LM4808MM National Semiconductor, LM4808MM Datasheet - Page 9

• Manufacturer Part Number: LM4808MM
• Description: IC,Audio Amplifier,DUAL,TSSOP,8PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM4808MM

• Dc: 1118
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
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 LM4808 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 LM4808 does
not require heat sinking over a large range of ambient tem-
perature. From Equation 1, assuming a 5V power supply and
a 32
per amplifier. Thus the maximum package dissipation point
is 80 mW. The maximum power dissipation point obtained
must not be greater than the power dissipation that results
from Equation 2:
For package MUA08A,
M08A,
pending on the ambient temperature, T
roundings, Equation 2 can be used to find the maximum in-
ternal 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
tion of a 5V power supply, with a 32
ambient temperature possible without violating the maximum
junction temperature is approximately 131.6˚C 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 Performance Character-
istics curves for power dissipation information for lower out-
put powers.
POWER SUPPLY BYPASSING
As with any power amplifer, proper supply bypassing is criti-
cal for low noise performance and high power supply rejec-
tion. 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 Characteristics sec-
tion, the effect of a larger half supply bypass capacitor is im-
proved low frequency PSRR due to increased half-supply
stability. Typical applications employ a 5V regulator with
10 µF and a 0.1 µF bypass capacitors which aid in supply
stability, but do not eliminate the need for bypassing the sup-
ply nodes of the LM4808. The selection of bypass capaci-
tors, especially C
quency PSRR, click and pop performance as explained in
the section, Proper Selection of External Components
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 LM4808 is tolerant of external com-
ponent combinations, consideration to component values
must be used to maximize overall system quality.
The LM4808 is unity gain stable and this gives a designer
maximum system flexibility. The LM4808 should be used in
load, the maximum power dissipation point is 40 mW
JA
= 170˚C/W. T
P
P
DMAX
DMAX
B
, is thus dependent upon desired low fre-
= (T
= (V
JMAX
DD
JMAX
A
JA
)
reduced. For the typical applica-
2
− T
= 210˚C/W, and for package
= 150˚C for the LM4808. De-
/ (2
A
) /
2
R
L
JA
)
A
load, the maximum
, of the system sur-
(1)
(2)
9
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 band-
width is dicated by the choice of external components shown
in Figure 1 . Both the input coupling capacitor, C
put coupling capacitor, C
which limit low frequency response. These values should be
chosen based on needed frequency response for a few dis-
tinct reasons.
Selection of Input and Output Capacitor Size
Large value input and output capacitors are both expensive
and space consuming for portable designs. Clearly a certain
sized capacitor is needed to couple in low frequencies with-
out 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
performance.
In addition to system cost and size, click and pop perfor-
mance is affected 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 ca-
pacitor 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
LM4808 turns on. The slower the LM4808’s outputs ramp to
their quiescent DC voltage (nominally 1/2 V
the turn on pop. While the device will function properly, (no
oscillations or motorboating), with C
vice will be much more susceptible to turn on clicks and
pops. Thus, a value of C
mended in all but the most cost sensitive designs.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 70mW/32
A designer must first determine the needed supply rail to ob-
tain the specified output power. Calculating the required sup-
ply rail involves knowing two parameters, V
the dropout voltage. The latter is typically 300mV and can be
found from the graphs in the Typical Performance Charac-
teristics. V
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
i
. A larger input coupling capacitor requires more charge to
OPEAK
can be determined from Equation 3.
B
o
B
, form first order high pass filters
equal to 1 µF or larger is recom-
is the most critical component to
Audio Amplifier
100 Hz–20 kHz
B
equal to 1 µF, the de-
DD
OPEAK
1 Vrms (max)
i
), the smaller
, and the out-
±
www.national.com
DD
0.50 dB
and also
70 mW
20 k
). This
32
(3)