Posted on Dec 16, 2012 8512
This 5-V circuit utilizes a 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 2 200 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 ICl"s positive input voltage. ICl"s output biases the LT1072 switching regulator"s Vc pin. The resistors connected to the regulator"s feedback pin, fb, 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 FB 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.