ADuM5402 input behavior

Document created by analog-archivist Employee on Feb 23, 2016
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We use the ADUM5402 on many of boards to generate the isolated 3V3 for our POE

As part of that circuit a 3V3 voltage is generated through a FET into Pin 12 of
ADUM5402 to indicate when POE power is applied. The voltage comes out Pin 5 to
power an led through a 100R resistor.

It works fine, but i am seeing issues in production, that the LED comes on when
no POE is applied.

I think maybe some leakage current is going through the FET and voltage going
into Pin 12 of ADUM5402 is floating (or at least floating between a logic 1 and
logic 0)

I have looked at the datasheets and other documentation on the your website to
try and find out what to do with an input pin that is not connected or even

i.e. would a 100K pull down on Pin 12 be ok?


The inputs are very high impedance, they will float to midrange and give
unpredictable behavior. A pull down would probably be a good solution, 100kohm
seems reasonable.