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LMR12020 with DAC for adjustable ouput

Paul Bar
Intellectual 375 points
Other Parts Discussed in Thread: LMR12020

Hi,

I am using a LMR12020 with a dual voltage ouput. Now I would like to control the output using a DAC which would be MCU controlled.

The module adjust its ouput so that the FB pin always sees 1V. Lets say I use a 9k resistor with a 1k resistor so that the output is stable at 10v. If I add a DAC ouput on the FB pin with 0.5V on in, it should decrease the voltage to 5V and increase the voltage ripple ~2x. Is this correct?

The output voltage ripple is not that important to me as this regulator feeds a linear current regulator.

Thanks for your help

  • 0
    TI__Guru 74475 points

    I have attached a drawing of what I think you want to do.  It should work, but like everything else you need to test it out fully.

    The ripple should not increase.  You need to be sure that the output impedance of the DAC is low out to high frequencies, so that it does not cause trouble with the stability.  You may need a small cap on the DAC output. 

  • 0 in reply to Frank De Stasi
    Intellectual 375 points
    I will be trying that. You place the DAC to a different place that I first thought but this is way better like you did.

    Why would I need some small cap? Usually OP AMP are less stable with output capacitance.
  • 0 in reply to Paul Bar
    TI__Guru 74475 points

    The regulator is expecting to see a low impedance to ground on the bottom FB resistor.  So you need to be sure that the DAC output is a low impedance.

    Yes, opamps do not like capacitive loads, that's why you need to check carefully.