At 07:51 PM 12/22/2009, you wrote:
>Good day to all.
>
>I'm searching for a low-cost negative 5V regulator that is reasonably
>accurate.  Something similar to National's LP2950A but for a negative
>rail would be perfect.
>
>I need only about 10mA and there is a -10V or -12V supply available
>to power the thing.  I need at least 1% accuracy, more accurate would
>be better.
>
>I'd use a 79L05 except for its poor accuracy.
>
>Anyone have any suggestions?
>
>Many thanks!
>
>dwayne

Assuming power consumption is not a big concern..
Resistor + shunt reference. As two-wire devices they don't care about
plus or minus, provided of course that they are connected the right 
way around.

(You could use an op-amp as suggested with a positive reference, but capacitive
loading on op-amp outputs requires care (e.g. an extra compensation network)
to avoid oscillation and tuned to ensure reasonable transient response if
that's a concern).

There are lots of references that would fit your stated requirements. 
You need to
pay attention to the series resistor value (worst case current 
through the shunt
reference must be high enough to maintain good regulation with heavy 
loading, and
must not exceed the maximum current with light (or no) loading.
Some parts are adjustable and will require a couple additional
accurate resistors, others are fixed 5V (or 2.5V, see below). Also the bypass
capacitor needs care-- to avoid oscillation etc.

One example would be the TL1431 (0.25% accuracy) with a couple of resistors
(or use two in series!) Or the LM4040-5 (no resistors, but only good for 15mA
forward current, so there is not much to play with on the high end).

If I keep tightening specs toward the ideal (ignoring price, which
you did not say was a concern) until just one part is left, we
are left with the LT1634-- 0.05% accuracy, 10ppm/K . 10uA to 20mA.
but USD $5+ a pop (likely more from Digikey). Or there is the very nice
AD588 (not a shunt reference) if you don't mind shipping $20 bills with every
product.

But I think the two TL1431s are the most elegant solution @ < $1. Min current
is 0.5mA and max is 100mA (check the datasheet to be sure acceptable 
regulation
is guaranteed at your current levels) so maybe 13-15mA nominal
through the resistor. 300 ohms would give you 15mA at 9.5V and 25mA at 12.6V-
crude but acceptable. Or use an LM317L with a single resistor to get a fairly
constant current and improve the regulation further! Eg. 100 ohms will give
12.5mA nominal. Excellent also if you chose to use a single LM4040-5.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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