Ok to parallel LM2940''s ?
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Posts: 46,084
Hi Jonathan,
I don't recall ever seeing two of the 3-pin voltage regulator ICs connected
in parallel to increase the available output current. Just glanced through
the LM2940 data sheet, and didn't find any mention of doing this. Seems
like the inevitable output voltage difference between the two might make one
"dominant" and cause unexpected effects. But I'm just guessing.
Here is something that might be interesting to try... One could connect
series diodes, such as 1N400x, to the output pins of each regulator - two
regulators, two diodes. Connect the anode of each diode to one of the
regulator output pins, and connect the two cathodes together for the
combined output. Now there would be no possibility that current from one
regulator could flow into the output pin of the other. But, but.... the
output voltage is now dropped by one diode drop (and that voltage will vary
somewhat in relation to the current flowing through the diode).
So... connect a series diode between each regulator ground pin and ground.
The voltage drop through the series diode will bump up the voltage on the
regulator's ground pin by one diode drop, causing the regulator's output
voltage to increase by a like amount. Even with perfectly matched diodes
the voltage drop through the output pin diodes would be greater than that of
the ground pin diodes, since there is (presumably) more current flowing
through them . So you would lose some of the rather tight and accurate
regulation from these nice regulators.
Also, the LM2940 has a special output capacitor ESR value requirement to
avoid possible oscillation, and I'm not sure if a connection like this would
make a problem more likely. I recently bought a capacitor ESR meter kit
with the intent to study the performance of the LM2940 with capacitors of
varying ESR. No results yet, though.
Overall I think the above "idea" with the diodes is a bit "out there" and
proves that I should be asleep right now! If you do try it, I would be
interested in hearing about the results.
There are circuits out there that use a power transistor connected to a
3-pin regulator so the regulator controls the voltage output and the power
transistor handles the current to the load. As I recall, the transistor in
this application is called a "pass transistor." This might be a better way
to increase the current output vs. a single LM2940. I'm not sure if the
LM2940 has any characteristics that would make it unsuitable for a "pass
transistor" regulator circuit application.
Okay... off to sleep now...
Randy
Original Message
From: "Jonathan Peakall" <jpeakall@m...>
To: <basicstamps@yahoogroups.com>
Sent: Friday, August 15, 2003 7:56 PM
Subject: [noparse][[/noparse]basicstamps] Ok to parallel LM2940's ?
> Hi All,
>
> Is it OK to parallel LM2940's for greater current output?
>
> Thanks,
>
> Jonathan
>
> www.madlabs.info
>
>
>
> 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.
>
>
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>
>
I don't recall ever seeing two of the 3-pin voltage regulator ICs connected
in parallel to increase the available output current. Just glanced through
the LM2940 data sheet, and didn't find any mention of doing this. Seems
like the inevitable output voltage difference between the two might make one
"dominant" and cause unexpected effects. But I'm just guessing.
Here is something that might be interesting to try... One could connect
series diodes, such as 1N400x, to the output pins of each regulator - two
regulators, two diodes. Connect the anode of each diode to one of the
regulator output pins, and connect the two cathodes together for the
combined output. Now there would be no possibility that current from one
regulator could flow into the output pin of the other. But, but.... the
output voltage is now dropped by one diode drop (and that voltage will vary
somewhat in relation to the current flowing through the diode).
So... connect a series diode between each regulator ground pin and ground.
The voltage drop through the series diode will bump up the voltage on the
regulator's ground pin by one diode drop, causing the regulator's output
voltage to increase by a like amount. Even with perfectly matched diodes
the voltage drop through the output pin diodes would be greater than that of
the ground pin diodes, since there is (presumably) more current flowing
through them . So you would lose some of the rather tight and accurate
regulation from these nice regulators.
Also, the LM2940 has a special output capacitor ESR value requirement to
avoid possible oscillation, and I'm not sure if a connection like this would
make a problem more likely. I recently bought a capacitor ESR meter kit
with the intent to study the performance of the LM2940 with capacitors of
varying ESR. No results yet, though.
Overall I think the above "idea" with the diodes is a bit "out there" and
proves that I should be asleep right now! If you do try it, I would be
interested in hearing about the results.
There are circuits out there that use a power transistor connected to a
3-pin regulator so the regulator controls the voltage output and the power
transistor handles the current to the load. As I recall, the transistor in
this application is called a "pass transistor." This might be a better way
to increase the current output vs. a single LM2940. I'm not sure if the
LM2940 has any characteristics that would make it unsuitable for a "pass
transistor" regulator circuit application.
Okay... off to sleep now...
Randy
Original Message
From: "Jonathan Peakall" <jpeakall@m...>
To: <basicstamps@yahoogroups.com>
Sent: Friday, August 15, 2003 7:56 PM
Subject: [noparse][[/noparse]basicstamps] Ok to parallel LM2940's ?
> Hi All,
>
> Is it OK to parallel LM2940's for greater current output?
>
> Thanks,
>
> Jonathan
>
> www.madlabs.info
>
>
>
> 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.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>