LM350\LM317 voltage loss

[Ivar Eriksen]

What is the voltage drop\loss that will come over an LM350 or LM317. (Like,
if i supply 12V, how much will i be able to get out of it)

I've tryed reading the datasheets, but i couldn't find anything about it.
I'm prob. just looking in the wrong place though, but it been ages since i
did any electronic work, and that was at school.

 

[Guest]

This is not a definitive answer - I don't have a data sheet either, but my
past experience with that series of Vregs brings to memory that an input to
output differential of about 3 volts is necessary in order to hold
regulation at the rated current of the device. I say if you supply 12v you
can get 9v regulated - keep in mind though that you have to maintain the 12V
in front of the regulator at the full current expected on the other end -
which may mean that you may need more than 12 v in front of the regulator.
It is dependent on many factors: the regulation factor of your power
transformer, filter & diode configuration and the current expectations of
the regulated supply.
TS

 

[John Muchow]

What is the voltage drop\loss that will come over an LM350 or LM317. (Like,

if i supply 12V, how much will i be able to get out of it)

For the National Semi. LM350 datasheet...
In the electrical characteristics table for Line Regulation, check out
the Conditions column. The regulation specs are based on a I/O
differential of at least 3V, but less than 35V.

You can have an I/O differential smaller than that, but the regulation
will get worse and worse as the input voltage goes down or the output
current goes up. This may or may not be important depending on how
much your input varies.

Check out the Current Limit and Minimum Operating Current graphs too.
You'll get an idea of what kind of current you can draw (and stay
within the regulation spec) and the minimum required current draw at
different I/O differential voltages.

My Texas Instruments LM317 datasheet mentions (in the Test Conditions
column) a minimum I/O differential of 3V.

In my own testing of a LM317 and LM317HV, the regulation disappeared
at about a 2.5V differential....YMMV.

John Muchow
-- remove SPAMMENOT for e-mail responses --

 

[happyhobit]

Hi Slegge,

What you're looking for is the 'Dropout Voltage'. See 'Figure 8' in the
LM317 datasheet. Worse case looks like about 2.5 volts @ 1.5 amps.

The LM350 is about 2.25 for 2 amps, a little more than 2.5 for 3 amps

Jay