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參數(shù)資料
| 型號： | LM4809MA |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | Dual 105mW Headphone Amplifier with Active-Low Shutdown Mode |
| 中文描述： | 0.105 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO8 |
| 封裝： | SO-8 |
| 文件頁數(shù)： | 12/18頁 |
| 文件大?。?/td> | 903K |
| 代理商： | LM4809MA |
Application Information
(Continued)
CODECs have outputs of 1V
(2.83V
P-P
). Please refer to
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
and C
in
Figure 1
).
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
has an effect on
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
and the input
capacitor, C
I
, produce a 3dB high pass filter cutoff fre-
quency that is found using Equation (3). In addition, the
output load R
L
, and the output capacitor C
O
, produce a -3db
high pass filter cutoff frequency defined by Equation (4).
f
I-3db
=1/2
π
R
I
C
I
(3)
f
O-3db
=1/2
π
R
L
C
O
(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
, the capacitor
connected to the BYPASS pin. Since C
determines how
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
), the smaller the turn-on pop. Choosing C
B
equal to 4.7μF along with a small value of C
(in the range of
0.1μF to 0.47μF), produces a click-less and pop-less shut-
down function. As discussed above, choosing C
i
no larger
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
.As soon as the voltage on
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
and C
. Ideally, the input and output track
the voltage applied to the BYPASS pin. During turn-on, the
buffer-configured amplifier output charges the input capaci-
tor, C
I
, through the input resistor, R
I
. This input resistor
delays the charging time of C
I
thereby causing the voltage
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
DD
) when
charged with a fixed current. Decreasing the value of C
I
and
R
I
will minimize turn-on pops at the expense of the desired
-3dB frequency.
Although the BYPASS pin current cannot be modified,
changing the size of C
alters the device’s turn-on time and
the magnitude of “clicks and pops”. Increasing the value of
C
reduces the magnitude of turn-on pops. However, this
presents a tradeoff: as the size of C
increases, the turn-on
time increases. There is a linear relationship between the
size of C
and the turn-on time. Here are some typical
turn-on times for various values of C
B
:
C
B
0.1μF
0.22μF
0.33μF
0.47μF
0.68μF
1.0μF
2.2μF
3.3μF
4.7μF
10μF
T
ON
80ms
170ms
270ms
370ms
490ms
920ms
1.8sec
2.8sec
3.4sec
7.7sec
In order eliminate “clicks and pops”, all capacitors must be
discharged before turn-on. Rapidly switching V
DD
may not
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
O
. This capacitor usually has a
high value. C
O
discharges through internal 20k
resistors.
Depending on the size of C
O
, the discharge time constant
can be relatively large. To reduce transients in single-ended
mode, an external 1k
–5k
resistor can be placed in par-
allel with the internal 20k
resistor. The tradeoff for using
this resistor is increased quiescent current.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 70mW/32
Audio Amplifier
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
70 mW
32
1 Vrms (max)
20k
100 Hz–20 kHz
±
0.50dB
L
www.national.com
12
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