(ya)-升(sheng)壓(ya)型穩(wen)(wen)壓(ya)器(qi)(qi)就可保持一個穩(wen)(wen)定的(de) 12V 輸(shu)出，從(cong)(cong)而使得(de) VSYS 能安全度過(guo)汽(qi)車冷車發動和(he)拋(pao)載(zai)等欠壓(ya)和(he)過(guo)壓(ya)情況(kuang)。當(dang)輸(shu)入(ru)電(dian)(dian)壓(ya)被中斷或脫離(li)了(le)該范(fan)(fan)圍時，基于 LTC3643 的(de)后備電(dian)(dian)源解決(jue)方案將保持 VSYS 系統電(dian)(dian)壓(ya)以(yi)提供(gong)短期的(de)數據備份。
“輕裝上陣”的電源保持解決方案
而對于那些要求相對較(jiao)短(duan)保持時(shi)(shi)間(jian)且對成(cheng)本(ben)敏感的(de)項目，下(xia)圖(tu)中的(de)解決(jue)方(fang)案可通過(guo)縮(suo)短(duan)保持時(shi)(shi)間(jian)來換取最低(di)的(de)組件成(cheng)本(ben)。該(gai)方(fang)案以一般情況下(xia)充當降(jiang)壓(ya)型轉換器的(de) LTC3649 為中心。但(dan)是在(zai)(zai)這里，LTC3649 在(zai)(zai)輸(shu)入電(dian)壓(ya)斷接(jie)時(shi)(shi)轉而執(zhi)行升壓(ya)轉換操作。LTC3649 通過(guo)對其自(zi)己(ji)的(de)輸(shu)出電(dian)容器進行放電(dian)以在(zai)(zai)關鍵負載的(de)端子上保持編(bian)程電(dian)壓(ya)。
LTC3649 “較廉(lian)價(jia)的” 備份解決方案，VIN 高達 60V
LTC3649是(shi)一款(kuan)具有(you)集成功率MOSFET的(de)(de)(de)單(dan)芯片降壓(ya)調節器。它具有(you)高(gao)效(xiao)率和低(di)靜態電(dian)(dian)流，這在許(xu)多電(dian)(dian)池(chi)供電(dian)(dian)系統中非常重要(yao)。它還具有(you)很(hen)高(gao)的(de)(de)(de)通用性、可編程頻率、高(gao)達(da)60 V的(de)(de)(de)寬(kuan) VIN 范圍(wei)以(yi)及低(di)至接地的(de)(de)(de)輸出電(dian)(dian)壓(ya)范圍(wei)。它簡化了汽車(che)和工業用品的(de)(de)(de)設(she)計，特(te)別是(shi)考慮到其作為保持電(dian)(dian)路的(de)(de)(de)潛質時。
結論
每當系統要求即使在(zai)主電(dian)(dian)(dian)源(yuan)發生故障的(de)(de)(de)(de)情(qing)況下也(ye)始終可(ke)用(yong)時，擁有后(hou)備(bei)(bei)電(dian)(dian)(dian)源(yuan)總是(shi)個好主意(yi)。而無論(lun)儲存介質是(shi)超級(ji)電(dian)(dian)(dian)容(rong)、電(dian)(dian)(dian)解電(dian)(dian)(dian)容(rong)、抑(yi)或是(shi)電(dian)(dian)(dian)池的(de)(de)(de)(de)情(qing)況下，ADI都(dou)能(neng)(neng)夠提供簡易后(hou)備(bei)(bei)電(dian)(dian)(dian)源(yuan)的(de)(de)(de)(de)可(ke)用(yong) IC選項。隨著電(dian)(dian)(dian)容(rong)及電(dian)(dian)(dian)池技術(shu)的(de)(de)(de)(de)進一步發展，電(dian)(dian)(dian)源(yuan)IC性能(neng)(neng)的(de)(de)(de)(de)繼(ji)續優(you)化(hua)也(ye)將為后(hou)備(bei)(bei)電(dian)(dian)(dian)源(yuan)增加更多(duo)的(de)(de)(de)(de)市場和應(ying)用(yong)潛(qian)能(neng)(neng)。
