Digital Power supply using LM2679 buck switcher

Discussion in 'The Projects Forum' started by james101, Jan 14, 2010.

  1. james101

    Thread Starter New Member

    Jan 12, 2010
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    Hello all,

    I have sucessfully designed a DC/DC buck switcher using the LM2679 IC. I could vary the output voltage from 3V to 10V using a potentiometer (the input voltage is 20V). Now, I want to be able to control the output voltage through a microcontroller. I thought I could easily control the output voltage by controlling the FEEDBACK pin with a variable voltage, but this doesn't seem to work.

    - I tried connecting a power supply directly to the FEEDBACK pin and vary the voltage. The IC appears to be in shutdown mode.
    - I also try connecting the power supply to the voltage divider, but the IC works intermittently. It's quite unstable.

    Am I missing something? How do power supply designer design their power supplies?

    Regards,
    James
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You need to replace the manual pot with an electronic pot, and control the electronic pot with your uC.

    If you simply try feeding a fixed voltage to the feedback input, you will be breaking the feedback loop. This is an un-good thing, as the LM2679 will either shut down completely, or be running wide open.
     
  3. james101

    Thread Starter New Member

    Jan 12, 2010
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    0
    Thank you for your reply. I think the pot option is not possible because I want to have a power supply from 3V to 16V. I thought the pot can only work up to 5v or so?

    What I really want to do is to have to MCU monitoring the DC/DC output voltage via a ADC and adjust the FEEDBACK voltage (via an Opamp or DAC) appropriately. The feedback loop is through the MCU.

    James
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Post your schematic; the circuit as it was when it was working.

    If you want the MCU to be able to do anything else besides control the output voltage, you should just let the LM2679 IC "do it's thing" for that, and you can sample the output using the MCU occasionally to make adjustments.

    If you want the MCU to have full control over the output, then you don't really need the LM2679; just use the appropriate MCU as a DC-DC switcher. It won't be able to do much of anything else, though.
     
  5. james101

    Thread Starter New Member

    Jan 12, 2010
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    The switcher schematic is attached. I want the MCU to have complete control over the output voltage. After all, it's a digital power supply!

    The MCU also control the graphic LCD and its GUI.

    James
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I would approach the problem by replacing R1 with a digital pot.

    Otherwise, your MCU will not have time to update the LCD, or do anything else.

    The IC will try to keep the FB voltage at (nominally) 1.21v.

    So, calculate R1/R2 to obtain a range where you stay within the current/voltage limits of a digital pot.

    Don't forget that you could have another resistor in parallel with R1 to reduce the maximum voltage/current required through R1.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Take it from someone who has done that, it's a pretty bad idea. Including the MCU within the feedback loop is very slow, you have ADC sampling times and other delays before you can compensate for voltage changes. It can work for really slow changing voltages like battery charging, but for a "power supply" its a bad idea as a PSU will need very fast response!

    The best system is to use the MCU to generate a reference voltage via PWM etc. Then use the SMPS IC to regulate the PSU voltage to match the reference voltage.
     
  8. james101

    Thread Starter New Member

    Jan 12, 2010
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    Hi SgtWookie,
    The LM2679 datasheet recommends to keep R1 at 1Kohms. I am not sure what's the reason behind it, but I will give it a try to see if it works reliably.

    Hi THE_RB,
    Which SMPS IC do you suggest? I need to a power supply up to about 16V @ 1.5A

    Best regards,
    James
     
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