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Digital LM317 Control

Discussion in 'Electronic Basics' started by Jay Davis, May 16, 2004.

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  1. Jay Davis

    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?
  2. UncleWobbly

    UncleWobbly Guest

    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)
  3. Jay Davis

    Jay Davis Guest

    How will this allow me to regulate over a full 1.2 to 30V range?
  4. The Al Bundy

    The Al Bundy Guest

    Something better and easier: use an opampl like the OPA549.

    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.

  5. R1 R2

    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
    | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

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

    Bob Monsen
  6. Jay Davis

    Jay Davis Guest

    Ah, thanks! That's what I was trying to do :)
  7. Jay Davis

    Jay Davis Guest

    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.
  8. John Larkin

    John Larkin Guest

    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!

  9. Jay Davis

    Jay Davis Guest

    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.
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