lm4549b National Semiconductor Corporation, lm4549b Datasheet - Page 25

• Manufacturer Part Number: lm4549b
• Description: Ac ?97 Rev 2.1 Multi-channel Audio Codec With Sample Rate Conversion And National 3d Sound
• Manufacturer: National Semiconductor Corporation

Low Power Modes
Improving System Performance
The audio codec is capable of dynamic range performance
in excess of 90 db., but the user must pay careful attention to
several factors to achieve this. A primary consideration is
keeping analog and digital grounds separate, and connect-
ing them together in only one place. Some designers show
the connection as a zero ohm resistor, which allows naming
the nets separately. Although it is possible to use a two layer
board, it is recommended that a minimum of four layers be
used, with the two inside layers being analog ground and
digital ground. If EMI is a system consideration, then as
many as eight layers have been successfully used. The 12
and 25 MHz. clocks can have significant harmonic content
depending on the rise and fall times. Bypass capacitors
should be very close to the package. The analog VDD pins
should be supplied from a separate regulator to reduce
noise. By operating the digital portion on 3.3V instead of 5V,
an additional 0.5-0.7 db improvement can be obtained.
The bandgap reference and the anti-pop slow turn-on circuit
were improved in the LM4549B. A pullup resistor is not
required on V
resistor can be left on the pc board, but the temperature
coefficient will improve with no resistor on this pin. In addi-
tion, the THD will improve by 0.2–0.5 dB. The external
capacitor is charged by an internal current source, ramping
the voltage slowly. This results in slow turn-on of the audio
stages, eliminating “pops and clicks”. Thus, turn-on perfor-
mance is also improved. The pullup resistor, in conjunction
with the internal impedance and the external capacitor, form
a frequency dependent divider from the analog supply. Noise
on the analog supply will be coupled into the audio path, with
approximately 30 dB. of attenuation. Although this is not a
large amount if the noise on the supply is tens of millivolts, it
will prevent SNR from exceeding 80 dB.
In Figure 1 and Figure 2, the input coupling capacitors are
shown as 1 µF capacitors. This is only necessary for extend-
ing the response down to 20 Hz. for music applications. For
telematics or voice applications, thelower 3 dB. point can be
much higher. Using a guaranteed input resistance of 10 kΩ,
(40 kΩ typical), a 0.1 µF capacitor may be used. The lower 3
REF
, pin 27. For an existing design, the 10 kΩ
(Continued)
FIGURE 8. AC Link Powerdown Timing
25
dB point will still be below 300 Hz. By using a smaller
capacitor, the package size may be reduced, leading to a
lower system cost.
Backwards Compatibility
The LM4549B is improved compared with the LM4549A. If it
is required to build a board that will use either part, a 10 kΩ
resistor must be added from the V
the LM4549A. It is not required for the LM4549B. Addition of
this resistor will slightly increase the temperature coefficient
of the internal bandgap reference and decrease the THD
performance, but overall performance will still be better than
the LM4549A. The LM4549A requires that pins 1 and 9
(DV
the 3.3 Volt digital supply and the bypass capacitors. The
inductor is not required for the LM4549B and should not be
used.
Multiple Codecs
EXTENDED AC LINK
Up to four codecs can be supported on the extended AC
Link. These multiple codec implementations should run off a
common BIT_CLK generated by the Primary Codec. All
codecs share the AC ’97 Digital Controller output signals,
SYNC, SDATA_OUT, and RESET#. Each codec, however,
supplies its own SDATA_IN signal back to the controller, with
the result that the controller requires one dedicated input pin
per codec. (Figure 9).
By definition there can be one Primary Codec and up to
three Secondary Codecs on an extended AC Link. The
Primary Codec has a Codec Identity = (ID1, ID0) = ID = 00
while Secondary Codecs may have identities equal to 01, 10
or 11. The Codec Identity is used as a chip select function.
This allows the Command and Status registers in any of the
codecs to be individually addressed although the access
mechanism for Secondary Codecs differs slightly from that
for a Primary.
The Identity control pins, ID1#, ID0# (pins 46 and 45) are
internally pulled up to DV
DD
) connect directly to a 27 nH. inductor before going to
DD
. The Codec may therefore be
REF
pin (pin 27) to AV
20123509
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DD
for