I am dealing with an old circuit that used a curious form of voltage regulator control. It relied on seven resistors that it would control individually, and the parallel combination of this resistance set the output of a voltage regulator. This was done using a L200, which is no longer obtainable, and was easily done because the "lower" feedback resistors could be switched in very easily. This can also be done with a LM317, however, one has to make the "upper" feedback resistor the parallel combination. A spice sketch shows the basic idea: Here, R4 sets the minimum output voltage. Then, one can increase the output voltage for each resistor that is added in parallel to R4. In this application, the feedback array consists of seven resistors, ranging in values from 55 ohms to 3k. The power supply runs at 36 V and logic supply is at 5 V. The resistor combination is updated once per second, so no high speed switching here.
I've learned that this is a sensitive part of the circuit, meaning the resistance and voltages can't be altered much before accuracy is lost. BJTs won't work because the base currents will alter the resistor voltages. FETs would be better but my circuit sketches have resulted in instability if the gate drive tries to relate to VOUT or VADJ. Analog switches with low Rds(on) that can go into a 36V application are hard to find and expensive. Regular mechanical relays could work as well but may run into physical size constraints, and maybe life span issues as well. Optocouplers seem to have trouble with getting a low enough "closed" impedance. Or maybe one of the above options works and I haven't found the right part or circuit yet, which brings me to the question:
My question for the group is if anyone could please recommend a control scheme for each parallel resistor that would cause less than one ohm of impedance across the switching element when engaged, and an effective open circuit when disengaged? The best answer would use commonly found discretes.