Behavior of Regulators Near and Below Drop-out
in [Design]
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From: John Fields on 20 Feb 2010 17:57 On Sat, 20 Feb 2010 15:36:23 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >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. --- In all fairness, you didn't indicate you were talking about LDO's and since "dropout voltage" applies to both beasts, who knew? Glad it got clarified though since, on a rainy Saturday afternoon in Austin, with nothing better to do than appreciate the difference between the bottle in front of me and a frontal lobotomy, I was heading for the soldering iron and the scope... ;) JF
From: Jim Thompson on 20 Feb 2010 18:13 On Sat, 20 Feb 2010 16:57:41 -0600, John Fields <jfields(a)austininstruments.com> wrote: >On Sat, 20 Feb 2010 15:36:23 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >>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. > >--- >In all fairness, you didn't indicate you were talking about LDO's and >since "dropout voltage" applies to both beasts, who knew? > >Glad it got clarified though since, on a rainy Saturday afternoon in >Austin, with nothing better to do than appreciate the difference between >the bottle in front of me and a frontal lobotomy, I was heading for the >soldering iron and the scope... ;) > >JF I checked the Tequila availability... enchiladas tonight ;-) I know there are various drop-out behaviors. JL was just trying to be rude and dismissive with his "Some people (?)". ...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: John Fields on 20 Feb 2010 19:20 On Sat, 20 Feb 2010 16:13:14 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Sat, 20 Feb 2010 16:57:41 -0600, John Fields ><jfields(a)austininstruments.com> wrote: > >>On Sat, 20 Feb 2010 15:36:23 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >> >>>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. >> >>--- >>In all fairness, you didn't indicate you were talking about LDO's and >>since "dropout voltage" applies to both beasts, who knew? >> >>Glad it got clarified though since, on a rainy Saturday afternoon in >>Austin, with nothing better to do than appreciate the difference between >>the bottle in front of me and a frontal lobotomy, I was heading for the >>soldering iron and the scope... ;) >> >>JF > >I checked the Tequila availability... enchiladas tonight ;-) --- Yum! We're having leftover "Chicken Tagine" over newly boiled Basmati rice tonight. The recipe called for dried apricots and Garam Masala, but since we didn't have any dried apricots we substituted Smucker's Apricot Preserves. Sweetart don't hurt chicken; what a nice surprise! I'll post the recipe if you like, since it seems recipes aren't off-topic here. ;) --- >I know there are various drop-out behaviors. JL was just trying to be >rude and dismissive with his "Some people (?)". --- I noticed that. PimPom just joined the fray recently and has kind of tried to flex his muscles with his "rule of thumb" stuff, and it seems that JL is passive-aggressively challenging him, but with no direct reference. What a chickenshit. JF
From: Jim Thompson on 20 Feb 2010 19:39 On Sat, 20 Feb 2010 18:20:47 -0600, John Fields <jfields(a)austininstruments.com> wrote: >On Sat, 20 Feb 2010 16:13:14 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >>On Sat, 20 Feb 2010 16:57:41 -0600, John Fields >><jfields(a)austininstruments.com> wrote: >> >>>On Sat, 20 Feb 2010 15:36:23 -0700, Jim Thompson >>><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>> >>>>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. >>> >>>--- >>>In all fairness, you didn't indicate you were talking about LDO's and >>>since "dropout voltage" applies to both beasts, who knew? >>> >>>Glad it got clarified though since, on a rainy Saturday afternoon in >>>Austin, with nothing better to do than appreciate the difference between >>>the bottle in front of me and a frontal lobotomy, I was heading for the >>>soldering iron and the scope... ;) >>> >>>JF >> >>I checked the Tequila availability... enchiladas tonight ;-) > >--- >Yum! > >We're having leftover "Chicken Tagine" over newly boiled Basmati rice >tonight. > >The recipe called for dried apricots and Garam Masala, but since we >didn't have any dried apricots we substituted Smucker's Apricot >Preserves. > >Sweetart don't hurt chicken; what a nice surprise! > >I'll post the recipe if you like, since it seems recipes aren't >off-topic here. ;) >--- > >>I know there are various drop-out behaviors. JL was just trying to be >>rude and dismissive with his "Some people (?)". > >--- >I noticed that. > >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! >and it seems that JL is >passive-aggressively challenging him, but with no direct reference. > >What a chickenshit. > >JF What a Chickenshit "Ol' Git" :-) ...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: miso on 21 Feb 2010 02:35 On Feb 20, 12:49 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- Web-Site.com> 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! > > 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 athttp://www.analog-innovations.com| 1962 | > > I love to cook with wine. Sometimes I even put it in the food. With a PNP, the pass device is saturated, implies high base current and thus lousy efficiency. [Micrel has parts like this.] There are "sat catcher" circuits to make this less of a feature. ;-) [Sat catcher, like body snatcher, will lead to all sorts of false google hits, but it is the term everyone in the biz uses.] With P-fet pass, it looks like a resistor (no efficiency issue), but the resistance is a function of gate drive. Maybe efficiency isn't the best term. Think of high quiescent current when the pass device is saturated. Needless to say, the P-fet device behaves more predictably. The sat catcher can effect stability. When I build board level stuff, I use the p-fet type LDOs, usually from TI.
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