SparkFun Forums

[brainwav]

Ok, I was just using 200ma as a max. I'll probably stay within 100ma. I got my datasheet from the sparkfun site http://www.sparkfun.com/datasheets/Components/LM317.pdf
I have the to-220 package and I bought the regulators from Sparkfun. Maybe I could put in 5.5 volts or something like that.

EDIT: After reading that datasheet you have, and the drop out voltage graph, I see what you are saying. So I can't use the LM317. What is I just put in 4.5 volts then. Have a voltage divider before the regulator? That would lower the in voltage and reduce the diff so I can use that Line.

[Philba]

I'm not a big fan of using a voltage divider to supply current as it won't regulate. think of your circuit as a big resistor. the bottom half of the divider now is 2 resistors. when your circuit's current pull changes, it's effective resistance changes and your divider's bottom half resistance will drop, thus dropping the voltage. If your current pulls a very steady current, you could be ok.

I suggest you work through the amount of current your circuit will reallly draw. it's kind of nice to not have to but it could make a huge difference in what you need to put it. if it's much under 100mA, there are lots of really nice solutions for that. Once you get close to 100mA, your options shrink.

By the way, I think that TI datasheet has to be wrong about the to220 - no way it can have a thetaja of 19.

[brainwav]

Ok, After seraching the net and talking with people at local electronic store, I have come up with a good LDO regulator that will do that I want it too. It's from Vishay, part number GS1117CT3.3. I can put 5V in and get 3.3 out and it's rated up to 800ma. of course I won't use that much. I says that the thetajc is 3.0C/W for the TO-220 package. What's the difference between thetaja and jc? This one does not list the ja.

[Philba]

jc is junction to case. its important but not very useful - the real issue is case to ambient which is the ability of the case to radiate heat. You see this jc as part of the junction to ambient rating which is junction to case times case to ambient (jc * ca). If they don't have the ja rating, I'd use 65C/W. If you stay under 100mA, you'll be just fine with the temp rise of about 11C.

I think the vishay part is a cross ref for the LD1117 that I pointed out earlier.

[brainwav]

yeah it looks like the same one. Do you know where I could get the datasheet for that one? One that has the ja?