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LM334SMX Datasheet(PDF) 9 Page  Texas Instruments 

9 page LM134, LM234, LM334 www.ti.com SNVS746E â€“ MARCH 2000 â€“ REVISED MAY 2013 (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%. If the estimate for the tempco of the diode's forward voltage drop was off, the tempco cancellation is still reasonably effective. Assume the tempco of the diode is 2.6mV/Â°C instead of 2.5mV/Â°C (an error of 4%). The tempco of the circuit is now: (11) A 1mA LM134 current source with no temperature compensation would have a set resistor of 68 Î© and a resulting tempco of (12) So even if the diode's tempco varies as much as Â±4% from its estimated value, the circuit still eliminates 98% of the LM134's inherent tempco. Copyright Â© 2000â€“2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM134 LM234 LM334 
