4_TO_20_mA_CURRENT_LOOP

Published:2009/7/9 23:49:00 Author:May | From:SeekIC

This 5-V circuit utilizesa servo-controlled dc/dc converter to generate the compliance voltage necessary for loop-current requirements. This circuit will drive 4 to 20 mA into loads as high as 2200Ω with 44 V of compliance. It is inherently short-circuit protected. A current source by definition limits current regardless of the load. The circuit's input voltage and the 4-mA trim network determine 101's positive input voltage. IC1's output biases the LT1072 switching regulator's VC pin, The resistors connected to the regulator's feedback pin, BB, prevent the circuit output from running away in the event that the load opens up. Normally, IC1 controls the loop. However, if the load opens, IC1 receives no feedback. Under this condition, the Fs pin becomes active when it equals 1.2 V and forces the loop to close locally around the regulator by activating IC2's internal amplifier. Thus, the circuit automatically changes from a current to a voltage regulator, thereby preventing excessive output voltages.



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