Cirrus Logic AN Integrating CobraNet into Audio Products - Manual (Page 6)

Integrating CobraNet Circuitry into PCBs P RELIMINARY
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CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004
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Integrating CobraNet Circuitry into PCBs
There are several advantages to integrating the CobraNet circuitry into your own PCB
rather than using one of the CobraNet modules. Signal quality, higher integration, and
lower cost are just some of them.
Integrating the CM-1 Circuit
The CM-1 is a challenging circuit to integrate. We do not recommend integrating this
circuit unless it is absolutely necessary. If design requirements necessitate integration, it
should be done with a great deal of care. It is not impossible to integrate, just very difficult
given the level of technology used. If integration of the CM-1 circuit is necessary,
contacting Cirrus Logic, Inc. support is highly recommended.
Integrating the CM-2 Circuit (using the CS18101)
Unlike the CM-1, the CM-2 (and thus, the CS18101) is very integration friendly. The circuit
is much simpler and has fewer high-speed signals. It most cases it makes more sense to
integrate the CM-2 than to use the module.
What follows are some design notes on integrating the CM-2 circuit into a PCB.
The CM-2 has a voltage regulator that converts +3.3 volts to +1.8 volts. The regulator on
the CM-2 is a high-efficiency switching regulator. This regulator is more expensive than a
basic low-dropout linear regulator, but dissipates less power (0.1 watts vs. 0.75 watts). In
a PCB design, a linear regulator can easily be used if cost is more important that power
consumption. Also, this voltage regulator can be omitted if a +1.8v power rail already
exists in the design.
The CM-2 contains a second Ethernet controller and connector. If these are not required
they can simply be removed from the design. In this case, all devices and circuits
referenced on the "Secondary Ethernet" schematic page can be omitted.
It is not recommended to replace the 10uF ceramic capacitors with tantalum or aluminum
electrolytic. The ceramic caps have superior performance and life span, as well as
smaller size than either tantalum or aluminum electrolytic capacitors. It's also important to
make sure that any voltage regulator used will work properly with these ceramic caps.
If power will not be supplied via the Ethernet cable then consider using a different
Ethernet connector that includes the termination resistors and caps in the connector. This
should lower EMI, reduce parts count, and reduce PCB size.
The CM-2 is designed with termination on only a few of the signals. This is possible
because the PCB layout keeps the signal traces short. If the design requires longer
traces, selectively terminating some of the signals may be required.
The CM-2 uses capacitor arrays rather than individual capacitors. This works well when
the caps are placed on the bottom of the PCB, below the ICs. When the caps are placed