LM4863
Application Information
(Continued)
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4863 is tolerant to a
variety of external component combinations, consideration
to component values must be used to maximize overall sys-
tem quality.
DS012881-24
FIGURE 2. Headphone Circuit
The LM4863 is unity-gain stable, giving the designer maxi-
mum system performance. The LM4863 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power. In-
put signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the sec-
tion,
Audio Power Amplifier Design,
for a more complete
explanation of proper gain selection.
Besides gain, one of the major considerations is the
closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components
shown in
Figure 1.
The input coupling capacitor, C
i
, forms a
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed fre-
quency response for a few distinct reasons.
CLICK AND POP CIRCUITRY
The LM4863 contains circuitry to minimize turn-on transients
or “clicks and pops”. In this case, turn-on refers to either
power supply turn-on or the device coming out of shutdown
mode. When the device is turning on, the amplifiers are inter-
nally configured as unity gain buffers. An internal current
source ramps up the voltage of the bypass pin. Both the in-
puts and outputs ideally track the voltage at the bypass pin.
The device will remain in buffer mode until the bypass pin
has reached its half supply voltage, 1/2 V
DD
. As soon as the
bypass node is stable, the device will become fully opera-
tional, where the gain is set by the external resistors.
Although the bypass pin current source cannot be modified,
the size of C
B
can be changed to alter the device turn-on
time and the amount of “clicks and pops”. By increasing
amount of turn-on pop can be reduced. However, the
tradeoff for using a larger bypass capacitor is an increase in
turn-on time for this device. There is a linear relationship be-
tween the size of C
B
and the turn-on time. Here are some
typical turn-on times for a given C
B
:
www.national.com
10
C
B
0.01 µF
0.1 µF
0.22 µF
0.47 µF
1.0 µF
T
ON
20 ms
200 ms
420 ms
840 ms
2 Sec
In order eliminate “clicks and pops”, all capacitors must be
discharged before turn-on. Rapid on/off switching of the de-
vice or the shutdown function may cause the “click and pop”
circuitry to not operate fully, resulting in increased “click and
pop” noise. In a single-ended configuration, the output cou-
pling capacitor, C
O
, is of particular concern. This capacitor
discharges through the internal 20 kΩ resistors. Depending
on the size of C
O
, the time constant can be relatively large.
To reduce transients in single-ended mode, 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 in-
crease in quiescent current.
The value of C
I
will also reflect turn-on pops. Clearly, a cer-
tain size for C
I
is needed to couple in low frequencies without
excessive attenuation. But in many cases, the speakers
used in portable systems, whether integral or external, have
little ability to reproduce signals below 100 Hz to 150 Hz. In
this case, using a large input and output capacitor may not
increase system performance. In most cases, choosing a
small value of C
I
in the range of 0.1 µF to 0.33 µF), along
with C
B
equal to 1.0 µF should produce a virtually clickless
and popless turn-on. In cases where C
I
is larger than
0.33 µF, it may be advantageous to increase the value of C
B
.
Again, it should be understood that increasing the value of
C
B
will reduce the “clicks and pops” at the expense of a
longer device turn-on time.
NSC [ NATIONAL SEMICONDUCTOR ]
