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LM334SM Datasheet(PDF) 8 Page - Texas Instruments

Part No. LM334SM
Description  LM134/LM234/LM334 3-Terminal Adjustable Current Sources
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM134, LM234, LM334
SNVS746C – MAY 2004 – 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
μV/°C as the tempco of the LM134 (which includes the IBIAS component), and −2.5 mV/°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 = 25°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
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%.
8
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Copyright © 2004–2005, Texas Instruments Incorporated
Product Folder Links: LM134 LM234 LM334




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