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

[Neil Johnson]

Hi Justin,

Justin Owen wrote:
> These guys: http://www.sdiy.org/juz/VRef_buffer.png
...
> Any clues?

The datasheets for the LM336 etc provide the clues.  Note they are
specified for Ir = 1mA, so I wouldn't run them below this.  Your 10k
resistors are running them at a shade under this, probably not a
problem.

As to how much current they can "supply" - they don't.  They regulate
the voltage by shunting excess current to ground.  So you really need
to know (a) how much average current your load takes, and (b) how much
variation.  So, for the LM336 the abs.max reverse current is 15mA, so
lets take a conservative 10mA.  We don't want to go below 1mA, so that
sets the regulation range to 9mA.  With that in mind you now need to
work out what your reference loads can demand.  Ideally they don't
change, but if they do then you need to work out what the min. and
max. are.  If the difference is more than 9mA then you'll need to use
some sort of buffer, like an op-amp.

On the subject of op-amp choice, if you're going to the trouble of
using a precision reference then you'd be better using the LT1013 as
that has much lower temperature drift.  An adjusted LM336 will have a
very low temperature change over the range 5 to 65 degrees C, so why
bugger it up with an op-amp that drifts all over the shop?

Hope that helps,
Neil
--
http://www.njohnson.co.uk