piclist 2002\10\31\211410a >
Thread: Power supply rework
www.piclist.com/techref/power/actodc.htm?key=power
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face BY : Russell McMahon email (remove spam text)



> In reading the data sheet for the LM317 and the LM388 the first time
> through, I was thinking in terms of a 7805-type regulator, where the
> output is maintained at X number of Volts above the reference pin.  I can
> see that's not the case.

This is more an aside in this instance, but worth noting for future
reference (no pun intended :-) ).
The LM317 and the 7805 essentially DO work identically - it's just that they
are usually *used* differently and are each optimised for their typical and
usually slightly different applications.
The 7805 seeks to maintain 5v between the output and reference pin. The load
is usually connected across the same points so it is true to say, as you do,
" ... the output is maintained at X number of Volts above the reference
pin."

Now the LM317 is a 1.2 volt regulator. It seeks to maintain the output pin
1.2v above the reference pin. However, 1.2v is not usually an especially
useful voltage so the regulator is used in configurations such that WHEN the
LM317 maintains this desired difference, the consequent voltage produced
across the load is equal to some other voltage - say 5v or 12v or whatever.
This is (usually) achieved by dividing the actual output resistively such
that a fraction of it appears across the output pins of the LM317. When Vout
is the desired voltage the LM317 sees its target voltage and all is well.
When the target output voltage drops or rises the LM317 sees a reduction or
increase in its 1.2v output level and adapts accordingly. So it is also true
to say for the LM317 that " ... the output is maintained at X number of
Volts above the reference pin." It's just that in this case, the reference
pin is NOT at earth, so the load sees this X volts plus the voltage from
reference pin to ground.

The LM317 designers did not, of course, set out to make a regulator for use
at 1.2v. They wanted to make a regulator that could be used for a wide range
of voltages by using division of the output as described above. The 1.2v
figure is simply a figure that is as low as possible while still providing
enough internal drop to allow the internal electronics to function. (It
needs to be low as the regulator can then be used for voltages equal to or
more than this voltage) The 7805 and any other similar 3 terminal regulator
CAN be used in a similar manner to produce a voltage higher than its design
voltage by dividing the load voltage so the target voltage is eg 5v when the
design voltage is at target voltage. (eg take a 7805 and place 1k from
output to its "ground" pin and 1k from this point to true ground and you
will get "about" 10 volts out.) Such regulators are usually less successful
than purpose designed ones such as the LM317 because they have not been
designed to take account of the effect of the resistor from reference to
ground. On eg 7805 the internal electronics use this path for their main
supply current and this affects the regulation as this current varies. In
the eg LM317 the internal electronics are largely powered from the in/out
voltage drop and the ref current is kept constant. This greatly improves
regulation.

None of which is too important in this case, but it's worth being aware of
for future reference.


       RM

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