LM3940 regulator not putting out 3.3V

[radiodude]

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.

 

[eng1]

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.

Are you using a suitable output capacitor? It's critical for LDO regulators.

 

[picbits]

And does it have a load on it ?

 

[mvs sarma]

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.

All said, do you mean to say that if 5V input is given you are getting 4.9xV and not 3.3?
please put a 10mA load of 330 ohms. please ensure that the filter caps at the input and output are correct. Ensure that the input and common connections are proper and the decoupling caps are close to the pins and not far away, and the values are correct by measurement if possible. Otherwise try replacing them.

 

[radiodude]

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.

 

[crutschow]

The capacitors are generally needed to stabilize the regulator and prevent it from oscillating.

 

[Roff]

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.

As others have said, there is a minimum required load (5mA).
[soapbox]It appears you read part of the datasheet (the pinout). Perhaps you should read the rest of it.[/soapbox].
OK, I got the sarcasm out of my system. I realize that datasheets look like gibberish to noobs. If that describes you - sorry.

 

[radiodude]

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/?).

 

[shaneshane1]

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/?).

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!

 

[Roff]

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!

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.

 

[shaneshane1]

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.

interesting stuff!

I'm use to trial and error when it comes to this sort of thing (anything caps) and to date my projects have not failed using this method!

mind you its about the only thing i don't have patience for, and may be very important for me to one day take the time to fully understand.

 

[crutschow]

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

Only if the (nano)meter were also 100 times as large.

 

[eng1]

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/?).

That capacitance can be increased. The critical parameter is the ESR of the capacitor. The better choice is a tantalum capacitor because of its lower and more stable ESR.

 

[radiodude]

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

 

[mvs sarma]

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

Put 3 of them one above the other and let item be soldered patiently

 

[radiodude]

Thanks, mvs sarma; didn't think of that. I guess a little extra height won't hurt.