Digital LM317 Control

J

Jay Davis

Guest
I'm trying to build a circuit that digitally controls a LM317 using a
(0-5V) DAC. I'm kind of confused about the math here - I can't see how
to change the range of the LM317 to anything other than a 5V range. I
can see how to vary it from say 5 to 10, or from 17 to 22, but I can't
see how to do more than a 5V swing. Right now, I was designing the
circuit like this:

1K 240
DAC OUT--/\/\/\--ADJ--/\/\/\---VOUT


I know that the LM317 creates a 1.2V reference across the 240 ohm
resistor, for a current of 5ma. So then I figure that (VOUT -
DACOUT)=5ma(1k) = range of only +/- 5V. Can someone set me straight here?

--
Jay
 
Why not just generate whjatever voltage you want from the DAC and feed it
into an emitter follower circuit using a power transistor (dont' forget to
account for the base voltage drop)


"Jay Davis" <test@windmeup.net> wrote in message
news:HMPpc.4630793$iA2.541089@news.easynews.com...
I'm trying to build a circuit that digitally controls a LM317 using a
(0-5V) DAC. I'm kind of confused about the math here - I can't see how
to change the range of the LM317 to anything other than a 5V range. I
can see how to vary it from say 5 to 10, or from 17 to 22, but I can't
see how to do more than a 5V swing. Right now, I was designing the
circuit like this:

1K 240
DAC OUT--/\/\/\--ADJ--/\/\/\---VOUT


I know that the LM317 creates a 1.2V reference across the 240 ohm
resistor, for a current of 5ma. So then I figure that (VOUT -
DACOUT)=5ma(1k) = range of only +/- 5V. Can someone set me straight here?

--
Jay
Click to expand...
 
UncleWobbly wrote:

Why not just generate whjatever voltage you want from the DAC and feed it
into an emitter follower circuit using a power transistor (dont' forget to
account for the base voltage drop)
Click to expand...
How will this allow me to regulate over a full 1.2 to 30V range?
 
"Jay Davis" <test@windmeup.net> wrote
news:W_Qpc.4634772$iA2.541518@news.easynews.com...
UncleWobbly wrote:

How will this allow me to regulate over a full 1.2 to 30V range?
Click to expand...
Something better and easier: use an opampl like the OPA549.
http://focus.ti.com/docs/prod/folders/print/opa549.html

You can make a voltage and current controlled powersupply with this opamp
and 2 DAC's.
And if you are handy enough you can even order some samples from TI.. :)

Just some idea.

Al
 
"Jay Davis" <test@windmeup.net> wrote in message
news:HMPpc.4630793$iA2.541089@news.easynews.com...
I'm trying to build a circuit that digitally controls a LM317 using
a
(0-5V) DAC. I'm kind of confused about the math here - I can't see
how
to change the range of the LM317 to anything other than a 5V range.
I
can see how to vary it from say 5 to 10, or from 17 to 22, but I
can't
see how to do more than a 5V swing. Right now, I was designing the
circuit like this:

1K 240
DAC OUT--/\/\/\--ADJ--/\/\/\---VOUT


I know that the LM317 creates a 1.2V reference across the 240 ohm
resistor, for a current of 5ma. So then I figure that (VOUT -
DACOUT)=5ma(1k) = range of only +/- 5V. Can someone set me straight
here?

--
Jay
Click to expand...
R1 R2
Vin---\/\/\---Vadj---\/\/\----Vout

Vout = 1.25*(240 + R2)/240 + Vin

In order to do a larger swing, you need a way to scale the output of
your DAC. One easy way would be an opamp circuit like this:
.----------.
IN | | OUT
--------------------| LM317 |----------------
| |
| ADJ |
'----------'
|\ |
+----|-\ |
.------. | | >------+-------'
| DAC +---|----|+/ |
| | | |/ .-. R1+R2
'------' | | |R1 OUT = 1.25 + DAC * -----
| | | R2
| '-'
| |
+--------------+
|
.-.
| |R2
| |
'-'
|
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Obviously, IN must be larger than OUT by at least 3V. Accuracy is
related to the tolarances of R1 and R2.

Regards,
Bob Monsen
 
Robert C Monsen wrote:

R1 R2
Vin---\/\/\---Vadj---\/\/\----Vout

Vout = 1.25*(240 + R2)/240 + Vin

In order to do a larger swing, you need a way to scale the output of
your DAC. One easy way would be an opamp circuit like this:
Click to expand...
Ah, thanks! That's what I was trying to do :)

--
Jay
 
The Al Bundy wrote:

Something better and easier: use an opampl like the OPA549.
http://focus.ti.com/docs/prod/folders/print/opa549.html
Click to expand...
That looks interesting. Since I'm just doing this as a "fun" project
and nothing real serious, I'll probably just stick with parts I have on
hand, but should I want do do anything "for real", I'll take a look at
that part - 8A, 60V - yikes.

--
Jay
 
On Sun, 16 May 2004 21:55:28 GMT, "Robert C Monsen"
<rcsurname@comcast.net> wrote:


In order to do a larger swing, you need a way to scale the output of
your DAC. One easy way would be an opamp circuit like this:
.----------.
IN | | OUT
--------------------| LM317 |----------------
| |
| ADJ |
'----------'
|\ |
+----|-\ |
.------. | | >------+-------'
| DAC +---|----|+/ |
| | | |/ .-. R1+R2
'------' | | |R1 OUT = 1.25 + DAC * -----
| | | R2
| '-'
| |
+--------------+
|
.-.
| |R2
| |
'-'
|
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Obviously, IN must be larger than OUT by at least 3V. Accuracy is
related to the tolarances of R1 and R2.

Regards,
Bob Monsen
Click to expand...
Use an LM1117 for lower dropout differential. Nice part.

And, as Win has pointed out, you can usually power the opamp off the
regulator output!

John
 
John Larkin wrote:

And, as Win has pointed out, you can usually power the opamp off the
regulator output!
Click to expand...
Yeah, I just realized this. Obviously I can't power the opamp off of a
5v supply and expect the circuit to be able to do the full 1.2 to 30V -
no wonder I couldn't get it to make sense in my head.

--
Jay
 
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