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Old 2nd August 2008, 12:08 AM   #1
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Post LM317 vs 7805 - 19 Different ICs Compared

I have been trying to figure out for a all around power supply is using a LM317 at Vo = 5V better then using a 7805. I have compiled a spreadsheet with data from 19 different ICs and it appears a 7805 is a better solution. I know this is discussed here a lot and thought you guys might like such a collection of information.

Does anyone have any real world experience with this subject or opinions in general?

Thanks,
Matt

P.S. A related post of mine with more information can be found at http://groups.yahoo.com/group/techlibdotcom/message/505
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Old 2nd August 2008, 12:41 AM   #2
anatech is offline anatech  Canada
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Hi Matt,
The data sheets are correct. Many experimenters have joined "the bigger capacitor is better" camp. This without doing any real studies beyond possibly a listening test.

One assumes that you will be supplying the regulator with filtered DC. Therefore, the capacitor immediately before the regulator is there to provide a low AC impedance at higher frequencies. Assuming that the regulator is within a few inches of your filters you are okay. If you have greater distance (read higher impedance) between the filters and the regulator, you should add a larger electrolytic close to the regulator as well. This occurs when you have local regulation on a card in a larger system.

The capacitor at the output does two things. It provides a low AC impedance for the load at higher frequencies and it supplies current if the load draws current with sudden surges. The output impedance of a regulator increases with frequency like an amplifier. Well, it does contain an amplifier so this should not be a surprise. Also, there is a fair amount of gain. It may oscillate if there is no bypassing close by (your small capacitors). Larger capacitors have inductance and do not generally work well at the higher frequencies we are concerned with. By omitting the bypass caps, they will probably have an oscillating regulator at some point in time. Electrolytic capacitors normally age with increasing internal resistance and a drop in capacitance.

Semiconductor companies spend a great deal of money and time designing and researching how to use these products. They will be right more often than wrong. They also seem to agree, so that means their engineers agree also. I'm betting they are dead on the money here.

Now, it is possible that a defect in PCB layout or circuit design may require larger capacitance on the output of a regulator. That does not mean that the smaller capacitors can be removed. It does mean that you run the danger of having your supply discharge backwards through the regulator when the power is turned off. This can and does happen. A reverse biased diode across the regulator is the protection from the that occurring.

-Chris
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Old 2nd August 2008, 03:04 PM   #3
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Here is the spreadsheet.
Attached Files
File Type: zip lm317 vs 7805.zip (4.0 KB, 522 views)
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Old 2nd August 2008, 03:11 PM   #4
paulb is offline paulb  Canada
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In your sheet, why are there duplicate sets of specs for the same device? The columns I pasted below show the ripple rejection is either 68.0 or 83.0.

ON MC7805B ON MC7805C
35 35
2.2 2.2
1.3 0.8
0.15 1.3
N/A N/A
3.2 3.2
50 50
68.0 83.0
2.0 2.0
125 125
65 65
-0.3 -0.3
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Old 2nd August 2008, 03:35 PM   #5
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Quote:
Originally posted by paulb
In your sheet, why are there duplicate sets of specs for the same device? The columns I pasted below show the ripple rejection is either 68.0 or 83.0.

ON MC7805B ON MC7805C
35 35
2.2 2.2
1.3 0.8
0.15 1.3
N/A N/A
3.2 3.2
50 50
68.0 83.0
2.0 2.0
125 125
65 65
-0.3 -0.3
Because one set is for the MC7805B and the for the MC7805C.
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Old 2nd August 2008, 03:48 PM   #6
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Sorry, It looks like I didn't make the column large enough to show the last letter. On the sheet there is a MC7805A, MC7805B, and MC7805C.

I have decided for the project I am doing that an adjustable regulator is the way to go. Over the next few days I am going to try to add to the adjustable part of the spreadsheet. My search criteria will be as follows.

1A - 2.5A output
Vo at least 5V - 15V
TO-220 package

Any ICs you guys want to see on the spreadsheet?
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Old 2nd August 2008, 04:07 PM   #7
paulb is offline paulb  Canada
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Ah, right, I didn't see the A/B/C on the spreadsheet but it sure showed up when I pasted it. Looks like ON has improved the old 7805.
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Old 2nd August 2008, 04:10 PM   #8
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I guess they are ON top of their game (pun intended)
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Old 2nd August 2008, 04:39 PM   #9
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Interesting work, thanks. What does the yellow highlighting represent?
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Old 2nd August 2008, 05:10 PM   #10
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I used the yellow highlighting to differentiate the better values. That way I could a get a quick visual of what IC is the overall best because no one IC was the best at everything.
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