switching regulators

InSilico

I'm attempting a foray into the world of switching regulators

I would like·this·to work·with a maximum voltage input of 24V, minimum 9V, voltage output of 7.5V and a maximum output current·of 2A.

I selected an adjustable LM2576, 1.74KOhm and·8.87KOhm resistors (to set the regulator at ~7.5V), a 220uH inductor and an·1N5821 diode. I'm still trying to figure out the capacitors.

Has anyone else had experience with switching regulators? What components and component values·would you use in this application? How picky is the switching regulator in terms of the external components? Would you use/suggest a different regulator?

For example, the·datasheet guidelines says I should get a 220uH inductor, but it seems I can get away with a 150uH inductor (which is cheaper to obtain at Mouser). Do the inductors have to come from Schott/Pulse/Renco?

I'm making a small network of microcontrollers (like 5-7 SXes) that may be in relatively remote locations. All of these remote nodes connect back to a central host via Cat3 or Cat5 cable. The idea I have is to have a switching regulator supply 7.5V from a 9V (or higher) source, and then the cables supply this to the remote nodes along with data. 7.5V is chosen to account for cable losses. Finally, a low-dropout linear regulator at the far end can further drop the voltage to 5V which can be used by the microcontroller/sensors/etc. at the far end.

Second opinions·are appreciated!

mctrivia

i have built a few switching regulators over the last month. They are not hard to do but you need to read the datasheet carefully. You will find pcb layout recommendations, formulas to calculate the ideal everything.

The value of the inductor does not need to be exact but the efficiency will suffer the farther away from the correct value you are(until it stops working all together)

For efficiency sake I would recommend just using a 5v regulator. with a few good caps and following the pcb layout recommendations the ripple current is not much.

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InSilico

Thanks, mctrivia, for your response.

There will be 5V linear regulators at the remote nodes. The cat3 or cat5 cables carry both data and power for the nodes. The cables will connect the nodes to a central station.

So not only will the station handle communication to the remote nodes, it will also serve as the power supply for the nodes connected to it. That's why I think there should be a switching regulator at the central point that provides power for all the nodes. I picked 7.5V to account for the voltage drop in the cable itself.

The current consumption of all the remote nodes put together adds up, and it gets to the point where too much heat is wasted trying to regulate 7.5V, even when using a LDO regulator (and in the first place, the input voltage may be 9V or higher). This is the primary reason why I want to try out using a switching regulator. Also, it's a good learning opportunity.

I have no trouble following through the datasheet. I just want to see how others implemented their switching regulator setup that works, like an example of component values and part #s.

Erik Friesen

ptn7800 from ti and the 7805 style from datel may do what you want. They are modules in the $12-15 range. Recom has some nice ones too. It will be hard to do it for much less by the time you buy the recommended caps.

Chris Savage

I too have designs a few switching regular circuits over the past couple of months and I find National's WebBench to help out greatly, especially when working with a new part. I have used it as a reference schematic and then tweaked the circuit from there, however you could in theory build the circuit from the reference design provided. Of course, I don't use the parts provided in their BOM, but again, they provide that to make it easier to get something built so you can use those parts to get going quickly.

www.national.com

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Chris Savage
Parallax Engineering

sam_sam_sam

InSilico

For example, the·datasheet guidelines says I should get a 220uH inductor, but it seems I can get away with a 150uH inductor (which is cheaper to obtain at Mouser). Do the inductors have to come from Schott/Pulse/Renco?

In the article· it explane all of this and more If you need me to I can Email you the article·

You can ·get a switching regulator kit from·Nuts Volts Store If you do not want to make your own

Kit includes an article reprint Switching Regulator please refer to the June 2008 issue.
·
http://store.nutsvolts.com/product.php?productid=16719&cat=379&page=1

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··Thanks for any··that you may have and all of your time finding them

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Sam

Post Edited (sam_sam_sam) : 4/29/2009 9:05:54 PM GMT

InSilico

Thanks for all the responses.

sam_sam_sam, if you can e-mail me that article it would be a great resource for me.

The only roadblock I have is the parts selection. Again, I don't know how picky the switching regulator is when it comes to the external components, or if there's anything specific I have to look for when I search for components on Mouser, Jameco, etc.

sam_sam_sam

Check Your Email I send It to you

Please Let me know if you all 6 pages of it· ·

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··Thanks for any··that you may have and all of your time finding them

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Sam

william chan

I had some difficulties with ripples using a 3.3v switching regulator from ON semi.
The datasheet recommended low ESR tantalum caps but those caps are expensive.

So I went with soem cheaper caps, but the ripples are unacceptable for my sensitive sensors.
Next time I will put more caps in parallel to reduce the effective ESR.

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sam_sam_sam

InSilico

Here are the part # that I used when I built my· Switching Regulator

These two are the one you have to be careful about

·Inductor is rated @ 3 amps this have to be rated for what load that you are going to have on the Regulator

One More big thing when building this type Regulator board the GROUND PLANE IS VERY IMPORTANT

If you are doing it on a bread board tie all (-) parts all in spot and use hook up wire for all the rest of the part

·Low Impedance Aluminum Electrolytic Capacitors

Mouser #: 673-PE-53117·
Mfr. #: PE-53117
Desc.: Inductors IND
$11.88·
·
Mouser #: 647-UPW1V331MPD6·
Desc.: Low Impedance Radial
Aluminum Electrolytic Capacitors
- 105 Degree 330uF 35V 105c 8x20 20% 3.5LS
Mfr. #: UPW1V331MPD6·

$0.55··


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··Thanks for any··that you may have and all of your time finding them

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Sam

Post Edited (sam_sam_sam) : 5/1/2009 3:35:06 AM GMT

InSilico

thanks, I think I have enough information to get started!