56
Tsi572 Hardware Manual
May 18, 2012
Integrated Device Technology
www.idt.com
A solid, low impedance plane must be provided for the SP_VDD 1.2V SerDes supply,
referenced to the VSS plane. This supply can be derived from the same power supply as VDD,
as long as a
Kelvin connection
is used. The preference however, is to use a separate power
supply.
The SP_AVDD 3.3V SerDes analogue supply also needs low impedance supply plane. This
supply voltage powers the RapidIO receivers and transmitters, and their associated PLLs.
Connect all of the SP_AVDD pins to this plane and decouple the plane directly to VSS. The
plane must be designed as a low impedance plane in order to minimize transmitter jitter and
maximize receiver sensitivity. Construction of this plane as a buried capacitance referenced to
VSS is suggested.
The REF_AVDD pins provide power to the S_CLK distribution circuits in the switch. The
voltage should be derived from the SP_VDD plane. One ferrite will suffice to isolate the
SP_VDD from the REF_AVDD. Two decoupling capacitors should be assigned to each pin.
Ti
p
The term
Kelvin connection
is used to describe a single point of contact so that
power from one power plane does not leak past the power supply pin into the other
power plane. The leakage can be prevented by the fact the output of a power supply
is a very low impedance point in order to be able to supply a large amount of
current. Because it is such a low impedance point, any noise presented to it by the
power plane is sent to ground.
A kelvin connection enables two power planes to be connected together at a single
point. Using this technique, the same power supply module can be used to provide
power to a noisy digital power plane (VDD), as well as a quiet analog power plane
(SP_VDD).
Figure 28: Kelvin Connection Example
VSS PLANE
VDD PLANE
SP_VDD PLANE
PWB TOP SIDE
Tsi57x
GND
GND
OUTPUT
OUTPUT
POL r
POL regula
egulator
or
Example of connection points described as a Kelvin Connection
VDD and SP_VDD planes are only connected to each other at the
POL regulator output pin.
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