[sdiy] Current sink/source specs LM336 vs. LM385

John Speth jspeth at avnera.com
Wed Mar 25 21:43:47 CET 2015


> These guys: http://www.sdiy.org/juz/VRef_buffer.png
>
> I'm trying to work out if there's a better VRef/Buffer combo between the
> LM336 2.5 vs. LM385 2.5 VRef and the TL072 vs LT1013 opamps as buffer.
>
> Part of that is working out how much current the two combinations can
> source/sink.
>
> I think both the TL072 and LT101 can sink/source between 20 and 25mA but
I
> can't work out what the equivalent values are for the VRefs.
>
> A VReg data sheet would generally give an 'Output Current' - but it
doesn't
> seem like VRef sheets do.

Both parts are diodes with a precision zener voltage realized by reverse
biasing the diode.  The adjust input seems to be for temp comp.  You must
select the reverse current so that it fits into the parameters defined in
the data sheet (Iz).   The LM366 specifies 1 mA.  The LM385 specifies 100
uA.  You can calculate the required resistor by using V=IR.

For example, if Vref = 2.5V, Vsupply = 12V, and your target current is 1
mA, then the resistor shall be (12V - 2.5V) / 1 mA = 9.5K ohms.

You're right that zener diodes have no drive current as they are just
voltage references, not active drivers.  Your opamp provides the drive.

JJS


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