Design Article
Calibrate a power supply with a digital potentiometer
David Fry, Strategic Applications Manager, Maxim Integrated Products
2/12/2012 1:50 PM EST
A resistive feedback network is often used to set the output voltage of a power supply. While a mechanical potentiometer (pot) conveniently solves the problem of adjusting a power supply, it can be replaced with a digital pot for easier automatic calibration.
This article presents a calibration solution that uses a digital pot, because pots are smaller, do not move with age or vibration, and can be recalibrated remotely. This proposed solution reduces the susceptibility of the system to the tolerance of the digital pot’s end-to-end resistance, making the solution optimal for designers.
The article also explains some of the equations required to calculate the resistor chain values and to use a digital pot in this way. Also, a spreadsheet with standard resistor values is available for easy calculations.
"Calibrate a power supply with a digital potentiometer" is presented in its entirety as a pdf document, for convenience.
About the Author
David Fry is the Strategic Applications Manager for the DACs, pots, and references product line at Maxim Integrated Products. He has over 25 years experience in the electronics industry, with the last 13 years working in semiconductor field applications, market development, and product-line applications. Prior to joining Maxim, David spent 11 years as a board-level designer working on RF designs and several years designing high-end broadcast video equipment. He earned a BSc in Electronics in 1984 from the University of Manchester Institute of Science and Technology.
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stricula
2/16/2012 3:54 PM EST
Had a colleague try this in a design. Worked well until ESD testing. Just wanted to present that potential pitfall. Ended up re-purposing that digital pot control output to PWM into an RC filter. The RC filter output was connected to a resistor which went into the feedback node of the regulator. The micro was always reading the voltage out of the regulator. It was an obvious solution once realized that the feedback node is ideally a fixed voltage; lower value than specified = more power commanded out. Makes for an easily configurable and adjustable power supply.
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sharps_eng
2/17/2012 3:05 PM EST
Agreed - replacing a hardware device with a digital pot which contains a bunch of registers, non-volatile or not, increases the state complexity of the system. It may make a better system, but it makes a different system which needs to be tested differently.
The ESD /EMI compliance issue is just one manifestation of this.
Also: make sure you choose the right version of the pot chip, you don't want your systems to be powered up first time, set at the wrong end of the pot when you want it to be set in the middle.
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BicycleBill
2/17/2012 4:18 PM EST
Certainly, it's not a solution to every design situation, but it gives you new degrees of freedom and flexibility--and along with that, comes responsibility!
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