3.3V AND 5V supply to Digital Isolator / Voltage Translator - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: June 29, 2019, 01:34:54 pm
content: 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 etc4) 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?

author: magic
author_rank: Super Contributor
timestamp: June 29, 2019, 03:39:38 pm
content: 5) use a generic LDO to get 3.3V from 5V that already exists and switch power to the slave by whatever meansIf 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.

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2019, 04:14:04 pm
content: 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.

author: max_torque
author_rank: Super Contributor
timestamp: June 29, 2019, 06:33:37 pm
content: 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?

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2019, 07:18:55 pm
content: 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.

author: DBecker
author_rank: Frequent Contributor
timestamp: June 30, 2019, 01:38:43 am
content: You might want to step back and describe the actual problem.Why are you using one of the ADUM chips that transfers power? It's primary advantage is that it transfers a tiny bit of power to the isolated side without additional components. If you aren't using that capability, there are faster, cheaper, more reliable solutions.I've designed several boards that use a commodity DC-DC converter, a Si8421 two channel isolater (or one of the larger parts with more channels) and a standard CAN transceiver. It's an approach that has proven to be simple and uneventful. I've tried to use the sexy ADUM parts, there is always something that don't quite fit: interface levels, not enough power out, too much heat generated, speed, etc.

author: max_torque
author_rank: Super Contributor
timestamp: June 30, 2019, 10:43:46 am
content: er, i'm not using the Digital isolators that have the built in DC/DCs (That's the "5" series parts)https://www.analog.com/media/en/technical-documentation/data-sheets/ADuM1200_1201.pdf^^ two channel, without DC/DChttps://www.analog.com/media/en/technical-documentation/data-sheets/ADuM5200_5201_5202.pdf^^ two channel with DC/DCI'm using a separate DC/DC because under some circumstances the interface actually powers the slave (for things like desktop programming etc) so i need much more power than the small DC/DC built into the isolator can provide. I'm therefore using the basic two channel device, that needs to take a small amount of external power to operate, and making use of the fact that the isolated side can be powered from two different potentials so as to achieve the ability to interface with both 5v and 3.3v systems!

author: Someone
author_rank: Super Contributor
timestamp: June 30, 2019, 11:17:52 am
content: Quote from: max_torque on June 29, 2019, 06:33:37 pmIf you wish to simplify further most 3.3V regulators can be bypassed with just a jumper/short/switch across their input-output pins. Recall all those application notes about reverse protection diodes? Single quadrant devices (most regulators) will happily sit there and output no current into a voltage above their set point and below the input voltage. The generic LD/LT/REG/LM/TLV/ZLDO 1117 regulators are ok with this, as are many other simple/cheap designs.

author: Ian.M
author_rank: Super Contributor
timestamp: June 30, 2019, 11:23:50 am
content: 4.8V - 4.9V is probably good enough for interfacing to 5V logic.Take an adjustable LDO with as low a dropout voltage as possible. *DON'T* use a 3 pin one - it needs to have a Gnd pin Supply it from Iso+5V and select the feedback potential divider for 3.3V out. Grround its Adj (feedback) pin using a switch or optocoupler to force its pass transistor hard on to get nearly 5V out (less its dropout voltage).If you want to do better, as 'Someone' has just pointed out, you could use a LDO that will tolerate up to Vin on its output, and put a P-MOSFET in parallel with it.

author: max_torque
author_rank: Super Contributor
timestamp: June 30, 2019, 04:16:47 pm
content: OK, possible change of plan.... I now have a micro on the isolated side (to deal with some other functionality) so it makes some sense to just use that ti run a basic buck converter. Most of the time the pass device will just be hard on (ie 5Vin -> 5Vout) but i can use a "spare" Pwm pin on the micro to adjust the duty cycle to get around 3.3V out (as mentioned, in this application, the exact voltage isn't that important it just needs to be low enough to enable the 3.3V TX line from the slave device to trip the "High" logic threshold). Means no messing around with switches or multiple regulators etc!