ADuM54xx: recommendations on how to put 2 ADUM54xx in parallel?

Q 

For redundancy purposes we need some recommendations on how to put 2 ADUM54xx
in parallel?

 

A 

It turns out to be a little tricky to use these parts in a redundant parallel
configuration.  The main issues are:

1) the power supply sections should be diode ORed so that if the fault is on
the Viso side, it can not drag both units down.  This also prevents the units
from back driving each other.

2) The data outputs can not simply be connected together.  Assuming one device
is "on" and one is "off".  A high output on the active device would be
connected to an inactive output on the device that is off.  This will power
the isolated side of the "off" device through the ESD protection diodes. Once
the secondary side of the "off" device comes up, its outputs will default to
low.  But this output is already being pulled high by the output of the "on"
device  This will cause large currents to flow, probably 30mA on the output
and an additional 90mA on the VDD1 line, and the output will go to an
undetermined state.  Probably half of Viso.
The solution is again to diode OR the outputs together with pull down
resistors to be sure it goes low when required.  The speed of the output will
depend on the size of the pull down resistor, the smaller the value, the
faster the output will slew low, but the more current the output will require.