switching regulator with mcu
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From: Nico Coesel on 22 Oct 2009 18:14 "michael nikolaou" <michaelnikolaou_remove_me_(a)yahoo.com> 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??? You mean some sort of pre-regulator before the linear? That might work, expecially if you make the the backlight is off until the pre-regulated voltage is withing range. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... "If it doesn't fit, use a bigger hammer!" --------------------------------------------------------------
From: dagmargoodboat on 22 Oct 2009 21:14 On Oct 21, 11:54 pm, Jon Elson <el...(a)pico-systems.com> wrote: > michael nikolaou wrote: > > Hi to newsgroup > > > I'm making a mcu based device which i want to be very small and low cost > > The design consumes 100..120 ma @ 3.3V > > The problem is the installation that requires 12 or 24 volts to operate. > > Check out the LM2575 series of regulator. There > should be a 3.3 V version > of it, I use the 5 V version in one of my > products. It needs a Schottky diode > and an inductor as the only additional parts other > than input and output > capacitors. No need for post regulators, it is > very clean. > > Jon That's a nice part, but the OP wants a tiny inductor, so the slow (52KHz) switching frequency's a problem for him. This part switches up to 1MHz and costs even less--only a dollar @ 1k compared to $1.13 for the LM2575T-3.3. http://www.national.com/pf/LM/LM2694.html It's PFM rather than PWM, so careful calculations are in order. I did none--just skimmed it. -- Cheers, James Arthur
From: michael nikolaou on 24 Oct 2009 13:15 Dagmar that is pretty good IC .Only the 150 uH spoils the picture . That needs 8x 8 mm minimum board size . The capacitors are fine. i think i will stick to that dagmargoodboat(a)yahoo.com wrote: > On Oct 21, 11:54 pm, Jon Elson <el...(a)pico-systems.com> wrote: >> michael nikolaou wrote: >>> Hi to newsgroup >>> I'm making a mcu based device which i want to be very small and low cost >>> The design consumes 100..120 ma @ 3.3V >>> The problem is the installation that requires 12 or 24 volts to operate. >> Check out the LM2575 series of regulator. There >> should be a 3.3 V version >> of it, I use the 5 V version in one of my >> products. It needs a Schottky diode >> and an inductor as the only additional parts other >> than input and output >> capacitors. No need for post regulators, it is >> very clean. >> >> Jon > > That's a nice part, but the OP wants a tiny inductor, so the slow > (52KHz) switching frequency's a problem for him. > > This part switches up to 1MHz and costs even less--only a dollar @ 1k > compared to $1.13 for the LM2575T-3.3. > > http://www.national.com/pf/LM/LM2694.html > > It's PFM rather than PWM, so careful calculations are in order. I did > none--just skimmed it. > > -- > Cheers, > James Arthur
From: who where on 24 Oct 2009 21:07 On Sat, 24 Oct 2009 20:15:47 +0300, michael nikolaou <michaelnikolaou_remove_(a)yahoo.com> wrote: >Dagmar > > >that is pretty good IC .Only the 150 uH spoils the picture . That >needs 8x 8 mm minimum board size . >The capacitors are fine. i think i will stick to that Unfortunately your design requirements create a conundrum - "cost/size/efficiency - choose any two" works but wanting all three makes it well nigh impossible. If cost is ignored, then a TO-220-ish integrated switcher like http://www.recom-international.com/pdf/Innoline/R-78AAxx-0.5_SMD.pdf would possibly fit the bill.
