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I need to build 2 active 4-20mA outputs from a 24V supply voltage with unknown load conditions. One basic solution could be like the schematic below. VIn is converted to a current through R1, converted to the "high side" via R2 and then mirrored to an output pin. The load RL can be anywhere between 0 and 1 kOhm, as long as the FET can drive it. Don´t worry about the components being used - it´s just a sketch.

schematic

simulate this circuit – Schematic created using CircuitLab

My main problem is the power burned on the PCB in the worst case. Given the maximum current of 20 mA and a load close to 0 Ohm, the whole 24V are shared between the output FET and R3. This leads to 24V*20mA ~ 0.5W of dissipated power. Times 2 channels, we end up with 1W of heat on the PCB which I do not like but can´t get rid of using linear components.

Is anyone aware of integrated solutions using switching regulators to either:

-> step down the 24V if not needed or

-> using a "precision" current source to directly drive the output

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  • \$\begingroup\$ What FETs are you actually using? Cooling 1 Watt without a heatsink isn't that big of a problem as long as you can get the power out of the components and onto the PCB efficiently. PCBs make for surprisingly good low-power heatsinks. \$\endgroup\$ Commented Jul 6, 2021 at 16:06
  • \$\begingroup\$ I know that 1 Watt might not sound much but the whole thing is hermetically sealed and should be as small as possible. \$\endgroup\$ Commented Jul 6, 2021 at 16:12
  • \$\begingroup\$ I would expect that circuit to oscillate. Much better idea to use a P Channel FET for M2 (source to R3) and swap the inputs on the op amp (OA4). ie. Take the feedback back to the op amp's inverting input. \$\endgroup\$
    – user173271
    Commented Jul 6, 2021 at 18:27
  • \$\begingroup\$ Can you put numbers to "small as possible"? Because the solution you've sketched there doesn't look very small to me, so I wonder whether our understanding of "small" and your understanding line up! Also, what's the ambient temperature you're dealing with? \$\endgroup\$ Commented Jul 6, 2021 at 23:56

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Any linear solution will have the same amount of power dissipation, and using a MOSFET or BJT to dissipate the power is easier than using an IC since they typically are happy with a high die temperature.

I don't think dynamically adjusting the voltage of a SMPS would be very good- noise and dynamic issues. It could be done, of course.

Maybe you could live with a 10V compliance and step the 24V down to 12V to halve the dissipation (using a buck converter and filter).

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