Behavior of Regulators Near and Below Drop-out
in [Design]
|
From: Jim Thompson on 20 Feb 2010 15:49 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! Thanks! ...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: pimpom on 20 Feb 2010 16:43 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.
From: John Larkin on 20 Feb 2010 16:56 On Sun, 21 Feb 2010 03:13:45 +0530, "pimpom" <pimpom(a)invalid.invalid> 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. > The internal schematics of classics like LM317 and LM1117 and such are on the data sheets. Some people (?) could deduce their behavior from that. John
From: Jim Thompson on 20 Feb 2010 17:36 On Sat, 20 Feb 2010 13:56:56 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sun, 21 Feb 2010 03:13:45 +0530, "pimpom" <pimpom(a)invalid.invalid> >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. >> > >The internal schematics of classics like LM317 and LM1117 and such are >on the data sheets. Some people (?) could deduce their behavior from >that. > >John Those aren't LDO's, they're NPN "followers"; not PNP or PMOS, whose behavior would be radically different, and quite process dependent. ...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: Tim Williams on 20 Feb 2010 17:40
"Jim Thompson" <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote in message news:iro0o5hmfbejuo9lt3udkodidv4nitjd1m(a)4ax.com... >>The internal schematics of classics like LM317 and LM1117 and such are >>on the data sheets. Some people (?) could deduce their behavior from >>that. > > Those aren't LDO's, they're NPN "followers"; not PNP or PMOS, whose > behavior would be radically different, and quite process dependent. LM337? In an earlier thread you noted it also uses an NPN output device. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |