Could you tell me,how to reduced this IC(ADP2381) EMI? thanks!
Most importantly for EMI reduction is to use a shielded inductor. Some inductors a more shielded then others and some things that manufacturers call shielded are simply drum cores that they painted some ferrite goo onto and called shielded. Parts like the XAL series from Coilcraft are good since they are just a winding dipped in magnetic material so you have a continuous flux path with no air gaps. There are plenty of other choices.
The next most important thing is good layout with multiple layers of continuous ground plane. A ground plane provides a magnetic "short circuit" for any flux flowing through the board. Please check out the pictures of the layout found in the datasheet and/or the ADISimpower design tool. Pay special attention to the placement of the input capacitance.
The source of much of the high frequency noise (10's of MHz and beyond) is from the switch node. This can be damped using a snubber. A detailed design technique is described in the attached document. In addition the rise time of the switching pulse can be slowed down by placing a resistor (10 ohms is a good start) in series with the Cbst. This will reduce efficiency somewhat, but probably less then an effective snubber would.
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