Behavior of Regulators Near and Below Drop-out
in [Design]
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From: Robert Baer on 21 Feb 2010 03:32 pimpom wrote: > Jim Thompson wrote: >> This rainy afternoon (East-coasters beware, that usually spells >> more >> snow for you), I was amusing myself trying to behavioral model >> a >> voltage regulator when you hit drop-out. >> >> Then I realized, I've never designed an integrated voltage >> regulator >> for general use, only those inside ASIC's where I can control >> all the >> conditions. >> >> Thus I'm clueless of behavior of commercial offerings at or >> below VDO. >> >> I'm guessing that output voltage drops linearly with VIN once >> the >> drop-out point is hit?? >> >> But what about current capability? Does it drop sharply, >> linearly, or >> linearly to some critical point then drop like a rock. >> >> Pointers/data appreciated! > > > I haven't done an in-depth study either, but I know that the > output voltage drops in an approximately linear manner down to a > certain level of Vin. I've observed input ripple reproduced > linearly at the output. I expect that behaviour below a critical > Vin level will be design-specific and will be hard to predict > without careful analysis. The critical level would be reached > when active devices can no longer be biased in the active region. > > I know even less about their actual behaviour regarding current > capability, but I do know that they do not drop sharply right > after dipping below Vdo. All this is assuming that we're talking > about common linear regulators like the 78xx series. > > .... and LDOs are real bitches; talk about how to build an oscillator - just design an amplifier.
From: pimpom on 21 Feb 2010 06:50 Jim Thompson wrote: > On Sat, 20 Feb 2010 18:20:47 -0600, John Fields > <jfields(a)austininstruments.com> wrote: > >> >> PimPom just joined the fray recently and has kind of tried to >> flex >> his muscles with his "rule of thumb" stuff, > > He's certainly a thinking young man! Thanks. But "young man"? You may be inching towards your 70th birthday but I'll have you know that, if I hadn't married late, I would be surrounded by grandchildren now :-)
From: John Fields on 21 Feb 2010 08:18 On Sun, 21 Feb 2010 17:07:07 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote: >I see you like ingredients of Indian origin in your recipes. I'm >in India, but I live far from where stuff are produced - both >culturally and geographically. Is Basmati rice grown in the west >or is it all imported from India? --- All the basmati rice here is imported, but there's a cross between basmati and American long-grain rice grown by a company called RiceTek in Alvin, Texas: http://www.ricetec.com/page.asp?id=144 and sold as "Texmati", "Delta Rose", and "Cajun Country Popcorn Rice", among others I'm sure. .. .. .. JF
From: Jim Thompson on 21 Feb 2010 10:02 On Sun, 21 Feb 2010 17:20:07 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote: >Jim Thompson wrote: >> On Sat, 20 Feb 2010 18:20:47 -0600, John Fields >> <jfields(a)austininstruments.com> wrote: >> >>> >>> PimPom just joined the fray recently and has kind of tried to >>> flex >>> his muscles with his "rule of thumb" stuff, >> >> He's certainly a thinking young man! > >Thanks. But "young man"? You may be inching towards your 70th >birthday but I'll have you know that, if I hadn't married late, I >would be surrounded by grandchildren now :-) > Everyone is young compared to me ;-) BTW, I have 8 grandchildren! ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Jim Thompson on 21 Feb 2010 10:04 On Sun, 21 Feb 2010 00:32:50 -0800, Robert Baer <robertbaer(a)localnet.com> wrote: >pimpom wrote: >> Jim Thompson wrote: >>> This rainy afternoon (East-coasters beware, that usually spells >>> more >>> snow for you), I was amusing myself trying to behavioral model >>> a >>> voltage regulator when you hit drop-out. >>> >>> Then I realized, I've never designed an integrated voltage >>> regulator >>> for general use, only those inside ASIC's where I can control >>> all the >>> conditions. >>> >>> Thus I'm clueless of behavior of commercial offerings at or >>> below VDO. >>> >>> I'm guessing that output voltage drops linearly with VIN once >>> the >>> drop-out point is hit?? >>> >>> But what about current capability? Does it drop sharply, >>> linearly, or >>> linearly to some critical point then drop like a rock. >>> >>> Pointers/data appreciated! >> >> >> I haven't done an in-depth study either, but I know that the >> output voltage drops in an approximately linear manner down to a >> certain level of Vin. I've observed input ripple reproduced >> linearly at the output. I expect that behaviour below a critical >> Vin level will be design-specific and will be hard to predict >> without careful analysis. The critical level would be reached >> when active devices can no longer be biased in the active region. >> >> I know even less about their actual behaviour regarding current >> capability, but I do know that they do not drop sharply right >> after dipping below Vdo. All this is assuming that we're talking >> about common linear regulators like the 78xx series. >> >> >... and LDOs are real bitches; talk about how to build an oscillator - >just design an amplifier. I'm intent on modeling these suckers accurately enough that oscillation can be predicted. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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