The set current (I

uting approximately 50% of the set current, and I

usually included in the I

used for calculations by 5.9%. (See CALCULATING R

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

diode (for best results, this value should be directly mea-

sured or obtained from the manufacturer of the diode).

With the R

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

across R

and R

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

tested over temperature. If the measured tempco of I

positive, R

negative, R

for use in this circuit is the 1N457 because its tempco is

centered at 11 times the tempco of the LM134, allowing R

10 R

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

The values of R

resistor values (R

Ω and R

Ω) with less than

a 0.75% error.

00569728

FIGURE 3. Zero Tempco Current Source

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