簡(jiǎn)單的增加一對(duì)電阻、一個(gè)電容、一個(gè)按鍵開(kāi)關(guān)轉(zhuǎn)換標(biāo)準(zhǔn)三端復(fù)位管理器為手動(dòng)復(fù)位

增加手動(dòng)復(fù)位功能通常需要手動(dòng)復(fù)位輸入的新電路。但是通過(guò)增加一對(duì)低值電阻，標(biāo)準(zhǔn)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(dòng)復(fù)位按鈕時(shí)刻起，確保純凈的信號(hào)。當(dāng)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(shí)，VCC電壓為R1/R2的電壓分壓）。這個(gè)動(dòng)作導(dǎo)致復(fù)位管理器輸出有效。松開(kāi)S1，VCC電壓恢復(fù)到高于最大復(fù)位限，其中一直有效到復(fù)位管理器完成time-out時(shí)間段。

S1不被按下時(shí)，復(fù)位管理器供電電流和輸出裝填會(huì)導(dǎo)致R2電壓降的情況。對(duì)絕大多數(shù)復(fù)位管理器，最大供電電流為50 &mICro;A。對(duì)絕大多數(shù)設(shè)計(jì)，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(shè)備接到，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(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(lái)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(cè)瞬時(shí)電壓的下降。R2 和C1的值決定時(shí)間常數(shù)，例如時(shí)間常數(shù)為100Ω×0.01 μF=1 μsec。這個(gè)公式顯然比可調(diào)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。電流大小滿足線性功率系統(tǒng)，但是不適合電池供電系統(tǒng)。通過(guò)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(huì)導(dǎo)致電壓降增加和瞬時(shí)響應(yīng)減緩。提醒注意的是，增加手動(dòng)復(fù)位電流只在手動(dòng)復(fù)位有效時(shí)發(fā)生，典型的系統(tǒng)電流下降在有效時(shí)才會(huì)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.