Linear Voltage Regulators vs. Switching Regulators

ElectricAye

I'm designing a stepper motor controller PCB that uses 24 volts to drive the motors, but the other on-board circuits (Propeller, other ICs, etc.) only use 3.3 volts and 5 volts. I suppose I can tap into the 24 volt power supply and then use a number of linear voltage regulators to step down the voltage to a final 3.3 volts, but I'm guessing there are right ways and wrong ways to make those steps. For example, is it wise to simply jump from 24 volts to, say, 9 volts all in one leap? Or is it better to step down to, say, 18 volts, then 12 volts, then 5 volts, then 3.3 volts? I'm guessing the bigger the leap, the more power that gets wasted, but I don't really know for sure.

Anybody know a good rule of thumb for deciding how to step down voltages?



many thanks,
Mark


Post Edited (ElectricAye) : 3/26/2010 2:12:01 AM GMT

erco

Linears act just like resistors to drop voltage, they dissipate power as heat. The power dissipated is the voltage dropped times the current drawn. The more voltage you drop, the hotter it gets. The more current you draw, the hotter it gets. That's just wasted energy. As long as you stay within your regulator's specs, it doesn't matter how many steps you take to drop the voltage down.

You might consider adding a 6-volt battery just to power your electronics. There will be much less power to dissipate, and many times, it's advantageous to isolate your motor batteries from your electronics batteries to avoid interference.

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·"If you build it, they will come."

John R.

Or bite the bullet and go with a switching regulator circuit. Some of the newer ones seem to be getting easier to integrate, and require fewer external components. There are also some "pre-packaged" solutions.

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John R.
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stamptrol

Use a switching reg for any big steps, say 24 down to 12 or 9. Have a look at the Power Trends (now TI, I think). Many of them have the same pinout as a linear and work beautifully.

Many times, a linear is fine as long as you pay attention to the V x I drop and provide adequate heat-sinking. At high current and high input voltage, you are fighting a losing battle, especially on battery operated equipment.

Cheers,

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Tom Sisk

http://www.siskconsult.com
·

Erik Friesen

Something like a ptn78000 built by TI is perfect. I like to step down to around 1 volt above a linear, that way you get a cleaner output.

Just don't use any until there are plenty around. I need them for my projects.

ElectricAye

Erik Friesen said...
S....

Just don't use any until there are plenty around. I need them for my projects.

I've never played with switching regs before. They sound intriguing. I have no idea how to spec them out so I was looking at the following:
www.national.com/ds/LM/LM1575.pdf


Thanks, you guys. This was something totally new to me.

icepuck

Heres a regulator thats the same pin out as the 78xx
www.dimensionengineering.com/DE-SWADJ3.htm

or this one can go from 30v down right to 3.3v.
www.dimensionengineering.com/DE-SW033.htm
-dan

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WBA Consulting

I second Erik's suggestion. I have used a few PTH12000WAH from TI to drop 12v to 5v for something that pulled ~4.5 amps. All that in a 0.5"x0.75" package! The PTN78060WAS would be a good fit. Set it for 5 volt output, then use a 3.3 volt linear regulator for the 3.3v rail. Yes, they are not cheap, but you get more than what you pay for in my opinion with these.

Erco stated things very well.

Z = (Vin - Vout) * current

Where Z is defined as all of the following: Dissipated power, wasted power, heat generated, size of heatsink needed, finger burnability, etc

Dropping 24 volts to 5 volts at only 300mA will still require a regulator/heatsink that can dissipate 5.7 watts. You could do that in steps to limit the amount of heat each regulator is dissipating, but the total wasted energy (dissipated heat) is the same.

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Andrew Williams
WBA Consulting
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PJAllen

Datel [noparse][[/noparse]muRata] 7805SR 0.5A switching reg, non-isol. DC-DC converter.· Same footprint as a linear 7805.· (There's a 3V unit, too.)· Practically form-fit-function except for a "big" cap that you may have to place·across the input.· Yep, it's always something.· Liking them a lot just the same.

There are lots of these, but they're not smoke-proof.

ElectricAye

WBA Consulting said...
...Yes, they are not cheap, but you get more than what you pay for in my opinion with these. ...

Dan, WBA, PJ,

thanks you guys for all the suggestions and the info on how the Linear Volt Regs enhance finger burnability. For my recent prototype I had to go with the linear regs but in my next version I'm definitely going to incorporate some of these highfalutin converters y'all have been telling me about. I only wish I'd asked the question earlier.


bless you all,
Mark

mctrivia

if running motors or servos off the same supply as your digital make sure you have a cap capable of powering your electronics during any stall currents. I like to stick an ultra cap in after the switching regulator. Total over kill but they are not to expensive and can power your electronics for several seconds of bad power.

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