At 08:08 PM 3/8/04 -0500, you wrote:
>Hi:
>I've found that using a 1K resistor between ADJ
>and ground and a 680 ohm resistor between ADJ and output gives me almost
>exactly 3 volts output.  While that would seem to indicate I've got the
>ratio about right, are these reasonable values to use in this
>application?  I'm using 1/4 watt resistors.
>

John,

Those resistors you tried will probably be fine.  The datasheet *does* tell
you what you need to know but it may not be obvious.

There are two things to keep in mind:

1) The LM317 regulates *best* when it is sourcing at least 10mA or more. It
will tend to go out of regulation with less current.  So the typical setup
is designed to have the two resistors pull 10mA. If you search long enough,
you will notice that 124 Ohms (or 120 for 5% values) is a commonly used
resistor.  You are fairly close so your values will probably work fine.  If
you always have some other load, you will not notice any regulation problems.

2) The quiescent current that flows out of the ADJUST pin is something like
50uA.  If you try large resistors, this current will make an additional
error to the result.

Hope this helps.
Tom

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