Parallel Voltage regulators
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Hi All,
I'm wondering if two voltage regulators in parallel, "LM2940" will
supply double the load current of one. Anyone tried this? Thanks.
-Dave
I'm wondering if two voltage regulators in parallel, "LM2940" will
supply double the load current of one. Anyone tried this? Thanks.
-Dave
Comments
I think this should work. But you will have to put a resitor of say
0.1 Ohm/5 Watt at the output of each LM2940 to cope with the
tolerance of the voltage regulators. However, you will lose some of
the voltage stability due to the resistors.
I did not try this with voltage regulators, but with power
transistors it works well.
Regards,
Klaus
O ja, Happy newyear !!
I have been told this isn't a good idea. One of the Vregs will wind up doing
most of the work. I would just use a regulator that will provide enough
current, or, as I did on my recent robot controller, use two, but have them
run different stuff and split the load up that way.
Jonathan
www.madlabs.info
Original Message
From: "nuclearspin2000" <nuclearspin2000@y...>
To: <basicstamps@yahoogroups.com>
Sent: Wednesday, December 31, 2003 4:30 PM
Subject: [noparse][[/noparse]basicstamps] Parallel Voltage regulators
> Hi All,
>
> I'm wondering if two voltage regulators in parallel, "LM2940" will
> supply double the load current of one. Anyone tried this? Thanks.
>
> -Dave
>
>
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>
>
>
> I'm wondering if two voltage regulators in parallel, "LM2940"
> will supply double the load current of one. Anyone tried this?
>
Never tried it, Dave, but it's in that category of things that can look like
they work until the worst possible time to fail.
Linear regulators use a feedback loop to maintain the target voltage. The
National app note <http://www.national.com/an/AN/AN-1148.pdf> describes how
they work if you're interested.
Putting two feedback loops face-to-face, so to speak, is generally a bad
idea. Almost always in fact. They won't do anything predictable -- including
fail. They tick along seeming to do just fine and suddenly go unstable when
both loops try to react to a condition change at the same time and they
create a positive feedback situation. Or they can fail at start-up. You'll
never know.
Depending on the particular chip, an LM2940 costs a buck or two. Going to an
LM1085 gives you 3A for only six bucks (At Digikey:
http://makeashorterlink.com/?T65452EE6). And you can get 1.5A or 2A for even
less if you don't need the headroom of an LM1085.
I'd do it with a single chip. The aggravation of an unpredictable
instability will outweigh the three or four bucks you're trying to save.
Gary
<http://wwwd.national.com/national/PowerMB.nsf/0/9cfa6057c64fbfb88825696
6006765c5?OpenDocument>
Have a Happy New year
Original Message
From: Gary W. Sims [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=nXU7qvMQ0fPLoWVI7OFz6zXnTLlqg37vOPSiy8ajnTiHhUodj66sJiXinYZhmG5yr1SZ1Vc1Rqx6Dyci8cHeH5U]simsgw@c...[/url
Sent: Wednesday, December 31, 2003 9:57 PM
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] Parallel Voltage regulators
From: "nuclearspin2000" <nuclearspin2000@y...>
>
> I'm wondering if two voltage regulators in parallel, "LM2940"
> will supply double the load current of one. Anyone tried this?
>
Never tried it, Dave, but it's in that category of things that can look
like
they work until the worst possible time to fail.
Linear regulators use a feedback loop to maintain the target voltage.
The
National app note <http://www.national.com/an/AN/AN-1148.pdf> describes
how
they work if you're interested.
Putting two feedback loops face-to-face, so to speak, is generally a bad
idea. Almost always in fact. They won't do anything predictable --
including
fail. They tick along seeming to do just fine and suddenly go unstable
when
both loops try to react to a condition change at the same time and they
create a positive feedback situation. Or they can fail at start-up.
You'll
never know.
Depending on the particular chip, an LM2940 costs a buck or two. Going
to an
LM1085 gives you 3A for only six bucks (At Digikey:
http://makeashorterlink.com/?T65452EE6). And you can get 1.5A or 2A for
even
less if you don't need the headroom of an LM1085.
I'd do it with a single chip. The aggravation of an unpredictable
instability will outweigh the three or four bucks you're trying to save.
Gary
To UNSUBSCRIBE, just send mail to:
basicstamps-unsubscribe@yahoogroups.com
from the same email address that you subscribed. Text in the Subject
and Body of the message will be ignored.
