ADUM2250 power on sequency & voltage

Hello,

   The sequence of the two 3.3V supplies is basically OK.  Since you didn't mention timing, remember to follow the startup criteria in the datasheet of both supplies being at least 2.5V and allowing at least 40us to elapse after both supplies have exceeded the internal 2.0V start up threshold.   

  Ignoring the bus supply voltage for the moment, having this 3rd supply for turn on afterwards should be OK if the application will tolerate the I2C bus lines SCL/SDA to both be low during the interval when the ADuM2250 has become active and the bus supply is below approximately 0.7V.  Depending on the timing, this scenario may not occur.

  Your reading of the datasheet is correct in that if the side 1 bus pin is above 700mV the ADuM2250 will identify this as a high input as long as the ADuM2250 sees the corresponding side 2 pin as also being high (the side 2 pin is above 0.3*VDD2).    If the ADuM2250 sees a low on side 2, then it will pull the side 1 bus pin to its output low voltage of approx 900mV. 

  Coming back to the question about the bus supply voltage of 1.8V, depending on the values of VIL for the other parts on this bus, you may have issues with them not recognizing the output low from the ADuM2250.  Normally, when the ADuM2250's side 1 VOL is greater than the other bus device's VIL, we'd suggest flipping the isolator around so that ADuM2250's side 2 is connected to the bus side with the low VIL devices.  Unfortunately, that won't work in your scenario since the side 2 input thresholds are scaled to their VDD and so would be greater than the 1.8V supply. 

  Our newest I2C isolator, the ADuM1252, supports 1.8V supply / bus voltage levels and could be a good solution.   At the present time, its only available in a narrow body package; however, a wide body package with similar isolation ratings to the ADuM2250 will be available very soon.  Note that this wide body package is an 8 pin package and so it will be 1/2 the size.

Eric

Thank you~