LM723 cap

[Paul Schreiber]

Every databook I have shows this as either 100pf (in low current designs) or 1000pf
(when using external pass transistor). 470pf is a good compromise, I suppose.

Paul Schreiber
Synthesis Technology


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From: 	<<<marjan>>>[SMTP:urekar.m at EUnet.yu]
Sent: 	Monday, September 21, 1998 11:49 AM
To: 	synth-diy at mailhost.bpa.nl
Subject: 	LM723  cap

Hi all

Here's something I found on the net (though I lost URL) 
that might be useful tip for your 723 voltage regulators :




" Power supply design with the LM723



  We had a big problem in this country with designs using the universal
LM723 regulator.   Some 25 or
so years ago, one of the popular electronics magazines published a
design in which the compensation
capacitor which should never have been more than 1nF (nanofarad) in
"Miller" mode, was quoted as
100nF, TWO HUNDRED TIMES the correct value. 

  The design would work, after a fashion, and was blindly followed by
hobbyist and "green" (No! Not
"enviro") designers in similar magazines over the next 15 years or more
with the result that 723s gained
a very unfortunate reputation; "hobbyist", but not what you'd use for a
lab supply. 

  Apart from very poor line interference rejection, the major problem
was that the resulting inability to
respond to transients (shorted output in particular) tended to defeat
the current limit circuitry, feeding
massive currents into the sense transistor (within the IC) whose E-B
junction was directly across the
(fractional-ohm in many cases) sense resistor.   Sooner or later, this
gave up the ghost and of course
there was now no short-circuit protection so the pass transistor,
current sense resistor, rectifier and
transformer suffered in turn.   Lots of smoke! 

   The correct fix, to those who knew and I had absolutely no success
getting any editors to admit to the
cock-up, was to use the correct capacitor value (470pF) as in the
professional designs.   Voilá!   A well-
behaved, rugged, reliable supply.   Using commonsense and including
resistance in series with the
current sense transistor (base) also helped reliability.   Since this
transistor had all three terminals
appearing as external pins, it was actually quite easy to substitute, or
indeed to add such a protective
resistor.  "




marjan