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LM334H Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM334H Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 14 page Application Hints (Continued) 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 ex- ample 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 cen- tered 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 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 1 and R2: The values of R 1 and R2 can be changed to standard 1% re- sistor values (R 1 = 133Ω and R2 = 1.33kΩ) 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 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 ef- fective. Assume the tempco of the diode is 2.6mV/˚C instead of 2.5mV/˚C (an error of 4%). The tempco of the circuit is now: A 1mA LM134 current source with no temperature compen- sation would have a set resistor of 68 Ω and a resulting tempco of So even if the diode’s tempco varies as much as ±4% from its estimated value, the circuit still eliminates 98% of the LM134’s inherent tempco. Typical Applications Ground Referred Fahrenheit Thermometer DS005697-15 *Select R3 = VREF/583µA. VREF may be any stable positive voltage ≥ 2V Trim R3 to calibrate www.national.com 7 |
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