Yahoo! Groups Links
To visit your group on the web, go to:
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To unsubscribe from this group, send an email to:
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Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
10 Amps out of two regulators you have to use two load sharing
resistors, if two 0.1 ohms resitors are used, at 5 amps the power
dissipation across each resistor would be 5x5*0.1 or 2.5 watts!.
Under overload or short circuit conditions, you will have a LARGE
power dissipated in those resistors.
Approach like that could be classified as a novice smoke test.
move to a bigger regulator. Happy New Year.
-Dave
--- In basicstamps@yahoogroups.com, "nuclearspin2000"
<nuclearspin2000@y...> wrote:
> Hi All,
>
> I'm wondering if two voltage regulators in parallel, "LM2940" will
> supply double the load current of one. Anyone tried this? Thanks.
>
> -Dave
>
> I'm wondering if two voltage regulators in parallel, "LM2940"
> will supply double the load current of one. Anyone tried this?
From: Gary W. Sims [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=UzQt9l4uBnMKf0zvrZYhg0JcWs0IUmBU9UgKVb1f83qQF8Zj8lFRpqxowXGOblGWiU9pHldPuj60XyaTn-9P7T4]simsgw@c...[/url
>
> Never tried it, Dave, but it's in that category of things that can look
> like they work until the worst possible time to fail.
> Linear regulators use [noparse][[/noparse]...]
From: "Earl Bollinger" <earlwbollinger@c...>
>
> According to National you can:
<http://wwwd.national.com/national/PowerMB.nsf/0/9cfa6057c64fbfb888256966006
765c5?OpenDocument>
>
Well-spotted, Earl. Here's a shorter version of that link for people having
trouble with line wrap: http://makeashorterlink.com/?A18951FE6
It's a National tech support response to someone who wanted to parallel two
power regulators. They said he could, with certain precautions. For those
not familiar with the issue, a power regulator chip has an internal feedback
circuit. This means the output voltage is compared to a reference voltage.
In the case of linear regulators, an error amplifier tries to force the
voltages at it's input to be equal. To do this, it sources current as
required to meet the load demand. The response technique is different with a
switching regulator.
My concern is that the discussion related to a buck regulator, that is, a
step-down switching regulator. Specifically, the LM2679 from National. Power
supply design is not my field, but I know that switchers and linear
regulators are two different breeds. The feedback loop is not the same in
the two different families, and even within linear regulators there is a
significant design difference between a conventional old-style linear and
the low-drop-out designs for battery operated systems. I wouldn't count on
that answer from National applying to low-drop-out linear regulators like
the LM2940 and LM1085 unless I got another specific answer from National.
This time addressing the problems it causes to tie two such LDO units
together.
National's answer did address the problem that arises with that particular
buck regulator if two are tied in parallel: one of them will take more and
more load until it goes into thermal shutdown -- at which point the other
will pick up the full load and then fail in turn. National tech support
described how one avoids that. That method might work for a linear regulator
also -- or it might not. I'm not qualified to judge even if I spent the time
to examine the specific implementation of the feedback loop used in the
LM2940.
Before I'd use the technique on an LM2940 I'd want to see a similarly
specific response from a manufacturer. The feedback loop in low drop-out
linear regulators is notoriously susceptible to instability. Ultra low
drop-out regulators are even more so. There is no reason not to use them if
you are aware of the problems and use the correct capacitors to prevent
oscillation across the full load and ambient temperature range -- but you
cannot blithely plug one in with a random electrolytic across the output. It
is courting disaster. For those who haven't smoked one yet, the critical
issue is the ESR of the capacitors used. That is, the Equivalent Series
Resistance of the capacitor. It can't be too high or too low, and it must
remain in the required band over the full range of operating temperatures.
This is not a big deal if you read the app notes carefully and choose the
right tantalum cap for the job, but as I said, you must pay attention to the
issue.
That characteristic of the control loop in a low drop-out regulator is my
reason for not trying this unless forced into a corner by the application --
and then I'd want a specific answer from National or the manufacturer in
question before I did it. And I'd put the answer in my design file to
protect myself from liability issues if my client's system failed.
Given all that, is it worth trying to save three bucks? At quantity one
prices, for all love?
That's just my own design approach, of course. I look forward to hearing the
result if someone wants to try it just for the heck of it. Remember to cycle
through the operating envelope, including load transients and ambient
temperature. Take videos in case anything happens the rest of us would find
entertaining<g>.
Gary