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LM334SMX Datasheet(PDF) 8 Page  Texas Instruments 

8 page LM134, LM234, LM334 SNVS746C â€“ MAY 2004 â€“ REVISED MARCH 2005 www.ti.com (5) The first step is to minimize the tempco of the circuit, using the following equations. An example is given using a value of +227 Î¼V/Â°C as the tempco of the LM134 (which includes the IBIAS component), and âˆ’2.5 mV/Â°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 R1 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 = 25Â°C is shown below, followed by an example that assumes the forward voltage drop across the diode (VD) is 0.6V, the voltage across R1 is 67.7mV (64 mV + 5.9% to account for IBIAS), and R2/R1 = 10 (from the previous calculations). (8) 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 ISET, however, the circuit should be built and tested over temperature. If the measured tempco of ISET is positive, R2 should be reduced. If the resulting tempco is negative, R2 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 R2 = 10 R1. 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 R1 and R2: (9) The values of R1 and R2 can be changed to standard 1% resistor values (R1 = 133Î© and R2 = 1.33kÎ©) with less than a 0.75% error. If the forward voltage drop of the diode was 0.65V instead of the estimate of 0.6V (an error of 8%), the actual set current will be (10) an error of less than 5%. 8 Submit Documentation Feedback Copyright Â© 2004â€“2005, Texas Instruments Incorporated Product Folder Links: LM134 LM234 LM334 
