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
23
2. Only one such LC filter is needed, per board (even in multi-channel applications).
However, multi-channel designs may still have to use multiple instantiations of
this LC filter due to large voltage drops across the inductor (due to the increased
amount of current draw of multiple XRT73L00 and XRT71D00 devices and the
dc resistance of the inductor), or because of the maximum operating current limit
of this inductor as well.
3. This LC filter may not be necessary if the 3.3V Power Supply is already filtered
elsewhere in the system.
COMPONENT SELECTION FOR L1 AND C5
The XRT73L00 device draws approximately 140mA and the XRT71D00 device draws
approximately 30mA. Therefore, this combined solution draws about 170mA. We
recommend that the user select a High-Current Inductor that has a small enough DC
resistance such that the voltage drop across the inductor will not exceed 50mV.
Therefore, the user should select a 15uH inductor that has a DC resistance of less than
0.294
Ω
.
An example of an acceptable inductor for L1 is the 4922-15L from API-Delevan. This
particular inductor has a maximum dc resistance of 0.089
Ω
, and has a maximum current
rating of 2.11A. Contact information for API-Delevan is presented in Appendix D, at the
end of this Applications Note.
The capacitor, C5, should be a 33uF 10V Tantalum capacitor, which is supplied by
various manufacturers. Digikey PCT2336CT-ND or equivalent would be acceptable.
STAGE 2 – SMALLER LC FILTERING FOR RECEIVE AND TRANSMIT
ANALOG VDD PINS
After the Power Supply signal passes through the large LC filter (consisting of L1 and
C5), it should then be routed to three different points, in parallel (in order to support both
the XRT73L00 and the XRT71D00 device).
•
Directly to the Digital VDD pins of the XRT73L00 and the XRT71D00 devices.
•
To an LC filter (consisting of L3 and C4), prior to being routed to the Receive Analog
VDD pin of the XRT73L00 LIU IC (pin 10) and the Analog VDD pin of the XRT71D00
Jitter Attenuator IC (pin 7).
•
To an LC filter (consisting of L2 and C3) prior to being routed to the Transmit Analog
VDD pins of the XRT73L00 LIU IC (pins 3 and 42).
The purpose of this LC filter is three-fold.
