On Thu, Dec 23, 2010 at 6:26 PM, V G wrote: > Goal: when the battery voltage (battery voltage is constantly dropping du= e > to usage), reaches a minimum (say, 4.4 V), output a logic low signal/open > collector/whatever, which is used to hold a switching supply's SHUTDOWN p= in > low (so it turns off). > > I searched around and then found this chip and example circuit on the > right: http://www.national.com/mpf/LM/LM431.html#Overview > > I see how they're "adjusting" the device with a resistor divider. Problem= is > - the supply voltage itself (battery voltage) constantly changes. Wouldn'= t > this also cause the resistor divider output to constantly drop, therefore > constantly adjusting the "breakdown" voltage as well? > > How would I go about doing this? The TL431 does the opposite of what you want: it pulls down whenever the input voltage *exceeds* 2.5v. You could pull down SHUTDOWN with a resistive divider and feed the TL431 into a common-emitter PNP to invert the signal. A better choice is to get a three-terminal voltage supervisor IC, which does everything you want inside one part. http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3DMCP100-4= 50DI/TO-ND Regards, Mark markrages@gmail --=20 Mark Rages, Engineer Midwest Telecine LLC markrages@midwesttelecine.com --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .