switching regulator with mcu
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From: dagmargoodboat on 21 Oct 2009 09:52 On Oct 21, 4:18 am, John Devereux <j...(a)devereux.me.uk> wrote: > "michael nikolaou" <michaelnikolaou_remove_...(a)yahoo.com> writes: > > > The 120 > > "Joerg" <inva...(a)invalid.invalid> wrote in message > >news:7k75l0F37imt0U1(a)mid.individual.net... > >> Martin Riddle wrote: > >>> "michael nikolaou" <michaelnikolaou_rem_o...(a)yahoo.com> wrote in message > >>>news:hbl267$2l80$1(a)ulysses.noc.ntua.gr... > >>>> Guys > > >>>> Thanks for all your replies > > >>>> My small research has found that switcher solutions are. > >>>> 1. simple and small sized switcher solutions are expensive. > >>>> 2. Mc33063 are ok in terms of price but the inductor plus the capacitors > >>>> are too much board estate > >>>> 3. Small size means high frequency and so you start with selective > >>>> components etc. > >>>> I NEED ONLY 120 ma is there nothing low cost and simple ?. > >>>> One switcher i located used 10 uH inductor , 47 uF output capacitor @ > >>>> 6.3V and 2.2uF input capacitor > >>>> but total cost was 1.8 euros. I's using one ARM7 mcu that costs 3 euro > >>>> and i don't want to spend as much for the PSU. > >>>> Some lower consumption devices @ 50 mA were using uA78M33 regulator with > >>>> a zener if 24 volts was used. > >>>> Joerg do you have schematic to study about the idea you are proposing ? > >>>> Any other simple ideas ??? > > >>>> "Joerg" <inva...(a)invalid.invalid> wrote in message > >>>>news:7k6faoF389blhU1(a)mid.individual.net... > > [...] > > >>>>> But mucho Dolares. I'd try using the MCU if possible if this is a high > >>>>> volume product. But it'll require lots of nifty engineering. For low > >>>>> volume, yeah, don't bother and use a chip. Then I'd use the MC34063 > >>>>> which costs under 20 cents. None of those high-faluting newfangled > >>>>> ritzy ones ;-) > > [...] > > >>> What about a LDO? National, LP2952. 30V input. Dissapation isnt that much > >>> at 120ma. > >>> You could always stick some resistance in the input pin to lower Vin, and > >>> device dissipation. > > >> Be careful. I don't know the LP2952 but I had one of the 29-series go > >> berserk when the source impedance became too large. It wasn't mentioned in > >> the datasheet ... > > >> Bottomline is I stay away from LDOs. Besides the above there are also > >> ESR-related stability issues. > > (top post corrected) > > > > > I need to make clear so we avoid confusions > > > The device uses a 2.2" tft lcd . This thing has a backlight and no matter > > what > > draws 60 ma @ 3.3V .That i can't avoid . So with 60 ma Arm7 consumption > > i end up to 120 ma. > > Now as i see whether i have a small switcher with huge inductor and > > capacitor > > so my board space is too large . > > The other solution is super fast switcher that has everything small but > > needs 2 euros > > at least . > > I imagine every engineer designing devices has faced this question . > > I'm sure though that a simple cpu controlled switcher could cope with the > > large dropout > > from 24V to 4-5 volts and then a simple linear regulator could take you to > > 3.3V so you play > > it safe.If things go wrong (cpu malfunction ) thermal shutdown from the > > linear regulator would > > reset the cpu and that would restart the system > > As i see it a simple fet switch with cpu pwm control should satisfy that . > > Any opinions or experience on that subject??? > > I have never had the nerve to try a CPU-controlled switcher - but I > never thought of a linear regulator as a "safety cutout" either. I'd say > go for it! You have a couple of issues: > > * startup - maybe just a trickle of current if the circuit has a very > low initial requirement > > * Configuration - either you need a high-side driver with a buck > configuration or an easier low-side one that needs a transformer > (SEPIC or flyback). Or I guess the simplest is inverting but that's > nasty if you have to connect to anything else since incoming "ground" > ends up being your positive rail. > > Also your "12-24V" sounds a bit automotive. If so be aware that there > can be some very nasty spikes and voltage surges, meaning that a strict > interpretation of the standards can require withstanding >>70V, not just > 24. > > -- > > John Devereux SEPIC's not bad... (view in fixed font) +12-24V -+- ======= | .-.-.-. C2 D1 +------+-----' ' ' '---+--||--+----|>|---+-----> +3.3v | | L1 | |_ | R1 | | )|| | | |/ Q1 | )|| L2 | +----| | _)|| | | |>. | | | | | ||--' === | .---' +---. ||<-. GND | Z1 ^ | | ||--+ Q2 | 4V | C1 --- | .---' | | | --- | | | | === | | | === | GND | | | GND | === '---)-------------------------' GND | | PWM from ARM7 >------' Real-estate-wise, L1 and L2 could share a core, and R1-Z1-Q1 could be replaced with an LDO. Though I kind of like Q1--it's tough and fast. Transient response with the MCU will suck--C1 will have to be large. Yes, I've seen SEPIC inductors sold by one of the major mfrs as such; can't recall who. It's the OP who has to make the classic tradeoffs between cost, complexity, size, performance, & design time--we don't have enough info. -- Cheers, James Arthur
