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3.3V AND 5V supply to Digital Isolator / Voltage Translator
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max_torque:
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?
magic:
5) use a generic LDO to get 3.3V from 5V that already exists and switch power to the slave by whatever means :-//

If you are worried about capacitance playing tricks on the LDO, there are LDOs rated for use with 1µF MLCCs, a cable probably won't be worse.
David Hess:
It is pretty easy to switch a variable regulator between two output voltages with a ground referred transistor at the bottom of the resistive divider.  Or an analog switch can select between two points on a divider and only see the feedback pin current.

This avoids the problem of driving a difficult load since a regulator will be intended for that but an operational amplifier driving an output pass transistor can do the same thing with some care but requires a reference.

A home made inverter can use primary side only regulation to generate switchable output voltages but it seems you already have an isolated supply.
max_torque:
The one regulator does two voltages trick would be perfect except i always need 5v for the CAN txcvr, and i would need around 6 or 7 volts for an LDO to output 5v (enough headroom on the input voltage)

It's looking like using some sort of analogue switch to select either the raw 5v, or 3.3v from an LDO running off that 5v is the way to go!



I have been wondering if i can actually find a single voltage that satisfies both target voltages?  High is usually around 0.7 x Vcc and low around 0.3 x Vcc, so with some series resistors and some steering diodes to prevent / clamp excessive current flow, perhaps around 4.2 volts might actually work for both?
David Hess:
So the first regulator operates on 7 volts and provides 5 volts and the second regulator operates on 7 volts and provides 5 or 3 volts depending on the selection.  The second regulator could be an operational amplifier using the 5 volt regulator's output as a reference.

Or do the inverter thing but that is not as straightforward.
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