Silicon Laboratories SI2494/39 Module Design and Application Considerations, Motherboard Design

AN93

Table 34. Layout Checklist (Continued)

P # Layout Items Required

25Minimize the area of the loop formed from U2 pin 4 to R9 to U2 pin 15.

26Cathode marking for Z1.

27Pin 1 marking for U1 and U2.

28Space and mounting holes to accommodate for fire enclosure if necessary.

29IGND does not extend under C3, D1, FB1, FB2, R15, R16, C8, C9, or RV1.

30Size Q1, Q3, Q4, and Q5 collector pads to safely dissipate 0.5 W (see text).

31Submit layout to Silicon Laboratories for review.

4.4.2.Module Design and Application Considerations

Modem modules are more susceptible to radiated fields and ESD discharges than modems routed directly on the motherboard because the module ground plane is discontinuous and elevated from the motherboard’s ground plane. This separation also creates the possibility of loops that couple interfering signals to the modem. Moreover, a poor motherboard layout can degrade the ESD and EMI performance of a well-designed module.

4.4.2.1. Module Design

Particular attention should be paid to power-supply bypassing and reset-line filtering when designing a modem module. Trace routing is normally very short on modules since they are generally designed to be as small as possible. Care should be taken to use ground and power planes in the low-voltage circuitry whenever possible and to minimize the number of vias in the ground and power traces. Ground and power should each be connected to the motherboard through only one pin so as not to create loops. Bypassing and filtering components should be placed as close to the modem chip as possible with the shortest possible traces to a solid ground. It is recommended that a pi filter be placed in series with the module VCC pin with a filter such as the one shown in Figure 22 on the reset line. This filter also provides a proper power-on reset to the modem. Careful module design is critical since the module designer often has little control over the motherboard design and the environment in which the module will be used.

4.4.2.2. Motherboard Design

Motherboard design is critical to proper modem module performance and immunity to EMI and ESD events. First and foremost, good design and layout practices must be followed. Use ground and power planes whenever possible. Keep all traces short and direct. Use ground fill on the top and bottom layers. Use adequate power supply bypassing, and use special precautions with the power and reset lines to the modem module. Bypass VCC right at the modem module connector. Be sure the modem module is connected to VCC through a single pin. Likewise, be sure ground is connected to the modem module through one pin connected to the motherboard ground plane. The modem reset line is sensitive and must be kept very short and routed well away from any circuitry or components that could be subjected to an ESD event. Finally, mount the modem module as close to the motherboard as possible. Avoid high-profile sockets that increase the separation between the modem module and the motherboard.

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