LM3940 regulator not putting out 3.3V

radiodude said:

I'm giving it exactly 5 volts directly from a DC Power Supply and the output voltage should be 3.3V but it's not.

It seems if
V_in < 1.5V, V_out = about 0 plus a couple millivolts
V_in >= 1.5V, V_out = V_in minus a couple millivolts

I am using an **broken link removed**. I've tried one from a box of them just to be sure I hadn't shorted the old one out. There's only 3 pins: V_in, Ground, and V_out....how can you mess that up?

**broken link removed**

I don't know what I am doing wrong as I am using some 7805s in my circuit which have no problem taking my 8volt DC and putting out 5volts.

Any suggestions? Thanks.

Click to expand...

radiodude said:

I'm giving it exactly 5 volts directly from a DC Power Supply and the output voltage should be 3.3V but it's not.

It seems if
V_in < 1.5V, V_out = about 0 plus a couple millivolts
V_in >= 1.5V, V_out = V_in minus a couple millivolts

I am using an **broken link removed**. I've tried one from a box of them just to be sure I hadn't shorted the old one out. There's only 3 pins: V_in, Ground, and V_out....how can you mess that up?

**broken link removed**

I don't know what I am doing wrong as I am using some 7805s in my circuit which have no problem taking my 8volt DC and putting out 5volts.

Any suggestions? Thanks.

Click to expand...

radiodude said:

No load, no capacitors. Eventually both aspects will be there, but I'm soldering piece by piece and just verifying voltages.

eng1, the LDO regulator will not work at all without capacitors? That's probably it then. I thought they were optional (i.e., if you wanted a smoother voltage). I'm guessing my 7805 is not an LDO as that works fine with no load or caps. Hmm.....

mvs sarma, you are correct. Vout seems to be Vin - X but X changes based on Vin.

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radiodude said:

I mean I read all of it over and over in search of what I could possibly be missing. It's just one of those cases where my intuition was wrong (caps were optional). I'm new at it so I'm gonna make mistakes...I can handle that.

One other question, do I have to use the values on the datasheet? I'm having a hard time finding a 33uF 0603-packaged capacitor. The board has already been designed and (printed/fabricated/?).

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shaneshane1 said:

well i use a 7805 5V regulator with a 0.1uF cap on the Vin and the Vout and everything works just great, eg: i run a servo motor @ over 500mA!

I've used other caps as well like 10uF, 47uF and even 470uF
and they all give relatively the same Vout with and without load!

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Roff said:

The LM7805 and the LM3940 are completely different animals, and their capacitor requirements are quite different.
You can use any value of output capacitance on the LM3940 so long as it is 33uF or more. The attachments below are out of the datasheet. I am posting them so you will know where the information came from.

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shaneshane1 said:

if we were over 100 times the size we are now, then wouldn't what we already have be nanotechnology!!!

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radiodude said:

One other question, do I have to use the values on the datasheet? I'm having a hard time finding a 33uF 0603-packaged capacitor. The board has already been designed and (printed/fabricated/?).

Click to expand...

radiodude said:

The problem is it seems 33uF is too big for an 0603 capacitor...The largest 0603 cap I can find is only 10uF.

Click to expand...