Echelon FT 3150 Smart Transceiver - Manual (Page 107)

FT 3120 / FT 3150 Smart Transceiver Data Book 101
ESD Design Issues
In summary, the following general rules and guidelines apply:
• The faster the FT Smart Transceiver clock speed, the higher the level of EMI.
• Better V
CC
decoupling quiets RF noise at the sources (the digital ICs), which lowers radiated EMI.
• The FT 3120 Smart Transceiver will generate less EMI than the FT 3150 Smart Transceiver because the
FT 3120 Smart Transceiver has no external memory interface lines.
• A four-layer PCB will generate less EMI than a two-layer PCB since the extra layers facilitate better V
CC
decoupling and more effective logic ground guarding.
• A two-layer PCB FT Smart Transceiver based-device should be able to meet FCC/CISPR level “B” EMC
if good decoupling and ground guarding are used.
• Ferrite beads in series with the network traces at the network connector, and ferrite chokes in series with
the power input traces at the power connector, can be used to help meet EMC requirements for devices
that have noisy application circuitry or special circuit requirements.
Early testing of prototype circuits at an outdoor EMI range should be used to determine the effectiveness of these
EMC techniques in a particular application.
ESD Design Issues
Electrostatic discharge (ESD) is encountered frequently in industrial and commercial use of electronic systems. In
addition, the European Community has adopted requirements for ESD testing.
Reliable system designs must consider the effects of ESD and take steps to protect sensitive components. Static
discharges occur frequently in low-humidity environments when operators touch electronic equipment. Keyboards,
connectors, and enclosures may provide paths for static discharges to reach ESD sensitive components such as the
FT Smart Transceiver. This section describes the issues involved with designing ESD immunity into FT Smart
Transceiver-based products.
There are two general methods that are used to ESD harden products. The first is to seal the product to prevent
static discharges from reaching the sensitive circuits inside the package. The second method involves designing
the grounding of a product so that ESD hits to user-accessible metal parts can be shunted around any sensitive
circuitry.
Since the network connector is user-accessible, it is not possible to seal FT Smart Transceiver based-devices
completely. However, the product's package should be designed to minimize the possibility of ESD hits arcing into
the device's circuit board. If the product's package is made of plastic, then the PCB should be supported in the
package so that unprotected circuitry on the PCB is not adjacent to any seams in the package. The PCB should not
touch the plastic of an enclosure near a seam, since a static discharge can creep along the surface of the plastic,
through the seam, and arc onto the PCB.
Once an ESD hit has arced to the product, the current from the discharge will flow through all possible paths back
to earth ground. The grounding of the PCB and the protection of user-accessible circuitry must allow these ESD
return currents to flow back to earth ground without disrupting normal circuit operation of the FT Smart
Transceiver or other device circuitry. Generally, this means that the ESD currents should be shunted to the center of
a star ground configuration (see Figure 4.4) and then out to the product's chassis or earth ground connection. If the
device is floating with respect to earth ground, the ESD current will return capacitively to earth via the network
wire, the power supply wires, and the PCB ground plane.
Designers of FT Smart Transceiver-based devices should follow the PCB layout guidelines presented earlier in this
chapter. In addition, external clamping of user-accessible circuitry is required to shunt ESD currents from that
circuitry into the center of the star ground on the PCB. For example, if the FT Smart Transceiver is scanning a
keypad with some of its I/O lines, then the I/O lines to that keypad will need to be diode-clamped as shown in
Figure 4.7. If a negative ESD hit discharges into the keypad, then the diode clamps to ground shunt the ESD current