EDN Access-- 6.22.95 40V current source operates from -40 to +85°

-June 22, 1995

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Design Ideas:June 22, 1995

40V current source operates from -40 to +85°C

Ray Scott,
SOR Inc, Lenexa, KS

Wide-output-current-range sources that accommodate a wide supply and temperature range require special consideration. The basic circuit of ( Fig 1) provides IOUT = VREF/RS; typically, VREF is 1.25V for the LM317L family. The LM317LZ is rated for 0 to +125°C; for lower temperature ranges, the LM317LBZ could be used (-40 to +150°C rating), but it is more costly and harder to obtain.

Fig 2's circuit uses commonly available components, including the LM317LZ, and can operate down to -40°C. It is designed for a current limit of 5 mA maximum. You select resistor R1 to be [appx]10 times RSET, making IOUT[appx=]1.10xISET. (In reality, IOUTISET+ IR1+IADJ, so a better approximation is IOUT1.15xISET). For IOUT of 5mA, ISET is 4.5 mA. RSET is VREF/ISET, so 1.25V/4.35mA=287 Ohms, and R110xRSET2.87 kOhms. In this case, R1 and RSET are standard 1% values and IOUT does equal 5 mA. In cases that are not exact, select R1 as close to the calculated value as possible and vary it to trim in the correct value of IOUT. (Don't stray too far from the 10:1 ratio of R1 to RSET, though.)

The temperature coefficient of D1 offsets that of the LM317LZ, so a temperature range of -40 to +85°C will alter the current limit less than ±100 µA for the 5-mA limit shown. This works for currents from 2.5 to over 20 mA with no special heatsinking and accommodates VIN-VOUT of 40V dc. To use the current source/limit, place an LED, zener diode, switch, etc. in series with the circuit as shown. VIN minimum should be 3.5V plus the voltage drop of the series device. Resistors of 1% tolerance were used for initial accuracy and repeatability, but you could use 5% values for reduced cost. (DI #1721)

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