Datasheet

LM134, LM234, LM334
www.ti.com SNVS746E –MARCH 2000–REVISED MAY 2013
(8)
This circuit will eliminate most of the LM134's temperature coefficient, and it does a good job even if the
estimates of the diode's characteristics are not accurate (as the following example will show). For lowest tempco
with a specific diode at the desired I
SET
, however, the circuit should be built and tested over temperature. If the
measured tempco of I
SET
is positive, R
2
should be reduced. If the resulting tempco is negative, R
2
should be
increased. The recommended diode for use in this circuit is the 1N457 because its tempco is centered at 11
times the tempco of the LM134, allowing R
2
= 10 R
1
. You can also use this circuit to create a current source with
non-zero tempcos by setting the tempco component of the tempco equation to the desired value instead of 0.
EXAMPLE: A 1mA, Zero-Tempco Current Source
First, solve for R
1
and R
2
:
(9)
The values of R
1
and R
2
can be changed to standard 1% resistor values (R
1
= 133Ω and R
2
= 1.33kΩ) with less
than a 0.75% error.
If the forward voltage drop of the diode was 0.65V instead of the estimate of 0.6V (an error of 8%), the actual set
current will be
(10)
an error of less than 5%.
If the estimate for the tempco of the diode's forward voltage drop was off, the tempco cancellation is still
reasonably effective. Assume the tempco of the diode is 2.6mV/°C instead of 2.5mV/°C (an error of 4%). The
tempco of the circuit is now:
(11)
A 1mA LM134 current source with no temperature compensation would have a set resistor of 68Ω and a
resulting tempco of
(12)
So even if the diode's tempco varies as much as ±4% from its estimated value, the circuit still eliminates 98% of
the LM134's inherent tempco.
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