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LM3940 3.3V Linear Voltage Regulator Gives 3.96V at 5V Supply.

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Pushpkant

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I have LM3940 voltage regulator, and it is 3.3V regulator. But it is giving 3.96V instead of 3.3V when input is 4.95V.
I have done all the design as per given in the data sheet.
But still I am not getting the voltage.
I have change the IC several times and tried to increase or decrease the load current still is not giving the 3.3V.
I have attached The datasheet.

Thanks for your help.
 

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  • LM3940IMP.pdf
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The output voltage is specified for a load current of 5mA to 1A. Are you measuring with at least 5mA current to the load?

If UNDER loaded it isn't unusual for the voltage to be a little higher. To get minimum 5mA at 3.3V try adding a resistor of 620 Ohms across the output.

Brian.
 

Although the output voltage isn't strictly specified for 0 output current, there are no indications in the datasheet that the regulator requires a minimal output current (in contrast to e.g. LM137). OP reported tests with different load current, by the way.

I rather expect something like
- pin connections wrong
- external source (if present in your circuit) is pulling up the output voltage
- regulator oscillating (bypass capacitance and ESR specifications not met)

Show your schematic or a photo.
 

Ensure that the decoupling caps are close to the pins and not far away, and the values are correct too. If still not working try replacing them. The capacitors are usually needed over there to stabilize the regulator and preventing from regulator oscillating. ESR of the capacitor is the critical parameter. It is better to use a tantalum capacitor because of its lower and more stable ESR. otherwise try to use two number of 100nF caps, one @ the input, one @ the output, and probably an electrolytic capacitor too.
 

Just a warning: cheap DVMs tend to show larger value than actually measured, when internal battery voltage drops.
 

My bet among all of the possible reasons is that the output capacitor has to high an ESR which will cause the output to oscillator at a very high frequency (50Mhz). Look at the output with an oscilloscope, use a 100uF tantalum output capacitor. Also don't forget the input capacitor and that both capacitors should be as close to the regulators pins as possible. Layout may also be an issue.
 

I have checked all of these but still it is giving 3.96V in the output, when input volt is 4.95V.
 

Make sure you follow and meet the output capacitor requirements for stability as stated under "Application Hints." Minimum capacitor value and ESR limits are essential to meet loop stability.
 

Till now I relay on the ESR data given by the manufacturer for the Capacitor.
Can you please tell me how to measure the ESR value for a capacitor. Everything else is just better than good in the design.
I have tried various kinds of capacitors with different values but the output is always the same. The IC has been change several times, but no better result.....
 

Have you checked for any oscillations on the output?
 

First of All If you are really sure that the voltage is not at all changing in-spite of required load connected on the output, Do one thing Isolate the load and connect an multi-turn higher wattage trimpot or preset and try calibrating the out put as per the regulation notes given in Datasheet, Once you achieve your required outage then subject the regulator output to the load and observe for changes in the output, If you find out that the voltage varies to an greater extent, then yes you will have to adjust input voltage too..
 

There is one silly reason why a LM3940 would give the incorrect output voltage, and that is if you fit it the wrong way around!!!
 

When you say the input voltage "is" 4.95V, do you mean
that a DMM reads 4.95V, or that you have verified that
it is in fact a fairly noise free, pertty much DC 4.95V?
A regulator that's fed a high ripple source and given a
light load, could easily be made to pump that load if its
output sink drive is small or none.

Also check minimum load current spec.
 

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