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Author Topic: Voltage regulator or DC/DC converter  (Read 2436 times)
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Ireland
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I'm almost finished a design using an embedded Atmega processor in an environment where I'll have a 24V power supply. I originally designed it with a fairly pricey 24V-5V DC/DC converter, then considered just using a 7805 instead.

My understanding of how a 7805 works is that it draws the same current from its input as it provides to its output, which means (unless I'm mistaken) that it would be running at 20% efficiency and burning the rest off as waste heat. Is this right?
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You are right. To a first approximation the 7805 regulator's efficiency is Vout/Vin, or in your case 5/24 = 20.8%.

You also have to be concerned with maximum current draw. The actual power dissipation of the regulator is (Vin-Vout)*Iout = (24-5)*Iout = 19*Iout. This total power dissipation heats up the regulator and if it gets hot enough, it shuts down. My rule of thumb is to keep power dissipation <1W for TO-220 packages. This would limit your current to about 50mA, enough for the Arduino circuitry but not much else.

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Yes. You'll need a decent heatsink also with the 7805.
Suggest you use a little switching step down regulator such as
http://www.pololu.com/catalog/product/2101
Higher current output parts are available also.
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Thanks for confirming my suspicions.

I just came across this, which looks like it might be a useful substitute, and still quite reasonably priced.
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The LM2575 is not a drop-in substitute -- it is a switching regulator controller, just one component of a switching regulator. You still need to add an inductor, diode, and capacitors around it, and route it properly on a PCB to keep the switching loop tight, etc.

The Pololu part suggested by CrossRoads IS a drop-in solution.

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Ireland
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The LM2575 is not a drop-in substitute -- it is a switching regulator controller, just one component of a switching regulator. You still need to add an inductor, diode, and capacitors around it, and route it properly on a PCB to keep the switching loop tight, etc.
Understood; I hadn't meant to suggest that it was a drop-in replacement. The data sheet makes it clear what extra components are required.
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Are you in US or in Europe? Should add to your profile for better source suggestions.
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Ireland
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Good call, I've done that now - I'm in Ireland.
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The data sheet makes it clear what extra components are required.
Just be aware that building a DC / DC converter is not simply a matter of putting the parts round it. It requires a PCB with a very critical layout requirement.
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Hmm, only 4 parts to support a fixed regulator per datasheet - that looks doable!
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Well I have had trained engineers working for me that have failed to get the layout right first time and some even second and third time and they knew what they were doing.
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That is pretty good incentive to just buy for a 1-time use.
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Just be aware that building a DC / DC converter is not simply a matter of putting the parts round it. It requires a PCB with a very critical layout requirement.

The datasheet suggests that a single ground point or ground plane be used - I'm using a ground plane anyway. It recommends keeping certain paths as short as possible - I have all the relevant components within a square of less than 1".

Are there other requirements beyond what the datasheet specifies? I could just buy something and kludge it into the circuit, but part of the reason for designing and building something like this is as a learning exercise.
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The datasheet suggests that ......
Yes that is a good starting point and all my engineers read and understood the data sheet and the recommendations. In practice however component placement current paths are crucial, 1" square is not particularly tight for this sort of circuit. Also decoupling into and out of the regulator are important as are a number of other factors like the load capacitance and even the load current. Sometimes you get oscillations at a certain load current, anything above or below is fine.

The point I am making is not that it is imposable to make your own it is just that it is not easy to get it right first time. Better men than me have failed at that.
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Ireland
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The point I am making is not that it is imposable to make your own it is just that it is not easy to get it right first time. Better men than me have failed at that.
That's discouraging. The implication is that there's no reliable way to design this board and be confident that it will work, other than the empirical (and pricey and time-consuming) approach of building it and seeing if it fails, then trying to figure out why.

Or pay the premium for a pre-packaged DC-DC converter, of course.
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