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. I don't want to use a switching regulator to make the voltage drop since its to much circuit involved and also not a zener since its to much heat involved . The mcu has A/D and PWM controller .Is it possible to use a simple mosfet switch with a capacitor to make a dropdown to half voltage if the mcu detects 24 volts input ?. Is there an easier way ?. Any help would be appreciated .
Michael, Probably not a good idea. You MCU will take a little while to come up to speed before it can monitor those voltages, and take action. At
24VDC, you chip is already fried by that time. That is a lot of voltage to drop with just a linear regulator, esp. if that 24VDC is not well regulated. It is possible, but it isn't simple...
Thanks fot the reply If i halve the duty cycle is should get from 6 to 12 volts input . This can be regulated to 3.3V with no problem . Is there no simple circuit, switched capacitor possibly , to make this voltage halving ????
Switched-capacitor voltage doublers work pretty well, but I doubt that a switched-capacitor voltage halver would be less complex than a switching regulator.
If your production volumes aren't too terribly high there are some nice switching regulator modules out there. TI makes a wide variety with a good size/power ratio, that'll take 12-24V easily. I'm sure that TI isn't the only one.
For that matter a switcher with an integrated switch and a fixed output of 3.3V could be had. This would require less real-estate then either a TO-220 with a heatsink or your switched capacitor circuit.
Search Digikey and you'll find some 3.3V switchers in a dpak only requiring a few external components and pretty well idiot proof. A small SOD Schottkey and a tiny smd cap and inductor would be the only external components you need.
You could build it on a separate PCB and mount it vertically to your main PCB. I've done that a few times. For 120 mA output you should be able to get it down around 1/2" x ½".
Mount the DPAK on the ground side put your 3 or 4 other components on the other side.
Resistor and zener to CPU but much higher zener voltage than the CPU uses in normal operation, meaning the MCU should have lots of headroom. Start MCU with bare minimum in code, basically doing nothing other than the switcher. Now fire up a buck or whatever switcher from 12-24VDC, MCU controlled hysteretic for example. When that gets into the green zone let the MCU commence its normal job because now that the switcher has started it can use all the power it wants.
If you can't spread zener voltage and normal operating voltage that far, add an extra transistor that turns off the zener after the switcher start process has run its course. This will cost another port pin.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Too much circuitry? Look at devices from TI like the TPS5410. Very small and it will run at a wide variety of input voltages.
--
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!"
--------------------------------------------------------------
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 ;-)
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
My small research has found that switcher solutions are.
simple and small sized switcher solutions are expensive.
Mc33063 are ok in terms of price but the inductor plus the capacitors are too much board estate
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 ???
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.
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.
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.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
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. 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 :-)
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.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
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.
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. ;)
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?"
Oh, I don't know. Last week I finished a project for a client. The board takes in 3W, of which half goes on to another part that this baord controls. Peak power is above 10W but brief and gets sunk into a metal buffer. The whole thing is slightly above a square inch and mounted to a metal flange. Works.
600mW is pretty easy to dissipate. A couple of SO8 chips could do it in air.
Maybe he grew up in the days of the Plymouth Fury :-)
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Nah, I wouldn't agree. Look at the MC34063. Costs about 15c in qties and can swing more than an amp. It's quite big inside. Same for FETs.
Problem is that only the slowpokes among switchers are cheap, for anything above 500kHz they want top Dollar. So guys like me often do those discrete. Until the prices come down to less than 50c.
[...]
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
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.
A switching pre-reg before the linear is s good idea, but recall that the O/P didn't even want a straight buck switcher as it is 'too much circuit(ry)".
For my money, a simple MC34063 buck switcher does the lot, 12 or 24V down to 3.3V. Not a lot of board area.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.