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Behavior of Regulators Near and Below Drop-out

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.
Reply to
Jim Thompson
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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.

Reply to
pimpom

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

Reply to
John Larkin

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.
Reply to
Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

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
Reply to
Tim Williams 

--
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
Reply to
John Fields

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.
Reply to
Jim Thompson 

--
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. ;)
Reply to
John Fields

He's certainly a thinking young man!

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.
Reply to
Jim Thompson

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

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.

Reply to
miso

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? I have an extremely limited knowledge of Hindi, but I do know that "garam' means 'hot' and masala is the word for spices. So "garam masala" literally means "hot spices".

I have no time for "my dick is bigger than your dick" stuff. Being entirely self-taught in a primitive environment without anyone to guide me, I'm aware that I have huge gaps in my knowledge. I try to contribute when I think can, answering questions in parts when I don't have a complete answer. I try to fill those gaps by discussing, asking questions or simply reading in these groups.

I've been lurking on and off in the sci.electronics groups for a little over a decade now, which is the length of time that Internet access has been available here. I made occasional posts but didn't always use the nym "pimpom". Flexing my muscles? I didn't think what I did could be interpreted as such. I was just trying to be helpful by sharing my experience.

Reply to
pimpom

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 :-)

Reply to
pimpom

--- 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:

formatting link

and sold as "Texmati", "Delta Rose", and "Cajun Country Popcorn Rice", among others I'm sure.

. . .

JF

Reply to
John Fields

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.
Reply to
Jim Thompson

"Jim Thompson" kirjoitti viestissä: snipped-for-privacy@4ax.com...

LP2950 has schematics on datasheet (National version).

-ek

Reply to
E

Thanks! I'll check that out. Although my past experiences in the "jelly bean" business is those schematics are usually "simplified" to hide IP.

This is getting to be a really amusing mental endeavor! For instance, how might an LM7805 behave with a zener inserted in its ground lead to boost the voltage ?:-) ...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.
Reply to
Jim Thompson

Thanks! Looks good! ...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.
Reply to
Jim Thompson

What are your assumptions about the source? A battery going flat might induce limit-cycle oscillations that wouldn't show up with a stiff source?? or not...

Reply to
mike

From my modeling point of view, the "source" is just "something" connected to the "IN" terminal.

BUT, The way I am envisioning the model, a flaky source, if you have a model for it, would induce the very behavior you want to see.

Maybe model "source" as a voltage source with a parameterized impedance rise?

Or get out a battery manual and model "Charge", a parameter that reflects both voltage and impedance effects? ...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.
Reply to
Jim Thompson

Overlay multiple un-synched signals to create a psuedo-random average modulation 'noise' in the drive signal that can be amplitude modulated to mimic anomalous source events.

Reply to
life imitates life

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