switching regulator with mcu

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.

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Joerg
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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! :)

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?

--

John Devereux
Reply to
John Devereux

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???

Reply to
michael nikolaou

[...]

[...]

(top post corrected)

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
Reply to
John Devereux

with just a cap and a mosfet? doiesn't seem possible,

linear regulator or buck regulator.

Reply to
Jasen Betts

SEPIC's not bad...

(view in fixed font)

+12-24V -+- ======= | .-.-.-. C2 D1 +------+-----' ' ' '---+--||--+----|>|---+-----> +3.3v | | L1 | |_ | R1 | | )|| | | |/ Q1 | )|| L2 | +----| | _)|| | | |>. | | | | | ||--' === | .---' +---. ||
Reply to
dagmargoodboat

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

Reply to
don

Here's one:

formatting link

Price? Dunno.

-- Cheers, James Arthur

Reply to
dagmargoodboat
[...]

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.

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.

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Reply to
Joerg

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.

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.

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 ;-)

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 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*

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Regards, Joerg

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Reply to
Joerg

Neither do Digikey, Mouser, Farnell or findchips.com :(

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John Devereux
Reply to
John Devereux

Of course, neat. Finally the famed Jeorgian discrete SMPS is starting to take shape :)

Anyone know how to do this for smaller quantities ( even though I could imagine those are also made in China. Coilcraft is

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John Devereux
Reply to
John Devereux

My first one took shape about 15 years ago, tons of them in use by consumers right now, still coming off the conveyor belt (in China). I don't know if it will outlast the VW Beetle in production but it might. It has already survived the client company's president :-(

Back then I learned a valuable lesson: Inductor prices in the western world must be taken with a grain of salt. A custom mfg place in Taiwan outbid a catalog (!) part from a western manufacturer, big time. And we even got an EMI-savvy toroid for our money versus the cheap open cores from mainstream distributors. Of course it does help not to have a middleman in the game.

Reply to
Joerg

With Coilcraft you have to become a "member". Sign up and then you get instant pricing access on their site. If you don't want to then request a quote, comes within minutes via email. Their pricing is actually quite favorable compared to other domestic suppliers, maybe because they do a lot of automotive.

--
Regards, Joerg

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Reply to
Joerg

[...]

[...]

Sorry, I meant bare PCB not stuffed. They advertise stuffing too - down to any quantity - but they said once they buy non-free issued parts from Digikey... :)

I just thought it strange that I can get small qtys of PCBs at 1/2 the price of local suppliers, but nobody does the same for parts. (Which ought to be much easier to supply).

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John Devereux
Reply to
John Devereux

You could talk to companies like this:

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But since 10k is a small qty mostly you'll have to start here:

formatting link

That won't give you much of an upside. I recently had a proto-run fabbed in the US (Aurora, Colorado), full turn-key, and was pleasantly surprised:

formatting link

They do, in places like Shenzen, if you let them purchase. But be careful that they don't substitute, say, an electrolytic for a more "economical" part.

--
Regards, Joerg

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Reply to
Joerg

[...]

formatting link

[...]

Thanks for the links; bookmarked. Will look into it further.

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John Devereux
Reply to
John Devereux

Yes, but the decrease in costs of the chip is compensated by a large inductor and capacitors.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
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Reply to
Nico Coesel

its

If you have the space those are cheap. Unfortunately Michael doesn't have the space. But he also doesn't have the BOM budget for a modern switcher chip :-(

IOW he is between a rock and a hard spot so the MCU or homebrew may be his only options. I don't know if his ARM7 MCU has good timers left in there. If not he's going to have to roll his own around a Schmitt inverter.

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Reply to
Joerg

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

Reply to
Jon Elson

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