How do you keep the Safety Rating Power below limits in an ADuM240x circuit under possible fault conditions ? What are Analog's recommended circuit guidelines for this ?
This is an interesting question, and it depends on what your system expects from our device. The fault condition that will effect our part is if the voltage rises above the absolute max. In that case the ADuM240x can go into latch-up. This will dissipate a lot of power on our device if there is an unrestricted supply. With enough power something will melt. If you expect the part to retain functionality, then the goal is to keep the power dissipation such that the junction temperature stays below 150C. If power is reset the part should come back.
If your goal is only to retain safety isolation, then the temperature of the die will have to rise above the decomposition temperature of the polyimide insulation, which is conservatively 300C.
You can figure out the power dissipation based allowed based on the Theta JA of the package, and figure the power allowed for each case above. Compare that to the available power from the supply. If you need additional limiting, you can add resistance to the VDD and data I/O lines.
We are working on a more formal recommendation, but this should get you started.
As a follow up question; the ADuM240x quotes a Junction to Case thermal resistance, but the ADuM225x gives a Junction to Ambient thermal resistance. I believe I need the Junction to Ambient and the Junction to Case for both the ADuM240x and ADuM225x to calculate the correct safe power dissipation ?
The supplemental question is if we have a time delay before the safety measure kicks in (e.g. a fuse) what is the maximum time the ADuM devices can operate in the fault condition ? I think we need to know the effective thermal inertia of the package ? For example if the fuse take tens on milliseconds to open, what is the maximum power that can be dissipated by the ADuM before this happens ? That is, it is not a continuous power dissipation issue.
Hello again, I think I need more data to be able to design a suitable protection circuit.
Firstly, the various isolator data sheets quote a mixture of Junction to Ambient (AduM2250) and Junction to Case (ADuM2400) thermal resistances. Then the safety current curve is plotted against case temperature and not ambient. As the case temperature is going to rise with power dissipation and I don't know the relationship between device current and power, how do I calculate the case temperature ?
I am currently thinking that a Murata posistor (PRG15BC330) in series with the signal pins (in fact two in series to cover the 'sleeping fault' scenario) with the power pin of the isolator zener clamped with two parallel 5.6V zeners should limit the power dissipation inside the isolator to the current allowed by the posistors and the voltage dropped across the input protection circuit of the isolator.
Therefore the additional question is will the input protection circuit pass the fault current out to the supply pin and limit the internal voltage drop to a known voltage ? Which should then allow me to calculate the worse case fault power dissipation from the maximum current that the posistors will allow into the isolator.
Retrieving data ...