Datasheet

LM134, LM234, LM334
SNVS746E –MARCH 2000–REVISED MAY 2013 www.ti.com
Application as a Zero Temperature Coefficent Current Source
Adding a diode and a resistor to the standard LM134 configuration can cancel the temperature-dependent
characteristic of the LM134. The circuit shown in Figure 15 balances the positive tempco of the LM134 (about
+0.23 mV/°C) with the negative tempco of a forward-biased silicon diode (about −2.5 mV/°C).
Figure 15. Zero Tempco Current Source
The set current (I
SET
) is the sum of I
1
and I
2
, each contributing 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
.)
(5)
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 measured or obtained from the manufacturer
of the diode).
(6)
(7)
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).
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