Dennis -
I KNEW we'd get someone who actually knew what they were talking about! :-)
I hadn't ever heard about the lead length in the input side. How "long"
is long? Does putting chip caps ON the regulator leads really buy you
anything? Certainly wouldn't hurt!?!
I understood the role of the output cap, I thought, and believed that it
shouldn't be TOO big, not so much because of inductance but to enable
the regulator to "regulate" the output voltage for varying loads.
"Fighting" the cap is a good description!
The Low Drop Out regs came along after I was out of engineering, so that
is good information.
The Fairchild data sheet I looked at for the LM78XX series showed a 0.33
uF on the output. Drake seems to have used a 1 uF tantalum (solid) on
ALL the "ins" and "outs". The tantalum cap should be ok for 3-30 MHz,
but is that TOO much capacitance. My experience says that a 0.33 disc
ceramic would be less expensive than the tantalums, so I would assume
that Drake thought it would be better.
Are you familiar with the 723 at all?
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Dennis Monticelli wrote:
My company makes IC voltage regulators. Let me pass along some
information.
The purpose of the "bypass" caps for a conventional NPN pass regulator
(317, 340, 7800, etc) s as follows. The input cap is to prevent
possible oscillations if the lead from the IC to the raw DC filter cap
is on the long side. It is just insurance. The output cap is to
improve the transient load regulation. IC regs use a feedback loop to
steady the output, however, the bandwidth of the loop is finite. The
output cap takes over for the loop beyond a certain frequency so as to
maintain good transient regulation.
A PNP pass regulator (a.k.a. a Low Drop Output regulator) uses the
input cap for the same purpose as the NPN types. But it's output cap
is not only there for transient load regulation, it is usually
mandatory for obtaining loop stability. Because of this special role,
you must pay careful attention to it's value and dielectric type as
recommended in the datasheet.
You generally do not bypass a regulator output to decrease internally
generated noise because the output Z is so low that it will fight the
cap. Sometimes an IC reg will give you a pin to bypass the internal
voltage reference portion of the chip. But this is not common and
optional to do anyway.
The presence of the input and output caps can help in keeping RF out
of the chip's internal circuitry. If you're worried about that use a
small value ceramic.
Dennis AE6C
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