PEX 8632-AA Quick Start Hardware Design Guide, Version 1.1
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© 2007 PLX Technology, Inc. All Rights Reserved.
VDD10A has a lower noise tolerance than the digital supplies. Therefore, VDD10A might require
additional filtering, depending upon the 1.0V ±5% power plane noise. The SerDes can tolerate ±5%
variance on the supply rails, due to noise and IR drop. VDD25 power is used for the single-ended I/O
buffers – Hot Plug, serial EEPROM, JTAG, I
2
C, and the Port Status indicators. Although power
consumption for this supply is relatively small, the output drivers have fast edge rates, and therefore,
require that adequate power de-coupling be provided, to supply transient current to the drivers. It is
preferred that VDD25 be implemented as a plane or partial plane, either on a signal layer or main power
plane layer. Provide 0.1 and/or 0.01 µF ceramic capacitors, along with one or more 10-µF tantalum
capacitors, to de-couple the VDD25 power balls. The number of capacitors required depends upon the
number of 2.5V I/O balls utilized in the design, and the existence or absence of an interplane capacitance
for the VDD25 rail.
VDD25A (and VSSA_PLL) are used to power the internal Reference Clock PLL. This ball might require
additional filtering circuitry, if the VDD25 plane is experiencing significant noise. VDD25A can tolerate
ripple from -100 to +100 mV, for frequencies above 10 MHz. If additional filtering circuitry is necessary,
a wide trace [0.254 to 0.381 mm (0.010 to 0.015 in.)] can be used to power this supply ball. Use a 0-ohm
resistor (0603 or 0805), in series with the main VDD25 supply, along with one or more 0.1 and/or 0.01 µF
capacitors after the resistor, near the ball. If the VDD25 plane couples significant noise into the VDD25A
supply, exchange the resistor for a ferrite bead, to aid in filtering the supply noise. In designs where
VDD25A ties directly to the VDD25 power plane, ensure that VDD25A has its own dedicated via to the
plane. Similarly, allow VSSA_PLL to have its own dedicated via to the main ground plane.
Note:
Placing ferrite beads in a power supply path is not a preferred method of filtering noise for supply
rails. Power supplies isolated through the use of ferrite beads typically have limited access to interplane
capacitance, which might have an adverse effect on a given supply rail.
1.9.2
Power Sequencing
There is no power sequencing requirement.
