bypass pin has reached its half supply voltage, 1/2 VDD. As
soon as the bypass node is stable, the device will become
fully operational.
Application Information
GROUNDING
Although the bypass pin current source cannot be modified,
the size of the bypass capacitor, CB, can be changed to alter
the device turn-on time and the amount of “click and pop”. By
increasing CB, the amount of turn-on pop can be reduced.
However, the trade-off for using a larger bypass capacitor is
an increase in the turn-on time for the device. Reducing CB
will decrease turn-on time and increase “click and pop”. If CB
is too small, the LM4832 can develop a low-frequency oscil-
lation (“motorboat”) when used at high gains.
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.
There is a linear relationship between the size of CB and the
turn-on time. Some typical turn-on times for different values
of CB are:
LAYOUT
As stated in the Grounding section, placement of ground re-
turn lines is critical for maintaining the highest level of sys-
tem 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.
Cb
TON
0.01 µF
0.1 µF
0.22 µF
20 ms
200 ms
420 ms
In order to eliminate “click and pop”, all capacitors must be
discharged before turn-on. Rapid on/off switching of the de-
vice or shutdown function may cause the “click and pop” cir-
cuitry to not operate fully, resulting in increased “click and
pop” noise. The output coupling cap, CO, is of particular con-
cern. This capacitor discharges through an internal 20 kΩ re-
sistor. Depending on the size of CO, the time constant can be
quite large. To reduce transients, an external 1 kΩ–5 kΩ re-
sistor can be placed in parallel with the internal 20 kΩ resis-
tor. The tradeoff for using this resistor is an increase in quies-
cent current.
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.
COUPLING CAPACITORS
Because the LM4832 is a single supply circuit, all audio sig-
nals must be capacitor coupled to the chip to remove the
2.5 VDC bias. All audio inputs have 20 kΩ input impedances,
so the AC-coupling capacitor will create a high-pass filter
with
If power supply bypass capacitors are not sufficiently large,
the current in the power supply leads, which is a rectified ver-
sion of the output current, may be fed back into internal cir-
cuitry. This internal feedback signal can cause high fre-
quency distortion and oscillation.
f−3dB = 1/(2π*20 kΩ*CIN).
If power supply lines to the chip are long, larger bypass ca-
pacitors could be required. Long power supply leads have in-
ductance and resistance associated with them, that could
prevent peak low frequency current demands from being
met. The extra bypass capacitance will reduce the peak cur-
rent requirements from the power supply lines.
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
f−3dB = 1/(2π*RLOAD*COUT).
POWER AMPLIFIER
The power amplifiers in the LM4832 are designed to drive
8Ω or 32Ω loads at 200 mW (continuous) and 75 mW (con-
tinuous), respectively, with 1% THD+N. As shown in the Typi-
cal Performance Characteristics, the power amplifiers typi-
cally 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.
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.
MICROPHONE AMPLIFIER
CLICK AND POP CIRCUITRY
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.
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 inter-
nally 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
13
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