LM134/LM234/LM334
Application Hints
(Continued)
00569728
FIGURE 3. Zero Tempco Current Source
The set current (I
SET
) is the sum of I
1
and I
2
, each contrib-
uting approximately 50% of the set current, and I
BIAS
. I
BIAS
is
usually included in the I
1
term by increasing the V
R
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).
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
:
With the R
1
to R
2
ratio determined, values for R
1
and R
2
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 I
BIAS
),
and R
2
/R
1
= 10 (from the previous calculations).
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.
7
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