Problem with LM3940IT-3.3

Other Hardware General Electronics
1

Hi,

I have a problem with a LM3940IT-3.3 which doesn't seem (I could have wired it up wrong) to work when trying to regulate my 5v MEGA output.

I only seem to be able to get 4.48v out of it.

Any ideas?

Thanks in advance.

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2

You have no capacitors on your regulator and its oscillating. If you read the datasheet
you will find that its crucial that this regulator has at least 33uF output capacitor of the
right ESR. Without the right component it is unstable.

All regulators need input and output capacitors, some are more fussy than others (low
drop-out regulators are usually fussiest).

3

Thanks for point that out. I'm just learning about electronics and done a bit of reading, so I have a couple of basic questions..

  1. The capacitors I have seem to have a voltage on it.. e.g. 470uF 16v.. do I need to have the same voltage capacitor to the regulator output (i.e. use a 3.3v capacitor on a 3.3v output regulator)?

  2. Is it OK to use capacitors with a higher capacity (e.g. 470uF where the regulator datasheet says 33uF or 0.47uF)?

  3. I assume that the - terminal of the capacitor needs to be connected to the ground of the regulator?

I have a bunch of capacitors which I'm just trying to work out which is best suited.. e.g. 470uF 16v, 16v47uF, 47uF25v, 16v10uF, 16v10uF.

4

The voltage rating should be >= 2x the voltage being filtered. So 6V for 3.3V output is okay, 16V is better.
Don't use too big of a cap - they need a large current surge to charge up when first powered. I typically use 10uF on inputs and 1uF on outputs.

5

OK so for the output the 16v47uF is probably a good fit.

For the input voltage, does that >=2x rule apply? i.e. 5v in = 10v capacitor? The 16v10uF would be fine for that regulator input capacitor? It just seems that 10uF when it only needs 0.47uF.

6

I'd be keen to know the answer to electer's last question, please? :slight_smile: