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This is a follow-up question which references to "DAC controlled LDO as current source.". I am now trying to use the recommended LTC6363-0.5 as seen in the schematics. This is a fully differential OP amp and it does not work the way I connected it to the LT3083. The OP amp does in fact produce an output voltage of 0.5 * input (DAC) voltage, but only if not connected to the LDO.

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I think the problem is that the output voltage levels of "SET" and "OUT" are about a few 100mV and the LDO - in the way I need it to work - needs an output voltage level at several volts. I think the key is the VOCM input but I have no clue how to connect it correctly.

I admit I do not understand OP amps well, especially this one, as I have never seen a differential OP amp before. Is it even possible for the LTC6363 to drive the LDO as a current source or do I need a non-differential OP amp? As a side note, the latter already works for me, but I have already bought several LTC6363 and I would like to use them if possible.

Furthermore, this OP amp can only be driven with a source of +/-5.5V max and the voltage level at the 1R resistor (referenced to GND) is usually about 6..7V which seems to be a problem already.

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  • \$\begingroup\$ The type of differential-output amplifier shown does not behave the way you think. The outputs do not have a high common-mode impedance/low differential impedance. Rather, both are low. There is a configuration that would work this way but you don't need it. \$\endgroup\$ – Spehro Pefhany Jan 8 at 16:50
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Instead of bringing OUT back to the negative output, bring it back to the negative input instead of ground. The negative output can remain disconnected. Using the -0.5 variant of the op amp gives you plenty of range on the input, but you're limited to 5V on the output...that can be solved by putting a series resistor between the op amp output and the set pin (82K should be about right). 8V supply with your load limits you to 2.5A, but replace the 2.2 ohm load with something lower impedance to solve that limitation. This should get your control loop functioning, some further calibration may be needed.

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