Using multiple digital potentiometers to set feedback ratio?

[Korken]

Hi all!

I have an step up/down switch-mode IC that I want to control the output of from a microcontroller.
How I have done this before is to have an I2C pot as feedback resistors so I can control the voltage divider.

However I have now come to the problem that I need more steps in the potentiometer, but the "high tap" ones get quite expensive.
So the idea is to use two "low tap" ones. What is the best way use multiple potentiometers to get higher resolution?

The only requirement is that the I2C pot only changes the resistance to ground as the voltage on the output can get above the maximum rating.

Or is there another way to change the feedback ratio?

Thanks!

[Marco]

Why not change the reference voltage with a dac instead?

[Korken]

Hmm, would you care to explain how you mean?
Or rather, how would you connect a DAC to do this? Offset the ground reference?

[dannyf]

No easy solution if you are going after full-range high resolution: digital pots are not meant for those applications.

One less-than-perfect solution is to pair two digital pots together - one pot is connected between the wiper and one terminal of another pot. It provides finer resolution on one end of the wiper but not on the other end.

[Korken]

I though a bit on the DAC part and I thought this would work (attached image).
I add in the DAC to provide a current sink/source to shift the output level.

Was this what you had in mind?

[Charles Creations]

I don't know what microcontroller your are using, but assuming it doesn't have a true hardware DAC, you can use a pwm function with a low pass filter to achieve a crude DAC. You are limited to your pwm resolution and you have to be sure that you don't load the filter too much or else its output will drop.

[Marco]

Was this what you had in mind?

No, that's better though ... should work.

[Korken]

Thanks for the replies!
All it takes is a new angle and great things happen.

I'd like to test it with the PWM as well, could be an easy solution. Thanks for the tip!