I'm building an isolated data interface, that needs to work with 5v and 3.3v slave systems. I'm using an OTS isolating voltage translator (Adum1201) that acts as the isolation barrier, and the voltage translation in one unit. One further complication is that i already have a 5v isolated DC/DC providing that voltage on the isolated side because the interface also includes an isolated CAN transceiver that requires 5v.
So, options as far as i can tell (in no particular order)
1) Swap to a higher output voltage for the DC|DC (say 7V), and fit two secondary fixed output linear regulators, 1 x 5v and 1 x 3.3v, and somehow swap between those to feed the Adum (using an analogue switch etc?) with the required supply voltage?
2) Swap to a higher output voltage DCDC, and fit one fixed 5v linear reg (for the CAN Txcvr) and one variable one for the Adum. A simple selectable resistive divider on the voltage setting pin of the LDO (via an optocoupler for example) would enable the output of that variable reg to be changed between voltages.
3) Use a "high current" low output impedance OP amp to create a selectable voltage rail from the existing 5v rail (could be an issue with stablity as their could be a fair amount of parasitic capacitance due to cable lengths etc
4) Use something like a diode chain or zener to drop the 5v to 3.3v, but bypass that chain with a mosfet that switches on to bypass?
The power requirement is a bit unknown. The Adum requires around 5mA, and the load of the slave could probably be anywhere from 1mA to 10mA, and the cable capacitance is likely to be the biggest issue, as cable lengths could be around 1 to 3m. Currently i am driving the slave via current limiting resistors that are capacitively bypassed to provide nice square edges to the (digital) signals. Ideally i'd have a nice wad of capacitance on the Adum's supply rail to help it meet those fast edge current requirements (say a few uF to 10uF at a guess)
Option 4 is nice and simple, but i wonder if i can get it to work across a range of output power drains?
Anyone done anything similar?