control LM2576HV with DAC

Discussion in 'General Electronics Chat' started by ecka333, Sep 14, 2011.

  1. ecka333

    Thread Starter Active Member

    Oct 1, 2009
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    Hello, i have buck converter IC LM2576HV. Chip's output voltage, according to manufacturers datasheet, can be controlled with potentiometer. My question is: can i control LM2576 output voltage with DAC converter? There is two types of DAC: voltage and current. Also converters may be buffered and without buffering. I have bought MAX531 DAC with parameters: buffered voltage output, power supply is +5v, The MAX531's internal op amp may be configured for a gain of +1 or +2. So can i control converter with this DAC, o i need current DAC?
     
  2. ecka333

    Thread Starter Active Member

    Oct 1, 2009
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    So nobody knows about DAC's?
     
  3. tom66

    Senior Member

    May 9, 2009
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    You could try applying a voltage to the bottom of the resistive divider. The output would then be DAC voltage + normal output voltage.
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You didn't mention the input voltage and what output voltage range you want to have.

    I hope you have bought the adjustable version as the fixed output version will expect to see the specified output voltage at its feedback pin.

    For the adjustable version, LM2576-ADJ expects to see 1.23V at its feedback pin. If it sees a lower voltage, it will keep increasing the switching duty cycle to try to raise the output voltage and vice verser.

    It is much simpler to use a digital potentiometer instead of a DAC. Connects the digital POT to acts as one leg of the actual output voltage divider, same as those in the fixed output versions, e.g. R1 in the figure below. Use a fixed resistor for R2.

    [​IMG]
     
  5. tom66

    Senior Member

    May 9, 2009
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    I've always wondered if a digital potentiometer would be okay, because many of them have a poor bandwidth of a few MHz. I've never tried it though, so wouldn't know if this is actually a problem.
     
  6. ecka333

    Thread Starter Active Member

    Oct 1, 2009
    76
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    I am planning to use LM2576HV-ADJ chip this is adjustable version. Power supply input will be about 26V, and output from 1,23V to 21V. According schematics, resistor R2 must be adjusted and R1 can be from from 1k to 5k. If i will adjust voltage of shematics part , where R1 is with DAC, my DAC will be burned, because it designed for 5V voltage, am i right?
     
    Last edited: Sep 19, 2011
  7. ecka333

    Thread Starter Active Member

    Oct 1, 2009
    76
    0
    So i decided to use resistors chain to control LM2576HV-ADJ voltage. But i dont know how can i implement resistor switches. Maybe there is way with multiplexer and transistors?
     
  8. PaulEE

    Member

    Dec 23, 2011
    423
    32
    The CD4066 is a quad bilateral switch. You can use a microcontroller or logic chips to switch between resistors. Each switch can run on +5v and the ON resistance is usually between 10 and 100 ohms; consult datasheet for details. Not a big deal if you're paralleling resistors of moderate to high values (>>10*100 ohms, as an example).
     
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