
Draft 6/5/00
A-24
Application Information
Copyright 2000 by LSI Logic Corporation. All rights reserved.
A.15 Power Supply Decoupling
There are six V
DD
pins on the L80223 and six GND pins.
All the V
DD
pins should be connected together as closely as possible to
the device with a large V
DD
plane. If the V
DD
pins vary in potential by
even a small amount, noise and latchup can result. The V
DD
pins should
be kept to within 50 mV of each other.
All the GND pins should also be connected together as closely as
possible to the device with a large ground plane. If the GND pins vary in
potential by even a small amount, noise and latchup can result. The V
DD
pins should be kept to within 50 mV of each other.
A 0.01–0.1 Fdecoupling capacitor should be connected between each
V
DD
/GND set as closely as possible to the device pins, preferably within
0.5 inches. The value should be chosen based on whether the noise
from V
DD
-GND is high- or low-frequency. A conservative approach would
be to use two decoupling capacitors on each V
DD
/GND set, one 0.1 ffor
low-frequency and one 0.001 ffor high-frequency noise on the power
supply.
The V
DD
connection to the transmit transformer center tap shown in
Figure A.1
through
Figure A.3
has to be well decoupled to minimize
common mode noise injection from the supply into the twisted-pair cable.
It is recommended that a 0.01 Fdecoupling capacitor be placed between
the center tap V
DD
and the GND plane. This decoupling capacitor should
be physically placed as close as possible to the transformer center tap,
preferably within 0.5"
The PCB layout and power supply decoupling discussed above should
provide sufficient decoupling to achieve the following when measured at
the device:
The resultant AC noise voltage measured across each V
DD
/GND set
should be less than 100 mV p-p
All V
DD
pins should be within 50 mV p-p of each other
All GND pins should be within 50 mV p-p of each other.