用標準三端復位管理器實(shí)現手動(dòng)復位功能

　　簡(jiǎn)單的增加一對電阻、一個(gè)電容、一個(gè)按鍵開(kāi)關(guān)轉換標準三端復位管理器為手動(dòng)復位

　　增加手動(dòng)復位功能通常需要手動(dòng)復位輸入的新電路。但是通過(guò)增加一對低值電阻，標準三端復位管理器能夠實(shí)現普遍應用。電路如圖1所示，從按下手動(dòng)復位按鈕時(shí)刻起，確保純凈的復位信號。當復位按鈕被觸發(fā)，VCC電壓降到復位管理器最小復位限（S1按下時(shí)，VCC電壓為R1/R2的電壓分壓）。這個(gè)動(dòng)作導致復位管理器復位輸出有效。松開(kāi)S1，VCC電壓恢復到高于最大復位限，其中復位一直有效到復位管理器完成time-out時(shí)間段。

　　S1不被按下時(shí)，復位管理器供電電流和復位輸出裝填會(huì )導致R2電壓降的情況。對絕大多數復位管理器，最大供電電流為50 µA。對絕大多數設計，復位輸出經(jīng)過(guò)一或兩個(gè)CMOS輸入，每個(gè)輸入需要10 µA。帶兩個(gè)CMOS的設備接到復位，經(jīng)過(guò)R2的總電流將為(2×10 µA)+50 µA=70 µA。經(jīng)過(guò)R2的電壓降為復位管理器復位限電壓加上70 µA×100Ω=7 mV的電壓和。

　　考慮替換方式來(lái)選擇R1, R2和C1的值。旁路電容C1的值應該足夠低 到允許復位管理器檢測瞬時(shí)電壓的下降。R2 和C1的值決定時(shí)間常數，例如時(shí)間常數為100Ω×0.01 µF=1 µsec。這個(gè)公式顯然比可調電源的衰減率更高。

　　S1觸發(fā)時(shí)，電流流過(guò)R1 和R2。在圖1的電路中，S1觸發(fā)時(shí)電流為3.3V/(100Ω+100Ω)=16.5 mA。電流大小滿(mǎn)足線(xiàn)性功率系統，但是不適合電池供電系統。通過(guò)增大R1值的方法減小電流，確保復位管理器VCC低于最小復位限。也可以增大R2，但是會(huì )導致電壓降增加和瞬時(shí)響應減緩。提醒注意的是，增加手動(dòng)復位電流只在手動(dòng)復位有效時(shí)發(fā)生，典型的系統電流下降在有效時(shí)復位才會(huì )出現。

　　附英文原文

　　Add a manual reset to a standard three-pin-reset supervisor

　　Simply adding a couple of resistors, a capacitor, and a pushbutton switch transforms a standard three-pin-reset supervisor into a manual reset.

　　Derek Vanditmars, Delta Controls, Surrey, BC, Canada; Edited by Charles H Small and Brad Thompson -- EDN, 4/12/2007

　　Adding a manual reset to a design usually involves designing in a new part with a manual-reset input. But, by adding a couple of low-value resistors, a standard three-pin-reset supervisor can work in most applications. The circuit in Figure 1 ensures a clean  signal during and after you have pressed the manual-reset button. When you activate the manual-reset button, the supply voltage drops below the reset supervisor’s minimum reset threshold because of the R1/R2 voltage divider formed when S1 is active. This action causes the reset supervisor to activate its  output. When you release S1, the supply voltage returns to above the reset-supervisor maximum-reset threshold, and  remains active for the time-out period of the reset supervisor.

　　When you do not press S1, R2 has a voltage drop arising from the reset supervisor’s supply current and  output loading. For most reset supervisors, the maximum supply current is 50 µA. For most designs, the  output goes to one or more CMOS inputs that require about 10 µA each. With two CMOS devices connected to  , the total current through R2 would be (2×10 µA)+50 µA=70 µA. The voltage drop across R2 due to the current flow effectively adds 70 µA×100Ω=7 mV to the reset supervisor’s reset-threshold voltage.

　　You should consider several trade-offs for the
selection of values for R1, R2, and C1. The value of the local bypass capacitor, C1, for the reset supervisor should be low enough to allow the reset supervisor to detect transient supply-voltage drops. The time constant of R2 and C1 determines this factor; in this example, the time constant is 100Ω×0.01 µF=1 µsec. This figure is typically much higher than the decay rate of a regulated power supply that has lost power.

　　When you activate S1, current flows through R1 and R2. In the circuit in Figure 1, the current flow when you activate S1 is 3.3V/(100Ω+100Ω)=16.5 mA. This amount of current would be OK for a line-powered system but may not be OK for a battery-powered system. You can reduce the current by increasing the value of R1 and ensuring that the reset supervisor’s supply voltage drops below the minimum reset threshold. You can also increase the value of R2, along with that of R1, but doing so will cause increased voltage drop and slower response to transients. Note that the increased current of the manual reset occurs only while the manual reset is active, and typical system current drops while  is active.

    英文原文地址：http://www.edn.com/article/CA6430341.html?industryid=47041