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Application Information
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4882 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown features turns the
amplifier off when a logic high is placed on the shutdown pin.
The trigger point between a logic low and logic high level is
typically half supply. It is best to switch between ground and
supply to provide maximum device performance. By switch-
ing the shutdown pin to the V
DD, the LM4882 supply current
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages less than V
DD, the idle
current may be greater than the typical value of 0.5 A. In
either case, the shutdown pin should be tied to a definite
voltage because leaving the pin floating may result in an
unwanted shutdown condition. In many applications, a mi-
crocontroller or microprocessor output is used to control the
shutdown circuitry which provides a quick smooth transition
into shutdown. Another solution is to use a single-pole,
single-throw switch in conjunction with an external pull-up
resistor. When the switch is closed, the shutdown pin is
connected to ground and enables the amplifier. If the switch
is open, then the external pull-up resistor will disable the
LM4882. This scheme guarantees that the shutdown pin will
not float which will prevent unwanted state changes.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a
successful design. Equation 1 states the maximum power
dissipation point for a single-ended amplifier operating at a
given supply voltage and driving a specified output load.
P
DMAX =(VDD)
2/(2
π2R
L)
(1)
Even with this internal power dissipation, the LM4882 does
not require heat sinking over a large range of ambient tem-
perature. From Equation 1, assuming a 5V power supply and
an 4
load, the maximum power dissipation point is
316 mW. The maximum power dissipation point obtained
must not be greater than the power dissipation that results
from Equation 2:
P
DMAX =(TJMAXT A)/
θ
JA
(2)
For the LM4882 surface mount package,
θ
JA = 210C/W and
T
JMAX = 150C. Depending on the ambient temperature, TA,
of the system surroundings, Equation 2 can be used to find
the maximum internal power dissipation supported by the IC
packaging. If the result of Equation 1 is greater than that of
Equation 2, then either the supply voltage must be de-
creased, the load impedance increased or T
A reduced. For
the typical application of a 5V power supply, with an 4
load,
the maximum ambient temperature possible without violating
the maximum junction temperature is approximately 83C
provided that device operation is around the maximum
power dissipation point. Power dissipation is a function of
output power and thus, if typical operation is not around the
maximum power dissipation point, the ambient temperature
may be increased accordingly. Refer to the Typical Perfor-
mance Characteristics curves for power dissipation infor-
mation for lower output powers.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. The capacitor location on both the bypass and
power supply pins should be as close to the device as
possible. As displayed in the Typical Performance Charac-
teristics section, the effect of a larger half supply bypass
capacitor is improved low frequency PSRR due to increased
half-supply stability. Typical applications employ a 5V regu-
lator with 10 F and a 0.1 F bypass capacitors which aid in
supply stability, but do not eliminate the need for bypassing
the supply nodes of the LM4882. The selection of bypass
capacitors, especially C
B, is thus dependent upon desired
low frequency PSRR, click and pop performance as ex-
plained in the section, Proper Selection of External Com-
ponents section, system cost, and size constraints.
PROPER SELECTION OF EXTERNAL COMPONENTS
Selection of external components when using integrated
power amplifiers is critical to optimize device and system
performance. While the LM4882 is tolerant of external com-
ponent combinations, consideration to component values
must be used to maximize overall system quality.
The LM4882 is unity gain stable and this gives a designer
maximum system flexibility. The LM4882 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configuartions
require large input signals to obtain a given output power.
Input signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the
section, Audio Power Amplifier Design, for a more com-
plete 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 band-
width is dictated by the choice of external components
shown in Figure 1. Both the input coupling capacitor, C
i, and
the output coupling capacitor, C
o, form first order high pass
filters which limit low frequency response. These values
should be chosen based on needed frequency response for
a few distinct reasons.
CLICK AND POP CIRCUITRY
The LM4882 contains circuitry to minimize turn-on and turn-
off 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 amplifi-
ers are internally muted. An internal current source ramps up
the voltage of the bypass pin. Both the inputs and outputs
track the voltage at the bypass pin. The device will remain
muted until the bypass pin has reached its half supply volt-
age, 1/2 V
DD. As soon as the bypass node is stable, the
device will become fully operational, 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 level of “clicks and pops.” By increasing the
value of C
B, the level of turn-on pop can be reduced.
However, the tradeoff for using a larger bypass capacitor is
an increase in turn-on time for the device. There is a linear
relationship between the size of C
B and the turn-on time.
Here are some typical turn-on times for a given C
B:
C
B
T
ON
0.01 F
20 ms
0.1 F
200 ms
0.22 F
420 ms
0.47 F
900 ms
In order to eliminate “clicks and pops,” all capacitors must be
discharged before turn-on. Rapid on/off switching of the
LM4882
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