lm4838mtx National Semiconductor Corporation, lm4838mtx Datasheet - Page 17

• Manufacturer Part Number: lm4838mtx
• Description: Stereo 2w Audio Power Amplifiers With Dc Volume Control And Selectable Gain
• Manufacturer: National Semiconductor Corporation

Application Information
amplifier’s half-supply bias voltage across the load. This
increases internal IC power dissipation and may perma-
nently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. Equation (2)
states the maximum power dissipation point for a single-
ended amplifier operating at a given supply voltage and
driving a specified output load.
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions.
The LM4838 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
4Ω load, the maximum single channel power dissipation is
1.27W or 2.54W for stereo operation.
The LM4838’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):
The LM4838’s T
to a DAP pad that expands to a copper area of 5in
PCB, the LM4838’s θ
soldered to a DAP pad that expands to a copper area of 2in
on a PCB, the LM4838MTE’s θ
LM4838MT package, θ
temperature T
nal power dissipation supported by the IC packaging. Rear-
ranging Equation (4) and substituting P
sults in Equation (5). This equation gives the maximum
ambient temperature that still allows maximum stereo power
dissipation without violating the LM4838’s maximum junction
temperature.
For a typical application with a 5V power supply and an 4Ω
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.
Equation (6) gives the maximum junction temperature
T
reduce the maximum junction temperature by reducing the
JMAX
. If the result violates the LM4838’s 150˚C T
P
P
DMAX
DMAX
A
, use Equation (4) to find the maximum inter-
= 4 * (V
JMAX
T
P
= (V
T
A
DMAX
JMAX
= T
DD
= 150˚C. In the LQ package soldered
JA
' = (T
JMAX
JA
DD
)
= P
2
/(2π
is 20˚C/W. In the MTE package
)
= 80˚C/W. At any given ambient
2
DMAX
/(2π
JMAX
– 2*P
2
R
2
L
R
) Single-Ended
θ
− T
DMAX
L
JA
) Bridge Mode
JA
A
+ T
)/θ
is 41˚C/W. For the
DMAX
θ
JA
A
JA
(Continued)
for P
DMAX
2
JMAX
on a
' re-
(2)
(3)
(4)
(5)
(6)
2
,
17
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 θ
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 θ
junction-to-case thermal impedance, θ
thermal impedance, and θ
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
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 1.0µF
tantalum bypass capacitance connected between the
LM4838’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
capacitors between the LM4838’s power supply pin and
ground as short as possible. Connecting a 1µF capacitor,
C
internal bias voltage’s stability and the amplifier’s PSRR. The
PSRR improvements increase as the BYPASS pin 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 capacitor values,
especially C
click and pop performance (as explained in the following
section, Selecting Proper External Components), system
cost, and size constraints.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4838’s performance requires properly se-
lecting external components. Though the LM4838 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4838 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
signals with greater voltage swings to achieve maximum
output power. Fortunately, many signal sources such as
audio CODECs have outputs of 1V
refer to the Audio Power Amplifier Design section for more
information on selecting the proper gain.
B
, between the BYPASS pin and ground improves the
JA
. The heat sink can be created using additional
B
, depends on desired PSRR requirements,
JA
is the sum of θ
SA
is the sink-to-ambient thermal
JC
, θ
RMS
CS
CS
, and θ
(2.83V
is the case-to-sink
SA
P-P
www.national.com
. (θ
). Please
JC
is the