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LM334M Datasheet(PDF) 9 Page - Texas Instruments

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Part No. LM334M
Description  LM134/LM234/LM334 3-Terminal Adjustable Current Sources
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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LM134, LM234, LM334
www.ti.com
SNVS746E – MARCH 2000 – 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
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 R1 and R2:
(9)
The values of R1 and R2 can be changed to standard 1% resistor values (R1 = 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
(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/°C instead of 2.5mV/°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
Ω and a
resulting tempco of
(12)
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.
Copyright © 2000–2013, Texas Instruments Incorporated
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Product Folder Links: LM134 LM234 LM334




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