Re: Lowish DO regulator


On Sat, 11 Oct 2008 10:22:05 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Jim Thompson wrote:
On Fri, 10 Oct 2008 20:35:22 -0700 (PDT), miso@xxxxxxxxx wrote:

On Oct 10, 11:48 am, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
wrote:
m...@xxxxxxxxx wrote:
On Oct 8, 11:07 am, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
wrote:
Jim Thompson wrote:
On Wed, 08 Oct 2008 07:55:46 -0700, Joerg
<notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx> wrote:
MooseFET wrote:
I have a 12V lead acid battery or maybe someone will connect this to a
voltage up to 35V. I need to make 8.3V. Some moron may connect the
power backwards.
Currently I am using a circuit with a P channel MOSFET to do the
reverse protection and an LM78XX type regulator. Does someone know of
a 78XX like regulator with a lower overhead number.
The load current is about 200mA max. I have a large heat sink /
chassis available.
I'd consider a switcher here. 35V minus 8.3V at 200mA is over 5W.
Personally I stay away from LDOs, seen too many not so well or not at
all documented instabilities. The topper was one that began rather wild
oscillations once the battery impedance rose above a certain level. Of
course none of this was mentioned in the data***. I did not like that
LDO in the circuit but a client's engineer wanted it. Call to the mfg,
more and more guys came into the office over there, lots of mumbling,
shoveling of papers, schematics etc. Suddenly someone at the mfg
exclaimed "Oh drat!". Then I knew we were in hot water.
BTW: Did you notice that in the debates nobody ever mentions the need
for better voltage regulators and op-amps. They aren't dealling with
any of the issues that really matter. :)
I'd vote for Prop 8888: A ban on LDOs :-)
Why not roll your own LDO out of discrete components?
Sure. But then I might as well roll my own switcher out of jelly bean
parts. Which I usually do :-)
I just did two more of those, neither one will make any PWM chip vendor
happy because there ain't none in there. Ok, one contains a PLL but I
could only bring myself to using a 15c logic part.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
I find it hard to believe once you integrate all the start-up and
protection circuits that this is cost effective.
That sometimes surprises clients as well. At first there is some gasping
when they look at the schematic. But once they see the BOM and learn
what it would take to coax a PWM chip to fulfill their particular
requirements that changes. For example most PWM chips feature only one
feedback path yet you usually have to curb power not just after reaching
target voltage but also for over-current, over-temp, external parameters
and such. Sure, chips like the TL494 offer two FB inputs but their
output drive, frequency and other parameters, well, let's just say we
can do better ;-)

However, the predominant benefit is freedom from a sole source. The
first such switcher design in my career was what sales guys call a
"design-out" of a PWM chip. A vendor (M....) just could not deliver
production quantities and my client had a hard and financially painful
line stop. That never, ever happened again. It was a very long time ago
and this design actually survived the company owner :-(

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Well maybe, but I've seen home brew switchers fail in the field, or
worse yet, do funny things like whack the battery when starting up.
Dell USED to design their own switchers. Note the emphasis on used.

If I don't use a LDO I designed myself, I just buy from TI. TI seems
to make cheap stuff that works. It's like National, but with quality.
Granted, stuff for your personal use is different than making a
product to sell.

I would need a damn good reason to result to using a switcher. Now I
do agree that 5W is really too much for a LDO. If the original poster
kept it reasonable (say 16V), I'd go for the LDO.

"Home brew" switchers used to be all that there was. I designed all
the off-line and DC-DC switchers for GenRad Portable Products
(Omnicomp) from 1977-1987. Never had one fail. And no I/C more
complex than an LM339.


Wasn't that where your boss came in, rocked the power switch back and
forth and it exploded?

That wasn't my boss, it was Ed Greenwood (*) the digital guru for
Omnicomp. And I fixed that situation so it could never happen again.
(It was the prototype BTW, not production yet.)

(*) Deceased now for some ~12 years, due to liver cancer, from
drinking 2-liters per day of a famous "diet" cola. (He weighed 330+
pounds.)

...Jim Thompson
--
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