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Old 16th April 2007, 05:57 PM   #1
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Question Equation for LM317 and LM337 +-15v

Hi,

I'm trying to find the proper equations for the LM317 and LM337 regulators. Is there an equation for voltage out for a given voltage in? Say I use 18v secondary, how can i get that down to 15v?

Do the LM317 and LM337 care what the input voltages are, beside the maximum possible voltage?

thanks,

Vince
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Old 16th April 2007, 06:57 PM   #2
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I know it sounds obvious but a good place to start is the National website. If you just put the part number in google the first couple of entries will usually have a data sheet included. The Nat site will almost certainly have suitabile application notes for the regulators.
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Old 16th April 2007, 07:23 PM   #3
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Here's a start: 3-pin regulators

Regards,
Dan
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Old 18th April 2007, 12:40 PM   #4
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I've looked at all the app notes before and again. But, I still have the same questions and a new one.

Does the LM317 and 337 care about the input voltatge? The equations don't ask for the known input voltage.

What does the Iadj(resistor) value mean? Is it asking for the current it I need it to run the cricuit or is it looking for a resistor value?

I would just go ahead and experiment until I got the results I need, but I'm using point to point and that will take me a long time.

Maybe I'm better off finding a known working schematic.
I'm defininately not using the fixed versions 7815 and 7915s.

Thanks anyway,

Vince
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Old 18th April 2007, 09:10 PM   #5
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Input voltage needs to exceed the dropout voltage+output voltage. Also, the difference between input and output voltage is lost as heat and needs to be dealt with.
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Old 19th April 2007, 05:59 AM   #6
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With an 18 volt nominal winding and a bridge rectifier, your input voltage is likely to be around 24 volts.

For your purpose you can ignore Iadj. R1=240, R2=2640 would work for +15 Volts.
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Old 20th April 2007, 08:01 PM   #7
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Quote:
Does the LM317 and 337 care about the input voltatge? The equations don't ask for the known input voltage.
No, but if you look at the later graph of 'dropout' voltage vs. load current you'd find the answer. These regs need a sustained input voltage about 3v above output at mximum current. Which means your raw supply needs to maintain +/-18v under full load for a regulated +/-15v output.

Quote:
What does the Iadj(resistor) value mean? Is it asking for the current it I need it to run the cricuit or is it looking for a resistor value?
It's a resistor value. The 317 defines a set vltage between the output and ADJ pins (of 1.25v). YOu put...any resistor here, call it R, and the resultant current through it will be (1.25/R). SO you then pick the 'lowr resistor, R2, so that [(1.25/R)*R2 + 1.25] equals your desired output set voltage. Simple 'Ohms Law' application when you think about it. It's a nice convention to pick R= 120ohms for two reasons:it makes Iadj about 10mA, which makes calculation easy, and because drawing a few mA is good practice to get the 317/337 regulating properly. Fo 15v output, then, you could pick R = 120ohm and R2 = 1320ohms, say 1K3. But R= 240, R2 = 2k7 would work just as well.

By the way - the output impedance of 317/337s drops the more current you draw, in fact, and this is a good thing: minimum output impedance is about the 300-500mA mark but this is and utterly impractical amount of power to waste in just the voltage set resistors
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Old 22nd April 2007, 01:45 PM   #8
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LM317 datasheet states Iadj as 50-100 microamps?
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Old 22nd April 2007, 01:57 PM   #9
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FWIW, here's an Excel sheet that will let you calculate the voltage range based on resistor tolerance and Iadj tolerance, or resistors from desired voltage. It's a bit cryptic, but useful.
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Old 25th April 2007, 05:15 PM   #10
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Thanks for everyones help.

This power supply will power a simple active crossover that draws very little current. It's based on Grey's active crossover with a single jfet.

There is now enough info here that i can get it to work.

Thanks for the explaination!

Vince
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