TAN-042
Designing the XRT71D00 and the XRT73L00 Devices to
operate in the Host Mode, and to be accessed via a single Chip
Select pin.
Preliminary
July 19, 2001
Revision 1.03
22
This section presents some guidelines on how to layout and filter the VDD and GND
signals that are fed to the XRT73L00 and XRT71D00 devices.
The XRT73L00 and XRT71D00 “GND” Pins
Tie all XRT73L00 and XRT71D00 “GND” pins to the system ground plane. In cases
where there are separate analog and digital ground planes, tie all “GND” pins to the
analog ground. Do not insert any impedance (in the form of an inductor or ferrite bead)
between the analog and digital ground pins of the XRT73L00 and XRT71D00 devices.
The XRT73L00 and XRT71D00 “POWER” Pins
The XRT73L00 device contains five (5) power supply pins. Pin 10 is the Receive
Analog Section power supply pin. Pins 3 and 42 are the Transmit Analog Section power
supply pins. Finally, pins 26 and 29 are the digital power supply pins.
Likewise, the XRT71D00 device contains three (3) power supply pins. Pin 7 is the
Analog power supply pin. Pins 27 and 30 are the digital power supply pins.
The Receive Analog Power supply pin (e.g., pin 10) of the LIU is the most critical, as it
powers the Clock Recovery PLL. Therefore, the power to this pin should be as clean as
possible.
It is also desirable to keep the Transmit Power Supply noise isolated from the Receive
Power supply. Further, it is also desirable to keep the other components, on the board
isolated from each other, in order to minimize cross-talk.
The attached schematics (e.g., Figures 4 and 6) illustrate Exar’s recommendation on how
to connect the Analog and Digital VDD pins (of the XRT73L00 and the XRT71D00
devices) to a 3.3V power supply. Exar’s approach recommends the use of two-stage LC
filtering.
STAGE 1 – A LARGE LC FILTER (consisting of L1 and C5)
This particular LC filter consists of a 15uH inductor and a 33uF capacitor. The purpose
of this LC filter is to eliminate much of the “DC-to-DC Converter-induced” low
frequency ripple, within the 3.3V power supply line, prior to being routed to any of the
VDD pins of the XRT73L00 and the XRT71D00 devices.
NOTES ABOUT THIS LC FILTER:
1. This LC filter should be placed close to the output of the DC-to-DC Converter.
