regulator thingie

John Larkin wrote:
> On Sat, 25 Jun 2011 07:54:24 -0700, Joerg <invalid@invalid.invalid>
> wrote:
> 
>> John Larkin wrote:
>>> On Wed, 22 Jun 2011 06:32:16 -0400, JW <none@dev.null> wrote:
>>>
>>>> On Tue, 21 Jun 2011 10:48:26 -0700 John Larkin
>>>> <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in Message id:
>>>> <ful107hc4if8pdn1eukhnbavctl33j7o4j@4ax.com>:
>>>>
>>>>> I'm making some replacement boards for a system I designed a while
>>>>> ago. 1980, actually. The power regulator board used a couple of
>>>>> LM376's, a nice little DIP8 voltage regulator that drives an off-board
>>>>> heatsunk PNP pass transistor.
>>>>>
>>>>> LM376 seems to be unavailable. But I can't find a substitute, namely a
>>>>> linear voltage regulator controller with foldback capability and
>>>>> external PNP pass transistor. Well, you could hack it with a 723, an
>>>>> even more ancient part.
>>>>>
>>>>> Strange that there's no equivalent, at least none I can find. Anybody
>>>>> know of one? I could make it out of opamps and stuff, I guess, or use
>>>>> an LM1117 in PNP-boost mode, with extra junk for the current liniting.
>>>>>
>>>>> The 376 was so elegant.
>>>> According to  http://www.national.com/kbase/category/Power.html#114
>>>> "LM376 was an LM305 in a plastic DIP package"
>>>>
>>>> Which can still be found, but not cheap by any means.
>>>> http://www.findchips.com/avail?part=LM305
>>>> http://www.oemstrade.com/search/LM305/
>>> I don't think any of them are still in production, and old parts from
>>> brokers are risky.
>>>
>>> Looks like a ua723 will do what I want, but it has a lot of
>>> constraints. I was just sort of surprised that nothing like the 376 is
>>> still made. What do people do when they need a big honking linear
>>> power supply? ...
>>
>> They use a 723 and some big honking transistors on a massive heat sink.
>> Take a look at linear bricks (PowerOne and such), those are a reason the
>> 723 is still around and will be for a long time.
>>
>>
>>>          ... Make the controller from a reference and a couple of
>>> opamps and a transistor or two, I guess... a lot of parts to replace
>>> one DIP8. Seems strange that the best, basically only, linear
>>> regulator controller left, the 723, is about 40 years old.
>>>
>> 40 years is no problem for a successful part, and the 723 clearly
>> qualifies as successful. Right up there with the LM317 or Bob Pease's
>> LM337. I remember when a Fairchild sales guy in the 90's wanted to
>> convince me that it would be futile if I designed a circuit with CD4000
>> logic because it'll go obsolete. His new logic parts were more expensive
>> and would have needed a regulator, so naturally I didn't listen.
>> Actually I had a mild ROFL episode about it later. Long story short that
>> unit still rolls off the production floor in quantities in Shenzhen today.
>>
>> There's app notes about the LM723. If you can't find anything I can go
>> through my stash (there's a reason I don't throw out the old printed app
>> notes and databooks). But it'll have to be later because today is the
>> day where SWMBO wants us to move a massive fountain to another spot.
>> Which of course is higher up and can only be reached via rocks. Anyone
>> have a Motrin?
> 
> The 723 is horribly under-specified. All the datasheets seem to be
> identical copies of the (Fairchild?) original. If you want to know the
> error amp bias current, you have to look at the schematic and estimate
> it, or measure it. It's not specified.
> 

National has a 1999 rev date on it. That doesn't qualify as original :-)

http://www.national.com/ds/LM/LM723.pdf


> Apparently the design was pre-bandgap, so it's full of zeners. So it
> won't work below 9.5 volts input. The reference out voltage exceeds
> the max allowed input differential voltage, so you need a voltage
> divider there. The current limit sense needs a full junction drop
> across the sense resistor.
> 
> National specs its ESD tolerance as 1200 volts!
> 
> It was brilliant 40 years ago. I'm just surprised that it's still
> about as good as you can buy.
> 

Well, there ain't no market to write home about. They are mostly used
when people need really low noise or in the linear bricks where the
vendors would like to not have to put any additional R&D whatsoever into
them.

If you want something more modern you'll have to go for a LT1575 or
similar chips:

http://cds.linear.com/docs/Datasheet/15757f.pdf

But they cost almost an order of magnitude more and aren't as versatile.

-- 
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.