# SW-Reg voltages at points

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

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