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Application Information
The maximum power dissipation point given by
Equation 3
must not exceed the power dissipation given
by:
P
DMAX
= (T
JMAX
- T
A
) /
θ
JA
(4)
The LM49100's T
JMAX
= 150°C. In the csBGA package, the LM49100's
θ
JA
is 50.2°C/W. At any given
ambient temperature T
A
, use
Equation 4
to find the maximum internal power dissipation supported by the
IC packaging. Rearranging
Equation 4
and substituting P
DMAX-TOTAL
for P
DMAX
results in
Equation 5
. This
equation gives the maximum ambient temperature that still allows maximum stereo power dissipation
without violating the LM49100's maximum junction temperature.
T
A
= T
JMAX
- P
DMAX-TOTAL
θ
JA
(5)
For a typical application with a 5V power supply and an 8
Ω
load, the maximum ambient temperature that
allows maximum mono power dissipation without exceeding the maximum junction temperature is
approximately 114°C for the csBGA package.
Equation 6
gives the maximum junction temperature T
JMAX
. If the result violates the LM49100's 150°C,
reduce the maximum junction temperature by reducing the power supply voltage or increasing the load
resistance. Further allowance should be made for increased ambient temperatures.
T
JMAX
= P
DMAX-TOTAL
θ
JA
+ T
A
(6)
The previous 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 3
is greater
than that of
Equation 4
, 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
JA
. The heat
sink can be created using additional copper area around the package, with connections to the ground
pin(s), supply pin and amplifier output pins.
15.9 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 1µF in parallel with a 0.1µF filter
capacitors 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 4.7µF tantalum
bypass capacitor and a parallel 0.1µF ceramic capacitor connected between the LM49100's supply pin
and ground. Keep the length of leads and traces that connect capacitors between the LM49100's power
supply pin and ground as short as possible.
15.10 Selecting External Components
15.10.1 Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value input coupling capacitor (C
IN
in
Figure 1
). A
high value capacitor can be expensive and may compromise space efficiency in portable designs. In many
cases, however, the loudspeakers used in portable systems, whether internal or external, have little ability
to reproduce signals below 150Hz. Applications using loudspeakers and headphones with this limited
frequency response reap little improvement by using large input capacitor.
The internal input resistor (R
i
), typical 12.5k
Ω
, and the input capacitor (C
IN
) produce a high pass filter
cutoff frequency that is found using:
f
c
= 1 / (2
π
R
i
C
IN
)
(7)
17
SNAA043A – October 2007 – Revised May 2013
AN-1622 LM49100 Evaluation Board»
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