lm4839mtx

Manufacturer Part Numberlm4839mtx
DescriptionStereo 2w Audio Power Amplifiers With Dc Volume Control, Bass Boost, And Input Mux
ManufacturerNational Semiconductor Corporation
lm4839mtx datasheet
 


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Application Information
The LM4839 has two operational amplifiers per channel. The
maximum internal power dissipation per channel operating in
the bridge mode is four times that of a single-ended ampli-
fier. From Equation (3), assuming a 5V power supply and a
4
load, the maximum single channel power dissipation is
1.27W or 2.54W for stereo operation.
2
2
P
= 4
*
(V
)
/(2
R
) Bridge Mode
DMAX
DD
L
The LM4839’s power dissipation is twice that given by Equa-
tion (2) or Equation (3) when operating in the single-ended
mode or bridge mode, respectively. Twice the maximum
power dissipation point given by Equation (3) must not ex-
ceed the power dissipation given by Equation (4):
P
' = (T
− T
)/
DMAX
JMAX
A
The LM4839’s T
= 150˚C. In the LQ package soldered
JMAX
to a DAP pad that expands to a copper area of 5in
PCB, the LM4839’s
is 20˚C/W. In the MTE package
JA
soldered to a DAP pad that expands to a copper area of 2in
on a PCB, the LM4839’s
is 41˚C/W. For the LM4839MT
JA
package,
= 80˚C/W. At any given ambient temperature
JA
T
, use Equation (4) to find the maximum internal power
A
dissipation supported by the IC packaging. Rearranging
Equation (4) and substituting P
for P
DMAX
Equation (5). This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipa-
tion without violating the LM4839’s maximum junction tem-
perature.
T
= T
– 2*P
A
JMAX
DMAX
For a typical application with a 5V power supply and an 4
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 99˚C for the LQ
package and 45˚C for the MTE package.
T
= P
+ T
JMAX
DMAX
JA
Equation (6) gives the maximum junction temperature
T
. If the result violates the LM4839’s T
JMAX
duce the maximum junction temperature by reducing the
power supply voltage or increasing the load resistance. Fur-
ther allowance should be made for increased ambient tem-
peratures.
The above examples assume that a device is a surface
mount part operating around the maximum power dissipation
point. Since internal power dissipation is a function of output
power, higher ambient temperatures are allowed as output
power or duty cycle decreases.
If the result of Equation (2) is greater than that of Equation
(3), then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. If these
measures are insufficient, a heat sink can be added to
reduce
. The heat sink can be created using additional
JA
copper area around the package, with connections to the
ground pin(s), supply pin and amplifier output pins. External,
solder attached SMT heatsinks such as the Thermalloy
7106D can also improve power dissipation. When adding a
heat sink, the
(Continued)
junction-to-case thermal impedance,
thermal impedance, and
impedance.) Refer to the Typical Performance Character-
istics curves for power dissipation information at lower out-
put power levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
(3)
rejection. Applications that employ a 5V regulator typically
use a 10 µF in parallel with a 0.1 µF filter capacitor to
stabilize the regulator’s output, reduce noise on the supply
line, and improve the supply’s transient response. However,
their presence does not eliminate the need for a local 1.0 µF
tantalum bypass capacitance connected between the
LM4839’s supply pins and ground. Do not substitute a ce-
ramic capacitor for the tantalum. Doing so may cause oscil-
lation. Keep the length of leads and traces that connect
(4)
JA
capacitors between the LM4839’s power supply pin and
ground as short as possible. Connecting a 1µF capacitor,
C
, between the BYPASS pin and ground improves the
B
2
on a
internal bias voltage’s stability and improves the amplifier’s
PSRR. The PSRR improvements increase as the bypass pin
2
capacitor value increases. Too large a capacitor, however,
increases turn-on time and can compromise the amplifier’s
click and pop performance. The selection of bypass capaci-
tor values, especially C
ments, click and pop performance (as explained in the sec-
' results in
tion, Proper Selection of External Components), system
DMAX
cost, and size constraints.
PROPER SELECTION OF EXTERNAL COMPONENTS
Optimizing the LM4839’s performance requires properly se-
lecting external components. Though the LM4839 operates
(5)
well when using external components with wide tolerances,
JA
best performance is achieved by optimizing component val-
ues.
The LM4839 is unity-gain stable, giving a designer maximum
design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain circuits demand input
(6)
signals with greater voltage swings to achieve maximum
A
output power. Fortunately, many signal sources such as
audio CODECs have outputs of 1V
refer to the Audio Power Amplifier Design section for more
150˚C, re-
JMAX
information on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor (0.33µF in Figure 1 ). A high
value capacitor can be expensive and may compromise
space efficiency in portable designs. In many cases, how-
ever, the speakers used in portable systems, whether inter-
nal or external, have little ability to reproduce signals below
150Hz. Applications using speakers with this limited fre-
quency response reap little improvement by using a large
input capacitor.
Besides effecting system cost and size, the input coupling
capacitor has an affect on the LM4835’s click and pop per-
formance. When the supply voltage is first applied, a tran-
sient (pop) is created as the charge on the input capacitor
changes from zero to a quiescent state. The magnitude of
the pop is directly proportional to the input capacitor’s size.
15
is the sum of
,
, and
. (
JA
JC
CS
SA
JC
is the case-to-sink
CS
is the sink-to-ambient thermal
SA
, depends on desired PSRR require-
B
(2.83V
). Please
RMS
P-P
www.national.com
is the