Appendix D Micro SMD Wafer
Level Chip Scale Package, PCB,
Layout, and Mounting
Considerations
Refer to Application Notes ( AN-1112) for more information
on Micro SMD Wafer Level Chip Scale Package.
Since National Semiconductor is constantly pursuing the
best package performance possible, please refer to the fol-
lowing web page for possible updates to the µSMD package
information
presented
in
Appendix
D:
<
http://
www.national.com/an/AN/AN-1112.pdf.
Appendix E Maximizing OCL Mode
Channel-to-Channel Separation
The OCL mode AC ground return (CNT_GND pin) is shared
by both amplifiers. As such, any resistance between the
CNT_GND pin and the load will create a voltage divider with
respect to the load resistance. In a typical circuit, the amount
of CNT_GND resistance can be very small, but still signifi-
cant. It is significant because of the relatively low load im-
pedances for which the LM4985 was designed to drive: 16
Ω
to 32
Ω
. The ratio of this voltage divider will determine the
magnitude of any residual signal present at the CNT_GND
pin. It is this residual signal that leads to channel-to-channel
separation (crosstalk) degradation.
For example, for a 60dB channel-to-channel separation
while driving a 16
Ω
load, the resistance between the
LM4985’s CNT_GND pin and the load must be less than
16m
Ω
. This is achieved by ensuring that the trace that
connects the CNT_GND pin to the headphone jack sleeve
should be as short and massive as possible, given the
physical constraints of any specific printed circuit board lay-
out and design.
AN-1452
www.national.com
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