From: JosephKK on 30 Oct 2009 20:06
On Tue, 20 Oct 2009 16:16:48 -0700, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Tue, 20 Oct 2009 15:49:59 -0700, Joerg <invalid(a)invalid.invalid> >wrote: > >>Jon Kirwan wrote: >>> On Tue, 20 Oct 2009 12:52:04 -0700, Joerg <invalid(a)invalid.invalid> >>> wrote: >>> >>>> michael nikolaou wrote: >>>>> 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. >>>> Not really, the passive parts are easy. But the challenge will be to >>>> find a mainstream (meaning inexpensive) small switcher chip. The cheap >>>> ones are all old and slow, 300kHz or less. This will take some time >>>> because you must always check pricing. You could start by looking at >>>> chips for the Power over Ethernet (PoE) market. >>>> >>>>> 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 ? >>>> No, that would be a little R&D project. Requires tight reigns on the >>>> firmware because the switcher must never skip a beat. But if you have a >>>> free timer in your ARM MCU it can be done. >>>> >>>>> Any other simple ideas ??? >>>> 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. >>> >>> This has been an interesting discussion. What is bothering me a lot, >>> in reading it, is the "ONLY 120mA" thing! 120mA? Only? Cripes. If >>> I were imagining being as space-constrained as the OP suggests, I'd IN >>> THE FIRST PLACE start asking myself why I actually need 120mA! Is it >>> the processor, itself? If so, look to get rid of it and find >>> something else. Doing so may put constraints on the application >>> itself (doing logarithms on a PIC16 is quite different in speed than >>> doing them on an ARM9, for example), but it may greatly relax the >>> power supply design issues. Everything is trade-off. But I'm >>> bothered by the casual acceptance of a 120mA spec as gospel when there >>> is a serious space and cost issue here. >>> >>> What is sucking that power? Can it be changed? If not, why not? >> >>I sure hope it's not the MCU alone because that would spell trouble. > >It sounds familiar. I've looked at these 'low power' ARM chips, >noticed that many seem to average 20-50mA, with the lower figure only >on a good day and many requiring more. It was NOT a shock to me to >see 120mA with ARMs. In fact, I'm just fine with that. But when the >OP writes, "I'm making a mcu based device which i want to be very >small and low cost" and then out the other side of the mouth says >"only 120mA" then I'm truly wondering. > >One of the BIG tradeoffs is power __AND__ heat. And by the time you >get anywhere near 120mA, you've often got both problems in spades. >It's a fundamentally different domain. > >I guess that's why I just went _white_ when I read those figures and >the OP's language in the same context. The only way 120mA is a >little, these days, is if you are used to x86 processors running at >GHz and requiring multiple power supply rails to help contain heat >problems better. > >If you live in that world, I can see it. But that sure isn't MY >embedded world perspective. I consider moving into the 120mA domain >as being akin to a "damn-the-torpedoes, devil-may-care" world. At >that point, you are already spending dollars, not pennies, and have >board room to spare. And if you are chugging 120mA, you NEED space, >anyway. > >>Often there are other things like ADCs, analog stuff etc. 120mA is >>really a piece of cake from a power converter point of view. After all, >>that's only 600mW if his VCC is 5V. Low enough for an energy star :-) > >Hehe. Yeah. If we are talking washing machines, 120mA is no problem. >But room isn't a problem, then, either. There's always a corner, plus >one HUGE heat sink, too. ;) > >>Michael may have to go with a SEPIC if he wants the FET to switch to GND >>but that's not a big deal either, just two parts more. Since the advent >>of PoE he's got plenty of options. > >I'd recommend that Michael rethink 120mA. I mean, jeeez! If you >nearing a watt already, with overhead, you need space and you expect >to spend something on the power supply, too. Or some serious, crafted >time. Or both. > >Almost two decades ago, I was worrying over a thermal cooling stack >with two Peltier stages and the bottom stage was consuming half a >watt. I was worried about that much heat. And the tiny micro device >plus analog circuits at the top was burning some 30mW. Now that's the >kind of thing you get with 120mA! 2-stage Peltier coolers AND a >micro. 20 years ago. > >This is crazy-making to think about 120mA! Yeah, if you are making an >iPhone or internet interface device with RF and all. I mean, you need >to actually broadcast maybe 1/4 watt or so. So yeah. But "I'm making >a mcu based device which i want to be very small and low cost?" > >The OP and I must come from different universes! > >Jon You may be on to something. OP seems to also be considering a linear regulator from 24 V to 3.3. Talk about power waste. |