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Is it possible to use a digital potentiometer with an SMPS, to adjust the voltage? I'm looking at one with a 2 MHz -3dB bandwidth. What does this mean for a 300 kHz switcher?

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Thomas, This question may be a bit too broad.

There are normally SMPS that will have a set resistance. This can be controlled by a digital pot, very effective at that.

If you are wanting to change components inside the control loop of the SMPS, I would suggest against it.

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  • \$\begingroup\$ I want to replace the feedback resistors with a digital pot. I don't think I can do this without making it unstable, but I'd like to know otherwise. \$\endgroup\$ – Thomas O Nov 10 '10 at 7:46
  • \$\begingroup\$ I am suggesting that you purchase an SMPS with a set resistance that is external of the loop and is designed for this feature. I am suggesting against modifying the control loop while running to change voltage. I am not saying it cannot be done, I am suggesting against it. \$\endgroup\$ – Kortuk Nov 10 '10 at 17:05
  • \$\begingroup\$ I'm building the supply myself, using an LT3757, to output 1A with a variable range of at least 9V-12V (if not 6.5V-15V.) \$\endgroup\$ – Thomas O Nov 10 '10 at 20:28
  • \$\begingroup\$ I have seen buck-boost chips that have a pin that you tie through a resistor to ground. This resistance can be varied, they often give a digital pot reference design, to change your output voltage over a wide range and they are designed for it. \$\endgroup\$ – Kortuk Nov 10 '10 at 21:23
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Bandwidth is usually not relevant when it comes to a power supply adjustment pot. The value is generally set at a single calibration point and isn't varied. Even if the pot were being varied (by a micro, for example) the adjustment speed has to be significantly lower than the switching frequency to maintain Nyquist stability criteria.

The bottom resistor of the feedback voltage divider isn't really part of the compensation formulae, so generally this is where adjustment is made. If you tweak this value, you can end up with lower (or higher) voltage, but there are many things that have to be considered with a change of output voltage (duty cycle, rectifier and switching device stress, capacitor voltage, etc.) so tread carefully.

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  • \$\begingroup\$ That's a good idea - only changing the bottom resistor. I will have to consider that. \$\endgroup\$ – Thomas O Nov 10 '10 at 7:46
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Bear in mind you will only be able to adjust output voltage over a limited range, maybe +/-25% before stability or other issues like popping output caps cause problems

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  • \$\begingroup\$ The SMPS will be built to handle the highest output voltage of 15V. \$\endgroup\$ – Thomas O Nov 10 '10 at 17:18

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