Need help find the proper resistor for a LDO regulator

I'm banging my head against the wall trying to figure out what resistor I need to use for a project I'm working on. Basically I'm using a UCC283 linear reg (http://www.ti.com/lit/ds/symlink/ucc283-adj.pdf) to drop a 5v input down to a 4v(Ish. Anything between 3.7v and 4.2v would be fine) output. I'm terrible at reading data sheets. Can someone please help?

 

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Chaos this is a wild ass guess from a plumber, use this info with caution.

Using this formula from the data sheet Vout = 1.25 x (R1 + R2) / R1. Looks like you could use 200 ohm at R1 and 470 ohm at R2 for 4.1875 volts.

200 + 470 = 670 x 1.25 = 837.5 divided by 200 = 4.1875

 

Huh. DocDave said it was 25K. Thank god for breadboards, and that resistors are cheap. LoL.

 

Are you using the 3V, 5V or Adj. version of that regulator?

 

Adj version. Has to be to get it down to 4v.

 

ok, just checking lol.

I see docdave only gave you one resistor size, do you already have one resistor that is set? (both R1 & R2 set the voltage out)

 

Just the one. I don't think both are necessary.

 

See I told you mine was a wild ass guess.

Just when you think your starting to understand this shit, another curve ball is thrown.

 

Who says I understand it? LoL. Anyways, I'm going to breadboard it and try both y'alls answers and see who's right.

 

I am curious to see who is right.... Plumber put the right formula according to the data sheet. If you look on page 5 right at the top their it is.

 

The resistance that I gave was for the ti 04050 , I have never used this one

 

Although your numbers work out right. I would use.

R1- 50k and R2- 110k for about 4V.

This is from the datasheet's block diagram.


Note in both instances, 3V and 5V TI has chosen to use 50k for R1. I'm sure this wasn't just a random choice, that's what I would use.