Datasheet

Data Sheet ADM1075
Rev. B | Page 27 of 52
be powered from the secondary side and can provide power
across the isolation barrier via the integrated dc-to-dc
converter. Therefore, the ADuM5404 can be used to power the
primary side of the ADuM1250 if both are used on the board.
Some extra care is required if using the ADuM5404 to power
the ADuM3200. If the power at the secondary side is enabled by
the ADM1075, the isoPower solution may not work. Because
isoPower is unpowered in this case, the ADuM3200 outputs are
in an undefined state. If the
SHDN
input comes from the
ADuM3200, it may be held low, and the ADM1075 never turns
on the FET or enables power at the secondary side.
isoPower uses high frequency switching elements to transfer
power through its transformer. Special precautions must be
taken during printed circuit board (PCB) layout to meet
emissions standards. See the AN-0971 Application Note for
board layout recommendations.
Powering the iCouplers from the secondary side is usually
straightforward because there is often a suitable voltage rail
available. However, there is not always a suitable voltage rail
available on the primary side (−48 V side). If the ADuM5404 is
not used on the system, the ADuM1250 can be powered on the
primary side in a number of different ways.
If a voltage rail is available on the primary side (3.3 V or 5 V
referenced to VEE), that can be used to power the chip directly.
Otherwise, the ADM1075 shunt voltage and/or the −48 V
supply can be regulated down to power the part. A simple
emitter follower circuit achieves this, as shown in Figure 54.
12V (SHUNT)
–48V RTN
–48V
–48V
5V AUX
20k
Ω
1kΩ
0.33W
1µF
2
0
k
6V
09312-148
Figure 54. Powering iCoupler from −48 V Supply