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I am needing a variable positive and negative output voltage ~(0-20)V with a 20+V input. To handle this, I am using the LT1074. I am using the two configurations from page 13.

To do this, I am planning on using a digipot on each to control the output. Unfortunately, they don't handle high voltages at pins well. Therefore, for + -> + configuration, i will need to swap out the R2 instead of R1

On the 2nd configuration, + -> - , I'm having a bit of trouble figuring out the voltages across R3 and R4. -Vout is -2.21*(R3/R4+1). My assumption is that Vfb is going to try to be -2.21, meaning R3, the larger resistor, is going to have a much smaller voltage drop then the smaller resistor, R4, which just feels wrong.

If any of you can help me with either the voltage drops, or what assumptions I make are fundamentally incorrect, i would appreciate it.

Thank you, James

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  • \$\begingroup\$ We love schematics. There's a button on the editor toolbar. \$\endgroup\$
    – Transistor
    May 26, 2016 at 21:30
  • \$\begingroup\$ I like this IC. I downloaded the pdf. I will let the OP cut and paste the diagram on page 13 of the pdf. \$\endgroup\$
    – user105652
    May 26, 2016 at 21:40

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maybe not as helpful as the comments (edit: was answers) above, but hopefully more constructive, I've been intending to do this myself recently (and might get round to it tomorrow...). Injecting a voltage into the junction of the voltage divider (node? ), will add an offset which will change the feedback voltage and hence the output voltage (I think). Using a digital pot to replace the divider is tricky, certainly the ones I looked at which have 0V as negative supply and generally 5V as positive, with protection diodes on the wiper which won't let you use them with negative rails or positive supplies outside of their own power supply range. using a DAC to provide a voltage (via a resistor) to the summing point, should, I hope, allow you to vary the FB voltage and the regulated output.

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