Looking for a digitally controlled buck-regulator, might it be via i2c, pwm, IO - Page 1
Forum section: electronics / projects

author: senso
author_rank: Frequent Contributor
timestamp: January 06, 2016, 03:04:08 pm
content: Good evening, I'm trying to find a buck regulator that will take about 17-19v and be able to output anything from 1v up to the Vin minus losses, current will be from 20mA(almost no load) to 1A worst case scenario.I have a prototype made using an LM2596 and a big 10 turn pot in the feedback, it works OK and its like all the sub 100€/$ Tattoo supplies that are not so called digital.After talking and measuring, a resolution of 0.1v is all about that is needed to provide a fine control.Now, I have had some ideas and wanted to make this a bit more advanced while also making the supply cheaper, the pot alone costs 11€, more than all the other parts and the pcb.Maybe I should use a digipot(or two, since almost all of them are 7 bits), but all of them can only see 5v at their terminals, and I dont really know how to slap a digi-pot into the feedback loop, maybe reduce the feedback voltage to under 5v, but I think it wont work into the low voltage range.I tried to find some i2c/PMBus buck regulators, and although they exist they are usually muti-phase power houses with tons of features in huge packages that I dont really need.Any ideas/inputs are welcome.Best regards.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 06, 2016, 03:41:54 pm
content: One or multiple digital pots will work. But you'll need to figure out the ratio of the resistors since digital pots only have a few choices. Not E192.I recently had a look at Richtek who seems to have some digital controlled regulators for more voltage than a CPU phase management ic.

author: ajb
author_rank: Super Contributor
timestamp: January 06, 2016, 04:26:10 pm
content: Quote from: senso on January 06, 2016, 03:04:08 pmYou could use the digipot as a rheostat in place of the bottom resistor in the feedback divider.Another option is to use a DAC to bias the feedback node of the converter via a third resistor. When the DAC is set to a voltage above the converter's ref voltage it pulls the feedback node up, and the converter will lower the output voltage in response, and vice versa when the DAC voltage is below the ref voltage it pulls the node down and the converter will raise the output voltage to compensate.

author: jt
author_rank: Regular Contributor
timestamp: January 07, 2016, 04:03:39 am
content: A lot of buck regulators support a voltage tracking function - these seem especially common for Linear Tech devices.As example, this component will track a voltage to the TR pin, which you could apply via an I2C DAC (you may or may not want to scale the DAC voltage down with a resistor divider to get full use of the DAC resolution and then voltage follow with a unity gain op-amp if input impedance is an issue). Technically it isn't tracking the voltage, but allowing you to override the internal reference; the affect is the same if you work out the math.http://cds.linear.com/docs/en/datasheet/8610fa.pdf

author: CarlG
author_rank: Regular Contributor
timestamp: January 08, 2016, 09:47:24 am
content: Quote from: EmmanuelFaure on January 06, 2016, 03:24:44 pmSorry, but isn't it the other way round? A digital pot connected as voltage divider Rout ranges from Rwiper up to Rtot/2+Rwiper? While the nice thing about R-2R is the constant output resistance?

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2016, 11:38:15 am
content: In these kind of situations I usually used PWM (with filter) to inject a DC offset into the feedback pin.

author: senso
author_rank: Frequent Contributor
timestamp: January 14, 2016, 03:18:18 pm
content: Thanks for the input, will add a couple pads into to prototype pcb and try either a dac, a digipot and some pwm injection.Sadly LT parts are usually no-go, because Mouser doesn't carry them, going to Digikey incurs in high customs, vats and shipping costs, and Farnell/RS have silly prices(sometimes twice as expensive versus Digikey).I will report back about how many LM's I have destroyed.Best regards.