On Sat, 20 Feb 2010 16:13:14 -0700, Jim Thompson
wrote:

On Sat, 20 Feb 2010 16:57:41 -0600, John Fields
wrote:

On Sat, 20 Feb 2010 15:36:23 -0700, Jim Thompson
wrote:

On Sat, 20 Feb 2010 13:56:56 -0800, John Larkin
wrote:

On Sun, 21 Feb 2010 03:13:45 +0530, "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.


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 chicken****.

JF