I am currently designing a product which will use the ADUM2250 to isolate across Live 230Vac mains and will need to comply with EN60065 as a Class1 product
I am trying to asses what I need to do about the safety limiting vaues and it seems confusing, starting with the values
1. The datasheet table6 says "supply current, IDD1 + IDD2, 550mA" but the graph plot says Idd1 only.
2. Your VDE document (VDE-0884-10-2019-3-19.pdf) says Max input current 265mA, and max output power dissipation 900mW.
Values aside, it seems to me that I have to put a resistor in series with the VDD1 and VDD2 pins to limit the current. (with a suitable bypass capacitor of course)
Lets say that I think I need 100R and the Vdd is 5V. That means I may have onltl 4.5V on the IC. I know that it works fine to 3V.
However, the pull-ups may be pulling to 5V. What are the consequences of pulling the I2C pins higher than the Vdd?
Thanks
Alex
Hello Alex,
The ADuM225x is a very old part and the understanding of safety limiting values has evolved over the years.
The safety limiting values are all about temperature rise in the device. Our isolators can be treated as a single chip internally from a power dissipation point of view. We have a Thermal constant Theta JA for the package and combined with a power dissipation will allow a temperature rise to be calculated. We have until now used 150degC as a the junction temperature of our devices and hence the maximum power dissipation can be calculated. In reality the max temperature should be related to the insulation not the silicon, and that max junction temperature is 300C where the mold compounds starts to deteriorate. So the calculations in the data sheet are overly conservative. We are changing this approach when we switch to the VDE-0884-11 standard.
The total power dissipation has to be kept under 2.755W at room temperature (under the way we calculate it now for 150C max junction). The graph should be labeled Idd1+Idd2. I would limit each side to half off that value (which is where the VDE certification document limit came from) with a series resistor or a current limit in the power supply.
The ADuM2250 does not have an ESD diode between the VDD and the open collector outputs, so it should not back power the part if the VDD drops below the pull-up voltage. the pull-up should limit any power by that path in any event.
Let me know if you have any other question.
MSCantrell