Miscellaneous Stuff

[Martin Czech]

SNIP
> 	Why would a 7815/7915 power supply be good inadequate for a
> modular??  Is there any reason why one should use a 723 or 317 regulator
> instead??
SNIP

A very late response from me, be nevertheless:

Good question. I had a look at the data sheets, for the 317 the values
are given in %, so i calculated them for 15V output. The noise spec
for the 7815 is for 10Hz-100kHz, whereas the 317 is only speced for
10kHz, assuming thermal noise I scaled that up with sqr(10):

                                LM7815                  LM317(@15V) 
line regulation        :        4mV(typ) 150mV(max)     1.2mV(typ) 5mV(max)   
load regulation        :        150mV(max)              75mV(max)
temperature stability  :        ?                       1%(0-75DEG)
noise                  :        28uVeff                 450uVeff
ripple rejection       :        70dB                    65dB

Line regulation: National claims better regulation for the 317, but the
maximum value seems to be suspiciously low, compared to the 7815. Maybe
some confusion about what maximum of the distribution is?  The typical
values are not that far apart, the 317 is 4 times better.  Load
regulation is twice as good on the 317.  No temperature data for the
7815, but via experiment i found out <1%.  Noise: if it is ok to scale
with bandwidth, the 7815 wins (more gain in the 317 regulation loop?)!
Also the ripple rejection is better, but both regulators need bypass
caps for higher frequency ripple.

Do you all aggree with these figures?

Assuming they are correct, one could say, that for the average analog
circuit, with allmost constant load (only little output currents) and
good psu with good passive ripple filtering a 7815 etc could well do the job.
Bypassing with caps is more important and a RC or even RLC filter at
the regulator input would also give an improvement for higher
frequencys.

For more demanding circuits, like preamps and the like, the 317 would be the better
choice, but also passive components are needed in this case. 

Digital or other current switching applications should hav a seperate regulator,
since even the 317 will go down 3V for 5us, if the output current switches from 10mA
to 1.5A. You'll never can regulate such transients (e.g.: vco discharger ?).

And: the 317 needs a minimum load of 10mA, whereas the 7815 does not.

So I'll go on using 78xx for the most stuff, and 317 only for "odd" voltages or
demanding applications. Passive ripple/transient rejection components can be 
of much more importance than the type of regulator.

m.c.