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LM334H Datasheet(PDF) 7 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM334H
Description  3-Terminal Adjustable Current Sources
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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Application Hints (Continued)
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 R2:
The values of R
1 and R2 can be changed to standard 1% re-
sistor values (R
1 = 133Ω and R2 = 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
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 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:
A 1mA LM134 current source with no temperature compen-
sation would have a set resistor of 68
Ω 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
DS005697-15
*Select R3 = VREF/583µA. VREF may be any stable positive voltage ≥ 2V
Trim R3 to calibrate
www.national.com
7




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