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4.4.9
82562GZ(GX) Layout Guidelines (Routing LAN 3.3 V When Using a
Copper Trace)
When 3.3 V for the LAN device is NOT supplied by a solid copper power plane, then it becomes
more important to make sure the power source will not carry system noise to the LAN device, and
to ensure that the power trace does not restrict current flow to the LAN device.
4.4.9.1
Avoid Restricting Current Flow
When a 3.3 V power trace is used to supply power to the LAN device, the power trace should be at
least 50 mils wide, to avoid restricting current flowing to the LAN device. On designs where there
is enough PCB space, a power trace that is 200 mils wide or wider, is better.
4.4.9.2
Power Trace Decoupling
If a power trace is more than an inch long, it should be decoupled at both the source and near the
destination.
Extremely long power traces should be decoupled every three to five inches. Placing a decoupling
cap on the power trace, once every few inches, greatly reduces the amount of noise that will reach
the LAN device. In-turn, this reduces radiated emissions, and improves IEEE PHY conformance.
Figure 11. Decoupled LAN Power Trace (Power Supplied by Trace)
Power rail between 30 - 50 mills wide
C
C
1. Big C >= 4.7 uF. Little c = 0.1 uF (or 0.01 uF).
2. Noise above 500 MHz might require a COG cap between 470 pF and 1000 pF.
3. To reduce noise in the LAN device and on the wire; If a power trace is more than an inch long, it should be
decoupled both at source and near destination. Also, a series Ferrite Bead near the destination can help.
4.4.9.3
Moat (a Space) Around LAN Power Trace, to Reduce Noise
From Adjacent Signals
To further reduce power supply noise, it is desirable to keep the LAN power trace or LAN power
fill separated from other signals and other voltage rails, by 30 mils, if possible. Separating power
traces and power-fills with 20 to 30 mil wide moats reduce signal and power noise from coupling
onto the LAN power trace or onto the LAN power-fill.
Application Note (AP-468)
82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
Pairs of caps are
4 inches apart
C
C
C
FB
C
C
C
Optional
Ferrite
Board
C
C
C
LAN/LOM IC
Power Rail
C
37
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