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Old 8th August 2011, 03:11 PM   #1
graygem is offline graygem  United States
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Default 15v to 3v question

I am building a small DAC using UDA1352TS. It calls for a 3v supply. I am using a 15v supply with a LM317 (TO-252) regulator with 220 and 330 ohm resistors. Also a 10uF input bypass cap. I'm wondering about heat dissipation of the LM317. Is there going to be too much heat for the LM317?
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Old 8th August 2011, 03:18 PM   #2
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How much current would it draw? Look at the datasheet, it has pretty clear graphs that shows you what you can get away with, with appropriate heatsinking.
My advice would be though to get a small PCB mount transformer, say 5 or 6VAC and regulate that down for greater efficiency and less heat.
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Old 9th August 2011, 02:02 AM   #3
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Quote:
Originally Posted by graygem View Post
Is there going to be too much heat for the LM317?
The DAC chip pulls around 13mA if you add up the analog and digital supply currents.

That gives you about 150 mW of heat for the regulator, at 12V across the regulator.

A TO-252 will get a little warm, but will be fine.
Tja = 103C/W, so expect a 16C rise in temperature for the regulator.
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Old 10th August 2011, 03:09 AM   #4
graygem is offline graygem  United States
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Thanks, that's what I had decided. I hooked my prototype up to a 15v supply. It ran a little warm but didn't really get hot. Thanks again to both.
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Old 10th August 2011, 10:26 AM   #5
AndrewT is online now AndrewT  Scotland
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I am not a digital Member and don't know how the DACs work, but quoting 13mA for the combined Digital and Analogue current draw does not seem right.
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Old 10th August 2011, 02:59 PM   #6
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I am not a digital Member and don't know how the DACs work, but quoting 13mA for the combined Digital and Analogue current draw does not seem right.
The numbers came from the datasheet:

http://www.nxp.com/documents/data_sheet/UDA1352TS.pdf

Something like 9.3mA for the digital section and 3.6mA for the analog section.

Last edited by discrete; 10th August 2011 at 03:01 PM.
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Old 10th August 2011, 03:18 PM   #7
AndrewT is online now AndrewT  Scotland
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The 3mA of maximum analogue output and the Pmax~38mW seem to confirm
your original post.
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