From: don on 21 Oct 2009 09:56 michael nikolaou wrote: > I need to make clear so we avoid confusions > > The device uses a 2.2" tft lcd . This thing has a backlight and no matter > what > draws 60 ma @ 3.3V .That i can't avoid . So with 60 ma Arm7 consumption > i end up to 120 ma. > Now as i see whether i have a small switcher with huge inductor and > capacitor > so my board space is too large . > The other solution is super fast switcher that has everything small but > needs 2 euros > at least . > I imagine every engineer designing devices has faced this question . > I'm sure though that a simple cpu controlled switcher could cope with the > large dropout > from 24V to 4-5 volts and then a simple linear regulator could take you to > 3.3V so you play > it safe.If things go wrong (cpu malfunction ) thermal shutdown from the > linear regulator would > reset the cpu and that would restart the system > As i see it a simple fet switch with cpu pwm control should satisfy that . > Any opinions or experience on that subject??? > Please post pics of the burned up chips. Management must be driving your decisions. Cost, cost, cost, forget about safety or reliability. good luck, you'll be back don
From: dagmargoodboat on 21 Oct 2009 11:04 On Oct 21, 8:52 am, dagmargoodb...(a)yahoo.com wrote: > Yes, I've seen SEPIC inductors sold by one of the major mfrs as such; > can't recall who. Here's one: http://www.coilcraft.com/lpr4012.cfm Price? Dunno. -- Cheers, James Arthur
From: Joerg on 21 Oct 2009 11:48 John Devereux wrote: > Joerg <invalid(a)invalid.invalid> writes: [...] >> Another option would be to use a CD40106 or something similar as a >> Schmitt oscillator, with its VCC capped/zenered around 6-8V. This can >> drive a little FET, a simple logic level device like a 2N7002 as long >> as doesn't cost much. Pipe Vref out of your MCU (hoping it has that >> ...) and use a cheap opamp to pull the Schmitt oscillator input "to >> the side" when the target voltage is reached. That reduces the duty >> cycle as much as needed to maintain regulation, pretty much like the >> throttle on a gasoline-powered generator. If the target voltage >> doesn't have to be very precise you could also use a NPN plus zener >> for that, without a reference source. Probably a TL431-type device >> would work as well and those are quite cheap, in the penny range. > > Hi Jeorg, > > Are you talking about a SEPIC/flyback arrangement here? I.e., 2N7001 > drives a transformer? How do you stop the opamp railing during startup > or a load step? The schmitt osc would stop and .. how does it go > ... Phut! :) > If you use an opamp a simple trick to avoid this is a diode. That way the enchilada can pull the Schmitt to one side but cannot push it to the other. If you are really brazen use a comparator with O/C -> no diode, saves 1-2 cents. If you use a transistor or TL431 the problem goes away on its own, sans diode. Now, set the max duty cycle to whatever worst case demand you calculated, at the minimum expected input voltage, plus 20% margin or so. > Also of course by the time he's actually bought a SEPIC transformer it > could all approach his 1.8 Euros. Or are there some super-cheap ones > now? > Yes, but only when you buy in Asia. For some reason you can't get them much under a buck inside the US even though I could imagine those are also made in China. Coilcraft is typically the best deal for coupled SEPIC coils if it has to be domestic. Using two individual inductors gets you around the problem. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 21 Oct 2009 12:05
michael nikolaou wrote: Best is to post underneath quoted text. Makes it easier to follow for people new to this thread, and they may also bring in ideas. > I need to make clear so we avoid confusions > > The device uses a 2.2" tft lcd . This thing has a backlight and no matter > what > draws 60 ma @ 3.3V .That i can't avoid . So with 60 ma Arm7 consumption > i end up to 120 ma. Sounds pretty normal. But check whether there are power surges after start-up or during certain phases in the code execution. I guess you don't want large electrolytics so the switcher would need proper reserves to deal with that. > Now as i see whether i have a small switcher with huge inductor and > capacitor > so my board space is too large . > The other solution is super fast switcher that has everything small but > needs 2 euros > at least . > I imagine every engineer designing devices has faced this question . Yep. And every time we go out and just price stuff out. Very tedious, but sometimes you end up finding a good deal on a 0.5-1MHz PWM chip. Check the EU companies as well for that, and definitely Japan, Korea etc. It's very similar to house hunting ;-) > I'm sure though that a simple cpu controlled switcher could cope with the > large dropout > from 24V to 4-5 volts and then a simple linear regulator could take you to > 3.3V so you play > it safe.If things go wrong (cpu malfunction ) thermal shutdown from the > linear regulator would > reset the cpu and that would restart the system > As i see it a simple fet switch with cpu pwm control should satisfy that . > Any opinions or experience on that subject??? > Personally I never use LDOs because many of those chips are IMHO not engineered well enough. Non-LDO regulators would need 6VDC or more, meaning way too much dissipation for a SC70 package. It will cost a lot more engineering hours but I'd first scour the market for cheap+fast switchers and if no luck I'd go straight for the CPU-only method. Directly from 12-24VDC to 3.3VDC. But the MCU code must be firmly under your control if you do that. If a rookie came in later and added some code, blissfully unaware of your hysteretic loop for the 3.3V, ... phut .... *BANG* No joke, this happened to me. Long story short: A really brazen designer had gone a lot farther, taking in rectified mains via a capacitor/resistor combo, rectified by <gasp> a substrate diode. "Regulation" was achieved by executing dummy code during idle phases so the load current was always the same and VCC would not run up. Nobody had told me about this so when I touched the X1 pin with the scope probe the oscillator briefly stopped and ... *KABLAM* -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |