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Using an AD5220 to vary a LM350??? — Parallax Forums

Using an AD5220 to vary a LM350???

HermieHermie Posts: 36
edited 2012-03-15 16:21 in BASIC Stamp
I've searched datasheets on both componates and can't determine if this will work. Has anyone ever tried this. According to the LM350 datasheet, voltage is varied via R2 and I'd like to control it with the AD5220 however I can't determine if the 5220 can handle it since the lm350 controls 3 amps. I'm sure I'm not the first to think of this, I just thought I'd shoot out the question before I try it and risk blowing up a chip.

Thanks in advance

Comments

  • davejamesdavejames Posts: 4,047
    edited 2012-03-14 08:45
    Well, think about where the AD5220 would be in the LM350 circuit - it's in the adjust path and not the 3A path.

    You'd need to find the LM350 spec for the amount of current through the adjust path, then see if the AD5220 could handle that (should be in uA).

    At first blush, I don't see a problem...but look up the specs to make absolutely sure.
  • DocedisonDocedison Posts: 2
    edited 2012-03-14 12:15
    The Necessary "Reference Current is 100 uA or so... most any op-amp can control the output. The Op-Amp must supply the required current sink to keep the Adj. pin @ Vref-Vout (1.25V - Vout) = Vadj
    My only thoughts would be to make sure that Vcc for the op-amp is well regulated and at least 2V greater than the required output voltage (assuming the op-amp Output Voltage range is Rail to Rail)
    Hope this helps, If you have more questions contact me and I will reply in this forum.

    Doc
  • HermieHermie Posts: 36
    edited 2012-03-14 19:36
    For davejames...I can't find or should I say, make sense of anything in the datasheet that deals with the current through R2, as it's only a circuit example.

    For Docedison...The AD5220 is a digital pot and the LM350 is an adjustable voltage regulator, not sure where an op-amp comes into play.


    Steve Green....If you read this....I'd love to get your take!

    Thanks....Hermie
  • davejamesdavejames Posts: 4,047
    edited 2012-03-15 09:37
    Hermie wrote: »
    For davejames...I can't find or should I say, make sense of anything in the datasheet that deals with the current through R2, as it's only a circuit example.

    Go here: http://www.ti.com/lit/ds/symlink/lm350-n.pdf

    Look on page 4 and you'll see a spec for "Adjustment Pin Current", which relates a current range of 50uA (typical) to 100uA (max).

    Will the AD5220 d-pot handle that?

    Also, look on pages 7 & 8 and you'll find the info on how to choose the correct resistance values for the adjust circuit.

    Go here: http://www.parallax.com/Portals/0/Downloads/docs/prod/compshop/AD5220.pdf

    Look on page 3 under "Absolute Maximum Ratings" and you'll find a spec for "Ax - Bx", which relates a current range of +/-20mA. Now I'm sure there is a maximum power dissapation for this part, so you probably couldn't have 20mA at some large voltage on the pot terminals without smoking the device. But I certainly don't see a problem for the d-pot to deal with 100uA as it would be in the LM350 adjust path.

    Make sense?
  • HermieHermie Posts: 36
    edited 2012-03-15 16:16
    Alright DaveJ based on your input, I'm going to give it a shot....it's only a $3 chip so it's not like I'll loose sleep if it blows up.....Thanks
  • davejamesdavejames Posts: 4,047
    edited 2012-03-15 16:21
    Hermie wrote: »
    Alright DaveJ based on your input, I'm going to give it a shot....it's only a $3 chip so it's not like I'll loose sleep if it blows up.....Thanks

    You're welcome.

    I hope there is some understanding in what you are attempting. If you have questions, please ask!

    Either way, lemme know how things turn out.

    Later...
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