
LM334SMX Datasheet(PDF) 7 Page  National Semiconductor (TI) 


7 page 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 +227ÂµV/ËšC as the tempco of the LM134 (which includes the I BIAS component), and âˆ’2.5 mV/ËšC 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 = 25ËšC 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 LM134â€™s temperature coefficient, and it does a good job even if the estimates of the diodeâ€™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 nonzero tempcos by setting the tempco component of the tempco equation to the desired value instead of 0. EXAMPLE: A 1mA, ZeroTempco 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 â„¦ and R 2 = 1.33k â„¦) with less than a 0.75% error. 00569728 FIGURE 3. Zero Tempco Current Source www.national.com 7 
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