LM4881M National Semiconductor, LM4881M Datasheet - Page 11

• Manufacturer Part Number: LM4881M
• Description: IC,Audio Amplifier,DUAL,BIPOLAR,SOP,8PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM4881M

• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 2-Channel Stereo
• Output Power (typ): 0.3x2@8OhmW
• Audio Amplifier Function: Headphone
• Input Offset Voltage: 50@5VmV
• Total Harmonic Distortion: 0.025%%
• Single Supply Voltage (typ): 3/5V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 50dB
• Single Supply Voltage (min): 2.7V
• Single Supply Voltage (max): 5.5V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 8
• Package Type: SOIC N
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4881 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature 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
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages less than V
current may be greater than the typical value of 0.7 µA. 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
LM4881. 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.
Since the LM4881 has two operational amplifiers in one
package, the maximum internal power dissipation point is
twice that of the number which results from Equation 1. Even
with the large internal power dissipation, the LM4881 does
not require heat sinking over a large range of ambient tem-
perature. From Equation 1, assuming a 5V power supply and
an 8Ω load, the maximum power dissipation point is 158 mW
per amplifier. Thus the maximum package dissipation point
is 317 mW. The maximum power dissipation point obtained
must not be greater than the power dissipation that results
from Equation 2:
For package MUA08A, θ
M08A, θ
107˚C/W. T
ambient temperature, T
tion 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 decreased, the load impedance
increased or T
power supply, with an 8Ω load, the maximum ambient tem-
perature possible without violating the maximum junction
temperature is approximately 96˚C provided that device op-
eration 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 dissipa-
tion point, the ambient temperature may be increased ac-
cordingly. Refer to the Typical Performance Characteris-
tics curves for power dissipation information for lower output
powers.
JA
JMAX
P
= 170˚C/W, and for package N08E, θ
P
DMAX
A
DMAX
reduced. For the typical application of a 5V
= 150˚C for the LM4881. Depending on the
= (T
= (V
A
JMAX
, of the system surroundings, Equa-
DD
JA
)
2
DD
− T
= 230˚C/W, and for package
/ (2π
, the LM4881 supply current
A
) / θ
2
R
L
JA
)
(1)
(2)
DD
, the idle
JA
=
11
POWER SUPPLY BYPASSING
As with any power amplifer, 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 µF and a 0.1 µF bypass capacitors which aid in
supply stability, but do not eliminate the need for bypassing
the supply nodes of the LM4881. The selection of bypass
capacitors, especially C
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 LM4881 is tolerant of external com-
ponent combinations, consideration to component values
must be used to maximize overall system quality.
The LM4881 is unity gain stable and this gives a designer
maximum system flexibility. The LM4881 should be used in
low gain configurations to minimize THD+N values, and
maximum the signal-to-noise ratio. Low gain configurations
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 dicated by the choice of external components shown
in Figure 1. Both the input coupling capacitor, C
output coupling capacitor, C
filters which limit low frequency response. These values
should be chosen based on needed frequency response for
a few distinct reasons.
Selection of Input and Output Capacitor Size
Large input and output capacitors are both expensive and
space hungry for portable designs. Clearly a certain sized
capacitor is needed to couple in low frequencies without
severe attenuation. But in many cases the speakers used in
portable systems, whether internal or external, have little
ability to reproduce signals below 150 Hz. Thus using large
input and output capacitors may not increase system perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
reach its quiescent DC voltage (nominally 1/2 V
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the
capacitor size based on necessary low frequency response,
turn on pops can be minimized.
Besides minimizing the input and output capacitor sizes,
careful consideration should be paid to the bypass capacitor
value. Bypass capacitor C
minimize turn on pops since it determines how fast the
LM4881 turns on. The slower the LM4881’s outputs ramp to
their quiescent DC voltage (nominally 1/2 V
the turn on pop. Thus choosing C
a small value of C
i
. A larger input coupling capacitor requires more charge to
i
(in the range of 0.1 µF to 0.39 µF), the
B
, is thus dependent upon desired
B
is the most critical component to
o
, form first order high pass
B
equal to 1.0 µF along with
DD
), the smaller
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i
, and the
DD
). This