How should I connect the XPort shield tabs in my board design? Should they be connected to frame ground, signal ground or left floating? What are the consequences if I don't have room for a 1-inch square heat sink on my board?

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The XPort shield is considered “chassis ground” and should separate from “signal ground”. Any ESD getting near the XPort at the panel opening will likely jump to the XPort’s shield.

We recommend using high voltage (~200V) low ESR capacitors to connect chassis ground to both signal ground and 3.3V. This will cause any voltage spike from ESD to be imparted equally to both signal ground and 3.3V with no net voltage increase between 3.3V and signal ground.

For the highest level of ESD protection of the XPort, it is recommended that the shield not be directly connected to signal GND. The metal shield fingers around the XPort’s RJ45 should physically contact the product housing when the housing is metal, or metallic coated.

The shield is also a heatsink for the EX chip. As in all heatsinking applications, the more copper connected to the heatsink the better. 1 inch square inch of copper is adequate to allow the XPort to work up to +85°C with at least 10°C of safety margin.

Having less that one square inch of copper in a board for the shield to will eliminate most of the safety margin above +85°C. If the application does not expect to see temperatures up to +85°C the heat sink may be smaller than 1 square inch and the ESD dissipation should be the major design consideration.

Remember, the more copper the better – use wider traces, etc.

If you are planning to run an XPort-03 in high performance mode to allow 920k baud rates a heat sink of at least 1 square inch of copper is highly recommended. This mode lowers the maximum operating temperature from +85°C to +75°C (with a 1 inch square heat sink) because the EX chip generates more heat when running at 88MHz than at 48MHz.

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[Originally Published On: 10/04/2005 10:00 AM]