用標準三端復(fù)位管理器實現(xiàn)手動復(fù)位功能

　　簡單的增加一對電阻、一個電容、一個按鍵開關(guān)轉(zhuǎn)換標準三端復(fù)位管理器為手動復(fù)位

　　增加手動復(fù)位功能通常需要手動復(fù)位輸入的新電路。但是通過增加一對低值電阻，標準三端復(fù)位管理器能夠?qū)崿F(xiàn)普遍應(yīng)用。電路如圖1所示，從按下手動復(fù)位按鈕時刻起，確保純凈的復(fù)位信號。當復(fù)位按鈕被觸發(fā)，VCC電壓降到復(fù)位管理器最小復(fù)位限（S1按下時，VCC電壓為R1/R2的電壓分壓）。這個動作導(dǎo)致復(fù)位管理器復(fù)位輸出有效。松開S1，VCC電壓恢復(fù)到高于最大復(fù)位限，其中復(fù)位一直有效到復(fù)位管理器完成time-out時間段。

　　S1不被按下時，復(fù)位管理器供電電流和復(fù)位輸出裝填會導(dǎo)致R2電壓降的情況。對絕大多數(shù)復(fù)位管理器，最大供電電流為50 µA。對絕大多數(shù)設(shè)計，復(fù)位輸出經(jīng)過一或兩個CMOS輸入，每個輸入需要10 µA。帶兩個CMOS的設(shè)備接到復(fù)位，經(jīng)過R2的總電流將為(2×10 µA)+50 µA=70 µA。經(jīng)過R2的電壓降為復(fù)位管理器復(fù)位限電壓加上70 µA×100Ω=7 mV的電壓和。

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

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

　　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