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LM2931-N: Output controlled by DAC

Alex Chitic
Prodigy 20 points
Part Number: LM2931-N

Hello,

I was hoping to use a resistor and a DAC to control the output of LM2931 using a microcontroller like it was described here: https://training.ti.com/voltage-regulator-margining-precision-dacs.

I was successful using this technique with a different LDO, but this one has a different formula for Vout (Vout=Vref(1+R2/R1) instead of Vout=Vref(1+R1/R2)).

This is the schematic that I used:

 

The nominal output voltage was calculated at 9V and R3 was calculated for an output voltage range of 1.2V to 10V while the voltage range of the DAC is 0.2V-3.1V.

I can measure an output voltage of 8.96V while the DAC is not connected, 1.49V for a DAC output of 0.2V and 4.54V for a DAC output of 3.1V.

These are the measured output voltage levels with different R3 values:

R3 56K2 9K09 4K22
Output at 0.2V 3.27V 1.53V 1.37V
Output at 3.1V 5.48V 4.57V 4.45V

Is there a different configuration that I might try in order to get the full desired output voltage range of 1.2V to 10.0V?

Thank you,

Alex Chitic

  • 0
    TI__Mastermind 49585 points

    Hi Alex,

    I think the inverted equation for calculating Vout is preventing this from working as-is. This is not a commonly used configuration so we do not have much insight here. You may need to redo the calculations in the video to account for the different equation.

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 20 points

    Hi Nick,

    Thanks for the answer! In the end I was able to use the same configuration and I was lucky that our application only needed an output of 9.2V to 11.2V.

    By changing the resistor values I was able to achieve an output between 9.2V and 11.2V from a DAC range of 0.2V to 3.1V.