lm49100gr National Semiconductor Corporation, lm49100gr Datasheet - Page 18

• Manufacturer Part Number: lm49100gr
• Description: Mono Class Ab Audio Subsystem With A True-ground Headphone Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
The LM49100's T
LM49100's θ
ture T
dissipation supported by the IC packaging. Rearranging
Equation (4) and substituting P
Equation (5). This equation gives the maximum ambient tem-
perature that still allows maximum stereo power dissipation
without violating the LM49100's maximum junction tempera-
ture.
For a typical application with a 5V power supply and an 8Ω
load, the maximum ambient temperature that allows maxi-
mum mono power dissipation without exceeding the maxi-
mum junction temperature is approximately 114°C for the GR
package.
Equation (6) gives the maximum junction temperature
T
maximum junction temperature by reducing the power supply
voltage or increasing the load resistance. Further allowance
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 (3) is greater
than that of Equation (4), then decrease the supply voltage,
increase the load impedance, or reduce the ambient temper-
ature. If these measures are insufficient, a heat sink can be
added to reduce θ
ditional copper area around the package, with connections to
the ground pin(s), supply pin and amplifier output pins.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is crit-
ical for low noise performance and high power supply rejec-
tion. Applications that employ a 5V regulator typically use a
JMAX
A
. If the result violates the LM49100's 150°C, reduce the
, use Equation (4) to find the maximum internal power
T
T
JA
JMAX
A
P
is 50.2°C/W. At any given ambient tempera-
DMAX
= T
JA
JMAX
= P
JMAX
. The heat sink can be created using ad-
= (T
DMAX-TOTAL
= 150°C. In the GR package, the
- P
JMAX
DMAX-TOTAL
- T
DMAX-TOTAL
A
θ
) / θ
JA
+ T
JA
θ
JA
A
for P
DMAX
results in
(4)
(5)
(6)
18
1µF in parallel with a 0.1µF filter capacitors to stabilize the
regulator's output, reduce noise on the supply line, and im-
prove the supply's transient response. However, their pres-
ence does not eliminate the need for a local 4.7µF tantalum
bypass capacitor and a parallel 0.1µF ceramic capacitor con-
nected between the LM49100's supply pin and ground. Keep
the length of leads and traces that connect capacitors be-
tween the LM49100's power supply pin and ground as short
as possible.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
pacitor can be expensive and may compromise space effi-
ciency 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
capacitor (C
is found using Equation (7).
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
to the BYPASS pin. Since C
LM49100 settles to quiescent operation, its value is critical
when minimizing turn-on pops. Choosing C
along with a small value of C
produces a click-less and pop-less shutdown function. As dis-
cussed above, choosing C
desired bandwidth helps minimize clicks and pops. C
should be in the range of 4 to 5 times the value of C
ensures that output transients are eliminated when power is
first applied or the LM49100 resumes operation after shut-
down.
IN
) produce a high pass filter cutoff frequency that
f
c
= 1 / (2πR
IN
IN
i
(in the range of 0.1µF to 0.33µF),
no larger than necessary for the
i
), typical 12.5kΩ, and the input
in Figure 1). A high value ca-
i
C
B
IN
determines how fast the
B
)
, the capacitor connected
B
equal to 2.2µF
B
IN
's value
. This
(7)