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How to calculate value of series resistor on DAC Output of LTC2974?

Category: Datasheet/Specs
Product Number: LTC2974, LT3045

Hi ADI Team,

Can you help us with below query?

For LT3045, how can we derive the value of R30? In datasheet of LTC2974, equations are available to derive these resistor values but considering the situation where R20 is available in design. But in case of LT3045, R20 is not included in design as output voltage can be configured with help of R10 itself.

Let us know your thoughts

Regards,

Purvi

  • Hi Purvi, the general answer to your question can be found in the DC2518 demo manual on pg 16.  I covered the case of managing a 12V rail which is probably a higher voltage than your case.  The concept is similar.  If your Vout voltage needs to be margined over a small range, you need to connect two series resistors from SET pin to GND.  The DAC is tied to the mid point of the resistors. When the DAC moves up/down, this translates directly to movement in Vout.  You don't need R10, R20, or R30.

    When driving a SET pin, the DAC should be set to non-inverting mode.

    Also you need the SET resistors because the DAC is high Z when the regulator powers up.  The resistors set the initial Vout voltage.

    Mike