internal power rails for DIY power supply - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: December 12, 2017, 08:48:58 pm
content: Hi guys,I'm designing a power supply. Internally it needs +20V, -5V (for opamps), +3.3Vdigital, +3.3Vanalog rails (for MCU). Analog rail is used for STM32's DAC and ADC.Questions:1) Can I join 3.3V rails together? Or digital noise will get through, even if I add an RC-filter to DAC output?2) My 3.3V reference (ADR4533) accepts maximum 16V, but I have ~20V unregulated. What should I do to supply ADR4533 from 20V? Can I add a (linear) pre-regulator? Can this potentially affect stability? (I heard chaining regulators is not the best idea). Can I supply two ADR4533 from the same pre-regulator?

author: danadak
author_rank: Super Contributor
timestamp: December 13, 2017, 01:02:49 pm
content: QuoteJoin the Rails at the source, do not chain (serialize) the loads on a single buss. Same canbe said of the grounds. In fact consider using chip Ls at source to each 3.3 V buss togenerate isolation between busses.http://www.analog.com/media/en/training-seminars/design-handbooks/Basic-Linear-Design/Chapter12.pdfhttps://www.maximintegrated.com/en/app-notes/index.mvp/id/5100http://www.cypress.com/documentation/application-notes/an57821-psoc-3-psoc-4-and-psoc-5lp-mixed-signal-circuit-board-layoutQuoteYes I would use a pre-regulator or a zener to affect the drop, latter cheaper and easier.Chaining regulator could affect stability, load dependent, Capacitor ESR issues and whatregulator gets what cap. Cap dump problems during power off. Try siming this, but I suspectmodels may not be adequate. You can do real time scope review looking for transient thatconvey phase margin is compromised, eg. lots of ringing.391 OpAmp CM range includes ground, any chance there is ground rail bounce such thatthe <= -.3V limit exceeded ? That could cause comparator phase reversal. Your DSOcan look at that. Use DSO in differential mode, see ap notes below.https://www.njr.com/semicon/design_support/faq/10088.htmlhttp://www.analog.com/media/en/training-seminars/tutorials/MT-036.pdfDSO differential / floating measurements (safety first) -http://download.tek.com/document/51W_10640_1.pdfhttp://www.newark.com/pdfs/techarticles/tektronix/FFM.pdfhttps://www.tiepie.com/en/classroom/Measurement_basics/DifferentialMeasurementshttps://www.google.com/search?ei=NygxWsq9L4Sl_QbLzp6oDQ&q=differntial+scope+measurments+tektronix&oq=differntial+scope+measurments+tektronix&gs_l=psy-ab.3...1825.4725.0.5264.12.12.0.0.0.0.98.886.11.11.0....0...1c.1.64.psy-ab..1.8.654...0i13i30k1j0i8i13i30k1j33i160k1j33i21k1.0.fB4yRdrzC90Regards, Dana.

author: exe
author_rank: Unknown
timestamp: December 13, 2017, 01:52:57 pm
content: Thank you, Dana, for detailed response. I'm digesting all the information you provided...One quick question: did you mean connecting zener in parallel or in series with the regulator? (I assume in series on the high side).

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2017, 07:04:18 pm
content: Quote from: exe on December 12, 2017, 08:48:58 pmAn RC or LCR filter may allow this however ground considerations are likely to cause just as much if not more of a problem unless a careful layout is used.QuotePractically any preregulator would work fine including a resistor and zener diode shunt regulator. Or a JFET or other cascode circuit could be used to control the input voltage to the reference from the output voltage. Depending on the range of the unregulated voltage, a series zener diode is another possibility depending on how good the line regulation of the reference is.