Hi Olin, I did a microcontroller-run boost dc/dc recently to get 70V @ miniscule current from a 4.2 to ~3V li-ion supply as part of an e-paper prototype. (The 70V is needed to erase the display). I used a simple resistor divider to sample the 70V rail with the A/D and turn on/off the PWM to the nfet switching the inductor to regulate. This all worked, but I had to manually "tune" the PWM signal for best efficiency and ensure it could boost to 70V @ the 3V minimum input voltage. By adjusting duty cycle and frequency one could see when the inductor was saturating, and how efficiency was poor at low frequency, then got pretty decent within a range, and then poor again as the frequency went up. All kind of fun, but I'd certainly rather have a set of equations to come up with duty cycle and frequency given voltage in range/out range/output current range/inductor value & current spec, etc. At least know good starting points for duty cycle/frequency and around what inductor specs are going to be required for a given design to know if cost/performance is going to be reasonable prior to building a board. I have to admit I spent only 1 or 2 evenings online reading and looking at dc/dc design info before just tuning it for the particular design I had in front of me. With the success of that design, I've been wanting to expand my bag of tricks to do things like... have a linear regulator with high input voltage spec be able to power a microcontroller initially, and then have the micro run a dc/dc to power itself and everything else, saving some $ and board space. Or run multiple dc/dcs for multiple rails, I do the same trick of using a dc/dc to make 5.5V and 3.7V from a 9V battery and post-LDO-regulate for a low-noise audio design. This requires a $5 chip, and there is no reason a micro couldn't do it, but I need more confidence before building these things. Long story short, if you have any online references or books regarding dc/dc design you can recommend for this microcontroller-controlled design route I'd certainly appreciate it. Thanks for sharing your schematic. J Olin Lathrop wrote: > David Duffy (AVD) wrote: >> For one of the projects we have on the go at present, I'm using a >> 78L05 regulator to drop the 24V incoming supply down to 5V for the >> PIC and MAX1485. The board's relay is a low power 24V type, so only >> draws about >> 8.5mA when energised. >> >> With the 78L05, the idle current draw from the 24V supply was about >> 4.7mA. Swapping it out for an LM2931-5 regulator dropped that to about >> 1.4mA, quite an improvement as the relay is off most of the time. >> >> That's good enough for this application, but it got me thinking about >> the few wasted mA * 19V (24V - 5V) in the linear regulator. This >> design is through hole, but had to be compact so I didn't even try to >> fit a switching regulator in. > > How much current does the 5V supply need when nothing is happening? A > linear regulator is quite inefficient when dropping 24V down to 5V, but it > can be really good at quiescient current. If most of the time is spent > doing nothing, a linear regulator may be the better answer. > >> Are there any small, low-current, hi-efficiency switching regulators >> out there that are still reasonably priced? I realise that a 78L05 is >> hard to beat price wise, but are there good alternatives out there? > > This is probably not appropriate for your case, but I'll mention it because > others might find it useful. I sometimes use a 10F204 as the controller for > a switching power supply. All the parts are dirt cheap except the inductor, > but that would be required either way. Nice switching power supply chips > are surprisingly expensive. It's also often useful to treat startup > differently than main operation and to be able to limit duty cycle. This > essentially makes the power supply current limiting without any direct > current sensing. It can usually be arranged to survive a short without > damage or excessive current draw all the while guaranteeing not to saturate > the inductor. This and intelligent startup are not easy to get from cheap > PWM chips with analog feedback. > > A neat trick I've used a bunch of times now is to put a PNP transistor > around a LDO linear post-regulator to detect the switcher output regulation > threshold. The B-E junction drop happens to be just the right target > voltage for the LDO input above its output. The MCP1700 LDO is a nice part > for this since it's pretty cheap, works with 0 ESR caps, and the B-E > junction drop above its output is solidly above its dropout voltage but > below its maximum input of 6V. > > Once example of this is on page 1 of http://www.embedinc.com/ioext/io2.pdf. > In this case the 5.6V output has to be cabable of 3/4 amp, so a few mA > quiescient current was no issue. Having the 5.6V supply availabe is useful > since you can give various subsections their own LDO, and other parts just a > diode if the 5V supply doesn't need to be too tight. It also makes it easy > to isolate supply segments so that one driven externally doesn't drive the > others. > > > ******************************************************************** > Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products > (978) 742-9014. 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