Thank You Russel! It was effectively the open collector of the LM393N that screwed it all up! After adding a 1k pullup to 5V the problems disappeared! Now I get a very decent linearity around 180mV per PWM tick +/- 10mV at the limits of the range which is approx. 4V to 38V (as measured by the Nano) I notice about 1% of AC content (via DVM) of the output which above 30 V rises up to 1.5%... Even that capacitor bridge you mention between output and FB does not (blindly) cure that, so I am shopping around for a scope of course! Of course I keep experimenting for improvements, but I guess an MPPT algorithm should be possible to even deal with this as given S2NR... Thanks for your time and research Russel, I owed those links to data sheets, you are right, but eventually I will get around publishing my circuit and code to keep it in the spirit of the open source! Ahm besides I hope to eventually get somebody to help me with the translation to code for an XLP PIC MCU! Cheers Tobias On 5/28/15, RussellMc wrote: > You are wandering around in the dark and the illumination you have provid= ed > does not allow us to see any except the broadest details. More illuminati= on > is needed. > > It's 'rather hard' to zero in on your problem(s) with no links to module > used or circuit diagram of what you started with. You have an informally > defined feedback phase response as you vary voltage and presumably are > getting positive feedback across the areas where output varies widely. It= 's > not obvious without knowing your circuit (both topology and component > values) why this causes full rail output at one end of the range - your > error amplifier apparently "lacks authority [tm] over the lower intended > Vout range but this could be for simple and obvious reasons or something > arcane. > > You are liable to be better off limiting the gain of the error amplifier. > The LM339 can be compelled to believe it is a linear amplieifer but an eg > LM324 is easier to convince. Also, the LM339 has open collector output so > is at the mercy of the passive pullup response if it ever wishes to take > V_FB high. As this is what turns the output off you MAY have asymmetric > on/off drive and depending on what you are using to pull FB up (and we ha= ve > no idea as [[repeat large hint follows ]] we do not know your circuit or > component values) this could be a major factor in your problems, or > unrelated to them. > > One of the many available software oscilloscopes - some of which were > recommended in prior discussions on this list - would probably help. > > PWM input should produce a DC reference that the error amplifier compares > to the divided Vout so response rate of the DC ref does not affect > stability - but ripple on it may. Your filter is probably OK. > > It probably is not a major factor in your loop instability, but adding a > small capacitor (maybe 1 nF or less) across the upper resistor in your > feedback divider from Vout will (may) aid step response. > > XL6009 datasheet here: > http://www.xlsemi.com/datasheet/XL6009%20datasheet.pdf > > Apparently shameless 95% copy here with irrelevant and interesting > occasional stylistic variations > http://www.haoyuelectronics.com/Attachment/XL6009/XL6009-DC-DC-Converter-= Datasheet.pdf > > Note that Vin max is shown as 32V and abs max is 36V. > What the spec is for your IC may or may not match. > > _________ > > Your XL6009 circuit **MAY** have looked like the one shown here > > > http://www.electroschematics.com/10625/12v-universal-laptop-notebook-char= ger/ > > Or some of the zillion here > https://www.google.co.nz/search?q=3Dxl6009&espv=3D2&biw=3D1280&bih=3D841&= tbm=3Disch&tbo=3Du&source=3Duniv&sa=3DX&ei=3DyWZmVbGNN6iymAX0g4Bg&ved=3D0CC= 4QsAQ > _________ > > Here is someone using one as a CC LED driver wit an LM358 as the current > sensing error amplifier, driving the FB pin. They use an amplifier gain o= f > 13 (12k feedback with 1k to ground on inverting input, and 1 uF (!) > negative feedback. > > http://uphotos.eepw.com.cn/l19900099/pics/e80845a690de29716e533b7cf871351= 6.png > > > >From here > https://earthelectro.wordpress.com/2014/10/30/the-new-dc-to-dc-circuit/ > > > What they are doing translates almost exactly into what you want if you > replace their CC sense voltage input with a resistive divider from Vout. > > > Russell > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 Tobias Gogolin Tel. D1 (49) 0151 5187 5210 Tel. D2 (49) 0152 0839 5060 skype: moontogo You develop Sustainable Ranch Technology at http://tech.groups.yahoo.com/group/SURA-TECH an Open Source Electric Motor/Alternator at http://groups.yahoo.com/group/Performance_Axial_Flux and an Open Source Motor Controller at http://groups.yahoo.com/group/GoBox --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .