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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)
The set current (I
SET) is the sum of I1 and I2, each contrib-
uting approximately 50% of the set current, and I
BIAS.IBIAS is
usually included in the I
1 term by increasing the VR value
used for calculations by 5.9%. (See CALCULATING R
SET.)
The first step is to minimize the tempco of the circuit, using
the following equations. An example is given using a value of
+227µV/˚C as the tempco of the LM134 (which includes the
I
BIAS component), and −2.5 mV/˚C as the tempco of the
diode (for best results, this value should be directly mea-
sured or obtained from the manufacturer of the diode).
With the R
1 to R2 ratio determined, values for R1 and R2
should be determined to give the desired set current. The
formula for calculating the set current at T = 25˚C is shown
below, followed by an example that assumes the forward
voltage drop across the diode (V
D) is 0.6V, the voltage
across R
1 is 67.7mV (64 mV + 5.9% to account for IBIAS),
and R
2/R1 = 10 (from the previous calculations).
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 R2:
The values of R
1 and R2 can be changed to standard 1%
resistor values (R
1 = 133
Ω and R
2 = 1.33k
Ω) with less than
a 0.75% error.
00569728
FIGURE 3. Zero Tempco Current Source
www.national.com
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