lm4832 National Semiconductor Corporation, lm4832 Datasheet - Page 15

• Manufacturer Part Number: lm4832
• Description: Digitally Controlled Tone And Volume Circuit With Stereo Audio Power Amplifier, Microphone Preamp Stage And National 3d Sound
• Manufacturer: National Semiconductor Corporation

Application Information
GROUNDING
In order to achieve the best possible performance, certain
grounding techniques should be followed. All input reference
grounds should be tied with their respective source grounds
and brought back to the power supply ground separately
from the output load ground returns. These input grounds
should also be tied in with the half-supply bypass ground.
Bringing the ground returns for the output loads back to the
supply separately will keep large signal currents from inter-
fering with the stable AC input ground references.
LAYOUT
As stated in the Grounding section, placement of ground
return lines is critical for maintaining the highest level of
system performance. It is not only important to route the
correct ground return lines together, but also important to be
aware of where those ground return lines are routed in
conjunction with each other. The output load ground returns
should be physically located as far as reasonably possible
from low signal level lines and their ground return lines.
Critical signal lines are those relating to the microphone
amplifier section, since these lines generally work at very low
signal levels.
SUPPLY BYPASSING
As with all op amps and power op amps, the LM4832 re-
quires the supplies to be bypassed to avoid oscillation. To
avoid high frequency instabilities, a 0.1 µF metallized-film or
ceramic capacitor should be used to bypass the supplies as
close to the chip as possible. For low frequency consider-
ations, a 10 µF or greater tantalum or electrolytic capacitor
should be paralleled with the high frequency bypass capaci-
tor.
If power supply bypass capacitors are not sufficiently large,
the current in the power supply leads, which is a rectified
version of the output current, may be fed back into internal
circuitry. This internal feedback signal can cause high fre-
quency distortion and oscillation.
If power supply lines to the chip are long, larger bypass
capacitors could be required. Long power supply leads have
inductance and resistance associated with them, that could
prevent peak low frequency current demands from being
met. The extra bypass capacitance will reduce the peak
current requirements from the power supply lines.
POWER-UP STATUS
On power-up or after a hard reset, the LM4832 registers will
be initialized with the default values listed in the truth tables.
By default, the LM4832 power and microphone outputs are
muted, the tone controls are all flat, National 3D Enhance is
off, the chip is in stereo mode, and the microphone input 1 is
selected with +30 dB of gain.
CLICK AND POP CIRCUITRY
The LM4832 contains circuitry to minimize turn-on transients
or “click and pops”. In this case, turn-on refers to either
power supply turn-on or the device coming out of shutdown
mode. When the deviceis turning on, the amplifiers are
internally configured as unity gain buffers. An internal current
source charges the bypass capacitor on the bypass pin.
Both the inputs and outputs ideally track the voltage at the
bypass pin. The device will remain in buffer mode until the
15
bypass pin has reached its half supply voltage, 1/2 V
soon as the bypass node is stable, the device will become
fully operational.
Although the bypass pin current source cannot be modified,
the size of the bypass capacitor, C
the device turn-on time and the amount of “click and pop”. By
increasing C
However, the trade-off for using a larger bypass capacitor is
an increase in the turn-on time for the device. Reducing C
will decrease turn-on time and increase “click and pop”. If C
is too small, the LM4832 can develop a low-frequency oscil-
lation (“motorboat”) when used at high gains.
There is a linear relationship between the size of C
turn-on time. Some typical turn-on times for different values
of C
In order to eliminate “click and pop”, all capacitors must be
discharged before turn-on. Rapid on/off switching of the
device or shutdown function may cause the “click and pop”
circuitry to not operate fully, resulting in increased “click and
pop” noise. The output coupling cap, C
concern. This capacitor discharges through an internal 20
kΩ resistor. Depending on the size of C
can be quite large. To reduce transients, an external 1 kΩ–5
kΩ resistor can be placed in parallel with the internal 20 kΩ
resistor. The tradeoff for using this resistor is an increase in
quiescent current.
COUPLING CAPACITORS
Because the LM4832 is a single supply circuit, all audio
signals must be capacitor coupled to the chip to remove the
2.5 V
so the AC-coupling capacitor will create a high-pass filter
with
The amplifier outputs also need to be AC-coupled to the
loads.The high-pass filter is comprised of the output load
and the coupling capacitor,where the filter cutoff is at
POWER AMPLIFIER
The power amplifiers in the LM4832 are designed to drive
8Ω or 32Ω loads at 200 mW (continuous) and 75 mW
(continuous), respectively, with 1% THD+N. As shown in the
Typical Performance Characteristics, the power amplifiers
typically drive 4Ω loads at 350 mW, but with a slight increase
in high-frequency THD. As discussed above, these outputs
should be AC-coupled to the output load.
MICROPHONE AMPLIFIER
The microphone preamplifier is intended to amplify low-level
signals for signal conditioning. The microphone inputs can
be directly connected to microphone networks. The micro-
phone amplifier has enough output capability to drive a 1 kΩ
load. All microphone inputs and outputs must be AC-
coupled.
B
DC
are:
bias. All audio inputs have 20 kΩ input impedances,
B
, the amount of turn-on pop can be reduced.
f
−3dB
f
−3dB
0.01 µF
0.22 µF
0.1 µF
= 1/(2π*R
C
= 1/(2π*20 kΩ*C
b
LOAD
200 ms
420 ms
20 ms
B
T
, can be changed to alter
ON
*C
OUT
IN
O
, the time constant
).
O
).
, is of particular
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B
and the
DD
. As
B
B