lm4835lq National Semiconductor Corporation, lm4835lq Datasheet - Page 16

• Manufacturer Part Number: lm4835lq
• Description: Stereo 2w Audio Power Amplifiers With Dc Volume Control And Selectable Gain
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
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 LM4835 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 LM4835’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 LM4835’s T
to a DAP pad that expands to a copper area of 5in
PCB, the LM4835’s θ
soldered to a DAP pad that expands to a copper area of 2in
on a PCB, the LM4835’s 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 LM4835’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
maximum junction temperature by reducing the power sup-
ply voltage or increasing the load resistance. Further allow-
ance should be made for increased ambient temperatures.
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
JMAX
. If the result violates the LM4835’s 150˚C, reduce the
P
DMAX
JA
. The heat sink can be created using additional
A
= 4
, use Equation (4) to find the maximum inter-
P
T
JMAX
DMAX
T
A
JMAX
*
= T
(V
= 150˚C. In the LQ package soldered
' = (T
JA
JMAX
DD
= P
)
JA
is 20˚C/W. In the MTE package
2
JMAX
DMAX
/ (2π
is 41˚C/W. At any given ambient
– 2*P
2
− T
θ
R
DMAX
JA
L
) Bridge Mode
A
+ T
) / θ
DMAX
θ
A
JA
JA
(Continued)
for P
DMAX
2
on a
' re-
(3)
(4)
(5)
(6)
2
16
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 10µF in parallel with a 0.1µF filter capacitors to stabi-
lize 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
LM4835’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 LM4835’s power supply pin and
ground as short as possible. Connecting a 1µF capacitor,
C
internal bias voltage’s stability and improves the amplifier’s
PSRR. The PSRR improvements increase as the bypass pin
capacitor value increases. Too large, however, increases
turn-on time and can compromise amplifier’s click and pop
performance. The selection of bypass capacitor values, es-
pecially C
and pop performance (as explained in the section, Proper
Selection of External Components), system cost, and size
constraints.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4835’s performance requires properly se-
lecting external components. Though the LM4835 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4835 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 demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio
CODECs have outputs of 1V
the Audio Power Amplifier Design section for more infor-
mation on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (0.33µF in Figure 2). A high value
capacitor can be expensive and may compromise space
efficiency in portable designs. In many cases, however, the
speakers used in portable systems, whether internal or ex-
ternal, have little ability to reproduce signals below 150 Hz.
Applications using speakers with this limited frequency re-
sponse reap little improvement by using large input capaci-
tor.
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
B
, between the BYPASS pin and ground improves the
B
, depends on desired PSRR requirements, click
JA
is the sum of θ
SA
RMS
is the sink-to-ambient thermal
JC
(2.83V
, θ
CS
CS
, and θ
P-P
is the case-to-sink
). Please refer to
SA
. (θ
JC
is the