LM134/LM234/LM334
Application Hints
(Continued)
If the estimate for the tempco of the diode’s forward voltage
drop was off, the tempco cancellation is still reasonably ef-
fective. 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:
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 ex-
ample 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 cen-
tered 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
:
A 1mA LM134 current source with no temperature compen-
sation would have a set resistor of 68Ω and a resulting
tempco of
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.
Typical Applications
Ground Referred Fahrenheit Thermometer
The values of R
1
and R
2
can be changed to standard 1% re-
sistor 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
DS005697-15
*
Select R3 = V
REF
/583µA. V
REF
may be any stable positive voltage
≥
2V
Trim R3 to calibrate
an error of less than 5%.
7
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