LM4809MA National Semiconductor, LM4809MA Datasheet - Page 12

• Manufacturer Part Number: LM4809MA
• Description: Dual 105mW Headphone Amplifier with Active-Low Shutdown Mode
• Manufacturer: National Semiconductor

www.national.com
Application Information
CODECs have outputs of 1V
the Audio Power Amplifier Design section for more infor-
mation on selecting the proper gain.
Input and Output Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input and output coupling capacitors (C
A high value capacitor can be expensive and may compro-
mise space efficiency in portable designs. In many cases,
however, the speakers used in portable systems, whether
internal or external, have little ability to reproduce signals
below 150Hz. Applications using speakers with this limited
frequency response reap little improvement by using high
value input and output capacitors.
Besides affecting system cost and size, C
the LM4809’s click and pop performance. The magnitude of
the pop is directly proportional to the input capacitor’s size.
Thus, pops can be minimized by selecting an input capacitor
value that is no higher than necessary to meet the desired
−3dB frequency. Please refer to the Optimizing Click and
Pop Reduction Performance section for a more detailed
discussion on click and pop performance.
As shown in Figure 1, the input resistor, R
capacitor, C
quency that is found using Equation (3). In addition, the
output load R
high pass filter cutoff frequency defined by Equation (4).
Also, careful consideration must be taken in selecting a
certain type of capacitor to be used in the system. Different
types of capacitors (tantalum, electrolytic, ceramic) have
unique performance characteristics and may affect overall
system performance.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the value of C
connected to the BYPASS pin. Since C
fast the LM4809 settles to quiescent operation, its value is
critical when minimizing turn-on pops. The slower the
LM4809’s outputs ramp to their quiescent DC voltage (nomi-
nally 1/2 V
equal to 4.7µF along with a small value of C
0.1µF to 0.47µF), produces a click-less and pop-less shut-
down function. As discussed above, choosing C
than necessary for the desired bandwith helps minimize
clicks and pops.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4809 contains circuitry that minimizes turn-on and
shutdown transients or “clicks and pop”. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. During turn-on, the
LM4809’s internal amplifiers are configured as unity gain
buffers. An internal current source charges up the capacitor
on the BYPASS pin in a controlled, linear manner. The gain
of the internal amplifiers remains unity until the voltage on
the BYPASS pin reaches 1/2 V
DD
I
, produce a −3dB high pass filter cutoff fre-
L
), the smaller the turn-on pop. Choosing C
, and the output capacitor C
f
O-3db
f
I-3db
=1/2πR
=1/2πR
RMS
DD
. As soon as the voltage on
(2.83V
L
I
C
C
I
O
I
and C
P-P
B
O
(Continued)
i
). Please refer to
has an effect on
, produce a -3db
B
determines how
i
I
, the capacitor
(in the range of
O
and the input
in Figure 1).
i
no larger
(3)
(4)
B
12
the BYPASS pin is stable, the device becomes fully opera-
tional. During device turn-on, a transient (pop) is created
from a voltage difference between the input and output of the
amplifier as the voltage on the BYPASS pin reaches 1/2 V
For this discussion, the input of the amplifier refers to the
node between R
the voltage applied to the BYPASS pin. During turn-on, the
buffer-configured amplifier output charges the input capaci-
tor, C
delays the charging time of C
difference between the input and output that results in a
transient (pop). Higher value capacitors need more time to
reach a quiescent DC voltage (usually 1/2 V
charged with a fixed current. Decreasing the value of C
R
-3dB frequency.
Although the BYPASS pin current cannot be modified,
changing the size of C
the magnitude of “clicks and pops”. Increasing the value of
C
presents a tradeoff: as the size of C
time increases. There is a linear relationship between the
size of C
turn-on times for various values of C
In order eliminate “clicks and pops”, all capacitors must be
discharged before turn-on. Rapidly switching V
allow the capacitors to fully discharge, which may cause
“clicks and pops”. In a single-ended configuration, the output
is coupled to the load by C
high value. C
Depending on the size of C
can be relatively large. To reduce transients in single-ended
mode, an external 1kΩ–5kΩ resistor can be placed in par-
allel with the internal 20kΩ resistor. The tradeoff for using
this resistor is increased quiescent current.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 70mW/32Ω Audio Amplifier
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
I
B
will minimize turn-on pops at the expense of the desired
reduces the magnitude of turn-on pops. However, this
I
, through the input resistor, R
B
and the turn-on time. Here are some typical
O
0.22µF
0.33µF
0.47µF
0.68µF
0.1µF
1.0µF
2.2µF
3.3µF
4.7µF
10µF
discharges through internal 20kΩ resistors.
I
C
and C
B
B
I
alters the device’s turn-on time and
. Ideally, the input and output track
O
O
. This capacitor usually has a
, the discharge time constant
I
thereby causing the voltage
100 Hz–20 kHz
B
B
increases, the turn-on
:
170ms
270ms
370ms
490ms
920ms
1.8sec
2.8sec
3.4sec
7.7sec
I
80ms
. This input resistor
T
ON
1 Vrms (max)
DD
±
DD
may not
70 mW
0.50dB
) when
20kΩ
32Ω
I
and
DD
.