LM3940-3.3 Linear Voltage Regulator With Polymer Capacitors

The LM3940-3.3 is a linear voltage regulator that whittles down 5V to 3.3V.  This part was introduced for the proliferation of 3.3V parts and logic in the early 1990's.

At first glance of the data sheet the minimum output capacitance for this part is 33uF. Unusually large for a linear voltage regulator but National Semiconductor must of had its reasons.

Using Capacitor #8 (33uF @ 10V) with an LM3940-3.3 did not work.

With an input voltage of 5V, the output voltage of the voltage regulator read 4.8V. This far exceeded the stipulated 3.3V.  The capacitor was confirmed to have a capacitance of 34uF with an ESR of 0.04 ohms using the Peak ESR+ meter.

I grabbed another LM3940-3.3 and connected it up but with no output filter capacitor. It too read 4.8V.

It was unlikely to be a whole bag of bad voltage regulators so I decided to place a load of 220R. The result?  No difference whatsoever.

The datasheet mentions that more output capacitance is acceptable.

From my previous designs that used the LM3940-3.3 I remembered that I used a 100uF capacitor.  So I soldered a temporary 330uF to see if it made a difference.  It does!

The output voltage is 3.3V as desired. This is not good because the voltage regulator is reliant upon the output capacitance.

If the capacitor loses its value the voltage may shoot back up to 4.8V damaging or destroying downstream components.

A closer examination of the data sheet says exactly that. 33uF is the minimum capacitance provided it has an ESR according to its graph.

The value for 0-120mA loads needs to be between about 0.09 and 0.6 ohms.  Capacitor #8 definitely does not!

For values 200mA and above the range is about 0.04 – 2.0 ohms.

The part size is handy for retrofitting into existing designs. The spacing works well for 100mil (aka 0.1 inch and 2.54mm) pitch spacings.

I used unusual (and terribly executed) soldering techniques to incorporate the surface mount packages into the vintage through hole design.

I removed the temporary cap and replaced it with the second capacitor #8 which has the parameters 33.56uF 0.09 ohms. Powered up and it works!