GX3290 Datasheet, PDF (41/47 Pages) Semtech Corporation – 290 x 290 3.5Gb/s Crosspoint Switch

5. Application Information
Note: Please refer to the following supplementary documents:
• Crosspoint Design Guide
• EB-GX3290 Schematics, PCB Layout and Bill of Materials
5.1 Power Supply Filtering and Recommendations
One of the most important steps that PCB designers can take to ensure power supply
integrity for the GX3290 device is to design the PCB layer stack-up to minimize power
plane pair inductance. Locating supply planes adjacent to ground planes, and separated
by minimum thickness dielectrics in the stack-up, will minimize plane-pair inductance,
and incidentally maximize the plane pair capacitance. Holes and cuts in the planes
should be avoided as much as possible. While such closely spaced plane pairs allow the
lowest inductance connections to supply pins of the GX3290 when they are closest to
the device mounting surface of the PCB, the need to balance PCB stack-ups will lead to
closely spaced layers on the far side of the PCB.
The supply currents drawn from the VDD_18 and VDDIO_D supplies are noisy and
activity dependent, and the corresponding supply/ground plane pairs should be treated
as for FPGA or CPU devices. Supply currents drawn from the VCC_25_A, VCC_IN1,
VCC_IN2, VCC_OUT1 and VCC_OUT2 supplies are continuous except under changes of
the high speed signal path configuration. The VCC_OUT1 and VCC_OUT2 supplies in
particular are subject to rapid steps in current under some configuration changes: the
maximum combined current draw of the VCC_OUT1 and VCC_OUT2 supplies, 8.52A,
can be switched in as little as 10ns. This current step may be reduced by appropriate
programming of the device.
Locating point of use voltage regulators close to the GX3290 device on the PCB will
maximize the regulation roll-off frequency. At the highest frequencies, the GX3290
package and mounting parasitics will limit the effectiveness of any measures on the PCB
to suppress voltage ripple on the device supplies. In between the voltage regulator roll
off frequencies and the frequencies where parasitics on each supply domain within the
GX3290 isolate the die from the PCB, decoupling capacitors on the PCB are effective.
PCB layout effort should be spent on details of the decoupling capacitor mounting
layout. Some simple layout measures can help to reduce the inductance of capacitor
mounting.
See the Crosspoint Design Guide for more detailed recommendations.
GX3290 290 x 290 3.5Gb/s Crosspoint
Final Data Sheet Rev. 2
GENDOC-053317 March 2013
www.semtech.com
41 of 47
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