On Nov 9, 4:36 am, Eeyore
wrote:
"robb" wrote in message
so i tested by connecting 5V/1A DC source between Vin and
GND-Pin3 and i checked the voltage with DVM between Vout and
GND-Pin3 and i was getting 3.47 V

Pointless. It won't operate correctly with only 5V in.

Agreed.

A linear voltage regulator has to have several volts more on the
input than the output to function correctly.

Some need 1-2V more, others around 1/2V (I think I've seen
LDOs down to 0.2V dropout [i.e. Vout-Vin]). I didn't find a spec
sheet for the L387, but the L387A has a max dropout of 0.8V
(and that's from 4.9V, so Vin[min] is 5.7V). Don't know whether
dropout is one of the improvements in the A version.

In most cases it's safe to test with Vin=Vout+2V, but with an
adjustable LDO operating near the top of the Vout range, that
may result in Vin exceeding spec. Checking the data sheet
first is always a good idea.

(LDO = Low DropOut, a category of linear regulator)

TM